CSE1720 Summer 2005 Lecture 8 / 1 Lecture No. 8 Communications.

CSE1720 Summer 2005 Lecture 8 / 1

Lecture No. 8Lecture No. 8

Communications


Lecture ObjectivesLecture Objectives

Today we are going to explore some of the aspects and features of

COMPUTER BASED DATA COMMUNICATIONS

and to obtain an overview of some of its forms, technology and terminology


CommunicationsCommunications

Telecommunications:

- Voice

- Data

- Images

Telephone Radio Television Any others ?

Teleprocessing:

Telecommunications + Computer Based Processing

Speeds : Bits per second

Frames per second

Speed Bands: Narrow Voice Broad



There has been an very large increase in both the Demand and Provision of Communication facilities in the past 5 years.

Australia’s $A 1.2 billion link to the U.S. (Southern Cross Cable Network) is to provide a 120 gigabits a second data capacity by the end of 2000.

The cable length is 29,000Km.

Any problems ?

And a link to Singapore has been developed

The Applications driving these are multimedia and digital television, and the Customers are

AAPT, Primus, OzEmail, C&W Optus (?SingTel)



Transmission Options:Asynchronous < 2000 bpsSynchronous > 2000 bps

Frame transmissions and ATM (Asynchronous Transfer Mode) are the current options.

Methods: Packet and Packet Switching Public Switched Network

- Dial up network- Leased lines

Private networksCircuit Switching Message Switching Packet Switching

Public Authorised Carrier - Protocols, Standards


Digital Data TransmissionDigital Data Transmission

Asynchronous serial transmission

Signal timing is unimportant.

Information is received and translated by agreed patterns - the receiving device can retrieve information without worrying about the rhythm.

Patterns are based on grouping the bit stream into bytes, and each byte is sent as a unit.

The receiver cannot use timing to predict when the next group will arrive

An extra bit is added to the beginning of each byte (normally 0) - known as the start bit. Another bit is added to the end of each byte (normally 1) - known as the stop bit.

The receiver counts bits to ensure content


Digital Data TransmissionDigital Data Transmission

Synchronous transmission.

The bit stream is combined into ‘frames’ of multiple bytes

Each byte is sent without there being a gap between bytes

Data is an unbroken stream of 0 and 1’s

There is no built-in device for synchronisation

Timing is essential - the receiver has only the count of bytes

Synchronous transmission is faster than asychronous transmission - It is more useful for high-speed applications (computer to computer links).

The data link layer synchronises the bytes.



System Components - there are 5

Message

Sender Medium Receiver

Protocol Protocol

Step 1Step 2Step 3…………

Step 1Step 2Step 3…………



The 5 components:

1.The Message is the data (information) to be communicated. It can be text, numbers, pictures, sound, video - or any mix of these

2. The Sender is the device which sends the data. It can be a computer, workstation, telephone handset, video camera …

3. The Receiver is the device which receives the message. It can be a computer, workstation, telephone handset, television …..



4. The Medium is the physical path by which the message moves from the Sender to the Receiver. It can be a twisted wire pair, coaxial cable, fibre-optic cable, laser, or radio waves (terrestrial or satellite microwave)

5. The Protocol is a set of rules which govern data communication.

It represents an agreement between the communicating devices.

The protocol is essential to ensure that connected devices can also communicate - and with no confusion, or misinterpretations


Components of a Data Communications SystemComponents of a Data Communications System

INPUT

OUTPUT

interface interface

COMPUTERComms ControlSoftware

Applications Software

Storage, etc

communications

channels

1

2 2

3

4

5


Communications - NetworksCommunications - Networks

A network is a set of devices (nodes) which are connected by media links.

Typically a node can be a computer, a printer, or any other device which is capable of sending and/or receiving data generated by other nodes on the network. (the links are called communication channels).

Networks use Distributed Processing. A task is divided amongst multiple computers. Instead of a single large machine being responsible for all aspects of a process, each separate computer (personal or workstation) handles a subset - electronic job sharing ?



What are the advantages of this ?

1. Security/encapsulation. The system designer (and manager) can limit the nature of of interactions of a given or all users)

2. Distributed Database. No one system needs to provide storage capacity for the entire database. Which is just as well, as the total information on the World Wide Web is beyond the capacity of any single device to store it.

3. Faster Problem Solving. Multiple computers working on parts of a problem can often result in the process being completed in a shorter time than a single machine could do

(many computers make light work ?)



Network Criteria. There are some interesting aspects associated with networks.

Data Communication

Network Criteria

Performance Reliability Security

Factors Involved:Number of UsersType of transmission mediumHardwareSoftware



Applications

• Marketing and Sales• Financial services• Manufacturing• Electronic messaging• Directory Services• Information services• Electronic data interchange• Teleconferencing• Cellular telephone• Cable television


Mobile and Wireless ApplicationsMobile and Wireless Applications

• Mobile Computing :• Personal Digital Assistants, cell phones - networked

communications and applications• Online transaction processing - order entry, customer

account charge• M-Commerce: Internet access, Information searches,

Collaborate and make joint decisions, but/sell from ???• Retail - inventory detail, sales transactions• Field service/sales - dispatching, online diagnostics, parts

ordering• Banking: Purchasing, selling, inquiry, brokerage


Standards OrganisationsStandards Organisations

On previous overheads (7 and 9), you saw the term ‘protocol’

Some of the Standards creation Committees you will meet are

The International Standards Organisation (ISO)

The International Telecommunications Union (ITU-T, which was formerly the CCITT)

The American National Standards Institute (ANSI)The Institute of Electrical and Electronics Engineers (IEEE, also known as ‘I triple e’) not to be confused with ‘I Claudius’

Bellcore (Bell Communications Research)

and a few others which popped up in the e-commerce lecture


Some ConceptsSome Concepts

What are some of the relationships between communicating devices ?

There are 5 general concepts:

1. Line configuration

2. Topology

3. Transmission mode

4. Categories of networks

5. Internetworks


Line ConfigurationsLine Configurations

point to point

multipoint


TopologiesTopologies

2 relationships are possible in a topology :

– (1) peer-to-peer (the devices share the link equally)

– (2) primary-secondary (one device controls traffic and the others must transmit through it)


Bands and SpeedsBands and Speeds

• Narrow Band - Low Speed Channel

Up to 300 bits per second

(Telex, Low speed printers)• Voice Band - Medium Speed channels

Up to 4800 bits per second

(Video Terminals, Microcomputers, Medium speed printers, )

Can be conditioned to 9600 bps• Broadband or High Speed Channels

- 20,000bps to 100Mbps

(high speed transmissions, Computer to Computer

or High speed packed data transmission)


Topologies - Star NetworkTopologies - Star Network

computer computer

computer

computer computer

central computer

Primary-secondary relationship


Topologies - Tree NetworkTopologies - Tree Network

mainframe

node 1node 2

node 3 node 4

node 5node 6

node 7node 8node 9

Primary-secondaryrelationship


Topologies - Bus NetworkTopologies - Bus Network

minicomputer minicomputer minicomputer

minicomputer

printer

Either peer-to-peer or primary-secondary


Topologies - Ring NetworkTopologies - Ring Network

computer

computer

computer

computercomputer

(Token Ring)

Peer-to-peer relationship


Topologies - MeshTopologies - Mesh

Every device hasa dedicated point-to-point link withevery other device.The link carriestraffic only betweenthe 2 devices itlinks.There are n(n-1)/2physical links forn devicesPeer-to-peer relationship


Transmission ModesTransmission Modes

Simplex - one-way only

Half-DuplexBoth ways, but only oneway at a time

Full-DuplexBoth ways simultaneously


Categories of NetworksCategories of Networks

There are 3 primary categories:

1. Local area networks (LAN)

2. Metropolitan area networks(MAN) also known as Medium Area networks

3. Wide area networks (WAN)


Local Area NetworkLocal Area Network

This is normally a privately owned network and links the devices in a single office, a building, a campus

A LAN can be 2 computers and a printer (many users have this at home)

It may extend through a Company and include voice, sound, and video peripherals.

Generally the cable reach is about 2 kilometres

It is common for one computer to be a ‘server’ - which will store all of the software and user accounts

Speeds are in the 4 to 16Mbps range. 100Mbps and nGbps are in sight


Medium Area NetworksMedium Area Networks

A medium area network is normally designed to extend over a city. It can be a single network (cable television network) or it can be a means of connecting a number of LANS so that resources may be shared LAN to LAN, as well as device to device.

A MAN may be wholly owned and operated by a private company, or it may be a service provided by a public company.


Wide Area NetworksWide Area Networks

These provide long-distance transmission of data, voice, image and video information over large geographical areas.

These may comprise a country, continent or the world.

WANS may utilise public, leased or private communications devices.

They have an ‘unlimited’ span.

A WAN which is wholly owned and used by a single Company is known as an enterprise network


InterNetworksInterNetworks

Two or more networks can be connected.

They then become an internetwork or internet.

Individual networks are joined into the internetworks by internetworking devices.

These devices include routers and gateways.

The term Internet (notice the I) is the name of a specific worldwide network


InterNetworksInterNetworks

R

R

R

R

R

R

RR

LAN LAN

LAN LAN

LAN MAN

WAN

R indicates Routers


Transmission MediaTransmission Media

Guided Media : A conduit from one device to another• Twisted pair Telephone wire• Co-Axial Cable - Baseband

all devices share 1 communications channel

- Broadband

provides 10 times the number of

baseband cable• Fibre Optics - Signals transmitted by light energy

12.5mm dia f.o. cable can carry up to

50,000 channels

- expensive


Transmission MediaTransmission Media

Unguided Media : No physical conductor• Communications Satellite

- Ground collectors

- Ground stations

Microwave signals to 200 Mbits/second

(expected increase to 1000 Mbits/second)• Cellular radio

- Divides geographic area into cells

(e.g. suburbs)

Each cell is serviced with a low power

transmitter / receiver


FrequenciesFrequencies

A few short words about ‘frequencies’ and ‘wavelengths’

Frequency is measured in cycles per second (called hertz).

Wave forms are described by their length and the common measures of wavelength are measured in kilometres, metres, millimeter and micrometers


Radio Communication BandRadio Communication Band

Frequency Range Wavelength

VLF Very low frequency 3KHz to 30 KHz 10km to 100km

LF Low frequency 30KHz to 300KHz 1km to 10km

MF Middle frequency 300KHz to 3 MHz 100m to 1km

HF High frequency 3MHz to 30MHz 10m to 100m

VHF Very high frequency 30MHz to 300MHz 1m to 10m

UHF Ultra high frequency 300MHz to 3GHz 10cm to 1m

SHF Super high frequency 3GHz to 30GHz 1cm to 10cm

EHF Extremely high frequency 30GHz to 300GHz 0.1mm to

10mm


Radio Communication BandRadio Communication Band

The characteristics of communications channels are (broadly)

Bandwidth - which is the range of frequencies a circuit can support

Transmission Rate - the number of bits per second which can be transmitted in a circuit

Direction of flow - (simplex, half duplex, duplex)

Medium or circuitry used - twisted pair, microwave, broadcast circuit

Circuit Conditioning - reduction (or attenuation) - filtering processes - as in a stereo which filters out turntable rumbling below 50MHz. A 10dB attenuation at 100MHz means that sounds are reduced by 10 decibel units in the 100MHz range



• Analogue - Based on Telephone (voice) system

Convertor needed to

1. change input device mode to channel carrier mode

2. change from channel carrier mode to receiving device mode

Known as Modulating and Demodulating = modem

(amplitude modulation

angle modulation - covers phase and frequency)

Typical speeds are 9.6KBps, 14.4KBps, 28.8KBps, 56KBps


ModemsModems

• Types

stand alone

board level - plug in

acoustic coupler

intelligent

Intelligent Modem facilities:

Simultaneous voice and data transmission

Transmission error detection

Automatic dialup

Conversion from ASCII to EBCDIC and reverse

Automatic testing

Automatic transmission speed selection


Analog / Digital ModulationAnalog / Digital Modulation

computer modem

digitalsignal

analogsignal

analog signal digital modem signal computer



• Digital Digital Communication Networks

offer - high speeds

- low error rate

- broad band (range of speeds)

- no modems

- accommodation of voice carrier systems• I.S.D.N. Integrated Services Digital Networks

Voice, Data, Image, Video, Text,

It is a circuit switched network

Telstra ISDN - Voice Data Text

speeds from 2400bps top 64000 bps (64KBps)

Hybrid channels offer 1536Kbps


I.S.D.N.I.S.D.N.

PABX PABX

ISDNNODE

ISDNNODE

SwitchedVoice

Switched Faxand Text

Dedicated Tie Line

CommonChannel Signalling

2Mb 2Mb

Common Carrier - Telstra, Optus


Satellite CommunicationsSatellite Communications

point to point link

satellite

earth station earth station

multiple receivers

transmitter

satellite



3 Satellite Constellations: (microwave, straight line signal propogation)

Low Earth Orbit (LEO) not synchronised

Medium Earth Orbit (MEO) not synchronised

Geosynchronous Earth Orbit (GEO) (35,200 km)

Mobile Satellite SystemsSatellite Personal Communication Systems Small portable handsets with access to anywhere on Earth

Estimated 100,000,000 waiting for telephone services (world base)



Definitely not to scale

Earth

Galileo

JUPITER Time of Flight Approx 6 Earth Years

Arrival Date 6/12/95Digital Images

Gas Emissions

..

A question : Who has the Mars Lander ?


So What is The Internet ?So What is The Internet ?

• It is a complex web of networks.• Each network can have millions of inter-connected

computers which use telephone infrastructure to communicate

• No one ‘owns’ the Internet - Its backbone, through which information flows, is owned by a number of private companies

• Messages are transmitted through the Internet via a computer language called ‘transmission control protocol - TCP/IP.

• Each message travels as a packet and has a coded address which tells the network its destination and also has a block of data content



• This packet is transmitted via variety of routes• Data travels more efficiently using digital techniques• Telstra has been (slowly) digitising its public switched

telephone network

How do home computers link to the Internet ?• Normally via an Internet Server Provider (ISP) - a monthly

fee is charged for connection and maintenance• These ISP’s are connected to a larger network service -

Network Service Providers (NSP’s) which are connected using fibre optic cable OR satellite links.



• There are about 700 ISPs operating in Australia

• Telstra has a large share of the Internet wholesale market - others are C&W Optus, OzEmail, connect.com

• Most ISPs are small and operate a single point of presence or ‘POP’ - this is a location which comprises modems and a network connection into which an ISP’s customers dial to make an Internet connection.



• High speed Internet requires ISDN - but there is a catch !

This is not possible (at present) as it cannot be provided between a customer’s premises and the exchange where the length of copper wire is greater than 4.5Km.

• Watch the outcomes of the Federal Government’s legislation under the Telecommunications (Consumer Protections and Service Standards) Act 1999

• It’s a very interesting Act.


This Might HelpThis Might Help

WholesaleInternet Provider

TelstraC&W OptusOzEmailNet.com

NSP NSP NSP

NetworkService Provider

Internet Service Provider

Modem and phone line link

Satellite


Communications HardwareCommunications Hardware

• Multiplexor– single channel to carry simultaneous transmissions

• Communications Control Unit– multiterminal use of channel– code and decode– error functions

• Concentrator– microprocessor - collects and bundles data for high

speed transmission to CPU and reverse• Front End Processor

– large systems (number of terminals, users, processing)– access controls - priorities– network statistics - message routing


MultiplexorMultiplexor

aaa aaa

bbb cbacbacba bbb

ccc ccc

terminals

multiplexor multiplexor

computer


ConcentratorConcentrator

terminals concentrator computer

buffer


Front End ProcessorFront End Processorterminals

concentrator

concentrator

front endprocessor

printer

disk unit computer


Data Communications SoftwareData Communications Software• Includes Communications Access Programs

establish connections between terminals and Computers AND the link between application programs and the communications network

• Network Control Programs

A communications network function manager which– Connects / Disconnects communications links– Detects and corrects errors– Polls terminals– Queues data and program tasks– Routes messages– Logs statistics - errors, users, usage, facilities


Network ArchitecturesNetwork Architectures• Combination of hardware and software used to implement

the communications function• Protocols : set of conventions which govern the exchange

of data between 2 entities

Syntax - data format, coding, signal levels

Semantics - control information for coordination and error handling

Timing - speed matching and sequencing

TCP Transmission Control Protocol

IP Internet Protocol ---- > TCP/IP - datagrams - 65,535 bit packets)

SNA, Novell IPX/SPX, Decnet

TELNET Provides a simple asynchronous terminal capability


Network Operating SystemsNetwork Operating Systems

Novell Netware (Versions … 4.01)

Banyan Vines

3 COM

EasyLan

Network OS


Distributed SystemsDistributed Systems

Processing

Distributed Data Base

Controls

Distributed Processing : Processes (jobs) to have processing capability at a number of sites

Some Options : Permanent, temporary Linkage

Controls required to Ensure:1. INTEGRITY 2. AVAILABILITY 3. SECURITY

4. RECOVERY 5. COMPATABILITY 6. CONTINUITY


Communications Interface FunctionsCommunications Interface Functions

• Error monitoring, Diagnosis and Correction• Modulation / Demodulation• Data Compression• Data Coding and Decoding• Protocol Conversion• Message Switching• Buffer Storage• Port Contention• Concentration


Communications Example1Communications Example1

Time DivisionMultiplexor

Ballarat

FootscrayRegional Computer

9600 bps

DDS Line

modem 1

modem 2

modem 3

modem n

signalcontrollers

300 bps dedicated

23 regional computer systems1600 traffic signal controllers

1995


Communications ExampleCommunications Example

City Link and the Tollway system

Possibly the Mitcham-Frankston Tollway ?

- Pick up of E-tag at designated points

- Registration numbers video’d and converted to digital image

- E-tags and registrations linked to

owner details

- Debiting system for E-tags

- Day trippers and other pass forms

- Reconciliation system for others

- Detection of ‘unauthorised’ users

and follow up



Telstra services and e-commerce

A 9600bps connection : ISP time-charged on time taken to down load - e.g. image.

The result : very slow and costly performance

In 1985/8, Telstra introduced the ‘pair-gain multiplexer’ facility over a single cable facility. (phone to exchange line ratio

is 4 : 1)

This means that instead of each connection having a copper wire to an exchange, 4 connections are ‘fed’ into one cable



The IP (Internet Protocol) requires as much bandwidth to an exchange as is available - with minimum multiplexers

Normally, 8 phone connection wires enter a ‘pair gain’ multiplexer - and 2 wires go back to the exchange

In practice this means that a 56,000bps modem can only provide at a maximum 28,800 bps

A 3rd user will achieve 9600bps (28,800/3 = 9600)

A 4th user would achieve 28,800/4 = 7,200bps



You have probably heard of

CDMA = Code Division Multiple Access (useful for mobile phones as a substitute for Analog transmission)

and the latest development

ASDL = Asymmetric Digital Subscriber Line (uses 1KHz to 4KHz for telephone service and 20KHz to 2.2MHz for data transfer over copper wire.)


Some TermsSome Terms

• A Network - A facility which provides a data transfer service among stations attached to a network

• An Internet A collection of communication networks interconnected by bridges or routers

• A subnetwork A member unit of a network.• IWU - Interworking Unit - A device used to connect two

subnetworks and which permits communication between end systems attached to different subnetworks

• Bridge An Interworking Unit which is used to connect two LANs which use identical protocols. No data modifications are incurred

• Router A device used to connect two networks which need not be similar


Definitions - Medical Definitions - Medical

• Artery - The study of Art• Caesarean Section - A neighbourhood of Rome• Colic - A sheep dog• Dilate - To live a long time• Fester - Quicker• Labour Pain - Getting hurt at work• Outpatient - A person who has fainted• Node - Was aware of• Recovery Room - A place to do upholstery• Seizure - A Roman Emperor• Postoperative - A letter carrier• Sciatic - An attic with a view of the sky


Some things to do duringthe (short) break until Monday!!

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CSE1720 Summer 2005 Lecture 8 / 1 Lecture No. 8 Communications.

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