TextBook Perspective+ 3rd+Ed

Data Communications

A Business Perspective

Third Edition

Derek Smith Adrie Stander

© 2001 by A Stander & D Smith. Published by Dept of Information Systems University of Cape Town Rondebosch 7701

All rights reserved. No part of this book may be reproduced, in any form or by any means, without permission in writing from the publisher.

Contents ChaptersError! Bookmark not defined.

ExercisesError! Bookmark not defined.

AcknowledgementsError! Bookmark not defined.

1: Introduction 7 Data Communication .......... 7

Communications Media......8

Networks ..............................9

Types of Networks........................ 10

Protocols and Standards ... 12

OSI Model .......................... 12

OSI Layers.....................................12

Communication Between Stacks ..14

Internet Model ..............................15

Applying The OSI Model... 16

Summary.............................17


2: Data Transmission 19 Character Encoding........... 19

Transmission Methods......20

RS232 ................................. 21

Analogue Signals ...............22

Amplitude..................................... 23

Frequency..................................... 23

Phase ............................................ 24

Digital Signals....................26

Modems..............................26

Bits Per Second ............................ 27

Bandwidth.................................... 27

Modulation methods......... 27

Amplitude Modulation ................ 27

Frequency Modulation................. 28

Phase Modulation ........................ 29

Transmission Methods......30

Protocols ...................................... 30

Asynchronous Transmission ........31

Synchronous Transmission..........32

Half / Full Duplex ........................33

Errors................................. 34

Error Prevention ............... 34

Vertical Redundancy .................... 35

Longitudinal Redundancy............ 35

Cyclical Redundancy Checking .... 35

Error Correction................ 36

Summary ........................... 36


3: Data Transmission Media 39

Guided Media: .....................39

Unguided Media: ...............39

Coaxial Cable ..................... 40

Twisted Pair Cable ............ 42

Unshielded Twisted Pair Cable ....42

Shielded Twisted Pair (STP)

Cable .............................................43

Fibre Optic Cable .............. 44

Infrared Systems............... 45

Radio Systems ................... 46

Microwave Systems........... 46

Satellite ...............................47

Circuit Switched Mobile ... 48

GPRS.................................. 48

EDGE ................................. 49

UMTS................................. 49

Summary ........................... 50

Exercises ............................ 50

4: Network Hardware 51 Servers ................................51

Network Computers...........51

Storage Devices ................. 52

Network Interface Cards .. 52

Hubs....................................53

Types of Hubs.................... 54

Passive Hubs ................................54

Active Hubs ..................................54

Intelligent Hubs ........................... 55

Repeaters ........................... 55

Bridges ...............................56

Switches ............................. 57

Routers ...............................58

BRouters ............................59

Gateways ............................60

Modems.............................. 61

Multiplexers....................... 61

Summary............................64

Exercises ............................64

5: Network Structures and Architectures 65

Bus Network Structure......65

Star Network Structure .....66

Ring Network Structure .... 67

Token Ring ................................... 68

Switched Networks............69

Complex Topologies ..........70

Wide Area Networks ..........71

Centralised Networks........ 72

Distributed Networks........ 72

Access Methodologies ....... 73

CSMA / CD................................... 73

Token Passing .............................. 74

Architectures...................... 74

Ethernet (IEEE 802.3)................. 74

Token Ring (IEEE 802.5) ............75

FDDI..............................................75

High Speed Architectures . 76

100 Base T.................................... 76

100VG AnyLANError! Bookmark not defined.

Summary............................ 77

Exercises ............................ 77

6: Wide Area Networks 79 WAN Switching ................. 79

WAN Transmission ...........80

PTSN ............................................80

ISDN..............................................81

ADSL ..................................82

Dedicated (Leased) Lines . 82

WAN Services.................... 83

X-25 ..............................................83

Frame Relay..................................84

Cell-Relay - ATM..........................84

Summary ........................... 85

Exercises ............................ 85

7: Internet 87 Internet Protocol (IP) ....... 87

Internet Service Providers 88

PPP and SLIP Connections ..........89

Internet Domain Names ..............90

Linking Up......................... 92

E-Mail ................................ 92

Message Formatting.....................93

Message Transfer .........................94

FTP..................................... 96

World Wide Web............... 96

Summary ............................97

Exercises .............................97

8: Network Security 99 Logical Security ................. 99

Passwords.....................................99

Encryption..................................100

Access Rights.............................. 101

Screen Savers.............................. 102

Time Scheduling......................... 102

Intruder Detection ..................... 102

Firewalls ..................................... 102

Backups & Disaster Recovery..... 103

Physical Security ..............103

Restricted Access Areas.............. 103

Active Measures ......................... 104

Passive Measures........................ 104

Minimum Policy Documentation104

Software Piracy ................105

Viruses ..............................105

Disaster Recovery Manual106

Security Policy Document106

Hacking............................ 106

Dictionary Attacks ..................... 106

Sniffers & Spikes .........................107

Telnet Software ...........................107

Renaming of Admin Accounts ....107

Modems.......................................107

Summary..........................108

Exercises ..........................108

9: Networks and e-Commerce 109

Data Interchange............. 109

XML............................................ 109

Value Added Networks.....110

Virtual Private Networks . 111

Payment Methods............. 111

SSL .............................................. 112

SET .............................................. 112

Small Payments........................... 113

Summary...........................114

Exercises ...........................114

10: Network Management115 Problem Analysis..............115

Conceptual and Physical

Design................................ 117

Selection of Hardware and

Software............................ 119

Building a Request for

Proposal (RFP).................120

Managing the Network .... 121

Selection of Staff ..............125

Operations Control ..........125

Network Tools ..................126

Security and Disaster

Recovery ...........................126

Network Trends ...............127

Hardware Trends ....................... 128

Software...................................... 128

Office Automation ...................... 128

Public Networks ......................... 129

The Electronic Cottage ............... 129

Summary ..........................130

Exercises ...........................130

Glossary 131

Websites 137 General ....................................... 137

Wireless ...................................... 137

Index 138

Chapter 1: Introduction 7

1: Introduction

From the earliest days, communication has formed an important part

of everyday life. As man scattered all over the world, face-to-face

communication was not enough and many innovative techniques

developed to keep in touch and exchange information.

Communication systems evolved from the beating of drums and

smoke signals to the sophistication of the systems we know today.

To communicate successfully with each other the communication

process must adhere to a strict set of rules. We have to speak the same

language for someone else to be able to understand us.

This book is about data communication technology and how we can

use it to develop successful communication infrastructures for

business.

Data Communication To share information we have to communicate. This communication

can be with someone close to us or with someone far away. Fig. 1-1

shows, just like in human communication, data communication needs

a sender or transmitter, a medium to convey the message and a

receiver.

Fig. 1-1 The Communication Process

Medium

Transmitter Receiver

8 Data Communications

Similarly a form of language that can be understood by both parties is

needed and if both parties cannot understand it, some form of

translator is needed. Just as a normal language has an agreed set of

rules, the language used by electronic equipment must also have an

agreed set of rules, otherwise communication will be impossible.

In data communication, bits in the form of 0's and 1's carry the

message. The bits are carried along some form of transmission

medium in the form of electrical or other signals that can be switched

on or off. Normally the off state will represent 0 while the on state will

represent 1.

The distance that needs to be covered determines the type of medium

that can be used. Not all media can deliver a quality signal over a long

distance. By a quality signal we mean a signal that is delivered to the

correct receiver, is unchanged from the original and is delivered in a

timely manner. Significant delays in transmission render the data

useless for use in video, audio and voice data systems such as

television broadcasts and telephone systems.

Communications Media Communications equipment can be connected in many different ways.

The communications media are the physical connections that tie

everything together. The characteristics of the media determine what

type of network the medium can be used for. Copper cables are

commonly used for older types of LAN's while fibre optic cabling is

used primarily to connect computers that require high-speed access to

networks on different floors and in different buildings. Where cabling

is difficult or expensive to use, wireless technologies can be used.

Many different types of media are found in the various parts of larger

networks.

The choice of transmission media is influenced by many factors. Cost

is an obvious factor while the ability to expand the network is also

important, as some types of media such as fibre optic cable are

difficult to alter after installation.

The amount of data that the medium can transfer could slow down a

network if it is not high enough. It is usually measured in kilobits or

megabits per second (Kbps or Mbps) and is also known as the

bandwidth of the medium.


Networks A network is a group of connected computers that allows people to

share information and equipment such as printers. A network can be

any size. A small network can have as few as two computers for

sharing files while a network for the exchange of information can

connect millions of computers all over the world.

Most organisations use permanent networks to transfer information.

These networks use cables to link the computers together. The

computers and cables stay connected and in place at all times. It is

also possible to create a temporary network for a brief time, such as

when an employee connects a computer at home to a computer at

work using a modem.

Any form of digital information can be shared over a network The

information can be in the form of word-processing documents,

database information, graphics or any other machine-readable data.

Computers connected to a network can also share devices, sometimes

called resources. The ability to share resources reduces the cost of

buying computer hardware, as it is not necessary to buy, for example,

a printer for each person on the network. Networks also allow users to

share a single copy of a software package stored on a central computer

from where it can be accessed by other computers on the network.

Networks enable people to communicate and exchange messages

easily and efficiently. This is very useful for people working on the

same project. Smaller computers, linked as networks, are often used to

replace large mainframe computers, as they are less expensive and

often easier to use.

Networks offer many benefits that can help streamline operations and

reduce costs for organisations. A network allows computers to share

resources like printers and hard drives, reducing the cost of buying

hardware. Since people can share information and communicate

with each other using the network, they can work more productively.

It is also possible for users to work from home by connecting to the

company's network by using a modem.

The central installation of application software on a server is more cost

effective than installing a copy of the software on each client machine.

As only the information on the server needs to be copied and saved,


the process is more reliable and easier than backing up each user's

data individually.

Security of information is also easier to control as most networks have

built in security programs that control access to the data. As only a few

computers are used to store most of the important data in an

organisation, it is easier to protect these machines by keeping them in

a secure, protected area such as a locked fireproof room.

Types of Networks As each business or organisation has its own unique needs, many

different types of networks are used. The size of the organisation often

determines the type of network to be used. Large networks are more

costly to build and also require specialised cabling and computers to

link users and devices that are separated by long distances.

Local Area Networks

A local area network (LAN), as shown in Fig 1-2, is the most common

type of network found in business. This type of network is used to

connect computers and devices in the same building or other relatively

small areas.

Fig. 1-2 Local Area Network (LAN)

User 1Printer

User 3

Data Storage

User 2

Printer

User 4


Metropolitan Area Networks

Metropolitan area networks (MANs), which are rare and contain

elements of both Local Area Networks and Wide Area Networks, as

shown in Fig. 1-3, connect computers located in the same geographical

area such as a city or town.

Fig.1-3 Metropolitan Area Network

Wide Area Networks

A wide area network (WAN), as shown in Fig. 1-4, connects local and

metropolitan area networks together nationally or internationally.

New York Network

London

Network

Communications Equipment

Mainframes in Sydney

Fig. 1-4 Wide Area Network

Public city network


These networks may be located throughout a country or even around

the world. A wide area network owned and controlled by a single

company is often referred to as an enterprise network.

Protocols and Standards Different information technology companies often manufacture the

hardware and software products needed to build a network. For a

network to function properly, an industry standard, to which all

manufacturers must adhere, is required. The sets of rules used by

computers to communicate with each other are known as protocols.

Just as we cannot understand each other when we speak in different

languages, computers need to speak the same language to be able to

understand each other. This language used by computers is

determined by the protocol used. Devices on a network must use the

same set of rules or protocol to communicate successfully.

OSI Model The International Standards Organisation (ISO) developed the

standard, the Open Systems Interconnect (OSI) model. The ISO is an

organisation that develops standards for the computer industry.

When manufacturers follow the OSI model, they ensure that their

products will communicate properly with other network devices.

OSI Layers The OSI model consists of seven layers (Fig 1-5), each of which is

responsible for a particular aspect of the communication process.

The application layer specifies the rules for communication between

application programs and other services running on the network such

as print or database services. It supports the activities appropriate for

the application, e.g. a command sequence to retrieve information from

a database or instructions to send an e-mail message.


Fig. 1- 5 The seven layers of the OSI model

The presentation layer formats information into a form that can be

read by software applications and ensures that the information is

formatted correctly for processing and display at the receiving end.

The session layer establishes connections (sessions) between two

computers and also monitors the process. It determines how the two

devices communicate and regulates the flow of messages between

them.

The transport layer is responsible for the end-to-end control of the

exchange of messages between two nodes and the correction of

transmission errors to ensure that information is delivered reliably.

The network layer identifies computers on a network and directs the

transfer of data over a network.

The data link layer groups the data into sets (packets) to prepare it for

transfer over the network.

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

DATA LINK

PHYSICAL


The physical layer determines how the computer connects to the

transmission media, such as a cable. It also specifies how electrical

signals transfer information on the medium.

Communication Between Stacks

Fig 1-6 Communication between the OSI stacks

For two computers to communicate on a network, they must both use

the same theoretical model, such as the OSI model. When a frame is

constructed at the sending node, it starts with the application layer at

the top of the stack of layers.

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

DATA LINK

PHYSICAL

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

DATA LINK

PHYSICAL


The frame information is sent to the presentation layer and continues

down the stack to the physical layer, where it is sent to the network as

a complete packet-carrying signal. (Fig 1-6).

The receiving node receives frames at the physical layer and then

sends each frame to be checked by the data link layer, which

determines if the frame is addressed to the workstation.

When the data link layer finds a frame addressed to that workstation,

it sends the frame to the network layer, which strips out information

intended for it and then sends the remaining information up the stack.

Each layer in the stack acts as a separate module performing one

primary type of function and has its own format of communication

instructions (protocol).

Information from one layer is transferred to the next by means of

commands called primitives. The information that is transferred is

called a protocol data unit (PDU). New control information is added as

the information progresses to the next layer.

Once the next layer receives the PDU, the control information and

transfer instructions are removed with a service data unit (SDU) as the

resulting packet. As the SDU travels from one layer to the next, each

layer adds its own control information.

Internet Model The Internet model is also known as the TCP/IP protocol suite, or

TCP/IP model. This communications architecture takes its name from

TCP/IP (Transmission control protocol/Internet protocol), the

standard protocols for open systems internetworking.

Similar to the OSI model, the TCP/IP model is a layered

communications architecture in which upper layers use the

functionality offered by the protocols of the lower layers. Each layer’s

protocols are able to operate independently of the protocols of other

layers. For example, protocols on a given layer can be updated or

modified without having to change protocols in all other layers, as was

done during the development of a new version of IP to solve the

problem of a pending shortage of IP addresses. This change is possible

without the need to change the other protocols in the TCP/IP model.


Figure 1-7 compares the seven-layer OSI model with the four-layer

TCP/IP model. In the case of the latter, the full functionality is divided

into four layers rather than seven.

A P P L I C A T I O N

P R E S E N T A T I O N

S E S S I O N

T R A N S P O R T

N E T W O R K

D A T A L I N K

P H Y S I C A L

A P P L I C A T I O N

T R A N S P O R T

N E T W O R K

I N T E R F A C E

O S I I N T E R N E T

Fig 1-7 The OSI and Internet models

Applying The OSI Model Accessing a drive on another computer via the network is an example

of how layered communications work.

The redirector at the application layer locates the shared drive. The

presentation layer ensures that the data format is the same for both

computers. The session layer establishes the link between the two

computers and ensures the link is not interrupted. The transport layer

guards against packet errors, ensuring that the data is interpreted in

the same order as it was sent.

The network layer makes sure packets are sent along the fastest route.

The data link layer creates packets and ensures that packets go to the


right workstations. Finally, the physical layer makes the data

transmissions possible by converting the information to electrical

signals that are placed on the communications medium.

Summary Data communication and networks offer many benefits that can help

to streamline operations and reduce costs for organisations. A network

allows computers to share resources like printers and hard drives

reducing the cost of buying hardware. Since people can share

information and communicate with each other via the network, they

can work more productively. It is also possible for users to work from

home by connecting to the company’s network.

The central installation of application software on a server is more cost

effective than installing a copy of the program on each client machine.

As only the information on the server needs backing up, the process is

more reliable and easier than backing up each user’s data individually.

Security of the information is also easier to control as most networks

have built in security programs that control access to the data. As only

a few computers are used to store most of the important data in an

organisation, it is easy to protect these machines by keeping them in a

secure, protected area.

The key benefit most often derived from the implementation of

networks is the flexibility of information delivery due to their

distributed nature. Local area networks make up a crucial part of the

overall enterprise network infrastructure that serves as the backbone

linking the many distributed components of information systems. As

companies transition their information systems from mainframe-

based to distributed systems, opportunities exist to reengineer

applications and the business processes they support. In order to

analyse and design effective information systems, it is essential to

understand overall logical and physical architectures that model these

systems.


Chapter 2: Data Transmission

19

2: Data

Transmission To study the field of data communications, it is important that we

understand the underlying concepts that will be used throughout this

book. In this chapter we will concentrate on the more general aspects

of data transmission, thereby building a foundation for the

understanding of future concepts.

Character Encoding Data is stored in a digital computer as a sequence of binary digits

(bits). Each bit can have a value of zero or one and to create a data

character a certain number of bits need to be grouped together. The

number of bits relates to the specific encoding scheme or translation

table that is being used.

The most common methods used today include ASCII and EBCDIC.

ASCII stands for American Standard Code for Information

Interchange. This is normally implemented as a 7-bit code and there

are many versions of the code although most are very similar. Slight

changes for different countries (for example, the British pound sign

versus the US dollar) are incorporated in the variations.

Apart from alphanumeric and special characters (for example !,@,#),

there are also other special characters (for example, NUL) which are

used in data communication and printer or display control. An eighth

bit known as a parity bit, is added to 7-bit ASCII for error detection.

Error detection and parity checking will be described later in this

chapter.

EBCDIC stands for Extended Binary Coded Decimal Interchange

Code. EBCDIC uses an 8-bit code to form a character and thus twice

as many different characters can be represented compared with a 7-bit

ASCII code. An extended ASCII code, based on 8-bits is available to

handle the increased requirements of text and graphics. Figure 2-1

illustrates the use of the ASCII and EBCDIC codes.


Readable

Form

ASCII EBCDIC

A 1000001 11000001

5 0110101 11110101

LF (line feed) 0001010 00100101

FIG 2-1 ASCII and EBCDIC coding

Transmission Methods The bits that represent the humanly readable characters can be

transmitted using one of two basic methods. They can be transmitted

one after the other (serial transmission) or simultaneously (parallel

transmission). Figure 2-2 illustrates how bits are transmitted, one bit

at a time, along a single conductor.

Fig 2-2 Serial Transmission

Serial transmission is slower than parallel transmission, but can be

used for transmission over longer distances. Typical applications of

serial transmission are to connect two computers or a computer to a

modem. It can also be used to connect a slow printer to a computer.

1 0 1 0 0 10 1


21

Fig 2-3 Parallel Transmission

Parallel transmission, where the bits are transmitted using a number

of conductors at the same time as shown in Figure 2-3, is used for

faster transmission over short distances. It is typically used for

transmission between components within a computer and to connect

high-speed printers to a computer.

RS232 With both parallel and serial communications it is important to

distinguish between the standards used for the physical connectors

and the standards used to describe the electrical characteristics or

protocols of the transmission methods.

The presence of an electrical charge on the pins of the connectors used

for serial data communications has a specific meaning. These standard

definitions are known as the RS-232 standard. As character encoding

needs characters to be represented by 1s and 0s, the role of the

transmission protocol is to present these 1s and 0s as electrical signals.

RS-232 defines a voltage of +5 to +15 Volt dc on a pin as a 0 while a

voltage of -5 to –15 represent a 1.

1

0

1

1

1

0

0

0


Fig 2-4 DB-25 Connector

In the case of the DB-25 connector all 25 pins are normally defined,

but the majority of serial transmission applications uses only 10 or

fewer pins. On many computers the DB-25 connectors are replaced

with DB-9 connectors that have only 9 pins.

Fig 2-5 DB-9 Connector

Cables fitted with the right type of connectors are required to link the

correct pin on the sender’s connector to the correct pin on the

receiver’s connector.

Analogue Signals Broadcast and phone transmission have conventionally used analogue

technology. Analogue means any fluctuating, evolving, or continually

changing process and is usually represented as a series of sine waves.

The term originated because the modulation of the carrier wave is

analogous to the fluctuations of the voice itself. A modem is used to

convert the digital information in your computer to analog signals for

your phone line and to convert analogue phone signals to digital

information for your computer

A waveform is a representation of how alternating current (AC) varies

with time. The most familiar AC waveform is the sine wave, which

derives its name from the fact that the current or voltage varies with

the sine of the elapsed time. Other common AC waveforms are the


23

square wave, the ramp, the sawtooth wave, and the triangular wave.

Fig 2-6 shows a typical sine wave.

Fig 2-6. Sine wave

The sine wave is unique in that it represents energy entirely

concentrated at a single frequency. An ideal signal has a sine

waveform, with a frequency usually measured in megahertz (MHz) or

gigahertz (GHz). Household utility current has a sine waveform with a

frequency of 50 Hz in South Africa, although in some countries it is 60

Hz

The three main characteristics of analogue signals are:

• Amplitude (volume)

• Frequency (pitch)

• Phase

Amplitude This is the strength of the signal. It can be expressed a number of

different ways (as volts, decibels). The higher the amplitude, the

stronger (louder) the signal. The decibel (named in honour of

Alexander Graham Bell) is a popular measure of signal strength.

Frequency For an oscillating or varying current, frequency is the number of

complete cycles per second in alternating current direction. A cycle is

one complete movement of the wave, from its original start position

and back to the same point again. The standard unit of frequency is

the hertz, abbreviated Hz. If a current completes one cycle per second,

then the frequency is 1 Hz; 50 cycles per second equals 50 Hz.


An example is sitting on the beach, counting the waves as they come in

on the shore. If we counted the number of waves that crash on the

beach over a minute period, then divided that number by 60 (because

there are 60 seconds in one minute), we would have the number of

waves per second... (i.e. frequency!).

Larger units of frequency include the kilohertz (kHz) representing

thousands (1,000's) of cycles per second, the megahertz (MHz)

representing millions (1,000,000's) of cycles per second, and the

gigahertz (GHz) representing billions (1,000,000,000's) of cycles per

second.

Computer clock speed is generally specified in megahertz. As of this

writing, new personal computers have clock speeds ranging from

about 166 MHz to more than 1 GHz.

Frequency is important in wireless communications, where the

frequency of a signal is mathematically related to the wavelength. If f

is the frequency of an electromagnetic field in free space as measured

in megahertz, and w is the wavelength as measured in meters, then

w = 300/f

and conversely

f = 300/w

Humans can hear from reasonably low frequency tones (about 100Hz)

all the way up to about 12KHz. As we get older, our ability to hear the

higher notes is lessened, due to the aging process making the bones in

the ear harder and less able to vibrate. In addition, human speech

contains a great deal of redundant information, and can be compacted

within the range of 300Hz to 3400Hz (the frequency of an analogue

telephone signal) whilst still retaining approximately 80% of its

content.

Phase In electronic signaling, phase is a measurement (in degrees) of the

position of a point in time (instant) on a waveform cycle. A complete

cycle is defined as 360 degrees of phase as shown in Fig 2-7 . Phase

can also be an expression of relative displacement between or among

waves having the same frequency.


25

Phase shift occurs when the cycle does not complete, and a new cycle

begins before the previous one has fully completed. In a practical

sense, imagine you are in the bath. If you drop the soap, it forms

ripples where the waves travel outwards towards the edge of the bath.

When the wave reaches the edge, it hits the wall of the bath and

bounces back. This is like phase shift, which is an abrupt change in the

signals relationship. The human ear is insensitive to phase shift, but

data signals are severely affected by it. Phase shift is caused by

imperfections cable media, such as joins and imperfect terminations.

Fig 2-7 Phase of a wave

Phase difference, also called phase angle, is conventionally defined as

a number greater than -180, and less than or equal to +180. Leading

phase refers to a wave that occurs "ahead" of another wave of the same

frequency. Lagging phase refers to a wave that occurs "behind"

another wave of the same frequency. When two signals differ in phase

by -90 or +90 degrees, they are said to be in phase quadrature. When


two waves differ in phase by 180 degrees (-180 is technically the same

as +180), the waves are said to be in phase opposition. Fig 2-7 shows

two waves that are in phase quadrature. The wave depicted by the

dashed line leads the wave represented by the solid line by 90 degrees.

Digital Signals Digital technology generates, stores, and processes data in terms of

two states: positive and non-positive. Positive is expressed or

represented by the number 1 and non-positive by the number 0.

Digital data, transmitted or stored, is expressed as a string of 0's and

1's. Each of these state digits is referred to as a binary digit (and a

string of bits that a computer can address individually as a group is

called a byte).

Analogue technology, conventionally used by broadcast and phone

transmission, conveys data as electronic signals of varying frequency

or amplitude that are added to carrier waves of a given frequency.

Digital transmission methods are primarily used with new physical

communications media, such as satellite and fiber optic cable, but

new transmission techniques are starting to use existing copper cable

for digital transmission.

Digital signals require greater bandwidth capacity than analogue

signals, and are thus more expensive to use.

Modems Modems are devices that allow digital data signals to be transmitted

across an analogue link. As has already been discussed, the

connections provided by telephone companies for the use of speech via

dial up telephones is analogue based. However, in recent times,

especially with the development of the Internet, it is now standard

practice to use a dial-up telephone connection to access these services.

As the computer uses digital information, and the telephone line uses

analogue signals, a device is needed which takes the digital data from

the computer and converts it to analogue tones (within the voice

channel range of 300Hz to 3400Hz) so that the signals can travel

across the dial up speech connection. At the receiving end, the signals

are converted back to digital by a similar modem.


27

Bits Per Second This is an expression of the speed of data transmitted via the

transmission medium. Bits per second are used to indicate the speed

of transmission, which is often specified as Kilobits (thousands of bits)

per second or Megabits (Millions of bits) per second. The terms are

abbreviated as Kbps or Mbps.

Bandwidth Bandwidth is used to indicate how fast data flows on a given

transmission path and the width of the range of frequencies that an

electronic signal occupies on a given transmission medium. Any digital

or analogue signal has a bandwidth.

Generally speaking, bandwidth is directly proportional to the amount

of data transmitted or received per unit time. For example, it takes

more bandwidth to download a photograph in one second than it takes

to download a page of text in one second. Large sound files, computer

programs, and animated videos require still more bandwidth for

acceptable download time.

Bandwidth is expressed as data speed in bits per second (bps). Thus, a

modem that works at 57,600 bps has twice the bandwidth of a modem

that works at 28,800 bps. In analogue systems, bandwidth is

expressed in terms of the difference between the highest-frequency

signal component and the lowest-frequency signal component. A

typical voice signal has a bandwidth of approximately three kilohertz

(3 kHz); an analogue television video signal has a bandwidth of six

megahertz (6 MHz) , 2000 times as wide as the voice signal.

Modulation methods This refers to how the digital signal is altered so that it can be sent via

the analogue telephone network. There are a number of different

methods. The more complex methods allow much higher transmission

rates (bits per second) than the simpler methods.

Amplitude Modulation The amplitude modulation technique uses two different amplitudes to

represent the 1 and 0 values of a data signal as shown in Fig 2-8. The

amplitude of the signal is constant during each bit period. It is


possible to use four different amplitude levels to represent two bits at

a time eg. One level for 00 and a level each for 01, 10 and 11. This

enables us to double the bps rate.

Fig 2-8 Amplitude modulation

Amplitude modulation is susceptible to electric interference, making it

difficult to distinguish accurately between more than a few amplitude

levels. This problem makes amplitude modulation one of the least

efficient modulation techniques with a maximum of 1200 bps over

normal telephone lines.

Frequency Modulation This modulation method uses different frequency ranges to represent

the different data values as is shown in Fig 2-9. The frequency of the

signal is constant during each bit period. Frequency modulation is not

affected by electrical spikes but intermodulation distortion can occur

when the frequencies of two different signals mix to create a new

frequency suitable for low speed transmission.

Fig 2-9 Frequency modulation


29

The frequency of the carrier signal determines the transmission speed.

Higher speeds are not possible. If you increase the number of bits per

second of the digital signal, the corresponding carrier tones are on for

a shorter duration. Eventually, the stage is reached where only a few

cycles of the carrier tone are being sent for each digital bit.

To reliably detect the carrier signal at the receiver, up to 5 complete

cycles are required. Less than this may cause the carrier signal to be

misinterpreted. To compensate for this, the frequency of the carrier

signals must be increased. You cannot raise the carrier frequency

higher as the bandwith limits of the voice channel will cut it off if

using a telephone line.

Phase Modulation Phase modulation is a method of impressing data onto an alternating-

current varying the instantaneous phase of the wave. This scheme can

be used with analog or digital data. Fig 2-10 shows how this is done.

Fig 2-10 Phase modulation

Phase modulation is a very accurate modulation technique and

therefore it is possible to use more than one phase angle to represent

data values eg. 45 degrees represent 11, 135 degrees 10, 225 degrees

represent 01 and 315 degrees 11. This is called quadrature phase

modulation (Fig 2-11). If this technique is combined with the use of

two different amplitudes, four bits can be represented by each signal

change.


Fig 2-11 Quadrature phase modulation

Transmission Methods This section briefly discusses the differences between two different

methods of serial transmission, namely, asynchronous and

synchronous. A protocol establishes a means of communicating

between two systems. As long as the sender and receiver each use the

same protocol, information can be reliably exchanged between them.

We shall look at two common protocols used in serial data

communications: the first is known as asynchronous, the second as

synchronous.

Protocols A protocol is a set of rules, which governs how data is sent from one

point to another. In data communications, there are widely accepted


31

protocols for sending data. Both the sender and receiver must use the

same protocol when communicating.

Asynchronous Transmission Asynchronous protocol evolved early in the history of telecom-

munications. It became popular with the invention of the early tele-

typewriters that were used to send telegrams around the world.

Asynchronous systems send data between the sender and receiver by

packaging the data in an envelope. This envelope helps transport the

data across the transmission link that separates the sender and

receiver. The transmitter creates the envelope, and the receiver uses

the envelope to extract the data. Each character (byte) that the sender

transmits is preceded by a start bit, and suffixed with a stop bit. These

extra bits synchronize the data between the receiver and the sender. In

asynchronous serial transmission, each character is packaged in an

envelope, and sent across a single communications line, bit by bit, to a

receiver.

Because no signal lines are used to convey clock (timing) information,

this method groups data together into a sequence of bits (5 - 8), then

prefixes them with a start bit and appends the data with a stop bit to

the data. The purpose of the start and stop bits was introduced for the

old electromechanical tele-typewriters. These used motors driving

cams that actuated solenoids that sampled the signal at specific time

intervals. The motors took a while to get up to speed, thus by prefixing

the first data bit with a start bit, this gave time for the motors to get up

to speed. The cams generate a reference point for the start of the first

data bit. At the end of the character sequence, a stop bit was used to

allow the motors/cams etc to get back to normal. In addition, it was

needed to fill in time in case the character was an end of line, when the

tele-typewriter would need to go to the beginning of a new line.

Without the stop character, the machine could not complete this

before the next character arrived.

This method of transmission is suitable for slow speeds less than

about 32000 bits per second (32 Kbps). In addition, the signal that is

sent does not contain any information that can be used to validate if it

was received without modification. This method does not contain

error detection information, and is susceptible to errors. In addition,

for every character that is sent, an additional two bits is also sent.

Consider the sending of a text document which contains 1000


characters. Each character is eight bits, thus the total number of bits

sent are 10000 (8 bits per character plus a start and stop bit for each

character). These 10000 bits are equivalent to 1250 characters,

meaning that an additional 250 equivalent characters are being sent

due to the start and stop bits. This represents a large overhead in

sending data, clearly making this method an inefficient means of

sending large amounts of data.

It's important to realize that the receiver and sender are re-

synchronized each time a character arrives. This means that the

motors/cams are restarted each time a start bit arrives at the receiver.

Nowadays, electronic clocks provide the timing sequences necessary to

decode the incoming signal.

Synchronous Transmission One of the problems associated with asynchronous transmission is the

high overhead associated with transmitting data. Another problem is

the complete lack of any form of error detection. This means the

sender has no method of knowing whether the receiver is correctly

recognizing the data being transmitted.

In synchronous transmission, greater efficiency is achieved by

grouping characters together, and doing away with the start and stop

bits for each character. We still envelop the information in a similar

way as before, but this time we send more characters between the start

and end sequences. In addition, the start and stop bits are replaced

with a new format that permits greater flexibility. An extra ending

sequence is added to perform error checking.

A start type sequence, called a header, prefixes each block of

characters, and a stop type sequence, called a tail, suffixes each block

of characters. The tail is expanded to include a check code, inserted by

the transmitter, and used by the receiver to determine if the data block

of characters was received without errors. In this way, synchronous

transmission overcomes the two main deficiencies of the

asynchronous method, that of inefficiency and lack of error detection.

There are variations of synchronous transmission, which are split into

two groups, namely character orientated and bit orientated. In

character orientated, information is encoded as characters. In bit

orientated, information is encoded using bits or combinations of bits,

and is thus more complex than the character orientated version.


33

In asynchronous transmission, if there was no data to transmit,

nothing was sent. We relied on the start bit to start the motor and thus

begin the preparation to decode the incoming character. However, in

synchronous transmission, because the start bit has been dropped, the

receiver must be kept in a state of readiness. This is achieved by

sending a special code by the transmitter even if it has no data to send.

The header field is used to convey address information (sender and

receiver), packet type and control data. The data field contains the

user’s data (if it can't fit in a single packet, then multiple packets are

used and numbered). Generally, the data field has a fixed size. The tail

field contains information that the receiver uses to check whether the

packet was corrupted during transmission. In bit orientated protocols,

the line idle state is changed to “7E” (hexadecimal), which

synchronizes the receiver to the sender. The start and stop bits are

removed, and each character is combined with others into a data

packet. User data is prefixed with a header field, and suffixed with a

trailer field, which includes a checksum value (used by the receiver to

check for errors in sending).

Half / Full Duplex The flow of data over a network must be controlled in some way.

There are three different methods of controlling data flow - simplex,

half duplex and full duplex transmission. Simplex transmission

means that the data can only flow in one direction. Radio and

television are examples of simplex transmission. Business

applications of this technique would include receive-only devices such

as printers and process control applications involving data capture

and transmission of data to a host computer or output screens in

airport lounges.

Half-duplex transmission allows data to travel in either direction but

only in one direction at a time. For example, a two-way radio uses the

same frequency for the sender and the receiver and both may send

information but not at the same time. There is a time delay between

the receipt of a message and the sending of another because the line

has to be synchronised to send data in the opposite direction. This

delay is called the line turnaround delay.

A full-duplex line allows messages to be transmitted in both directions

simultaneously thereby eliminating the line turnaround delay. This

approach requires two separate channels as opposed to one for half-

duplex transmission.


Errors Data being transmitted is always subject to errors and the volume of

errors will vary according to the media being used. Natural noise

occurs randomly on a data communications network and can occur as

either white noise or impulse noise. An example of white noise would

be the hissing sound heard in the background during a telephone

conversation.

Although noise cannot be removed, a partial solution would be to send

the data signal at a different amplitude to the amplitude of the white

noise thus not confusing the information with the noise. Impulse

noise has greater consequences and is characterised by a spike in the

signal caused by lightning strikes or other more local changes in

electrical impulses. On a voice circuit this can be heard as a clicking or

crackling sound of a relatively short duration. The outcome is the

possible changing of several bits of data leading to data errors.

Impulse noise can be removed or reduced by creating a dedicated data

circuit.

Other sources of error relate to network noise and manifest

themselves in the form of crosstalk and echoes. Crosstalk occurs when

signals from one channel distort signals from a different channel. In a

telephone conversation, crosstalk is when other parties are conversing

in the background. Echoes occur when the listener hears the echo of

his own voice after speaking.

Error Prevention As it is not possible to eliminate all noise, certain error prevention

techniques are required to reduce the possibility of data corruption.

The most common form of error prevention is that of line

conditioning. This means that the telecommunications supplier can

provide a line that has superior electrical characteristics to a voice-

grade line.

Another way of preventing errors is to lower the transmission speed.

Significantly fewer errors occur when data is transmitted at lower

speeds. It is not unreasonable to have an objective of less than one

error per one hundred thousand bits of data transferred. Many data


35

communications networks are designed for fewer than one error per

one million bits transmitted.

Vertical Redundancy As errors cannot be totally eliminated it is necessary to detect when

errors occur. One of the most common error detecting approaches is

parity checking (also called vertical redundancy checking - VRC). This

involves adding an extra bit, known as a parity bit, to each character

that is being transmitted (this would be added to the ASCII or EBCDIC

characters). This extra bit is required to bring the total number of ON

bits in the code to either an even number (even parity) or to an odd

number (odd parity). On receipt of each character the number of ON

bits is checked to see if they are either an even or odd number.

Notice that this technique only allows the user to detect whether one,

three, five or seven bits have been altered in transmission but will not

detect whether even numbers of bits have been altered. When using

high transmission rates this limitation is highly significant.

Longitudinal Redundancy Error detection using VRC can be improved by using it in conjunction

with longitudinal redundancy checking (LRC). LRC uses a redundant

character called a block check character (BCC) which is added to a

block of transmitted characters. The first bit of the BCC is in fact a

parity check of all of the first bits of all the characters in the block. The

second bit of the BCC acts as a parity check for all the second bits in

the block and so on.

Cyclical Redundancy Checking Using LRC with VRC will provide us with a superior method of

detecting errors. However, the combined methods will still not detect

all errors.

A more sophisticated error detection technique is cyclic redundancy

checking (CRC). The transmitting station calculates a block checking

character for the data using a complex polynomial function. The

receiving station separately calculates a BCC, based on the bit stream

it has received, and compares this with the one received. If the two

are equal then the block is assumed to be error free. For transmitting

blocks of data the CRC method is becoming a standard way of

detecting errors. Note that this does not guarantee the absence of an


error as two or more errors in the bit stream may compensate for each

other. However, this is a rare event.

In large data networks, the data being transmitted is typically split

into a number of packets. These packets may be routed in different

directions and may not arrive at the destination in the same order. In

such systems it is important to assign a sequence number to each

packet so that the receiving station can carry out sequence checking.

These sequence numbers can also be used for error control. If a block

of data arrives and is faulty or does not arrive at all, the receiver can

ask for it to be re-transmitted.

Some systems use character echoing where the characters transmitted

are then echoed back to the user to check that they are correct. This

method has the major disadvantage of doubling the chances of

obtaining an error since the message has to be received at both ends of

the system.

Error Correction Having detected an error, the hardware needs to correct it. When the

receiving station receives the data it can reply with a positive or a

negative acknowledgement known as an ACK (positive) or an NAK

(negative) respectively. This advises the sender that the data was

received successfully or unsuccessfully and the sender will either send

another message or re-send a previous message.

The number of times a message is resent must be limited to avoid

wastage of resources. This limit, typically anything up to one hundred,

avoids sending unproductive messages on a permanently faulty

network. Should messages not be received at all over a certain time

period, then the receiving station will issue a time-out, which means it

has acknowledged the fact that a permanent problem has occurred on

the line.

Summary In this chapter we have looked at encoding schemes to enable a

combination of 1’s and 0’s to represent a large number of human

readable characters. The characteristics of electromagnetic signals and

how these characteristics can influence the transfer of data signals

were also covered. We also looked at different transmission methods


37

and modulation methods to enable the various types of signals to carry

data. Lastly we looked at ways to detect and prevent data

transmission errors and also at ways to correct errors if they occur.

Chapter 3: Data Transmission Media

39

3: Data

Transmission

Media

The communications media on a network are the physical connections

that tie everything together. The media used in network communi-

cations include the following:

Guided Media: • Coaxial cable

• Shielded twisted pair cable

• Unshielded twisted pair cable

• Fibre optic cable

Unguided Media: • Radio

• Infrared

• Microwave

• Satellite

• Cellular

In the next sections each medium, including its advantages and

disadvantages, will be discussed. The different characteristics of the

media types make them suitable for different types of networks. The

most common is twisted pair. Coaxial cable, though still common, is

used mainly for older LAN’s. Fibre optic cabling is used primarily to

connect computers that demand high-speed access to networks on


different floors and in different buildings. Wireless technologies are

used in situations where it is difficult or too expensive to use cable.

Smaller networks often use only one type of medium, but many

different types of media are found in the various parts of larger

networks.

There are many factors that influence the choice of transmission

media. Cost is an obvious factor while the ability of the medium to

accommodate expansion is also important, as some types of media

such as fibre optic cable are difficult to alter after installation.

The bandwidth or the amount of data that the medium can transfer

per second could slow down a network if not high enough. Bandwidth

is usually measured in megabits per second (Mbps).

Coaxial Cable Coaxial cable, also known as coax, consists of a copper core

surrounded by insulation. Figure 3-1 shows how the insulation is

surrounded by another conducting material, such as braided wire,

which is covered by an outer, insulating material.

Fig 3-1 Coaxial Cable

Insulation

(PVC, Teflon)

Braided shielding

Conducting

core

Outer cover


41

Coaxial cable comes in two varieties, thick and thin. The thick version

was used in early networks, mainly as backbones to connect different

networks, but is used infrequently today, as there are better

alternatives such as fibre-optic cable.

Thin coaxial cable, which has a much smaller diameter, is used to

connect desktop workstations. It resembles television cable. The cable

has an impedance of 50 ohms. Impedance is the resistance to the flow

of current influencing how fast data can travel through the conductive

material.

Thick coaxial cable normally has a maximum cable length (or run) of

500 metres. This is often referred to as 10 base 5. The 10 indicates

that the cable transmission rate is 10Mbps. Base means that

baseband partition transmission is used. Baseband uses the full

capacity of the medium to transmit a signal. This means that only one

node can transmit at a time. The 5 indicates 5 times 100 metres for

the longest cable run.

Thin coaxial cable is referred to as 10 base 2 which means it has a

maximum theoretical network speed of 10Mbps, uses baseband type

transmission and can have wire runs of a up to 200 metres.

Coaxial cable is attached to BNC (British Naval Connector)

connectors, which are then connected to T-connectors. The middle of

the T-connector is connected to the network card that connects the

computer to the network. If the computer or device is the last node on

the cable, a terminator is connected to the one end of the T. The

female BNC connector (Fig 3-2) has two small pins that attach to two

slots in the male connectors. The connectors are twisted together for a

connection.

Fig 3-2 BNC Connector


Thin coaxial cable is cheaper and easier to install than thick coaxial

cable. Twisted pair cable is more flexible and that is one reason why

coaxial is now used on a limited basis. An advantage of coaxial cable is

that it is resistant to electrical interference.

Twisted Pair Cable Twisted-Pair cable resembles telephone wire. It is more flexible than

coaxial cable for running through walls and around corners. This

cable can be adapted for high-speed communications of 100Mbps or

faster. Normally the maximum length for twisted pair cable is 100

metres. Twisted pair cabling is connected with RJ-45 connectors (Fig

3-3), which is a wider form of the connectors used on telephones. This

type of connector is less expensive than T-connectors and also easy to

connect.

Fig 3-3 RJ-45 Connector

Unshielded Twisted Pair Cable The two kinds of twisted pair cable are shielded and unshielded.

Unshielded cable is often preferred because of its lower cost and high

reliability.

Unshielded twisted pair cable (UTP) (Fig 3-4) is very light and

flexible, and therefore easy to install. It comes in five categories that

determine the bandwidth of the cable. Categories 1 and 2 are used for

transfer speeds up to 4 Megabits per second (Mbps), Category 3 up to

16 Mbps, category 4 up to 20 Mbps and category 5 up to 100 Mbps.


43

Fig 3-4 Unshielded Twisted Pair Cable

Shielded Twisted Pair (STP) Cable

Shielded twisted pair cable is similar to unshielded twisted pair cable,

except that the wires are surrounded by a braided metal wire or foil

covering. The purpose of the cover is to protect the wires from

electrical interference. Shielded twisted pair cable is capable of

transmission speeds of up to 150 Mbps.

Fig 3-5 Shielded Twisted Pair Cable

Colour-coded

insulation

Plastic

encasement

Copper wire

conductor


Shielded twisted pair cable is more difficult to install than unshielded

cable as it is thicker than UTP and not very flexible. The connectors

used for shielded twisted pair cable are also bulky and difficult to

install.

Fibre Optic Cable

Fig 3-6 Fibre optic Cable

Fibre optic cable is another communication medium used in place of

wire-cable in data communications networks. Light pulses are

transmitted down hair-thin strands of pure glass or plastic (Fig 3-6),

which are sheathed. Light-emitting diodes and lasers are used to

transmit light at a specific frequency through the fibre. As illustrated

in Fig 3-7, some rays will bounce off the cladding at different angles

while the cladding will absorb others. This causes distortion that limits

the transmission speed of the cable. In spite of this, fibre optic cable

can still achieve transmission speed of up to 3 Gigabits per second

(Gbps). The high transmission speeds make fibre optic cable ideal for

use as the main cable, or backbone, of a network.

This type of cable is very difficult to tap into and therefore new

additions to the network are difficult. This has the advantage of

making fibre optic cable one of the most secure of all data

transmission media. It has also been significantly more expensive than

Optical fiber (core)

Protective outer

sheath (jacket)

Glass cladding


45

coaxial cable and twisted-pair in the past although this situation is

changing now that the technology is more firmly established.

Fig 3-7 Reflection of Light in Fibre Optic Cable

A major benefit over the equivalent copper wire is the very low error

rates achieved, as there is no electrical interference with this medium.

Infrared Systems Infrared systems use infrared light to carry information between

devices, using the same technology as household remote controls.

Physical barriers between two infrared devices cause the biggest

problem with infrared transmission, so that all transmissions need a

line of sight between devices. Infrared signals cannot travel far without

weakening and even if a clear line of sight exist between two devices,

the signal may start to degrade if the distance is beyond a few metres.

Infrared systems can be used for data transfer at about 4 Mbps.

Although well below the capabilities of wire cable, infrared systems

may be suitable for networks that do not transfer a high volume of

data. Infrared systems are more expensive than cable based systems

and long distances can only be covered by using high-powered

transmitters. However they are being used widely for laptop and

cellphone communications.

Core

Jacket

Cladding

-Glass or plastic core

-Laser or light-emitting diode

-Specially designed jacket

-Small size and weight

Light at less than critical

angle is absorbed in

jacket

Angle of Incidence Angle of reflection


Radio Systems Radio systems can be used for any size of network. They are most

often used to connect devices spread over wide areas, such as a city.

Radio systems allow devices on a network to be more mobile and are

well suited for connecting portable computers to a network.

Radio systems transmit on a limited number of radio frequencies, but

the systems are usually capable of detecting which frequencies are

clear before starting to transmit, so that interference from other

transmissions is usually not a problem.

Radio systems transmit less information than many types of cable

based networks with speeds usually limited to about 2 Mbps.

Compared to cable based systems, radio systems are relatively

expensive but are useful in situations where it is difficult to lay cables.

Microwave Systems For long distance data communication, microwave radio technology is

now utilised in many countries. Microwave signals travel in a straight

line and therefore the transmitter and receiver must be in each other's

line of sight. It is therefore necessary to have microwave stations

("dishes" and relays) at distances of approximately fifty kilometres

apart. Each relay receives a focused radio frequency beam which it

regenerates and re-transmits to the next relay. These stations can

often be seen on tops of mountains.

The wide bandwidth of a microwave channel is subdivided into many

sub-channels which can be used for voice or data links. Microwave

transmission is faster than some of the alternatives allowing speeds up

to 45 Mbps. Apart from its speed advantage it is also easy to

implement.


47

Fig 3-8 Microwave Link

Satellite Satellite radio transmission is similar to microwave radio transmission

in that data is transferred on very high frequency radio waves. Both

media also need a line-of-sight (Fig 3-8) transmission between

stations. The differences between the two media are in the location of

the stations and the means of transmitting.

Microwave uses relay stations across land and satellite transmission

uses land stations and orbiting satellites. As the satellite is positioned

thirty-six thousand kilometres above the earth in a stationary position

(Fig 3-9), we can now introduce a further concept - the amount of time

it takes for a signal to travel from its source to its destination. This is

called propagation delay.

Fig 3-9 Satellite Link

Building in Europe Building in USA

Microwave dish Microwave dish

Satellite

Building A Building B

Microwave dish Microwave dish


This delay is relatively insignificant on earth but when utilising

satellites, it becomes significant and can range up to one half of a

second.

Circuit Switched Mobile

Cellular data transmission uses a circuit-switched network. This is a

type of network such as the regular voice telephone network in which

the communication circuit (path) for the call is set up and dedicated to

the participants in that call. For the duration of the connection, all

resources on that circuit are unavailable to other users. The data

transmission is similar to data transmission over a normal telephone

line and requires modems on both sides. The transmission is

relatively slow with speeds from 9600kbps to 28.8 kbps with high

speed circuit switched data (HSCD) transmissions.

GPRS GPRS (General Packet Radio Services) is a packet-based wireless

communication service. Packet-switched describes the type of network

in which relatively small units of data called packets are routed

through a network based on the destination address contained within

each packet. Breaking communication down into packets allows the

same data path to be shared among many users in the network. This

type of communication between sender and receiver is known as

connectionless (rather than dedicated). Most traffic over the Internet

uses packet switching and the Internet is basically a connectionless

network.

GPRS delivers data rates from 56 Kbps up to 114 Kbps and continuous

connection to the Internet for mobile phone and computer users. The

higher data rates allow users to take part in video conferences and to

interact with multimedia Web sites and similar applications using

mobile handheld devices as well as notebook computer computers.

GPRS is based on Global System for Mobile (GSM) communication

and will complement existing services such circuit-switched cellular

phone connections.

The GPRS should cost users less than circuit-switched services since

communication channels are being used on a shared-use, as-packets-

are-needed basis rather than dedicated only to one user at a time. It


49

should also be easier to make applications available to mobile users

because the faster data rate means that software currently needed to

adapt applications to the slower speed of wireless systems will no

longer be needed.

GPRS is an evolutionary step toward Enhanced Data GSM

Environment (EDGE) and Universal Mobile Telephone Service

(UMTS), which will both be explained in the next sections.

EDGE

EDGE (Enhanced Data GSM Environment), a faster version of the

Global System for Mobile (GSM) wireless service, is designed to

deliver data at rates up to 384 Kbps and enable the delivery of

multimedia and other broadband applications to mobile phone and

computer users. The EDGE standard is built on the existing GSM

standard, using the same time-division multiple access (TDMS) frame

structure and existing cell arrangements.

EDGE is regarded as an evolutionary standard on the way to Universal

Mobile Telephone Service (UMTS).

UMTS

UMTS (Universal Mobile Telephone Service) is a so-called third-

generation (3G), packet-based transmission of text, digitized voice,

video, and multimedia at data rates up to and possibly higher than 2

megabits per second (Mbps), offering a consistent set of services to

mobile computer and phone users no matter where they are located in

the world. Based on the Global System for Mobile (GSM)

communication standard, UMTS, endorsed by major standards bodies

and manufacturers, is the planned standard for mobile users around

the world by 2002.

Once UMTS is fully implemented, computer and phone users can be

constantly attached to the Internet as they travel and have the same

set of capabilities no matter where they travel to. Users will have

access through a combination of terrestrial wireless and satellite

transmissions. Until UMTS is fully implemented, users can have

multi-mode devices that switch to the currently available technology

(such as GSM 900 and 1800) where UMTS is not yet available.


Today's cellular telephone systems are mainly circuit-switched, with

connections always dependent on circuit availability. Packet –switched

connection, using the Internet Protocol (IP), means that a virtual

connection is always available to any other end point in the network. It

will also make it possible to provide new services, such as alternative

billing methods (pay-per-bit or session). The higher bandwidth of

UMTS also promises new services, such as video conferencing. UMTS

promises to realize the Virtual Home Environment in which a roaming

user can have the same services to which the user is accustomed when

at home or in the office.

Summary In this chapter we have looked at the different types of media that can

be used to link data communications equipment. We have looked at

guided media such as the different types of networking cable and

optical fibre, as well as unguided media such as infrared, microwave

and radio systems. We have also looked at mobile data

communications that are rapidly evolving from circuit switched

transmissions to high-speed packet based transmissions as used by the

third generation networks of the near future.

Exercises

1. Discuss the advantages and disadvantages of the different types of

networking cable.

2. Explain the difference between circuit switched and packet switched

transmission.

3. Discuss why and when you would use optical fibre for data

communications.

4. Discuss the different options available for mobile data

communications.

5. What are the current shortcomings of mobile data transfer and how

will they be eliminated in the near future?

Chapter 4: Network Hardware 51

4: Network

Hardware We cannot assemble a car if we do not know the function of the

various components. It is also not possible to design or even

understand the functioning of a computer network if we are not

familiar with the hardware components that act as building blocks for

the network. It is therefore necessary to understand the background of

some of the more important pieces of hardware used to build

computer networks.

Servers Servers are computers that perform specific tasks on the network.

Typical tasks include file services, printer services and web services.

Servers usually are powerful computers with fast processors and large

amounts of memory. Servers often have access to large storage devices

that enable them to store a lot of data.

It is important that servers should be reliable and therefore additional

facilities are often used to ensure reliability. These may include

measures such as extra power supplies, RAID disks etc.

Network Computers Sometimes very basic computers called Network Computers (NC’s) are

used as workstations on a network. As most of the applications are

stored on a central applications server it is not necessary for the

network computers to contain any but the most basic hardware.

All that is needed by the Network Computer is a processor and enough

random access memory (RAM) to connect to the network and run

programs written in Java. Java is a computer language that can be

used to develop applications such as word processors that will run on

most operating platforms. By doing so the cost can be reduced as all

the users on the network can work with the same program, and it is


not necessary to purchase a copy of the application program for each

operating system.

Storage Devices The file server is often the most important device on a network, as it is

used to store the information on the network. To achieve this, large,

fast and reliable hard drives are usually connected directly to the file

server. To prevent loss of data from these hard drives, regular backups

must be made.

The most popular medium to use for backups is streamer tape, as this

can back up very large volumes of data relatively quickly. This process

can often take place automatically at off peak times. Compact disks

can also be used for backup purposes, but this medium is mostly used

for retrieval of information such as reference or historical information.

Network Interface Cards To enable a computer to connect to a network, a Network Interface

Card (NIC) is needed. The task of this device is to translate the signals

generated by the computer into a form that can be transmitted on the

transmission medium of the network. The NIC also contains the

correct type of connectors for connecting to the transmission medium.

Another task of the NIC is to identify the computer on the network.

When manufactured, a unique hardware address, the MAC (Media

Access Control) address, is set for each network interface card.

As network computers often do not contain disk drives to boot from,

the network interface cards in these machines often contain a boot

ROM that contains the software that enables the computer to boot up

and connect to the network.

Network Interface Cards also need special software (called a driver),

which will allow the operating system of the computer to communicate

with the hardware of the network card. Each type of network card

requires a specific driver that must be installed on the computer when

the network card is installed.


Hubs As its name implies, a hub is a centre of activity. A hub or

concentrator is a common wiring point for star networks(where all the

computers are wired to a central point). Many network topologies use

hubs to connect different cable runs and to distribute data across the

various segments of a network. Hubs basically act as signal splitters.

They take all of the signals they receive and redistribute them to all the

computers on the network. Some hubs regenerate weak signals before

retransmitting them. Hubs for fibre optic cable actually use mirrors to

distribute the light beam among the various ports.

Fig 4-1 Hub based network

On a typical 10baseT (10 Mbit baseband) network all devices will be

wired to a hub using unshielded twisted-pair (UTP) cabling.

Sometimes it is necessary to connect multiple hubs. In that case one

Workstation

Workstation

Workstation

Workstation

WorkstationHub

File serverWorkstation


of the higher speed ports on the hub can be used to build a backbone

for the network. Each hub should connect directly to the backbone.

Types of Hubs There are many types of hubs. This section will look at the standard

features found in most hubs and the differences between passive,

active and intelligent hubs.

The features of hubs are determined in part by the types of cabling

that run to the hub. Since hubs are electronic devices that take a single

signal and broadcast it to multiple ports, hubs need a power source.

Most have light emitting diodes (LED’s) to monitor power and the

active connections at the various ports. Some may also have LED’s to

monitor traffic on a particular port, as well as monitoring packet

collisions on the LAN in general.

Passive Hubs Passive hubs simply take all of the packets they receive on a single port

and rebroadcast them across all ports. Passive hubs commonly have

one higher speed port in addition to the RJ-45 connectors that connect

each LAN device. This higher speed port can be used to connect to the

network backbone.

Active Hubs Active hubs generally have all the features of passive hubs. They also

monitor the data being sent out and provide certain diagnostic

capabilities. Active hubs repair certain damaged packets and will

retime the distribution of other packets.

If an active hub receives a weak but still readable signal, it will restore

the signal to a stronger state before rebroadcasting it. This feature

allows devices that are not operating within optimal parameters to

continue to function on the LAN. Some active hubs will report devices

that are not fully functional.

When cables experience electromagnetic disturbances that prevent

packets from reaching the hub or other devices in timely fashion,

active hubs will retime and resynchronise the packets when they are

being retransmitted. In situations where packets do not reach the

destination at all, active hubs can compensate by retransmitting


packets on individual ports and retiming packet delivery for slower

connections.

Intelligent Hubs In addition to the features found in active hubs, intelligent hubs allow

the ability to manage the network from one central location. If the

hub cannot remedy a problem by itself, it will provide diagnostic

information to easily identify and remedy network problems.

Intelligent hubs also offer flexible transmission rates to various

devices. This means that the network can gradually be upgraded to

higher speed connections or simply deliver faster transmission speeds

to devices that need faster services. Intelligent hubs also have

incorporated support for technologies such as bridges, routers and

switches.

Advanced features often include redundant power supplies, redundant

cooling fans and automatic termination of coaxial cable. The built in

redundancy enables the automatic take-over of one unit should

another one fail.

Repeaters As a signal travels along a cable, it will get weaker. This degradation is

referred to as attenuation. To fix this problem, a connector known as a

repeater can be used. A repeater receives a signal and retransmits it

again at full strength, thereby allowing signals to travel farther along

the network.

Repeaters are useful in areas where long lengths of cable are needed

such as a network in a large building, as for example Fig 4-2.

Repeaters are very simple devices that amplify the signal before

retransmitting it. To avoid the amplification and retransmission of

interference on the network, most repeaters have filters to remove any

distortion before they amplify and retransmit the signal.


Repeater

BUSINESS

OFFICE FACTORY

Fig 4-2 Repeater

Bridges Bridges are used to connect two or more LAN’s together and forward

data according to the MAC (media access control, a hardware address

built into the network interface card) address. A bridge can be seen as

a low-level router, as routers use forwarding by addresses such as IP

(Internet Protocol) addresses.

Bridge

Server

Server

Server

Fig 4-3 Bridge

Bridges determine if information is destined for the same network or

for the network on the other side of the bridge and only information


destined for the other side of the bridge will be forwarded to that

network thereby reducing the traffic on that sector of the network.

Bridges must know the address of every computer on the network.

Learning bridges can read and store the address of every computer

that transmits data on the network.

As bridges cannot change information in any way, bridges can only be

used to connect networks using the same transmission technique.

A remote bridge connects two remote LAN’s over a slow link such as a

telephone line, while a local bridge connects two locally adjacent

LAN’s together.

Switches Like bridges, switches subdivide larger networks and prevent the

unnecessary flow of network traffic from one segment to another.

Switches direct data only across segments containing the source and

destination hosts. In the nonswitched situation, each time a

particular device broadcasts on the network, other devices become

incapable of accessing the network, thus preventing collisions. This

slows down the overall network speed.

Switches help to create additional network access opportunities for

attached devices by restricting data flows to local segments unless data

are destined for a host located on another segment. The switch

examines the destination address and forward the requisite data only

across the destination segment, leaving all additional segments free

from that particular broadcast.

Segment switches are able to handle the traffic from an entire network

segment on each port, allowing a higher number of workstations to be

connected. Segment switches are also capable of handling single

workstations on each port. This allows machine arrangements

requiring only intermittent network access along the same segment,

sharing a relatively low speed connection. At the same time, faster

devices can be connected with one device per port so that each one

seems to have its own dedicated path to the network.


Routers Routers are used to link different networks together. On large

networks there may be more than one route that data can follow to the

destination. Routers can route data to the correct destination. If a

certain path malfunctions, then another functioning path can be used.

Some routers, called intelligent routers can automatically detect if a

certain path is slow or not working. The router can then determine the

best route for the data to take and redirect it around the problem area.

Routers contain fast processors to prevent the analysis and routing

process from slowing down the network. Information about the

networks attached to a router must be entered into a router to enable

it to route data. Most routers can be linked to a computer for this task.

With the older static type of router all information about the routes

that data could take had to be entered by the network administrator.

Dynamic routers can automatically configure the available routes on a

network.

Routers use mathematical algorithms to determine the best route for

data. These algorithms use variables such as the speed of the network

segments and the distance. The network is divided into segments with

individual addresses. Routers use this segment address and the

address of the destination computer to route the data through the

network.

Routers have the ability to translate the protocol of one network type

into the protocol of another type of network and are often used to

connect different types of networks. A popular use of a router is to

connect local area networks to a Wide Area Network.


TCP/IP

basedserver

Server

Router

Transmitted

frame

Frame

forwarded

to the right

network by

the router

Frame

reaches the

designated

workstation

Fig 4-4 Routing of Packets

BRouters

BRouters are connectors that combine the characteristics of a bridge

and a router and are often used like a bridge, but to link two dissimilar

networks.


Gateways A gateway is another type of connector device that is used to link two

dissimilar networks. It often takes the form of a computer with two

types of network cards installed. The gateway would then read the

data from the first network card using a specific protocol and transmit

the same data on the second network card using the protocol of the

second network card. All this is achieved by special software in the

gateway computer. The gateway may also be a specialised device (not a

computer).

SNA

Gateway

NetWare

Fig 4-5 Gateway

If a network does not recognise the address of a destination computer,

the data is passed to the default gateway on the next network. The data

is passed from default gateway to default gateway until the address is

recognised. Default gateways are most common on the Internet.


Modems Modems are an inexpensive way to connect a computer to a network

over analogue telephone lines. Always used in pairs, one modem will

convert the digital pulse into an analogue signal, that can be

transmitted over a telephone line. At the receiving end, the other

modem in the pair will convert the analogue signal back to a digital

pulse. Modems are capable of modulating digital pulses into sound

and demodulating it back to digital pulses, hence the name modem

(MOdulator – DEmodulator)

Fig 4-5 Modems

Wide Area Networks (WANs) often use modems to connect areas of

the network that only need to transfer small amounts of information

over a telephone line. Modems are relatively slow. A fast modem may

only transfer data at 56 kilobits per second (Kbps), while some

network devices can transfer data at speeds of 10 000 kilobits per

second (Kbps). Modems are widely used for home connections to

Internet Service Providers (ISPs).

Multiplexers Using a dedicated line to connect to each remote device is obviously

inefficient and expensive. Multiplexing is a very common way to allow

several devices to share a communications line. Several devices are

connected to a multiplexer (Mux for short) and this device

consolidates the data from a number of devices and transmits it down

one line to a similar mux at the other end where the data is separated.

Modems are still used to convert the signal into analogue format. Each

device thinks it has its own high-speed point-to-point link.

Modem

Telephone

network

Modem

Analog signalsDigital Signals Digital Signals


There are two common multiplexing techniques in use. These are

Frequency Division Multiplexing and Time Division Multiplexing.

With Frequency Division Multiplexing (FDM) the band width of the

circuit is split into several subchannels as in Fig 4-6. Each subchannel

must have a guard band to prevent accidental leakage of data from one

subchannel to another. This leakage, which is similar to crosstalk

during a telephone conversation, can never be completely eliminated.

Because the guard bands need some of the frequency of the channel,

there is a reduction in channel capacity.

Fig 4-6 Frequency Division Multiplexing

Although Frequency Division Multiplexing is very popular because it is

easy to understand and implement, it has a further disadvantage. If a

device is inactive, the frequency is unused and capacity is lost. Time

Division Multiplexing (TDM) overcomes some of the inefficiencies of

FDM. Based on a time-sharing approach, TDM allocates each device a

time slot during which it can transmit data. Each terminal has the

entire bandwidth for a specific period of time. The data enters the

TDM from the devices and is placed in a buffer from which it is

removed using character interleaving (a character from line one

followed from line two etc.) or bit interleaving where a transmission

block is formed from a bit from each line.

f6

Channel 1

Channel 2

Channel 3

Channel 4

Channel 5

Channel 6

f1

f2

f3

f4

f5


Fig 4-7 Time Division Multiplexing

There is no loss in line capacity as experienced with FDM. However,

an inactive device will mean that some of the blocks will be empty.

This is still a vast improvement in efficiency over FDM.

The Statistical Time Division multiplexer (STDM or StatMux)

overcomes the inefficiencies of TDM by allocating band width only to

those devices that have something to send. Thus no bandwidth is

wasted and the terminals that generate heavy traffic have more use of

the line than the other terminals. StatMuxes have built-in memory

and processing power and can carry out functions such as data

compression and mixed line speeds, as well as having integrated

modems and sophisticated performance and error diagnostic facilities.

Dense Wavelength Division Multiplexing (DWDM) is an optical

technology used to increase bandwidth over existing fiber optic

backbones. DWDM works by combining and transmitting multiple

signals simultaneously at different wavelengths (colours) on the same

fiber. In effect, one fiber is transformed into multiple virtual fibers.

By multiplexing eight signals into one fiber, the carrying capacity of

the fiber is increased from 2.5 Gb/s to 20 Gb/s. Currently, because of

DWDM, single fibers have been able to transmit data at speeds up to

400Gb/s. As more channels are added to each fiber terabit capacity

will soon be available.

Channel 1

Channel 2

Channel 3

Channel 4

Channel 5

Channel 6

Channel 1

Channel 2

Channel 3

Channel 4

etc.


A key advantage to DWDM is that it is protocol and bit-rate

independent and therefore DWDM-based networks can carry

different types of traffic at different speeds over an optical channel.

Summary In this chapter we have looked at a number of the devices available to

connect computer systems and networks. In many cases the

functionality of two or more of these devices may be combined in a

single package . It is important to know the specific application of a

device eg. to connect two similar networks or to connect two different

types of networks

We have also looked at ways of saving cost by combining a number of

signals onto a single cable or optical fibre by the use of different

multiplexing techniques.

Exercises 1. List the hardware discussed in this chapter along with the main

function of each item.

2. Explain how you would link your organisation’s network to the

Internet by using some of the equipment discussed in this chapter.

3. Discuss the advantages and disadvantages of the various multiplexing

techniques.

4. Design a network for your company (with branches in three different

cities) without using the Internet to link the branches.

5. Repeat the exercise above but this time use the Internet.

Chapter 5: Network Structures and Architectures

65

5: Network

Structures and

Architectures

The topology or structure of a network has both a physical and a

logical dimension.

The physical structure refers to the parts of a network that actually

exist, such as the computers, printers and cables. This structure refers

to where the computers on a network are located and how the devices

that form the network are physically connected.

The logical structure of a network refers to how data flows through a

network. This is the path followed by electrical signals when

transferred through the transmission media.

The same structure is not necessarily used by the physical and logical

topologies. For example the signals on a physical star (all computers

connected to a central point) network may follow a path that

resembles a ring around the network.

Bus Network Structure With LANs, bus topology means a shared link along which nodes are

attached. A LAN bus can be a single line or a more complex structure

in a tree-branch configuration.

Fig 5-1 shows how terminators are used to absorb the signals at the

end of the cable preventing the reflection of signals back along the

cable.


Fig 5-1 Bus Network Topology

All the devices connected to the network listen to the transmissions on

the network but only accept the transmissions addressed to them. This

type of network is simple and inexpensive, but the simplicity limits it

in terms of distance and scalability.

Star Network Structure The star network topology has connections that radiate out from a

central hub (Fig 5-2) to the networked devices. Each of the connected

devices can share the media independently, not influencing other

devices on the network when problems are experienced.

Workstation

Printer

Laptop computer

Workstation

File server

File server

Terminator

Terminator

Chapter 5: Network Structures and Architectures 67

Workstation

Workstation

Workstation

Workstation

WorkstationHub

File server

Workstation

Fig 5-2 Star Network Topology

Star topologies have become very popular for contemporary LAN’s.

They are flexible, scalable and relatively inexpensive.

It is easy to find misbehaving devices on a star network. Such devices

will not influence other devices on the network, thereby enabling the

uninterrupted operation of the rest of the network. The star topology

also forms the basis for the switched network topology discussed later

in this chapter.

Ring Network Structure The ring topology shown in Fig 5-3 initially featured peer-to-peer

connections, where each network device had a connection to each of

its nearest neighbours. The connections had to form a physical loop,

with data being transmitted in one direction around the ring. Every

device acted as a repeater accepting packets of data addressed for it


and passing on the other packets to the next device. The network was

disabled if there was any break in the ring.

Ring

Workstation Workstation

Ring

Workstation

Fig 5-3 Ring Network Topology

Token Ring A later form of the ring topology, the token ring topology, uses a small

packet of data, called a token that is passed along the network. A

station wanting to transmit must wait for the available token as it

passes by. It then takes the token and adds the data to it. The unit of

data on the ring will make a round trip to the receiving station, that

reads the data, and back to the transmitting station. The token and

data are then absorbed by the transmitting station. The transmitting

station will insert a new token on the ring so that another station can

get a chance to transmit data.

Chapter 5: Network Structures and Architectures

69

To prevent network failures caused by a break in the network, token

ring networks are normally implemented as a physical star structure

with a central hub, with the token passing from device to device in a

circular fashion. Workstations connect to central hubs called

multistation access units (MAUs). If a workstation fails, the MAU

immediately bypasses the station to maintain the ring of the network.

Often cable with multiple wire pairs is used so that when a break in the

cable occurs, the ring can revert back on itself with signals travelling in

the opposite direction.

When the network load is low, there is some inefficiency with token

ring networks, as a station must wait for the token to come around

before transmitting. Under heavy loads, the ring functions in an

efficient and fair round-robin fashion.

Switched Networks A switch is a device that learns the hardware addresses of the devices

connected to it and stores them in a table. It then forms a temporary

switched path directly between the originator and the recipient and

forwards frames along that path.

Devices are connected to a switched hub (Fig 5-4) in a similar fashion

to a conventional hub. With the switching hub however, each device

has full access to the available bandwidth, and by eliminating the

sharing of bandwidth, switching hubs can improve LAN performance

substantially.


Workstation

Workstation

Workstation

Workstation

WorkstationSwitched Hub

Workstation File server

Fig 5-4 Switched Network Topology

Complex Topologies More complex topologies often include a number of different network

structures in the same network. As shown by Fig 5-5 it is possible to

have a bus topology, a star topology and a ring topology linked

together to form a mixed or hybrid network.


Fig 5-5 Hybrid Network Topology

Wide Area Networks The most common example of such networks is a Wide Area Network

(WAN). A typical WAN, such as shown in Fig 5-6, can link together a

number of local networks with other networks situated throughout the

country or even around the world. The local networks can use fixed

media such as optical fibre for the backbones that link them together,

while for the long-distance links a number of different options can be

used such as satellite, microwave or telephone links.

Hub

RING

STAR

BUS


New York Network

London

Network

Communications Equipment

Mainframes in Sydney

Fig 5-6 Wide Area Network

Centralised Networks In a centralised network the important network resources such as the

servers are kept in a location relatively central to the rest of the

network. This structure makes it easy to protect valuable resources

and to perform maintenance tasks such as backups and capacity

planning.

A negative aspect of this structure is that when the central server fails,

many users will be affected. To prevent this, a duplicate server, called

a mirror server is often used to take over the functions of the main

server, should it fail.

Distributed Networks Sometimes the important resources of a network may be distributed

throughout the network. For example, each branch of a company or


each department may have its own servers. It can be difficult to

administer such a network structure as tasks such as backups and user

maintenance must often be done on a local level.

An advantage of this structure is that when a specific server fails, not

all the users of the network will be affected.

Access Methodologies

CSMA / CD Carrier-Sense Multiple Access/Collision Detection or CSMA/CD forms

the basis of the Ethernet access method.

The term carrier-sense indicates that nodes check the channel to make

sure that no one else is transmitting on it before they attempt to

transmit.

Multiple access indicates that nodes share the network medium and

have access to all network transmissions.

Collision detection indicates that there is a mechanism for resolving

the problem when two nodes attempt to transmit over the shared

network medium at the same time.

In CSMA/CD networks, a node checks the channel before sending. If

there is no traffic for some predetermined amount of time, the node

can transmit. If the channel is busy, the node monitors the channel

until it is clear and then attempts to transmit. Because another node

may have also been monitoring the channel to send, a collision may

occur.

The node monitors the channel while it is sending so that it can tell

when a collision occurs. If another transmission is colliding with its

own transmission,

the node sends out a signal indicating that any received packet should

be discarded and then waits to send again.


If both nodes back–off for the same time they will just collide again,

therefore a random back-off period is set, so that the collision does not

occur again. Collisions are therefore a bigger problem on busy

networks than on lightly loaded networks.

Token Passing The token passing method uses a token that moves between the

computers on the network in a predetermined sequence. A computer

with a message to transmit waits until it receives what is called a free

token, that is, one available for use. The computer then changes the

free token into a busy token and attaches its message to it.

The computer retransmits the token and the message on the circuit to

the next computer in the sequence. Should that computer want to

transmit a message, it must wait, because the token is busy. It simply

passes on the message with the busy token to the next computer in

sequence. When the token with the message arrive at the destination

computer, it copies the data in the message, sets the acknowledgement

part of the message, and the message continues around the ring, back

to the transmitting computer. The transmitting computer then

removes the message and inserts a new free token on the ring.

Architectures Ethernet (IEEE 802.3) A high percentage of LANs use Ethernet. The basic layout of the

network, the way in which the nodes of the network are

interconnected, uses a bus topology. All nodes are connected to one

circuit running the length of the network that is called the bus. All

messages from any computer flow onto the central cable (or bus) and

through it to all computers on the LAN. Every computer on the bus re-

ceives all the messages sent on the bus, even those intended for other

computers. Before processing an incoming message, the Ethernet

software checks the address and only processes those messages

addressed to that computer.

When more than one computer shares the same communication

circuit, it is important to control their access to the media. Ethernet

uses a contention based media access control technique called

Collision Sense Multiple Access with Collision Detection (CSMA /CD).


It is a very simple concept: wait until the bus is free and then transmit.

Computers wait until no other devices are transmitting, and then

transmit their data.

There are many different types of Ethernet, classified as either

baseband or broadband. Baseband treats the cable as one single chan-

nel, so it can carry only a single transmission at any one moment.

Broadband uses analogue signalling and splits the cable into many

different channels (using frequency division multiplexing) so more

than one transmission can occupy the LAN cable at the same time.

Networks with speeds up to 10 Mbps are normally seen as standard

Ethernet while the term Fast Ethernet is used for networks with

speeds of up to 100 Mbps.

Token Ring (IEEE 802.5) Another popular type of LAN is token ring. As the name suggests,

token ring uses a ring topology. A ring topology connects all

computers on the LAN in one closed loop circuit with each computer

linked to the next. Messages pass around the ring in one direction only

to each computer in turn. Each computer receives messages intended

for all computers, but only processes those addressed to it and

transmits messages to the next computer in the ring.

All computers in a token ring LAN share the same common circuit, so

it is essential that access to the media be controlled. This is done using

a technique called token passing.

Token passing methods restrict the maximum number of messages

that a computer can transmit before it must issue a free token and

permit other computers to send messages. This ensures that no one

computer monopolises the circuit.

Speeds between 4 Mbps and 16 Mbps are typical of token ring

networks.

FDDI Fibre Distributed Data Interface is also a high-speed LAN technology

providing speeds up to 100Mbps. It is not generally used for direct

connection to desktop computers, but rather as a backbone

technology. A backbone connects two or more LAN segments to

provide a path for transmitting packets among them. A simple


backbone might connect two servers through a high-speed link

consisting of network adapter cards and cable.

FDDI networks have a dual, counter-rotating ring topology. This

topology consists of two logical closed signal paths called “rings.”

Signals on the rings travel in opposite directions from each other.

Although both rings can carry data, the primary ring usually carries

data while the secondary ring serves as a backup.

If the cable is cut or a link between nodes fails, the secondary ring

keeps the network functioning.

FDDI uses token passing and is implemented using fiber-optic cable.

It is a fast, reliable standard with the dual, counter-rotating ring

topology increasing the network’s reliability by keeping the network

functioning even if a cable is damaged.

FDDI’s main disadvantage is price. FDDI adapter cards and fiber-optic

cable are both relatively expensive compared to other technologies

offering the same speed. Fiber-optic cable installation also requires

more expert technicians.

High Speed Architectures In this section we discuss two types of faster Ethernet .

100 Base T 100Base-T is a high-speed LAN technology providing data transfer

rates as high as 100 Mbps. Like l0Base-T Ethernet, l00Base-T uses

CSMA/CD as the media access control method. Changes such as

decreased transmission distances between nodes produce a reliable

data transfer rate of 100 Mbps, 10 times faster than traditional l0Base-

T Ethernet.

100Base-T supports unshielded twisted-pair (UTP) wiring, shielded

twisted-pair wiring, and fibre-optic cable. It is simple to upgrade to

100base-T from 10Base-T. It is widely available and as it is similar to

10Base-T there is no need for expensive retraining. Adapter cards are

not significantly more expensive than l0Base-T cards and it uses

relatively inexpensive UTP cable that is often already installed in many

organisations.


100Base-T reduces the maximum network size compared to l0Base-T

because of the shorter transmission distances between nodes. As

CSMA-CD is a shared media contention scheme, collisions may occur,

especially under maximum loads resulting in reduced data

throughput.

Summary In this chapter we discussed the main types of physical layout used for

networks. We looked at bus, star and ring networks, some adaptions

thereof and switched networks. More complex versions and

combinations of these topologies such as hybrid and Wide Area

Network were also covered.

The logical flow of data through the different types of network was also

discussed, as the electrical signals don’t always follow the physical

layout of the network.

Under media access , the CSMA / CD and Token passing methods

were discussed.

Ethernet, Token ring, FDDI and some high speed developments based

on these architectures were covered in the final section of this chapter.

Exercises 1. Discuss and compare the bus, star and ring network topologies.

2. Define CSMA / CD and explain how it works.

3. Explain how the token passing access methodology works.

4. Discuss the differences between 100base T and 100 VG AnyLAN.

Chapter 6: Wide Area Networks 79

6: Wide Area

Networks With the growth of the Internet and global business, it has become

very important to be able to connect computers over long distances.

Many different technologies have become available over the years to

enable us to achieve reliable high-speed links. In this chapter we look

at a few of these.

WAN Switching Switching is a technique for sending a signal with information from

one point to another on a network but along different paths, much like

trains are switched over multiple tracks.

Circuit switching involves creating a dedicated physical circuit

between two nodes that is used to transmit information between them

without interruption, similar to making a telephone call between two

parties. The transmission channel remains in service until the two

nodes disconnect.

Packet switching is a data transmission technique that establishes a

logical channel between two transmitting nodes but uses several

different paths of transmission to continually find the best routes to

the destination. It is a combination of circuit and message switching as

it establishes a dedicated circuit between the two nodes, but the circuit

is a logical connection not a physical one.

A number of physical routes may be used in the session, but each node

is aware only that there is a dedicated channel. The best route can be

established for the type of data and the amount of data sent, creating

an opportunity to make the transmissions high speed.

For any packet switch to process any packet of data bound for

anywhere, it is essential that packet address information be included

in each packet. Each packet switch then reads and processes each

packet by making routing and forwarding decisions based upon the

packet’s destination address and current network conditions.


As a data message is broken up into numerous pieces by the packet

assembler, these message pieces may actually arrive out of order at the

message destination due to the speed of the alternate paths. The data

message must be pieced back together in proper order. These self-

sufficient packets containing full source and destination address

information plus a message segment are known as datagrams.

A switching methodology in which each datagram is handled and

routed to its ultimate destination on an individual basis resulting in

the possibility of packets traveling over a variety of physical paths on

the way to their destination is known as connectionless because

packets do not follow one another, in order, down a particular path

through the network.

There are no error detection or flow control techniques applied by a

datagram based or connectionless packet-switched network with no

guarantee of their safe arrival.

In contrast to the connectionless packet networks, connection oriented

packet networks establish virtual circuits enabling message packets to

follow one another, in sequence, down the same connection or

physical circuit. This connection from source to destination is set up

by special set-up packets. Once the call set-up packets have

determined the best path from the source to the destination and

established the virtual circuit, the message-bearing packets follow one

another in sequence along the virtual circuit from source to

destination.

Because of the virtual circuit, a connection-oriented service can offer

check sum error detection with retransmission and flow control.

WAN Transmission PTSN The Public Telephone Switched Network (PTSN) is an interconnection

of switching centres and connections to subscribers, that offers voice

dial-up between any two subscribers connected to the PTSN. Overhead

or underground multi-strand cables are used to connect subscribers to

the exchange or switching centre. Each switching centre is

interconnected to the next. The development of automatic exchanges

has simplified the interconnection of subscribers.


Earlier in the evolution of data communications, the only connection

mechanisms available were via dial-up lines provided over the

switched telephone network. These dial-up lines were suitable for

voice use, but severely affected the transfer of digital data. They were

prone to noise from the electro-mechanical switches used to

interconnect subscribers, and they placed limits on the speed at which

data could be transmitted. The computerisation of switching centres

has reduced the interference and noise associated with voice channels,

and high grade (low error rate), high speed (9600bps or higher)

communication is now possible over most dial-up voice circuits.

Special end-to-end circuits that are suitable for data are available. No

switching is involved as they bypass the switching mechanism in the

exchange. These leased lines are suitable for data communications at a

number of different speeds. A typical use of leased lines is to provide a

permanent connection to the Internet.

ISDN The Integrated Services Digital Network (ISDN) is a network design

that allows a network to support voice, data, video and other data over

twisted pair cable. It is fully digital, and requires voice users to convert

speech into digital signals before transmission. Rather than a company

having three separate lines to handle three machines like phone, fax

and computer, it can now have a single physical line and connect all

three simultaneously.

The single line is a digital line that connects into the telephone

company’s digital network. ISDN is not the same as the PTSN. Where

the PTSN is an analogue network designed for speech, the ISDN is a

digital network designed for transferring digital data.

The basic ISDN connection is a 2B + D connection. That is, 2 B

channels each of 64Kbps, and a single D channel of 16Kbps. The B

channels are designed to carry user data, whilst the D channel is

meant to carry control and signaling information. This is known as the

Basic Rate Interface (BRI).

The Primary Rate Interface (PRI) offers 23B channels and one D

channel at 64Kbps each in North America and Japan and 30B

channels and one D channel for Europe, Australia and some other

parts of the world.


ISDN uses the existing telecommunications dial-up infrastructure,

though special ISDN connection interface boxes are required at the

user’s premises. Each B channel can be used separately or combined

with other B channels to achieve higher speeds. Calls are set up to a

destination on demand, just as in the PSTN. Once the connection is

made, the user has access to the full digital capacity of the link. Users

are often charged on a combination of time and the amount of data

transferred.

ISDN offers reduced cost, reduced error rates, access to different

services over a single link and a standard network interface. It is

typically used for desktop conferencing, video conferencing, high

quality audio connections for sports events and temporary Internet

connections for retrieving and sending email.

ADSL As access to the Internet and associated applications like multi-media,

teleconferencing and on demand video become pervasive, the speed of

the local loop (from the subscriber to the telephone company) is now a

limiting factor with analogue modem connection rates up to 56Kbps,

which is too slow for most applications.

Asymmetric Digital Subscriber Line (ADSL) is a technology for

transmitting digital information at a high bandwidth on existing

phone lines to homes and businesses. Unlike regular dialup phone

service, ADSL provides a continuously-available, "always on"

connection. ADSL is asymmetric in that it uses most of the channel to

transmit downstream to the user and only a small part to receive

information from the user. ADSL simultaneously accommodates

analogue (voice) information on the same line. ADSL is generally

offered at downstream data rates from 512 Kbps to about 6 Mbps.

To implement ADSL, a terminating device is required at each end of

the cable, which accepts the digital data and converts it to analogue

signals for transmission over the copper cable. In this respect, it is very

similar to modem technology.

Dedicated (Leased) Lines Dedicated lines are fixed high speed connections between two desired

locations, at speeds ranging from as low as 9600bps to as high as


45Mbps. The permanent end to end connections do not involve

dialling. The higher the speed, the greater the cost, which is usually a

fixed monthly rental charge.

The connection is suited to companies that want permanent

connections between their office branches, or perhaps to a company

that wants a permanent connection to the Internet.

The basic unit of measurement for dedicated lines is a T1 connection,

which supports 1.544Mbps. A T3 connection supports 45Mbps.

Fractional T1 circuits are available in units of 64Kbps, with

connections of 384Kbps, 512Kbps and 768Kbps being common. This is

not available in South Africa.

WAN Services X-25 Packet switching is an established technology which sends data across

a packet switched network in small parcels called packets. If the data

packets travel the same path to the destination, this is called virtual

circuit; if packets can travel any path, not necessarily the same as each

other, this is called datagram.

Packet switched connections are normally in the speed range of

19.2Kbps to 64Kbps, though some higher speed connections may be

available in certain areas. As it is a dial-up switched connection, it is

not suitable for permanent connections.

X.25 was designed to be implemented over noisy analogue phone

lines. Thus it has a lot of built in error control. An X.25 connection

supports several virtual circuits that are numbered. These represent a

time division of the available bandwidth of the connection. This

division into virtual circuits allows each virtual circuit to support a

single device. The virtual circuit is a full duplex connection that is

established for the duration of the call.

Devices which do not have built in packet switched support can be

interfaced to a packet switched network using a Packet

Assembly/Disassembly (PAD) unit. This allows existing computers or

terminals to be connected.


Frame Relay Frame relay was introduced to meet the demands of high-volume,

high-bandwidth WANs and has become very popular with large

organisations.

The concept behind this technology is similar to X.25, in that frame

relay uses packet switching and virtual circuit techniques. Frame relay

is designed to interface with modern networks that do their own error

checking. It achieves high-speed data transmission by not doing

extensive error checking. It is used with TCP/IP or IPX—based

networks, where those protocols handle the end-to-end error

checking.

Frame relay looks for bad frame check sequences. If it detects errors

that were not discovered by intermediate nodes, it discards the bad

packets. It also discards packets if it detects heavy network congestion,

a disadvantage that should be considered.

Cell-Relay - ATM Cell relay is a technology that takes frame relay a step further. Because

frame relay breaks data into packets designed for switched store-and-

forward transmission, it is not suitable for voice and video

applications. Cell relay creates large fixed-length data entities called

cells. When transmitted, a cell may be empty or full of data. The cell

relay technology enables information to be sent without the packet-

switching delays of frame relay.

In ATM (Asynchronous Transfer Mode)networks, all information is

formatted into fixed-length cells consisting of 48 bytes of payload and

a 5 byte cell header. The fixed cell size ensures that time-critical

information such as voice or video is not adversely affected by long

data frames or packets. The header is organized for efficient switching

in high-speed hardware implementations and carries payload-type

information, virtual-circuit identifiers, and header error check.

ATM standards defined two types of ATM connections: virtual path

connections (VPCs), which contain virtual channel connections

(VCCs). A virtual channel connection (or virtual circuit) is the basic

unit, which carries a single stream of cells, in order, from user to user.

A collection of virtual circuits can be bundled together into a virtual

path connection. A virtual path connection can be created from end-

to-end across an ATM network. In this case, the ATM network routes


all cells belonging to a particular virtual path along the same way

through the ATM network.

ATM handles data, voice, and video transmissions. It can be used for

LAN or WAN communications eliminating the need for separate

short- and long-distance networks and allowing high speeds over

Ethernet, token ring, Fast Ethernet, FDDI, and other kinds of

networks. ATM can handle transmission speeds that range from 155

Mbps to 622 Mbps, with a theoretical limit of 1.2 Gbps.

Summary In this chapter we looked at the various options for connecting

networks over long distances. The options vary widely in the range of

speeds and services that they can deliver. Some of the newer

technologies have only recently started to catch on due to lack of

suitable infrastructure in many parts of the world.

Exercises 1. Use the Internet to compile a summary of the data transmission

services offered by Telkom.

2. Research the latest ADSL developments and describe how they work.

3. Discuss the implementation of ISDN with the advantages and

disadvantages related to it.

Chapter 7: Internet 87

7: Internet

The Internet grew out of a project begun in the late I960s by the

Advanced Research Projects Agency (ARPA) of the USA Department

of Defense. The goal of the project was to create a fault-tolerant

computer network that could continue functioning even if one

computer site were destroyed.

The result was ARPAnet, as the first big network project was called,

which explored new ways of creating a linked network. Stanford

University, UCLA, UC Santa Barbara, and the University of Utah were

selected as the four original ARPAnet nodes.

Decentralization is what made this network special. Instead of the

network cables dictating the flow of information, each computer was

assigned equal responsibility for receiving, interpreting, and

transmitting network information. This was also the first

implementation of the Internet Protocol (IP).

Internet Protocol (IP) To understand IP, consider the scenario where a letter that you have

written is taken to the nearest post office, which sends your letter,

along with a bunch of other mail headed for that postal code, to the

airport to be flown to a regional post office in that area, which then

sends your letter to the closest city post office, which routes it to a mail

carrier, who delivers the letter to its destination

Many intermediaries would be involved in the process, and a letter

going from one point to another might take a different route from time

to time. On its next trip, the plane leaving Cape Town might stop in

Johannesburg to consolidate the Durban mailbags into another plane

headed for that airport.

IP works in a similar way in that it establishes connections from one

point to another on a network via a route that is determined by the

networked computers. The protocol part of IP is a set of rules for

determining how a process should be carried out, in this case, the rules


pertain to how information is sent efficiently and dynamically over a

network. Messages, divided into small segments for easier handling,

are put inside an IP packet, which is addressed and transmitted. Like

the post office example, the message packets may make several “hops”

from source to destination. The fewer the number of hops, the faster

the message travels.

During the 1970s, ARPAnet grew as other universities and nonmilitary

researchers were permitted to join. So many sites across the USA

joined that the original standards and communications protocols

could no longer support the traffic. A new standard, called

Transmission Control Protocol (TCP), was proposed in 1979 as a

supplement to IP, and it was adopted in 1983 as TCP/IP.

The IP portion of the joint protocol takes care of routing the packets to

their destination. The TCP part performs integrity checks on the data

and enhances the reconstruction of the packets into the original

message or file at the destination end.

Today, TCP/IP forms the basis for over 100 different protocols for

data and message transmission among computers on a network. A

driving force behind this development was the desire to allow different

kinds of computers to communicate with one another over a common

network. Any computer that can use TCP/IP can communicate over

the Internet.

In 1983, the Internet was formed when the military operations portion

of ARPAnet split into its own network, MILnet, unburdening ARPAnet

of some of its traffic load, yet allowing it to continue to serve as a

research communications tool and information highway. Computers

were assigned IP addresses, and gateways were set up to handle

TCP/IP packet forwarding between ARPAnet and MILnet networks.

As new networks and individual nodes were added to the Internet,

they adhered to TCP/IP network conventions.

Internet Service Providers

Before the power from high voltage power lines is connected to your

home, the power is fed through a series of step-down transformers so

that when you plug a kettle into the socket, it is connecting to 220


volts. You could not use, you do not need, and you would not want to

pay for the higher level of power. In a similar way, an ISP provides the

level of bandwidth that is required for your personal or professional

needs.

Bandwidth refers to the speed with which data travels, measured in

bits per second (bps). Over a network connection, bandwidth makes

an enormous difference in how much data you can send and receive in

a given period of time. Bandwidth becomes increasingly significant as

you exchange not just text messages, but more space-demanding types

of data, such as graphics, audio, and video. While a brief e-mail

message takes up only 2.5KB, a typical scanned picture occupies

300KB, audio takes up about 475KB per minute, and one minute of

video can require upwards of 1,800KB.

Other important issues to take into account when selecting a service

provider are:

• Does the ISP support the type of computer(s) that will be

connected?

• Does the provider have multiple points of presence?

• Does the provider place any restrictions on business uses of

your account?

• What is the ISP’s maintenance schedule?

• How will the ISP respond to service problems?

• What are the startup and monthly costs for using the Internet

connection?

• Can your ISP meet your future needs?

PPP and SLIP Connections Point-to-Point Protocol (PPP) and Serial Line Internet Protocol (SLIP)

are communication standards that allow a computer connected via a

modem to act as a node on the Internet network. In addition to

supporting TCP/IP, a modem connection from your PC to the Internet

must support the underlying protocol for communicating over a serial

line, such as is found telephone lines. The serial line protocol is

responsible for error control, packetizing the data, and other activities

that allow TCP/IP programs to work together. PPP is the newer


protocol, and it is considered to be more robust than SLIP. PPP is

built in to most modern operating systems.

Internet Domain Names One of the first service matters you will conduct with your ISP is to get

an Internet address so others can contact you. A fully qualified address

includes the user ID and a complete domain description. For example,

the address [email protected] is a fully qualified address. The

user ID, the part to the left of the “at” symbol (@), describes the

account owner; the part of the address after the “at” symbol is the

domain name.

Having your own domain name will make it easier for customers,

contacts, suppliers, and others who want to reach you to remember

your address and associate you with your organization. Individuals, on

the other hand, may not wish to go to the trouble of applying for their

own domain name, in which case they will be able to use the ISP’s

domain name.

A domain name has multiple parts, separated by a period. The

rightmost portion of the name is used to indicate the highest level of

the domain, known as the top level. Working backwards from that

portion of the name, you get more and more specific information

about the host. Take the address “commerce.uct.ac.za” for instance.

Reading from the right, the za indicates South Africa, the ac indicates

an educational institution, uct Cape Town University and commerce

the commerce faculty server machine.

Country codes are being added to the list of top-level domains. There

are over 300 country codes, and most of those have Internet

connectivity at present. Recent changes in the way domain names are

assigned open up “geography” domains as opposed to “category”

domains. For example many domain names now end with a country

code (eg. za) as opposed to com or some other category domain.

Associated with each domain name is a numeric address, known as the

IP address. IP addresses are 32-bit numbers divided into four parts.

Though no central authority runs the Internet, various committees

and sponsored organizations manage various aspects. Among those

organizations is the Internet Network Information Center, or InterNIC

for short. The InterNIC currently leases domain names at a fixed price

per year.


Previously, the names and IP addresses of machines could be kept in a

single file maintained by the InterNIC, and that file was distributed to

every host on the network. But as the size of the network grew, so too

did the size of the file, not to mention the time to process domain

name registrations. Now, the process is more decentralized. Instead of

assigning individual names, the InterNIC assign an Internet service

provider a block of IP addresses, from which they assign a number to a

unique domain name. These blocks of addresses are given out in three

sizes. A Class C block is the smallest and most common. It would look

similar to 192.3.44*, and would allow a network administrator to

assign up to 254 IP addresses. A small business might be interested in

obtaining one or more Class C addresses. A large organization or a

regional ISP would be better off with a Class B address, which would

give them the 128.3.*.* number space of 64,516 nodes. Very rarely are

Class A blocks assigned, since they take the form of 0.*.*.* and hold

the potential for over 16 million IP addresses. Class A addresses start

with a number between 0 and 127, Class B addresses start with a

number between 128 and 191, and Class C addresses start with a

number between 192 and 255.

Generally domain names are easier to understand than IP numbers.

Computers, however, translate the name into an IP address so they

can process it faster. This translation process takes place automatically

when you request a service of a host computer via the Domain Name

System.

Sometimes you will request an Internet service such as a Web page or

FTP directly and not be able to use it. Several reasons may account for

this, including the following:

• The host providing the service is offline.

• Too many other people are using the service at this time, and

you will have to wait for an open port.

• A network connection is having problems.

• A specific IP number or domain name has been declined access

because the administrators of the host no longer wish to offer

public service due to resource abuse or excessive traffic loads.


Linking Up You will need two kinds of software when using the Internet over a

dial-in connection. First, you will need communications software to

establish a PPP connection. Once that connection is established, your

machine will be recognized as a node on the Internet and you will be

able to run TCP/IP-based software (like a Web browser) for the

particular Internet services you wish to use. Users with a dedicated

connection will have a TCP/IP connection established already, and will

be more concerned with providing client software and setting up

Internet servers.

The client software is generally responsible for providing the user with

the interface, connection, and the translation. Client software is

responsible for opening and maintaining a connection to the server,

translating your commands into instructions suitable for the server,

and interpreting the results of the server’s work into a useful format.

Many current Internet programs employ a graphical user interface

(GUI), complete with icons, pull-down menus, toolbars, scrolling

windows, and mouse control. These functions are handled by the

client application as they would be for any other applications you run

on your computer.

E-Mail Internet messaging services must define certain requirements and

standards if they are to he successful. These requirements include:

• A message format where all messages are easily identifiable

as messages.

• A framework for moving messages from one system to

another, across the network.

• A framework for delivering messages to users.

• A mechanism for resolving message addresses, usually

expressed in human-friendly formats, into machine-friendly

addresses to facilitate delivery.


Message Formatting We address three levels of message formatting issues here: data

representation, message and header formatting, and enclosure

formatting.

Data Representation

Anything that is sent out onto the Internet as a message must use the

same kinds of bits to represent the same kinds of information.

We have already got a standard for Internet data representation. It is

called 7-bit ASCII or US-ASCII, and it uses the first seven bits of each

byte to represent one of 128 values; the eighth bit is always set to 0.

Internet messages must use 7-bit ASCII for their headers, at least, and

Internet messaging systems must treat non-ASCII data in a way that is

consistent with other systems expecting to process only 7-bit ASCII

data.

Message Syntax

Messages may consist of ASCII data, but that data must be organized

in a way that makes it recognizable as a message as opposed to a

simple stream of characters. Each message consists of an envelope and

contents. The envelope contains whatever information is necessary to

deliver the contents. The structure and format of the envelope are not

specified by current standards. However, the envelope may be created

by a messaging application by extracting certain information from the

contents of the message.

The Internet Message Format Standard (MFS) specifies that messages

consist of a set of header fields, which are grouped together into the

message header, and an optional message body. You can send a

message with no body, but you can’t send a message with no header.

Some header fields are requiredand others are optional but all must

adhere to the proper syntax. messaging applications may peek into the

message contents and use the data in message header fields to create

the message envelope.

The format of the message envelope is determined by the application

handling message delivery, which may be the Simple Mail Transfer

Protocol (SMTP)or some other message transfer protocol. MFS also

limits itself to defining messages entirely consisting of US-ASCII


characters and leaves specification of binary data enclosures to other

standards, notably MIME.

Enclosure Formatting

The Multipurpose Internet Mail Extensions (MIME) standards define

mechanisms that allow senders to enclose, and recipients to detach,

non-ASCII objects that have been sent with standard Internet

messages.

MIME is a specification that defines a mechanism for defining special

types of data. MIME can be used for binary and multimedia data,

including programs, graphics, audio, video, and any other non-ASCII

data. MIME can also he used for ASCII data that carries its own

formatting. For example, a rich text format (RTF) word processing file

consists of ASCII text and formatting tags. You could encapsulate an

RTF document in a MIME enclosure that would allow a MIME-

enabled client to display it.

MIME uses two sets of headers to do most of its work. The content-

type header identifies what kind of material is being enclosed and the

cortent-transfer-encoding header identifies how the content is

encoded.

Secure MIME (S/MIME) is a proposed Internet standard that defines

a mechanism for consistently sending and receiving MIME data

securely. S/MIME uses digital signatures to do authentication, data

integrity, and nonrepudiation of origin on MIME enclosures. It also

uses encryption to provide data security for MIME enclosures.

Message Transfer The message transfer protocols are concerned with the addressing and

delivery information stored on the message envelope.

Message transfer protocols define how messages are forwarded from

one system to another across the Internet or other networks. Message

transfer protocols do not address the question of how to deliver

messages to end users. The Simple Mail Transfer Protocol (SMTP)

protocol defines SMTP functions, including SMTP client and server as

well as SMTP relays and gateways.

The flow of messages within the SMTP-defined transport is from a

message store (a file system) on one server, across the network, to


another message store on the destination server. A fully capable

SMTP server is able to act as an SMTP client and initiate requests to

transfer messages to another SMTP server, as well as act as an SMTP

server and respond to requests from SMTP clients. Some SMTP

servers may act only as relays, accepting messages that are placed in

its message store and then forwarding them to another SMTP server,

regardless of the ultimate delivery destination.

Email client software usually behaves as a less capable SMTP server,

submitting messages from the user to a more capable SMTP server for

forwarding. An organization might also use a relay SMTP server to

collect outbound messages from users and then forward them outside

a firewall to another SMTP server that has Internet connectivity.

A gateway SMTP server acts as a protocol gateway, meaning that the

gateway accepts messages using SMTP but then converts those

messages into some other message delivery protocol (eg. Lotus

CC:Mail) for local distribution.

Message Delivery

The message forwarding protocols describe how messages can be

moved from one server’s message store to another server’s message

store, but leave out the problem of how those messages arrived in the

originating server’s message store and how they are retrieved from the

receiving server’s message store.

Three common mechanisms place messages in front of users and

submit messages from users. The simplest is to implement an SMTP

server that has a user interface for receiving messages and allow the

user to read those messages with some utility such as a text processor

or email client. The problem with this approach is that it requires that

the user’s system be available at all times to receive incoming

messages. It cannot be turned off at night, nor should it be brought

down for any length of time.

A further mechanism defines a protocol that allows a server to act as a

post office for a client, fulfilling a function similar to the way a brick-

and-mortar post office offers post office box services. The email Post

Office Protocol (POP) collects messages by acting as a receiving SMTP

system and putting the messages in a data store. The email client

checks the user’s electronic mailbox every so often to see if any

messages are there. If there are, the client downloads the messages


and puts them on a local file store for the user to read. The post office

server is always up and always ready to accept messages. The users

can check their mailboxes whenever they want and the server will

deliver all current messages. The Post Office Protocol version 3 is the

standard used by the Internet.

FTP FTP stands for File Transfer Protocol. The abbreviation stands for

both the network protocol as well as the service it provides. With FTP,

you can search for and download public domain, shareware, and

private files from all over the world. FTP has the ability to transfer

both text and binary files across different computer platforms and

networks.

World Wide Web The World Wide Web (WWW) was created by physicists who were

looking for a better way to cross-reference scientific research papers at

the Swiss European Laboratory for Particle Physics (also known as

CERN) in 1993. Since that time, it has become the fastest-growing

Internet service. It was estimated that nearly 400 million Web pages

exist, as of early 1999.

Web users access information resources by following hyperlinks to

different information resources and services. The most popular Web

clients are MS Internet Explorer and Netscape Communicator, which

have versions available for PC, Mac, and UNIX platforms.

By entering a Web address, known as a Uniform Resource Locator

(URL), a Web client can provide hypertext links to text, graphics,

sound, and movies. URLs are given in a format that lists the service

first and then the host name and path. Services include http (for

WWW), FTP, and mailto (for e-mail). Web pages also offer the ability

to collect information from users through the use of forms.

Creating basic Web pages can be done on any platform using a simple

text editor by using HTML (Hypertext Markup Language). It is

characterized by simple tags bracketing text and giving it special

properties when read by a WWW browser.


Web pages are published by putting them on a server so that others

can access them via their URL. Many software packages such as

Netscape Enterprise Server, Microsoft Internet Information Server

and the free Apache Web server support this activity.

Summary It is a challenge to keep up with the rapid changes that take place on

the Internet and users of this technology who take the time to do so

will have a clear advantage in understanding this new environment

and enjoying its advantages.

Organizations can use the Internet to inform the world about their

products and services. Web pages can bring a whole new meaning to

the concepts of marketing and customer service while information

exchange with other parts of the organization, outside technical

support and current and potential customers can be facilitated

through the use of the Internet.

The Internet is not just about hooking up computers, but also about

allowing people to make connections in new and interesting ways. The

Internet is a network of networks for computers, people, and

information, it is becoming as standard a part of business equipment

as the telephone and fax machine.

Exercises 1. Create a simple web page using a web page editor or a simple text

editor. Make sure it has a heading, some links and at least one image.

2. Describe how an e-mail travels from one user to another.

3. Determine the IP address of any workstation connected to the

Internet. Is it a Class B or a Class C address ?

4. Use FTP to download a file from a FTP server to your computer.

Chapter 8: Network Security 99

8: Network

Security

Security in a network / data communications environment can be

grouped into two basic categories: logical security, and physical

security. In this chapter we will be discussing both of these categories

in more detail. We will look at the minimum management policies

that an organisation should have in place to minimise its security risk,

and touch on topical issues such as hacking, viruses, backups, software

piracy and disaster recovery.

The amount of money spent on security, normally depends on both the

risk profile of the organisation / industry, and on the implication s of a

security breach. For instance, in an organisation where data is mostly

of public value, the only real risk would be that of data corruption.

This can easily be catered for with a good backup and restore system.

It does not make business sense to spend more money on data

protection than the data or its implications of misuse is worth. If, on

the other hand, the organisation could be held accountable for data

misuse, the risk profile (and possible cost) changes considerably, and

more money should be invested in security measures.

Logical Security Logical security refers to all methods used to secure information, data,

and data flows.

Passwords One of the most basic techniques for restricting access to data, or to

authenticate users on a network, is with the use of passwords.

Passwords should be changed on a regular basis (recommended every

forty-five days) and should contain non-alphabetic characters.

Passwords should be a minimum of five characters in length, and

when asked for a password change, the system should not allow

passwords that have been recently used.


The best form of password is to use nonsensical words that have no

meaning except to you. A good example is ‘HlTm9qRs2’. Some

systems are case sensitive and it is advisable to use multiple-case

passwords to enhance security in these systems.

Encryption Most network operating systems can use encryption techniques to

enhance data security during transmission and storage. For example:

Novell NetWare ™ uses the RSA™ public key encryption scheme.

Public Key Encryption

The public key infrastructure (PKI) assumes the use of public key

cryptography, which is the most common method on the Internet for

authenticating a message sender or encrypting a message. Traditional

cryptography has usually involved the creation and sharing of a secret

key for the encryption and decryption of messages. This secret or

private key system has the significant flaw that if the key is discovered

or intercepted by someone else, messages can easily be decrypted. For

this reason, public key cryptography and the public key infrastructure

is the preferred approach on the Internet.

A public key infrastructure consists of:

• A certificate authority (CA) that issues and verifies digital

certificates. A certificate includes the public key or information

about the public key.

• A registration authority (RA) that acts as the verifier for the

certificate authority before a digital certificate is issued to a

requestor.

• One or more directories where the certificates (with their

public keys) are held.

• A certificate management system.

In public key cryptography, public and private keys are created

simultaneously using the same algorithm (a popular one is known as

RSA) by a certificate authority (CA). The private key is given only to

the requesting party and the public key is made publicly available (as

part of a digital certificate) in a directory that all parties can access.

The private key is never shared with anyone or sent across the

Internet. You use the private key to decrypt text that has been

encrypted with your public key by someone else (who can find out


what your public key is from a public directory). Thus, if I send you a

message, I can find out your public key (but not your private key) from

a central administrator and encrypt a message to you using your

public key. When you receive it, you decrypt it with your private key.

In addition to encrypting messages, you can authenticate yourself to

me by using your private key to encrypt a digital certificate. When I

receive it, I can use your public key to decrypt it.

A number of products are offered that enable a company or group of

companies to implement a PKI. The acceleration of e-commerce and

business-to-business commerce over the Internet has increased the

demand for PKI solutions such as RSA, which has developed the main

algorithms used by PKI vendors.

Pretty Good Privacy

For e-mail, the Pretty Good Privacy (PGP) product lets you encrypt a

message to anyone who has a public key. You encrypt it with their

public key and they then decrypt it with their private key. PGP users

share a directory of public keys that is called a key ring. If you are

sending a message to someone that doesn't have access to the key ring,

you can't send them an encrypted message. As another option, PGP

lets you "sign" your note with a digital signature using your private

key. The recipient can then get your public key from the key ring and

decrypt your signature to see whether it was really you who sent the

message.

Password Encryption

User passwords are normally encrypted at the workstation network

client, and sent, in encrypted form, to the server for authentication.

Some vendors (Novell, Microsoft) do not decrypt the password at all,

but compare it in encrypted form to an encrypted password table on

the server.

Access Rights User authentication is normally of a ‘go’, ‘no-go’ nature. It is often

necessary to restrict users on more levels to allow some users only to

read, some users to read and update, and other users to only see data

that they themselves have created. To this effect, most network

operating systems have some form of access control rights associated

with the user and different data areas. The system / user


administrator can normally change these access rights to reflect the

needs of the user in the security context of the organisation.

Screen Savers Also known as ‘Console Locking’, passworded screen savers are an

important part of network and data security. Employees often leave

their workstations for short periods of time during the normal working

day. They normally do not log out of the network, but leave the

machine available to any unauthorised person who happens to have

physical access to their desk. Passworded screen savers automatically

execute after a pre-set period of inactivity and prevent possible abuse

from unauthorised persons. Screen savers should auto-execute after

three to five minutes at maximum, to be useful.

Time Scheduling In environments that require a high degree of transaction security (for

instance financial institutions), it is often a good idea to restrict access

to network and data resources to reflect the normal working day.

Most network operating systems allow for users to be automatically

logged off, and disallowed outside of pre-set hours. It is a good idea to

either restrict access (based on a time buffer), or at a minimum to log

and report on exceptions for these types of organisations.

Intruder Detection Most network operating systems have the ability to log and detect

incorrect user logins. An incorrect login is where the wrong username

and password combination is entered to try to authenticate to the

network. It is a good policy to set the system to automatically disable

the account after three successive wrong login attempts. The system

can automatically re-enable the account after thirty minutes to an

hour. This ensures that unauthorised persons do not try to guess

usernames and passwords.

Firewalls Firewalls are logical or physical devices that restrict network access

based on machine location and / or machine identification. A typical

firewall could be set up to allow only a certain user to authenticate

from a specific machine / location, or to allow authentication from a

certain group of machines or users only. This feature is especially

useful in an environment where security is of paramount importance,


and should be considered with the simultaneous implementation of

time scheduling.

A general use for firewalls is for protecting the organisation from

illegal access through the internet. Such a firewall would normally be

set up to only allow incoming traffic to specific applications such as

FTP and WWW servers, on specific machines.

Backups & Disaster Recovery A good backup strategy is an essential part of ensuring data security

and integrity. The most important function of a backup system is that

of data recovery. This function should be tested on a regular basis to

ensure that equipment and software are performing as intended.

There are two basic backup strategies. A full backup strategy would do

a complete backup of all applications and data, with every backup run.

An incremental strategy would do a full backup only once in a cycle,

and would then do incremental backups (changed files only) during

the rest of the cycle. Your choice of strategy would depend on the

amount of data, the size of your backup device, and the size of your

backup time window.

A backup cycle normally involves a daily backup tape-set for each

working day of the week, one tape-set for the end of each week in the

month, and one set for each month of a six month period. Each set is

re-used at the end of its period, for the following period. For instance,

the Monday tape set would be re-used every Monday, the End of Week

1 set at the end of every first week in the month, etc. It is

recommended that backup tape sets should be replaced at regular

intervals to ensure optimal storage integrity. These backup tapes

should be held “off-site” in case of major physical disasters.

Physical Security Physical security categorises all methods to secure physical machinery,

cabling, and related peripherals.

Restricted Access Areas One of the best methods of physical equipment security is to restrict

access to the areas where equipment is located. The most effective

version of a secure area remains a locked door with a known /


controlled set of keys. Other secure area mechanisms include card

readers, and security clearance of on-site personnel. Modern access

control methods for high security areas also often make use of

biometrics as an authentication mechanism.

Biometrics access control is based on using aspects unique to the

individual, such as fingerprint identification, facial recognition and

voice recognition, to control access to facilities.

Active Measures Active security measures include the use of monitored security

cameras, regular security patrols, and documented and enforced

procedures for checking equipment for home use, or bringing your

own equipment in from outside the organisation.

Passive Measures Passive security measures include unmonitored video recording

systems, passive alarms and infrared detection devices, smart tags,

and visual markings on equipment.

Smart tags allow the triggering of alarms if tagged items are removed

from specified locations.

Minimum Policy Documentation Every organisation should have a minimum set of policies documented

and available to staff to ensure data security and integrity.

Minimum documentation should include:

• documented daily, weekly, and monthly operational

procedures

• documented policies on software piracy

• documented policies on virus detection and prevention

• documented backup procedures

• disaster recovery documentation

• documented security policy.


Software Piracy Software piracy is both unethical and illegal. Software groups such as

the Business Software Alliance (BSA) encourage employees to report

the illegal use of software. Legal proceedings often follow, and the

possible total cost in fines and legal costs can be staggering. These

groups do not necessarily distinguish between software used by an

organisation, and software used in a private capacity by employees of

the organisation while on the premises of the organisation.

To safeguard itself against legal action organisations should have a

written policy on software piracy that is available to staff. This policy

should state that the organisation does not condone its employees

illegally copying or using software, and should spell out the

recriminations that would result if employees make themselves guilty

of such acts. Procedures should also be in place to regularly audit

software on equipment owned by the organisation. In brief, the

organisation must show that it is actively trying to prevent the illegal

usage of software on its premises and equipment.

Viruses A virus is a malicious software program designed to infiltrate and

propagate through connected networks, by file copying, data access, or

email. Viruses normally carry a payload that attempts to corrupt or

destroy data. It is of vital importance that your organisation has up-

to-date anti-virus software installed to safeguard its data resources.

New viruses are released every day, so anti-virus software must be

updated continually.

Documented procedures should be made available to staff to specify

what actions should be taken when data or applications that originated

external to the organisation, are accessed. These procedures should

include instructions to remove viruses from infected sources.

All personal computers in an organisation should be checked for

viruses regularly.

It is good idea to automate anti-virus software distribution throughout

the organisation. Some organisations also implement anti-virus

software on shared data and application servers, to ensure that all data

and applications which are served to workstations remain virus free.


Disaster Recovery Manual Disaster recovery procedures should be well-documented and

available to staff. These procedures should include telephone lists of

people to contact, scenario work flows of what to do for specific

disasters, and contact details of relevant vendors / consultants with

the applicable product lists and approximate costs to replace / lease

equipment etc. damaged during a disaster.

Staff should be trained in the use of these manuals, and the policies

and lists in the manuals should be checked regularly to ensure they

remain up-to-date and relevant.

Security Policy Document Every organisation should have a document detailing its security

procedures. This document should include policies controlling the use

and sharing of usernames and passwords, the organisational view of

data privacy and misuse, remote data access policies and procedures

governing general use of technology resources. This document should

be available to staff.

Hacking A hacker is a person or group that actively attacks your network and

data security with the aim of gaining access to restricted data.

Organisations can protect themselves from hackers by implementing

basic security measures.

Dictionary Attacks Hackers often use ‘dictionary attacks’ to try to guess passwords.

During a dictionary attack, a hacker would have a software program

supply a dictionary of possible passwords, one at a time, to the request

from the operating system for authentication information. By using

nonsensical passwords and by enabling intruder detection (see logical

security), this type of attack can be countered effectively.


Sniffers & Spikes Hackers sometimes make use of protocol analysers called sniffers, or

physical taps into network cabling called spikes, to analyse data

flowing through the network. They employ this technique to try to

find passwords. By enabling password encryption and by changing

passwords regularly, this method of attack can be effectively

combated.

Telnet Software Another method of entry is to load a server application on a user’s

machine. This can be done via a virus type of application, or by

physically gaining access to the users’ machine while the user is still

authenticated. By restricting access to the organisation’s premises,

loading passworded screen savers, and anti-virus detection software,

this form of attack can be effectively combated.

Renaming of Admin Accounts Hackers often target known supervisor accounts. Most network

operating systems have standard naming conventions for default

supervisor accounts. It is good security practice to rename these

accounts where possible, to guard against this form of attack. It is also

advisable not to use the supervisor accounts directly, but to create

supervisor equivalent accounts. This facilitates tracking changes

through the use of an audit trail, and ensures that multiple users do

not know the same password. It is good security policy to document

administrator passwords in a sealed envelope, and to store them in a

controlled safe.

Modems Modems are often a convenient and unprotected entry point to your

organisation’s network. Employees often connect modems to their

own personal computers for data sharing from home. A number of

hacking software programs exist that take lists of telephone numbers,

and dial each in turn to note which numbers are connected to

modems. Hackers use this information to mount attacks on these

normally unprotected entry points. As part of your organisation’s

security policy, staff should be made aware that all privately owned

modems need to be set up in ‘dial-back’ mode. When a staff member

dials in and identifies him/herself correctly to the modem software,


the software should hang up, and redial a pre-configured number to

re-establish the connection. Even if a hacker manages to guess the

correct username and password combination, the connection is

broken as soon as entry is gained.

Summary In this chapter we discussed the measures that can be implemented to

protect data and data flows from breaks in security. Logical security

was discussed under the headings of passwords, encryption, access

rights, screen savers, time scheduling, intruder detection, firewalls and

disaster recovery. We also looked at physical security and the creation

of security policies. Finally a number of methods that can be used by

hackers were explained.

Exercises

1. Using the Internet, find some recent examples of network security

breaches.

2. Explain how Public Key Encryption works.

3. How would you protect your password?

4. Discuss the measures you would implement to secure the data and

data flows in an organization.

Chapter 9: Networks and e-commerce

109

9: Networks and

e-Commerce E-commerce is conducted over a complex infrastructure. Much of this

has already been covered in previous chapters and therefore we will

attempt to illustrate in this chapter how other communication forms,

such as Electronic Data Interchange, stand in relation to the World

Wide Web.

Data Interchange Developers attempt to integrate systems so that all types of data can be

shared via the Web. At the moment some types can be accessed using

special plug-in software automatically invoked by browsers, but some

less compatible types still require extensive reformatting in order to be

available to other systems.

To share data between companies using enterprise software, or even

between components of systems within companies that use different

data formats, a common interface can be written to automatically

interpret the data. This interpreting software is called middleware. As

middleware is needed for each type of enterprise system, many

programmes have to be written to make the data available.

XML As HTML cannot display all types of data needed for business

purposes, a new specification for the way data is presented by software

programs called Extensible Markup Language (XML) was developed.

XML provides a structure to the data for interpretation by computer

applications across the Internet. This is achieved by tagging with

meta-data, describing characteristics related to each data field. So

when you type in a field, the meta-data will define what that field of

information is supposed to be describing and how it relates to other

fields.

The XML specification covers the following:


• How tabular data can be imported into other applications,

columns can be sorted and switched, etc.

• How EDI data sets can be represented more flexibly using the

XML standard, allowing more users to participate in data

interchanges and importing the data to internal company

systems.

• XML provides integration dialogs for ERP applications,

allowing interoperability between systems.

• Content tagging with XML tags allows information to be

categorized.

• Wireless Markup Language (WML) uses XML specifications to

present data to mobile devices.

Value Added Networks For many years Electronic Data Interchange has taken place between

large companies over Value Added Networks (VANs).

Usually, a large customer, such as a retailer, will set up a VAN hub which

is a sophisticated network infrastructure using a mailbox system for

transacting with each of the trading partners. Unlike the Internet, which is

a network designed to be accessible to the public, a VAN is specifically

designed to support the exacting demands of business transactions, such

as security and non-repudiation (backing out of a transaction by taking

advantage of the system’s inability to prove it took place).

The architecture of a VAN is able to provide

• Security

• Audit trails

• Acknowledgements

• Authentication

The large customer hosting the hub, can then connect to a number of

suppliers using the spokes of the VAN and use EDI to automatically

conduct transactions. In this way, items scanned in at the point of sale can

indicate a reduction of stock to the supplier who will automatically

Chapter 9: Networks and e-commerce 111

replenish the stock according to a pre-arranged Trade Partner Agreement

between the two parties.

EDI has brought many advantages including lower costs and faster

reaction times but is very expensive to set up and often difficult to change.

VANs have now absorbed Internet standards so as to be almost

indistinguishable from the Internet but have kept their security standards

and become known as Virtual Private Networks.

Virtual Private Networks A Virtual Private Network is a network infrastructure owned and managed

by a company. It utilises the existing Internet backbone, with its

common protocols and packet switching methodology, with the addition

of hardware and/or software to provide the required security.

This method encloses a piece of the Internet you want to keep private by

providing tunnels, gateways, or firewalls, at each point of access to the

virtual private network thus created. Access to the VPN is conditional on

the identity of the user .

Payment Methods Secure transactions need to comply with the following security aspects:

• PRIVACY - The privacy of the information of all parties must be

maintained.

• CONFIDENTIALITY – The information must be kept private

between the two parties communicating with each other.

• INTEGRITY — to prove that information has not been

manipulated in the process of transfer, or storage.

• AUTHENTICATION — to prove the identity of a party taking part

in the transaction.

• NON-REPUDIATION - to ensure that information cannot be

disowned eg. denial of taking part in a transaction.

The success of transactions depends on the trust between the two

parties to the transaction and on the security of the messaging process

between the two parties. In practice, the majority of B2C purchases are


made using the credit card system, so we will first look at how this

system is being made secure over the Internet.

SSL Around 98 percent of all transactions are secured using the Secure

Sockets Layer (SSL)protocol developed by Netscape. SSL uses the

public key encryption method to secure messages between merchants

and customers. Merchants wishing to take credit card details from

customers need to obtain a certificate (a public/private key pairing)

containing their key from a Certification Authority. A CA is a trusted

third party who generates secure certificates. These Certificate

Authorities are appointed and recognised by the major browsers, such

as Netscape Navigator and Microsoft Explorer.

The advantage of the public key model is that only merchants need the

certificate to conduct a secure session. The process is automatically

performed through the browser interface. When purchasers need to

send card details to the merchant, their browsers locate the

merchant’s public key from the merchant’s website and uses it to

encrypt the information. The merchant’s server has the private part of

the key and can decrypt the message thus received. During the initial

handshake between the merchant and the customer, the customer’s

browser does a check on the merchant’s certificate against the

approved list of Certificate Authorities. Having established the

merchant’s bona fides, SSL constructs a secure session using the

public key process. Once this is established, individual messages are

sent back and forth using conventional secret-key encryption. The

simplicity of the SSL protocol has been the main reasons for its

popularity.

SET The SSL protocol has become a standard for securing credit card

transactions over the Internet because it is supported and automated

by the major browsers, and simple to use. The public key encryption

model built into the SSL protocol is by no means secure. Customers

can fake their identity, and back out of transactions by claiming not to

have made the transaction in the first place.

The SET (Secure Electronic Transaction) Consortium was created to

put a single standard in place that would secure card transactions over

the Internet, and solve the security issues, including keeping the

Chapter 9: Networks and e-commerce 113

cardholder’s information private from the merchant and solving the

repudiation problem.

The adoption of SET is proving to be slow as it is a complex system

and expensive to implement, and SSL has gained a substantial head

start, particularly in the United States. A SET transaction can only

take place when four different parties are certified: the purchaser, who

needs a SET wallet containing a private key and third party

certification data, the merchant, a payment gateway and a Certificate

Authority. While SET has important backing from software vendors

and the credit card companies, its expense and complexity have

resulted in a slow start to actual implementation.

Small Payments Both SET and SSL are inappropriate for small transactions and a

number of payment forms have been developed for this purpose, some

of which are included here:

• Ecash — The purchaser installs, loads up his electronic ‘wallet’

with ecash from a participating bank and spends down until

time to reload. Due to the reluctancy of many banks to get

involved this method is not very popular.

• Coupons — Businesses issue redeemable coupons for loyalty

shown by customers.

• Utility Accounts - An alternative form of payment for small

transactions is to run up an account with a utility provider,

such as an ISP or phone company who act as a clearing house

for vendors.

• Smart Cards — A stored value, e-cash card can be inserted into

a cellular phone and used to download e-cash over the GSM

network. This can then be used to pay for goods in stores with

facilities to accept such payment. If the card is able to

download money, then payment for goods and services is likely

also to be possible from WAP sites.

• Bill presentment and payment - It is possible to pay accounts

using the Website of a bank. The next step is for the banks to

act as consolidators on behalf of merchants by presenting

customers with various merchants’ bills. This represents a

huge saving for the merchant. For consumers, the most


convenient method is to have only one website where all bill

presentment and payment takes place.

Summary In this chapter we have looked at the involvement of networks in e-

commerce. We have looked at how Value Added Networks paved the

way for Virtual Private Networks by creating a secure platform for

Electronic Data Interchange. We also looked at two methods used for

securing e-commerce transactions and some innovative methods of

payment for smaller transactions.

Exercises 1. Download an e-commerce store generation program from the Web

and use it to generate a simple e-commerce website.

2. Explain the various ways of securing e-commerce transactions.

3. What is the difference between a VAN and a VPN?

4. Describe how you would create a VPN by using the networking

facilities built into the MS-Windows operating system.

Chapter 10: Network Management 115

10: Network

Management

Designing and managing a network requires considerable planning.

The vast array of technology and business solutions continues to grow.

Planning a network is a complex task requiring considerable time and

resources and should be undertaken within the overall strategic plan

of the organisation. This is particularly important when the plan

includes the cabling of a building, which is an essential part of the

building infrastructure and is expected to have a life of 15-20 years.

This chapter will discuss a framework for planning and designing a

network, and will also look at critical management aspects to ensure

effective network operation

Problem Analysis When planning a new data communications network, enhancing a

current one, or using the public network, the systems approach should

be used. A successful network implementation is highly dependent on

the amount of planning done prior to ordering the equipment. The

systems approach ensures that management and users are heavily

involved in the discussion and decision making process. Involvement

and acceptance are essential features of any system. The systems

approach involves the following steps:

• Carry out a feasibility study

• Prepare a project plan

• Understand the current system

• Design the network

• Identify the geographical scope

• Analyse the transactions


• Determine the traffic load

• Develop a controls plan

• Develop network configurations

• Evaluate software

• Evaluate hardware

• Complete a cost benefit analysis

• Present findings to management and implement the

network

These will now be discussed in detail.

The first step, carrying out a feasibility study, identifies the purpose

and objectives of the proposed network and requires senior

management input. This step is especially important in companies

where networks are strategically important e.g. airlines, banks and

building societies. Once the problems (or opportunities) have been

defined, the purpose and objectives of the new system can be

identified and a short feasibility report is constructed showing broad-

brush benefits and costs and any other information needed to make a

decision.

The next step is to prepare a detailed project plan, which will include

an assessment of the technical feasibility, operational feasibility and

economic feasibility. These factors should be linked to the major goals

of the network. For example, the major goal of a new network may be

to speed up order entry and improve cash flow and the project plan

will be developed around these goals. The project must be planned,

monitored and controlled just as any other information systems

project.

The next step is to understand the current system. A detailed

understanding of how data is currently moving around the

organisation is essential. Detailed investigations of legal requirements

and user requirements are carried out in this step. Volumes of

transactions and all documentation relating to the system must be

collected. This is normally done by interviewing the relevant user

personnel. This step completes the problem analysis which is based

on a top-down approach first looking at broad company policy and


then getting down to the detail of operational systems. The next steps

in the process are the conceptual and physical design.

Conceptual and Physical Design

Having developed an overview of the functions that will be required in

the network, the next step is to design the network in detail. The

requirements should be placed in a priority order from "essential" to

"nice-to-have". These requirements are based on transactions and

systems and not specific hardware and software solutions at this stage.

The next step is to identify the geographical scope of the network,

whether it would include international requirements, national

boundaries, the cosmopolitan area or just the local facility. This

identifies tentative locations for terminals and circuits and could even

be detailed to specific terminal locations.

The transactions are then analysed to determine the traffic passing

between devices and along circuits. The transaction length and the

volume of the transactions is analysed over a period of time ensuring

that peak periods are catered for. The most important figure is the

average number of characters in a transaction. This obviously

includes the control characters which could well be as much as 20

characters per transaction.

Thus statistics of transactions over a typical day can be built up in

graphical form. It is important at this stage to consider future growth

in transactions, which were identified in the early stages of the study.

Based on these volumes, the number of characters transmitted per day

(both average and peak) will be identified.

The next step is to determine the traffic and circuit loading based on

the above figures.

This is done in conjunction with the geographical map already

produced. Transmission speeds on particular links can now be

established as well as the number of lines that will be required

between the different centres.


Before developing the physical network it is essential to consider

controls at this stage. Because an organisation's networks are often

strategic to their business, all security and control mechanisms should

be designed and installed when the network goes live. The network

needs to be protected from threats such as errors and omissions,

message loss, disasters and disruptions, breach of privacy, security

and theft, reliability, incorrect recovery restart for error handling and

poor data validation. All these need to be considered prior to physical

design and management should be appraised of the consequences and

costs of such actions.

The next step is to develop the physical design of the network. The

links between user devices and host computers are configured and

specific hardware and software is evaluated simultaneously.

Major considerations in this step are efficiency and the creation of a

cost-effective network. This is done by minimising the number of

circuits whilst providing adequate capacity and reasonable response

time. The terms "adequate" and "reasonable" need to carefully

quantified and should be understood and accepted by users and

management. Various network solutions can be produced and these

can be evaluated against each other. These alternatives are likely to

have different cost implications and also to solve the original business

problem to a different extent. The "best" solution is based on a sound

business decision and is not just a technical decision.

At the same time software and hardware will be evaluated. Decisions

regarding whether to use packet switching or bit/byte orientated

protocols such as SDLC/BSC as well as the systems software packages

on the host computer are all considered and evaluated as are the

software tools to diagnose and resolve network problems.

The hardware that needs to be evaluated will include the kind of

terminals will be required; the modems and multiplexers that will be

used; whether protocol converters are required; whether a PBX should

be integrated into the network; and the interaction between the front-

end processor and the host computer. Software testing equipment will

also be investigated and costed.

A cost benefit analysis is then performed. Cost categories are

developed in the areas of line costs, hardware costs, software costs,

network management costs and personnel costs. These costs should

all be clearly identified or at least accurately estimated. These costs


can be structured into three categories viz. implementation costs,

which are the one time costs of installing all the hardware and

software and network equipment; investment cost, is the one time

capital cost of acquiring hardware and software; and the operating

costs are the recurring outlays which are required to run the network.

How these costs are utilised depends on the company's procedures but

a simple cost benefit analysis and payback period might be useful or

an internal rate of return (IRR) calculation can be done. It is essential

to capture these costs to make the management investment decision.

Also, in many organisations, costs of the network are charged back to

the user.

The final step in designing a network is to sell the product to

management and implement it. Before implementation, both

management and users must accept that the costs and performance of

the proposed network meets the goals and objectives of the

organisation. Following successful selling of the product, the

implementation process begins. This is a major endeavour involving

equipment and people and generally requires a detailed

implementation plan. Often a specific project implementation team is

set up and a phased implementation is carried out, testing stage by

stage. The implementation plan will include user training and

documentation as well as testing backup recovery systems in case of

failure.

The system should be reviewed after a settling in period of up to six

months. This review of problems should be carried out at a high level

to ensure that the original objectives of the network have been

achieved and that operational problems are being properly identified

and resolved.

The above steps are fairly comprehensive and are based on the design

of a new network. Where a current network is being enhanced, some

of the steps may not be required but nevertheless an orderly project

plan is essential for success.

Selection of Hardware and Software

The move to online applications requires new hardware, software,

services and personnel. Where major acquisitions are involved,


evaluation of different offerings from suppliers is necessary. Selecting

a hardware and software system involves not only selecting technology

but also selecting a supplier who is capable of maintaining, enhancing

and expanding the system as it develops. It is important therefore to

consider the components and the supplier with equal emphasis and

care.

Building a Request for Proposal (RFP)

In small organisations an informal approach is normally taken to

system selection. However, in large organisations (such as government

establishments), the selection should be based on a fair appraisal of

the marketplace. The creation and issuing of a request for proposal

(RFP) which is sometimes called a request for quotation (RFQ) or a

tender document does this. The objective of the process is to produce

a document, which describes the problem to be solved, and requests

qualified suppliers to submit plans and costs to solve the problem.

The content of the RFP can be as small as a few pages for a small low

cost upgrade, ranging up to several hundreds of pages for a major

upgrade in hardware and software.

The format and content of the RFP is not rigid, although typically it

would contain an introduction, the response requirements (the time

and place of the proposal submission) and the evaluation criteria that

will be used to measure the different solutions. Some background on

the company’s characteristics is required followed by a problem

description, which will give the supplier enough information to

respond. References to other customers of the supplier are often

required as well as benchmark performance statistics of the supplier's

hardware and software offerings.

It is usual to take the responses to the RFP and after an initial

evaluation to reduce the number to somewhere between three and five

finalists. These suppliers will then be contacted and asked to run

specific benchmarks or to produce further technical detail of their

proposed solution. At this stage the supplier's references are also

contacted and questioned about support and performance. THE

supplier's contracts are examined, possibly by the user's attorneys to

ensure that any non-standard components of the contract are


acceptable. Issues such as penalties, support and maintenance are

resolved at this stage. Hidden costs for education and installation are

generally uncovered as well. This final and thorough evaluation allows

both the supplier and the user to ensure the final solution has been

optimised as far as possible.

The final selection is also influenced by non-technical issues such as

politics and supplier financial credibility and business history.

Managing the Network Online systems, specifically those with enquiry facilities, are intended

for use throughout a working day. An online system must be

continuously available during this time. This is called the uptime of

the network and in some critical systems like an airways online

reservation system the uptime should be very close to one hundred

percent.

The user's terminal is connected to a string of devices and any of these

devices can have a problem, reducing the availability of the service.

These devices include the multiplexers, the modems, the Post Office

lines and services, the front-end processor and the software in the

different devices. The problem is worsened by the fact that isolating

and diagnosing and correcting faults may be very difficult due to the

many different suppliers involved in supplying network equipment. A

further problem is the fact that online systems run at peaks during the

day and spare computer capacity has to be provided to cope with these

fluctuations in the workload.

Because the user interfaces directly with the online system through his

terminal, the implication is that the user is often the first one to

become aware of any malfunction of an online system. It is therefore

critical to an organisation running online systems to manage the

performance of the network; to manage changes made to the network;

and to manage the growth of new applications coming onto the

network.

The user of the online system requires a lot more support than his

predecessor who used a batch computer system. A higher degree of

training will be required and also immediate assistance with problems

that occur from time to time. Because the user is remote from the

computer site, his interaction with computer operations staff has to be

formalised. A user who has been properly trained will be able to solve


many of his own problems and an effective training system

shouldprovide the user with enough knowledge to understand the

hardware, the software and the people interactions that are required to

run the system effectively. Good user documentation to backup the

training, and continual liaison between the user and the network

personnel, are essential ingredients for success.

To measure and manage the availability of a network, statistics of

uptime have to be recorded. When a new online system is designed, a

theoretical expected availability figure should be calculated. This

should be discussed and agreed by the users. The availability level

should be constantly monitored and measured over monthly

accounting cycles. Actual availability figures can then be measured

against expected, planned figures and corrective action taken by the

computer centre management as required. The availability of a

network in a bank would be more critical than in a manufacturing

company and the reviews of availability could well happen daily as

opposed to monthly. Measuring performance on its own is inadequate

as this does not necessarily mean that the online system will meet the

specified requirements and that the procedures laid down are being

followed. Regular system audits are required to ensure compliance.

The performance of a network can be totally disrupted if changes

being made to the hardware, software and procedures are not properly

managed. In principle, all changes should be managed in terms of

planning, testing, implementing, monitoring and revising. Any

hardware changes being made to the system will degrade

performance. The objective of change management is to minimise this

degradation. Changes should be made at times when there will be

least disruption to the network and the users. Sufficient time must be

allowed for testing the equipment before introducing it as the live

system. The performance of the new system should be carefully

assessed to ensure compliance to specification.

Software changes to online systems are very complex and require

careful planning. The different software packages that are required on

a large mainframe are continually upgraded by the supplier and it is in

the company's interest to keep these releases current. These new

software products have to be tested on-site by software, operations

and network personnel to ensure that they function correctly when

implemented. This may involve considerable effort and expense, but

should be weighed against the possible disruption to the network and

to an organisation's business. Once implemented, the new software


should be monitored to ensure side effects are minimal. Standard

handover procedures should be developed for software and hardware

to ensure that all involved parties cooperate. To minimise the "small

change syndrome", where continual changes are made to the system

making it very hard to manage, it may be possible to accumulate small

changes and implement them together at monthly intervals. Change

management is a critical function requiring the following activities:

• All changes take place in a controlled fashion,

• Changes are planned as far ahead as possible,

• Users are totally informed of the timing and implication of the

changes being made,

• All changes are thoroughly user-tested before implementation,

• Procedures are correctly followed when making changes,

• All procedures are properly documented,

• All changes are properly recorded for future reference and

management documentation,

• A recovery plan is available should changes cause problems

and the change has to be backed-out,

• An up-to-date list of all the components of the network is kept,

and

• All changes that take place are properly authorised.

Availability means that all components are operating when a user

requires them, which for a terminal operator would mean the

terminal, all cables and connectors, modems, the transport medium,

controllers, processors and software.

Three factors influence the availability - mean time between failure

(MTBF), the mean time to repair (MTTR) and operational

considerations. Operational considerations may mean that certain

aspects of the network may have to be taken out of service e.g. the

online system may have to be taken down to give priority to other

events such as the urgent maintenance of new hardware and software.

Mean time between failure is the average period of time that a

component will operate before failing e.g. if a user terminal has a

MTBF of two thousand hours and it operates an average of eight hours


a day twenty three days a month it would be expected to fail once every

2000/(8 x 23) = 10.86 months. The mean time to repair is the average

amount of time required to get a failed component back into service.

For some components this time will be simply the time to replace e.g.

a spare modem can be used to replace a failed modem. For the

mainframe CPU, this will possibly be longer and have greater

ramifications. The availability is worked out using a probability

function. For a terminal with a MTBF of two thousand hours and an

MTTR of 0.5 hours, the availability for an eight hour period would be

approximately 0.9997. This means that a user can expect the terminal

to be unavailable three times every ten thousand tries. Where there

are multiple components in a network (which is normal), then if there

were five components in the network the availability would be A = 5 x

0.9995 = 0.995. The user would thus find the system unavailable once

every two hundred attempts. This availability factor and the need to

maintain a very high availability will determine the management effort

required and the stock holding of spare parts.

As the use of the network increases so capacity planning must be done

to ensure constant performance. Measuring capacity requires keeping

fairly straightforward statistics but also using hardware and software

monitors to measure internal machine functions. Simple statistics

include response time, number of transactions per unit of time and

transaction mix. An increasing response time indicates an increasing

workload which means that the network components are being utilised

more and more. Capacity problems often relate to queues that are

built up in the network. This can occur in multidrop configurations

where terminals are queuing up to send data or in statistical

multiplexers which have queues of messages to send, or in the CPU

where processes are waiting to be completed. The network can

therefore degrade in performance if the application and data base in

the mainframe are not working efficiently. When these degradation

problems occur, it is important to analyse the various statistics to

determine what can be done immediately. This might involve minor

tuning of the system or the acquisition of new equipment such as more

memory or more communications lines. Note that a tuning exercise is

a change in itself and should be managed to avoid further problems.

A management tool that is used effectively in large complex network s

is the Service Level Agreement (SLA). When a new system is

designed, certain parameters regarding performance are set, these

parameters are built into a service contract which binds the computer


operations department to provide a user service which has been

specified. This is then an effective way for both the user and the

computer department to decide on the problem areas using consistent

and objective measures.

Selection of Staff To run an effective online system four key responsibilities are

required. These include quality control, communications control,

technical control and user services. These responsibilities normally

come under the computer operations manager or computer facilities

manager. Many of these jobs now require communication skills as

well as technical ones. The job requires continual interaction with

users and management.

In an organisation where the network is a key strategic service in the

company there may well be a network manager reporting to the IS

executive and jobs such as change control manager will be elevated in

status accordingly. A typical network manager requires different skills

to do his job. These include technical skills, so that he can understand

data transmission, data communications software and data

communications hardware and the application systems that are

running; inter-personal skills, so that he can effectively liase with user

staff, suppliers and other DP managers; managerial skills, to ensure

that system changes are properly controlled, faults are properly

recorded and sorted out and to ensure that people under his control

are working effectively.

Operations Control The structures and procedures mentioned in previous sections may

make problem solving and correction time consuming. Clearly the

intention is to make these actions effective and efficient. To solve this

problem a Network Communications Centre (NCC) is set up in large

companies to provide a consistent one stop problem solving area. All

operational network functions are handled here including problem

reporting and problem solving as well as equipment installation and

removal. By employing the right people and procedures in such an

environment, network management can be made more effective.

Many software and hardware tools are available to capture statistics

and monitor the lines and devices that are being used. This will


include measuring the overall quality of the network and monitoring

whether faults are new, cleared or outstanding. The network control

centre can also be made responsible for measuring system availability

and mainframe utilisation, line utilisation and message throughput.

Network Tools Line monitors are used to diagnose problems. These can be for a

digital signal or an analogue signal i.e. in front of or behind the

modem. These measure and display signals to ensure that they are

correct. Performance monitors provide snapshots of how a system is

functioning, measuring queues and limit response times. This

information collected over a period of time helps the network

management team to spot trends in the use of the network and also to

monitor where improper use has been made of various functions.

Thus the number of transactions input into a particular system can be

monitored (or the number of users playing a computer game during

their lunch time can be reported).

Log files are a valuable tool in monitoring a network. Log files of

system messages can be held and interrogated on a continuing basis

and can then be used for diagnosing problems and predicting certain

activities, e.g. a line trace which is a log of the activity on a particular

line, can be turned on and off dynamically using software which allows

the network manager to interrogate that particular part of the

network.

Security and Disaster Recovery

Online systems allow greater access to data by a host of users. This

increases the concern for security and recovery. A breach of security is

a loss of availability, integrity or confidentiality and results from the

accidental or deliberate occurrence of a threat. The total loss of a

computer installation can be a major setback for organisations.

Replacing a destroyed data centre is a major task because not only

does the mainframe hardware and software have to be replaced but

also all online networks.


Security can be handled by considering risk management, contingency

planning and auditing. Risk management involves making up an

inventory of all assets including their value and function. All threats to

the assets must be identified and each threat must be estimated in

terms of frequency of occurrence. Countermeasures to these threats

must be identified and, where appropriate, implemented effectively.

One such measure could be insurance but it would not be cost effective

to insure against every possible threat. By careful analysis, many

threats can be avoided or eliminated or their effect minimised by

taking appropriate measures which are relatively inexpensive.

Contingency planning refers to the preparation of recovery plans.

Whenever a normal system is disrupted it should be possible to

introduce a standby plan of operation immediately. This plan will

have been prepared and tested out beforehand. When there is a major

disaster the standby plan brought into action would probably require a

management committee to carry out the various activities needed to

recover from the problem (e.g. a fire or flood or strike by employees).

The nature of the emergency will decide the level of the recovery plan

that is required. Good recovery plans must be understood, well

documented and fully tested.

Auditing is also required on a regular basis to ensure that the network

is meeting the objectives for which it was developed in a cost effective

way. An audit should ensure that controls to detect and prevent

misuse and malfunction of the network have been put into place and

are being used properly. These controls are especially necessary

during disruption to normal operation when the system is particularly

vulnerable.

Network Trends Data communications and networking is the most dynamic area of the

Information Technology industry and will continue to be so for the

rest of the decade. Many organisations are totally reliant on their

networks to shape their future business. It is therefore essential to

understand the trends in the network industry, which will shape the

organisation in the future and alter the way office automation

progresses. This section will identify the trends in the technology and

project some of the implications of these changes.


Hardware Trends The use of high-speed transmission media such as optical fibre cable

and satellite will continue to grow at an ever-increasing pace. This

means that channel capacity as well as line transmission speed will

grow, making it easier to integrate image, voice, data and perhaps even

video into one network environment.

The new generations of the PBX will enhance office telephone systems

and also allow voice, data and video images to be used in this area

especially with the rapid evolution beyond the ISDN standard.

Personal computers will continue to grow in power and be used

extensively in local area networks. At the department level the move

to distributed data processing and distributed databases will require

careful planning and control of sophisticated networks. Digital data

transmission will grow in popularity providing users with higher data

transmission rates and vastly reduced error rates coupled with the

lower price of modems and other network equipment. This will make

data communications an integral part of our work and our private

lives, especially with the growth of the Internet and mobile phones.

Software The wide acceptance of the OSI model will lead suppliers to move

closer to the layered structure. However, although the development of

the standards is complete, the implementation of them is not. Full

interconnection between computers and terminals will continue to

undergo difficulties in the near future. The domination of IBM and its

Systems Network Architecture will lead many suppliers to look for

ways to connect their systems via gateways into IBM architecture.

Local area network software will continue to evolve and become more

sophisticated in the area of management and control.

Office Automation The use of personal computers, the Internet and electronic PBXs has

already changed many offices. This is only the start of a trend which

will lead to more integrated services being offered in the areas of

electronic mail, image processing, voice services and video transfer

both within an organisation, nationally and internationally.


Public Networks The current move by Telkom to digitise all data networks will continue

for the foreseeable future. As this upgrade extends to the smaller

centres and the smaller PBXs providing a vastly improved service,

organisations will make far greater use of the network facilities

available in the country. Deregulation and privatisation of Telkom

will also add a healthy competitive spirit to the business.

The Electronic Cottage Experiments with telecommuting have already begun with people

working from home. For certain job categories such as word

processing staff and computer programmers this seems a reasonable

option. Although there are advantages and disadvantages to such

working conditions it is clear that there are many possibilities that can

reshape our work and our play. One of the key aspects is the fact that

the use of networks can change the way we compete. Decision-making

can be speeded up considerably by having all decision-makers at the

ends of network channels. The time frame required to make decisions

can therefore be reduced considerably by the use of network

technology and this will become a competitive feature in the future.

A network connecting to the home or the individual (via a mobile

phone) would change the way we do our banking, shopping, holiday

planning, social communication and entertaining.

The data communications technology industry has already come a

long way but is still in its infancy. Successful organisations are using

networks to re-shape the way they deal with their suppliers and the

way they compete with their competitors. Companies are using

networks to improve access to information and to improve data, voice

and image communication. With transfer speeds increasing

dramatically and technology costs reducing, the rate of growth of

value-added services will be exponential. Key factors in the network

revolution will not only be the technology but also the people and

procedures involved with that technology. The challenge is not just to

design and implement efficient networks but to make people more

efficient and effective in their jobs. Many of the future changes in our

work and our society will be as a result of the way we develop and use

future computer networks.


Summary Designing a network is a complex, expensive exercise. The wiring in a

building is often installed to last 15-20 years.

A stepwise systems approach to the implementation of a network is

essential to obtain the correct level of commitment and involvement in

the decision-making process.

To ensure a high level of availability and a good response time, the

network must be effectively managed. This includes problem

management, change management and capacity planning.

The ongoing running of the network requires a properly staffed

Network Control Centre with the necessary tools for monitoring and

reporting.

A well designed and tested disaster-recovery plan is a necessity.

Exercises 1. Draw up a structure for a typical Request For Proposal.

2. Discuss the systems approach to network implementation.

3. Discuss the network design process.

4. What would you include in a disaster recovery plan for your business?

5. Use information from the Internet to create a short summary of the

latest (last six months) developments in the data communications

field.

Glossary 131

Glossary Backbone

The part of a network used as the primary path for transporting traffic between network segments

Bandwidth

Maximum rate of data f1ow for a given media type. Usually measured in bits per second (bps).

Baseband

Transmission scheme in which the entire bandwidth, or data-carrying capacity, of a medium (such as a coaxial cable) is used to carry a single digital pulse, or signal between multiple users. Because digital signals are not modulated, only one kind of data can he transmitted at a time.

Bridge

A device that connects two networks together and operates at the Data Link layer.

Broadcast

A packet destined for all stations on the network or subnet.

Collision

Two stations start transmitting on Ethernet at about the same time, resulting in garbled information.

Cyclic Redundancy Check (CRC)

The CRC is used to detect errors in data on the local media that occur during transmission.

CSMA/CD

Carrier Sense Multiple Access with Collision Detection. A method of dealing with contention on a broadcast network. Stations listen for a quiet network (no carrier signal) before attempting to transmit. If a station’s transceiver detects a collision while transmitting, it will back-off and retry.

Data Link Layer

Layer 2 of the OSI reference model. Provides synchronisation, hardware addressability and error control over a physical link.


Domain Name Service (DNS)

A way for a node to retrieve an IP address associated with a domain name such as www.netgroup.com

Ethernet

A contention based broadcast media network that formed the basis for the IEEE 802.3 CSMA/CD standard. Invented by Xerox and formalised by Dec/Intel/Xerox (DIX).

Fast Ethernet

An industry term for Ethernet that operates at 100Mbps (1OOBASE-T).

Firewall

Isolation of LAN segments from each other to protect data resources and help manage traffic.

Frame

The block of data containing a PDU (Protocol Data Unit) transmitted at the Data Link layer. Used interchangeably with the term packet although some use frames to refer to the DLC layer and packets for everything above that.

File Transfer Protocol (FTP)

A popular protocol in IP networks used to transfer hulk data. Operates over the TCP protocol.

Gigabit Ethernet

An industry standard (1000Mbps Ethernet)

Hub

Common name for a repeater.

HTTP

Hypertext Transfer Protocol

IEEE

Institute of Electrical and Electronics Engineers. A professional engineering organisation responsible for many of the LAN standards included the 802.3 CSMA/CD. 802.4 Token Bus and 802.5 Token Ring standards. Also responsible for the 802.2 Logical Link Control (LLC) standard and assigning OUIs.

Glossary 133

Internet

A collection of networks and gateways all using TCP/IP protocols: this could be the Internet or an enterprise or service provider’s IP network.

Internetwork

A collection of networks interconnected by routers that function as a single network..

Internet Protocol (IP)

A connectionless best-effort network layer protocol

Internet Packet Exchange (IPX)

The networking layer protocol used by NetWare.

IP Telephony

The transmission of Voice over an Internet Protocol (IP) network.

ISO International Organisation for Standardisation. International institution that drafts and specifies standards for network protocols. Best known for its seven layer OSI model.

Kbps

(Kilo) (thousands) bits per second

Local Area Network (LAN) Any network topology designed to span short distances at high speeds and low latency. Ethernet, token ring and FDDI are some examples.

Media Access Control (MAC)

Au IEEE reference to the sublayer used to access a physical medium. For example token ring, Ethernet, FDDI etc. The term MAC address is synonymous with DLC address.

Metropolitan Area Network (MAN)

A high speed network usually spanning a city. The size of this network falls between a LAN and a WAN.


MAC Layer Address

Also called hardware address or physical address. A datalink layer address associated with a particular network device. Contrasts with network or protocol address which is a network layer address.

Mbps

Mega (million) bits per second.

Network Address

Also called a protocol address. A network layer address referring to a logical, rather than a physical, network device.

Network Interface Card (NIC)

The circuit board or other hardware that provides the interface between a communicating DTE and the network.

Network Layer

Layer 3 of the OSI reference model. Adds datagram (connectionless) services, end-to-end addressability and routing.

Overhead

The extra control error handling and routing or switching information added to riser data for transmission in the form of headers and trailers.

Open Systems Interconnection (OSI)

A reference to ISO protocol standards for the interconnection of dissimilar computers. Often referenced for comparing non ISO protocols.

Packet

The data unit used by the network layer to communicate with peers. This term is more loosely defined as any block of data sent across a network.

Packet Switching

A technology for sending data over a network - the Internet for example. The data that emerges from a connected device is broken into chunks called packets. Each packet contains the address of its origin (source) and the address of its destination. Data packets from many different sources can travel along the same lines and be sorted and directed through different routes by routers alone the way. When all the packets forming a message arrive at the destination, they are recompiled into the original message.

Peer-to-Peer

Communication between the same layers of protocol in a network.

Glossary 135

Physical Layer

Layer 1 of the 0SI reference model. The transmission of a bit stream over a physical media along with the access technique for a physical medium.

Protocol

A formal set of rules governing communications between peers. A protocol may define or negotiate elements such as acknowledgement algorithms, addressing, MTU, etc.

Remote Procedure Call (RPC)

A network API for an application to call a service across a network.

Router

A device that connects two or more network segments together and forwards packets based on network addresses and a set of one or more metrics.

Session Layer

Layer 5 of the OSI reference model. Establishes a dialog based on a logical name rather than an address.

Simple Mail Transport Protocol (SMTP)

A protocol that provides electronic mail services for the Internet.

Sequenced Packet Exchange (SPX)

A transport protocol that provides connection—orientated services over IPX.

Transmission Control Protocol (TCP)

A connection-oriented, transport layer protocol that provides reliable, full duplex communication between processes on separate stations over IP.

TELNET

TCP/ IP virtual terminal protocol. Allows a user to logon to a remote host and interact as if directly connected to that host.

Throughput

Rate of data flow achieved. Usually measured in bits per second (bps).

Transport Layer

Layer 4 of the OSI reference model. Provides reliable transfer of information between two end nodes by implementing error recovery and flow control.


Wide Area Network (WAN)

A network which encompasses interconnectivity between devices over a wide geographic area. Such networks would require public rights -of-way and operate over long distance such as countries and continents.

Websites 137

Websites

General

http://www.commweb.com/tutorials

http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/index.htm

http://whatis.com/index.htm

http://www.adsl.com/

http://www.verisign.com/enterprise/rsc/gd/secure-ext/

http://v90.com/

http://www.atmforum.com/

http://www.frforum.com/

http://www.rad.com/networks/tutorial.htm

Wireless

http://www.cellular.co.za/


Index 10 base.................................................. 43 100 Base T............................................80 100VG AnyLAN ....................................81 Access Methodologies...........................77 Access Rights ......................................105 Active Hubs.......................................... 58 Amplitude ...................................... 25, 30 Analogue .............................................. 24 application layer ...................... 14, 16, 18 Architectures........................................ 78 ASCII .............................21, 22, 37, 97, 98 Asynchronous Transmission............... 33 ATM................................................88, 89 bandwidth .10, 28, 29, 42, 44, 48, 52, 66, 67, 73, 81, 86, 87, 88, 93, 137

Bandwidth.........................29, 42, 93, 137 Bits Per Second .................................... 29 Bridges ........................................... 60, 61 BRouters .............................................. 63 Bus Network Structure ........................ 69 carrier.....................24, 28, 31, 77, 91, 137 Cell-Relay ............................................. 88 Centralised Networks ...........................76 Character Encoding ..............................21 Circuit Switched................................... 50 client.............. 11, 19, 96, 98, 99, 100, 105 Coaxial.......................................41, 42, 43 Complex Topologies .............................74 Cyclical Redundancy............................ 38 data link layer ...........................15, 17, 18 DB-25 ................................................... 24 DB-9 ..................................................... 24 Dense Wavelength Division Multiplexing...........................................................67

Design.................................................. 121 Dictionary Attacks .............................. 110 Digital transmission ............................ 28 Disaster Recovery ................ 107, 110, 131 Distributed Networks ...........................76 Domain Names .................................... 94 EBCDIC .....................................21, 22, 37 EDGE.....................................................51 electrical interference .............. 44, 45, 47 Electronic Cottage...............................133 E-Mail................................................... 96 Enclosure Formatting.......................... 98 Encryption........................... 104, 105, 112 envelope ............................ 33, 97, 98, 111 Error Correction .................................. 38

Error Prevention .................................. 37 Errors.................................................... 36 Ethernet... 77, 78, 79, 80, 81, 82, 89, 137, 138, 139

FDDI..................................................... 79 fibre optic ........................... 10, 42, 46, 57 Fibre Optic Cable ........................... 46, 47 file server .............................................. 56 Firewalls ............................................. 106 Frame Relay .........................................88 Frequency..................... 25, 26, 30, 31, 66 Frequency Division Multiplexing........ 66 FTP ........................ 95, 100, 101, 107, 138 Gateways .............................................. 64 gigahertz ......................................... 25, 26

Guided Media...................................41 Hacking ...............................................110 Half / Full Duplex ................................ 35 hertz...................................................... 25 hub.................................57, 58, 70, 73, 81 Hubs ..................................................... 57 Infrared ................................................ 47 Intelligent Hubs ................................... 59 Internet...7, 17, 18, 20, 28, 50, 51, 52, 60, 64, 65, 68, 83, 85, 86, 87, 89, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 104, 105, 112, 113, 114, 115, 116, 117, 132, 133, 139, 140, 141

Internet Message Format Standard .... 97 Internet Service Providers ............. 65, 92 Intruder Detection ............................. 106 IP 17, 52, 60, 91, 92, 93, 94, 95, 96, 101, 138, 139, 141

ISDN.................................85, 86, 89, 132 ISO.........................................14, 139, 140 LAN. 10, 12, 20, 41, 58, 60, 61, 69, 71, 73, 78, 79, 80, 81, 89, 138, 139

Leased...................................................86 Longitudinal Redundancy ................... 37 mainframe computers........................... 11 megahertz.................................25, 26, 29 Message Transfer .................................98 Metropolitan area networks .................13 MIME ...................................................98 modem.......11, 22, 24, 29, 65, 86, 93, 111, 128, 130

Modems................................... 28, 65, 111 Modulation............................... 29, 30, 31

Index 139

Multiplexers ......................................... 65 Network Computers..............................55 Network Interface Cards ..................... 56 network layer ............ 15, 17, 18, 139, 140 Network Management ........................ 119 Network Tools.....................................130 Operations Control .............................130 OSI.... 14, 15, 16, 17, 18, 20, 132, 137, 139, 140, 141

OSI model .............................................14 parallel transmission ........................... 22 parity checking................................21, 37 Passive Hubs ........................................ 58 Passive Measures ............................... 108 Passwords............................................103 Payment Methods............................... 115 Phase .............................25, 26, 27, 31, 32 physical layer ...................... 15, 16, 17, 18 POP....................................................... 99 PPP ....................................................... 93 presentation layer .....................15, 16, 18 Pretty Good Privacy ............................105 Problem Analysis ................................ 119 Protocols ...................................14, 33, 99 quadrature phase modulation............. 32 random access memory........................55 Repeaters ............................................. 59 Request for Proposal ..........................124 Ring Network Structure ....................... 71 Routers ................................................. 62 RS232 ................................................... 23 RS-232.................................................. 23 Satellite................................................. 49 Screen Savers ..................................... 106 Security ....7, 8, 12, 19, 103, 107, 110, 114, 131

Selection of Hardware and Software .124 serial transmission............. 22, 24, 32, 33 Servers...................................................55 session layer ................................... 15, 18 SET ...................................................... 116

Shielded Twisted Pair .......................... 45 SLIP ...................................................... 93 Small Payments................................... 117 SMTP .................................97, 98, 99, 141 Sniffers & Spikes ................................. 111 Software Piracy .................................. 109 SSL................................................116, 117 Staff..................................................... 129 Star Network Structure........................ 70 Storage Devices .................................... 56 Switched Networks .............................. 73 Switches.................................................61 Synchronous Transmission ................. 34 TCP/IP.................. 17, 88, 92, 93, 96, 139 Telnet................................................... 111 Time Division Multiplexing........... 66, 67 Time Scheduling ................................ 106 Token Passing ...................................... 78 Token Ring ..............................72, 79, 138 transmission media10, 15, 42, 46, 69, 81, 132

transmitter ...........................9, 33, 35, 48 transport layer ........................15, 18, 141 Trends................................................. 132 UMTS.....................................................51

Unguided Media .............................41 Unshielded Twisted Pair Cable ..... 44, 45 Value Added Networks .......................114 Vertical Redundancy............................ 37 Virtual Private Networks .............115, 118 Viruses................................................ 109 WAN . 13, 20, 75, 83, 84, 87, 89, 139, 142 waveform..................................24, 25, 26 wide area network...........................13, 14 Wide Area Networks .......7, 13, 65, 75, 83 Wireless ................................. 42, 114, 143 World Wide Web.........................100, 113 X-25 ...................................................... 87 XML..............................................113, 114

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