COM Study Guide 2012 - Complete

8/2/2019 COM Study Guide 2012 - Complete

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COMComputer Communications

Written by: N.Sadiq Ali

Unit Assessor: Rachakonda Balaraju

Unit Information


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1

Welcome to Students

Congratulations on being offered a place in a Diploma in Information Technology (DIT) at

Institute of Business Studies. If this is your first year with us, and you have selected this

unit as one of your initial units, I would like to extend a special welcome to you and wish

you every success in your studies.

This unit will give you a fundamental understanding of computer communications

technology, the underlying principles of data transmission, the electronic equipment and

computers that provide the connectivity. You will also study standard and the role of

standards in data communication.

As a practical component of this unit, you will also gain skills in the use of the worlds

largest network, the Internet.

To successfully complete this unit of study, it will be necessary to have access to a

personal computer and the Internet. To ensure that you are up-to-date, with respect to

your studies, you should cover one topic each week and try not to fall behind.

Wishing you a very successful year!

N. SADIQ ALI, B.Sc, M.C.A, PGDHRD, MACS, FPNGCS, MPNGHRI, MPNGID,

DIRECTOR


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UNIT STATEMENT

Unit Title: Computer Communications

Unit Code: COM

Type of Unit: Core Unit Diploma in Information Technology

Prerequisites: Nil

Mode of Study: Internal

Semester: 1

Teaching Unit: School of Computing

Staffing: Mr. Shaik Dawood Ansari

Aims

Global networks, wireless data communication, and client-server systems are just some of

the telecommunication concepts driving industry today. The aim of this unit is to provide a

unified overview of the broad field of data and computer communications and explore the

networking components.

Objectives

On successful completion of this subject, candidates should be able to

understand the principles of computer communications with emphasis on the

roles played by different layers of a communication architecture;

describe different transmission media and data transmission techniques;

understand operation principles of common local area networks and

communication protocols;

describe and use of common Internet applications;

realize the potential power of global computer networks and the ethical issues

this raises.

Handbook Entry

This unit serves as an introductory course in data and computer communication. The

scope of this unit is broad, covering three general areas: data communication, networking

and protocols. Data communication deals with the transmission of signals in a reliable and

efficient manner, which covers single transmission, transmission media, encoding,

interfacing, and data link control. Networking deals with the technology and architecture ofthe communications network used to interconnect communication devices. A discussion of


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communication protocols includes a treatment of protocol architectures as well as an

analysis of individual protocols at various layers of the architecture.

By studying this unit students would attain the operational knowledge on networks,

Internet, communication protocols and communication architecture. This unit will givestudents the skills to analyse the needs and specify network requirements.

Syllabus

Topic 1 Introduction

Topic 2 Protocols and Architecture

Topic 3 Data Transmission

Topic 4 Transmission MediaTopic 5 Data Encoding

Topic 6 The Data Communication Interface

Topic 7 Data Link Control

Topic 8 Circuit Switching

Topic 9 Packet Switching

Topic 10 LAN Technology

Topic 11 LAN Systems

Topic 12 Internet Protocols

Prescribed Text and Materials

William Stallings, Data and Computer Communications, Prentice Hall, Sixth edition, 2000.

Recommended reference materials

Tanenbaum, A. S., Computer Networks, 3rd edition, Prentice Hall, 1996.

Forouzan, B. A., Data Communications and Networking, 2nd edition, McGraw-Hill, 2001.


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Students Assessment Requirements

The assessment for this unit comprises two assignments and a final three-hour

examination. The allocation of marks is as follows:

Revision Test 1 5%

Revision Test 2 5%Attendance 5%

Assignment 1 10%

Assignment 2 15%

Final Examination 60%

Total 100%

To pass this unit, a total mark of 50% or more is required.

Final Examination

All topics are assessable. To pass this unit, a total mark of 50% or more is required.

Students should note that final results in this unit might be scaled in accordance with

Institute policy on grade distributions.

Study Time Expectations

All topics in this unit require engaging in approximately 60 hours of learning throughout the

semester. This averages out to around 4 hours per week for 15 weeks but some weeks

will always be heavier than others.


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Program Schedule

WEEK /

COMMENCING

TOPIC

NO.

TOPICS KEY

DATES0

13.02.2012 Induction Week

120.02.2012

1 Introduction

227.02.2012

2 Protocols and Architecture

305.03.2012

3 Data Transmission

412.03.2012

4 Transmission Media Revision Test 1

519.03.2012

5 Data EncodingAssignment 1

due on 23.03.126

26.03.20126 The Data Communication Interface

702.04.2012

7 Data Link Control

809.04.2012

8 Multiplexing

916.04.2012

9 Switching Revision Test 2

1023.04.2012

10 Congestion Control Assignment 2due on 04.05.1211

30.04.201211 Local Area Network

1207.05.2012

12 Protocols

1314.05.2012 Mock Exam

Exam Dates tobe advised

14 & 1524.05.12 02-06-12 Examination Period

Exam Dates tobe advised


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SAMPLE EXAM QUESTIONSComputer Communications

Answer all the five- (5) questions. All questions are worth equal marks.Time allowed: Reading 15 minutes Writing 3 hours

Question 1 (Total marks 12)

Define or briefly describe the following terms.

(i) Layered communication architecture (2 marks)

(ii) Synchronous data transmission (2 marks)

(iii) Packet switched network (2marks)

(iv) Internet Protocol (IP) (3 marks)

(v) Multimedia communication (3marks)


(a) (i) What is a communication protocol?

(ii) Briefly describe three key elements of a communication protocol. (6 marks)

(b) What are the similarities and differences between the ISO Open Systems

Interconnection Reference Model and the Internet architecture? (6 marks)


(a) List three types of transmission media and describe their respective advantages and

disadvantages. (4 marks)

(b) Use an example to describe an error correction technique for data transmission.

(4 marks)

(c) (i)What is Manchester coding? (2 marks)

(ii) What is its advantage? (2 marks)


(a) (i) What is the main difference between flow control and congestion control?

(ii) Briefly describe a (any) flow control mechanism / technique. (3 marks)

(b) In a network that has a maximum packet size of 100 bytes, a maximum packet lifetime

of 10 seconds, and an 10-bit packet sequence number, what is the maximum data rate per


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connection (assuming that each packet over a connection must be uniquely identified

using a sequence number)? (3 marks)

(c) Transmission Control Protocol (TCP) is one of the main protocols of the Internet. Briefly

describe the main functions of the TCP. (6 marks)


(a) (i) What is MIME?

(ii) What are the main functions of MIME? (3 marks)

(b) One of the main security services is privacy. How does public key encryption achieve

privacy? (3 marks)

(c) Briefly describe the basic operation principle of the World Wide Web (WWW).

(6 marks)


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COMComputer Communications

Written by: N.Sadiq Ali

Unit Assessor: Rachakonda Balaraju

StudyGuide


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COMPUTERCOMMUNICATIONS

1

INTRODUCTION


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COM: Computer Communications 1.1

TOPIC 1

1.0 INTRODUCTION

We begin our study of computer communications with a topic called

introduction to communications. The exchange of information between

computers for the purpose of cooperative action is generally referred to as

computer communications. Similarly, when two or more computers are

interconnected via a communication network, the set The basic task of a

communication system is exchange of data or information between two

parties or two connected devices.

1.1 Communication model

The fundamental purpose of any communication system is to transfer

the data from one location to another location. E.g. Exchange of voice

signals between two telephones over the same network. The simple

communication model uses the following key elements:

Source generates data to be transmitted Transmitter converts data into transmittable signals Transmission system carries data Receiver converts received signals into data Destination takes incoming dataThe key tasks that must be performed in a data communication system

are: transmission system utilization, interfacing, signal generation,

synchronization, exchange management, error detection and correction,

flow control, addressing, routing, recovery, message formatting, security

and network management.

Textbook: Pages 4 7

This section gives you brief description of the communication model and

the tasks performed in a data communication system.

1.2 Data Communication Networking

In data communication point-to-point connection between two computers

or devices is not usually practical because of one for the following reasons:

Devices are too far apart. Large set of devices would need impractical number of

connections.


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COM: Computer Communications1.2

The solution to this problem is a communication network that is to

attach each device to a communication network. Communication networks

are traditionally classified into the following two major categorizes:

LAN Local Area Network

WAN Wide Area Network

1.2.1 Wide Area Networks

Wide Area Networks is the connection between the computers or

devices, which covers a large geographical area and crossing public rights

of way. A WAN consists of a number of interconnected switching nodes.

A transmission from any one device is routed through these internal nodes

to the specified destination device and it relies in part on common carriercircuits. The alternative technologies are:

Circuit Switching - dedicated communications path established for the

duration of the conversation. E.g. telephone network.

Packet Switching

Data sent out of sequence, small chunks (packets) of data at a time Packets passed from node to node between source and destination Used for terminal to computer and computer to computer

communicationsFrame Relay

Packet switching systems have large overheads to compensate forerrors.

Modern systems are more reliable, errors can be caught in endsystem and most overhead for error control is stripped out

Asynchronous Transfer Mode (ATM)

Evolution of frame relay and has little overhead for error control. Fixed packet (called cell) length, anything from 10Mbps to Gbps. Constant data rate using packet switching technique

Integrated Services Digital Network (ISDN)

Designed to replace public telecom system Wide variety of services; entirely digital domain


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1.2.2 Local Area Networks

Local Area Network is a communications network that interconnects a

variety of devices and provides a means for information exchange among

those devices in smaller scope, that is within a building or small campus.

Usually owned by same organization as attached devices Data rates much higher Usually broadcast systems Now some switched systems and ATM are being introduced

Textbook: Pages 7 12

This section gives you brief description of data communication

networking, LAN, WAN and the technologies used in WAN.

1.3 ProtocolsA protocol is used for communication between entities in different

systems. Here entities refer to user applications, e-mail facilities,

terminals, database management systems and file transfer packages.

Systems refer to computer, terminal and remote sensor. An entity is

anything capable of sending or receiving information and a system is

physically distinct object that contains one or more entities. If two entities

communicate successfully then they must speak same language. The key

elements of a Protocol are:

Syntax - data formats, signal levels Semantics - control information, error handling Timing - speed matching, sequencing

1.3.1 Protocol Architecture

Task of communication broken up into modules For example file transfer could use three modules

File transfer application

Communication service module Network access module

1.3.2 A Three Layer Model

Communications can be said to involve three agents: applications,

computers and networks. The transfer of data from one application to

another involves first getting the data to the computer in which the

application resides and then getting it to the intended application using


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networks. With this concept the communication tasks can be organised

into the following three independent layers:

a) Network Access Layer

Exchange of data between the computer and the network Sending computer provides address of destination May invoke levels of service Dependent on type of network used (LAN, packet switched etc.)

b) Transport Layer

Reliable data exchange Independent of network being used Independent of application

c) Application Layer

Support for different user applications. E.g. e-mail, file transfer1.3.2.1 Addressing Requirements

Each computer on the network must have a unique address; this allows

the network to deliver data to the proper computer. Each application on the

computer must have an address that is unique within the computer; this

allows the transport layer to support multiple applications at each

computer. This is also known as service access point or SAP.

1.3.2.2 Protocol Data Units (PDU)

The combination of data and the control information is known as a

protocol data unit (PDU).

At each layer, protocols are used to communicate Control information is added to user data at each layer Transport layer may fragment user data Each fragment has a transport header added

Destination SAP Sequence number Error detection code

The Network PDU adds the following to the network header:

network address for destination computer Facilities requests


This section gives you brief description of protocols, protocol architecture

and the three-layer communication model.


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1.4 TCP/IP Protocol Architecture

The TCP/IP protocol architecture was developed by the US

Defense Advanced Research Project Agency (DARPA) for its packet

switched network (ARPANET) and now it is used by the global Internet. It

has no official model but a working one but the communication task can

be organised into the following five independent layers:

a) Application Layer - support for user applications. E.g. http, SMPT.

b) Transport Layer (TCP) or Host-to-host layer

Reliable delivery of data Ordering of delivery

c) Internet Layer (IP)

Systems may be attached to different networks Routing functions across multiple networks Implemented in end systems and routers

d) Network Access Layer

Exchange of data between end system and network Destination address provision Invoking services like priority

e) Physical Layer

Physical interface between data transmission device (e.g.computer) and transmission medium or network

Characteristics of transmission medium are signal levels, data rates,etc.


This section gives you brief description of Transmission Control Protocol /

Internet Protocol architecture and its layers.

1.5 OSI Model

The Open Systems Interconnection (OSI) model was developed by

the International Organization for Standardization (ISO) as a model for

computer communication architecture. It consists of the following seven

layers: Application, Presentation, Session, Transport, Network, Data Link,

and Physical


This section gives you brief description of Open Systems Interconnection

model and its layers.


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2

PROTOCOLS &

ARCHITECTURE


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TOPIC 2

2.0 PROTOCOLS AND ARCHITECTURE

Protocol architecture is the layered structure of hardware and software

that supports the exchange of data between systems and supports

distributed applications, such as electronic mail and file transfer. At each

layer of protocol architecture, one or more common protocols are

implemented in communicating systems. Each protocol provides a set of

rules for the exchange of data between systems.

2.1 Characteristics of protocols

Some important characteristics are as follows:

Direct

Systems share a point to point link or Systems share a multi-point link Data can pass without intervening active agent

Indirect

Switched networks or Internetworks or internets Data transfer depend on other entities

Monolithic or Structured

Communications is a complex task To complex for single unit Structured design breaks down problem into smaller units Layered structure

Symmetric - Communication between peer entities

Asymmetric - Client/server

Standard or Nonstandard Nonstandard protocols built for specific computers and tasks K sources and L receivers leads to K*L protocols and 2*K*L

implementations

If common protocol used, K + L implementations neededTextbook: Pages 32 55

This section gives you brief description of protocols and its

characteristics.


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2.2 Protocol Functions

In the communication architecture a small set of functions forms the

basis of the protocols and not all protocols have all functions. But in many

instances the same type of function will present in protocols at different

levels. The functions can be categorised as follows:

Encapsulation The addition of control information to data is generally

referred as encapsulation. The control information falls into the following

three categories.

Address information Error-detecting code Protocol control

Segmentation (Fragmentation)

A protocol is concerned with exchanging streams of data between

two entities. Data blocks are of bounded size. Application layer messages

may be large and Network packets may be smaller. Splitting larger blocks

into smaller ones is segmentation (or fragmentation in TCP/IP).

ATM blocks (cells) are 53 octets long Ethernet blocks (frames) are up to 1526 octets long

Why Fragment?

Advantages

More efficient error control More equitable access to network facilities Shorter delays Smaller buffers needed

Disadvantages

Overheads Increased interrupts at receiver & More processing time

Connection Control

Connection Establishment Data transfer Connection termination May be connection interruption and recovery Sequence numbers used for

Ordered delivery, Flow control, Error control


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Ordered Delivery

PDUs may traverse different paths through network PDUs may arrive out of order Sequentially number PDUs to allow for ordering

Flow Control

Done by receiving entity Limit amount or rate of data Stop and wait Credit systems - Sliding window Needed at application as well as network layers

Error Control

Guard against loss or damage Error detection

Sender inserts error detecting bits Receiver checks these bits If OK, acknowledge If error, discard packet

Retransmission - If no acknowledge in given time, re-transmit Performed at various levelsAddressing

Addressing level Addressing scope Connection identifiers Addressing mode

Addressing level

Level in architecture at which entity is named Unique address for each end system (computer) and router Network level address

IP or internet address (TCP/IP) Network service access point or NSAP (OSI)

Process within the system Port number (TCP/IP), Service access point or SAP (OSI)

Addressing Scope

Global nonambiguity Global address identifies unique system

There is only one system with address X


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Global applicability It is possible at any system (any address) to identify anyother system (address) by the global address of the other

system

Address X identifies that system from anywhere on thenetwork. e.g. MAC address on IEEE 802 networks

Connection Identifiers

Connection oriented data transfer (virtual circuits) Allocate a connection name during the transfer phase

Reduced overhead as connection identifiers are shorter thanglobal addresses

Routing may be fixed and identified by connection name Entities may want multiple connections - multiplexing State information

Addressing Mode

Usually an address refers to a single system Unicast address, Sent to one machine or person

May address all entities within a domain Broadcast Sent to all machines or users

May address a subset of the entities in a domain

Multicast, Sent to some machines or a group of usersMultiplexing

Supporting multiple connections on one machine Mapping of multiple connections at one level to a single

connection at another

Carrying a number of connections on one fiber optic cable Aggregating or bonding ISDN lines to gain bandwidth

Transmission Services

Priority - e.g. control messages Quality of service - Minimum acceptable throughput, Maximum

acceptable delay

Security - Access restrictionsTextbook: Pages 35 44

This section gives you brief description of protocol functions.


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2.3 OSI - The Model

A layer model Each layer performs a subset of the required communication

functions

Each layer relies on the next lower layer to perform more primitivefunctions

Each layer provides services to the next higher layer Changes in one layer should not require changes in other layers

Elements of Standardization

Protocol specification Operates between the same layer on two systems May involve different operating system Protocol specification must be precise

Format of data units, Semantics of all fields allowable sequence of PCUs

Service definition - Functional description of what is provided Addressing - Referenced by SAPs

OSI Layers (1)

Physical - Physical interface between devices Mechanical, Electrical, Functional, Procedural

Data Link - Means of activating, maintaining and deactivating areliable link

Error detection and control Higher layers may assume error free transmission

OSI Layers (2)

Network - Transport of information Higher layers do not need to know about underlying

technology

Not needed on direct links Transport - Exchange of data between end systems

Error free, In sequence, No losses No duplicates & Quality of service

OSI Layers (3)

Session - Control of dialogues between applications Dialogue discipline, Grouping & Recovery

Presentation Data formats and coding, Data compression & Encryption


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Application - Means for applications to access OSI environmentTextbook: Pages 44 54

This section gives you brief description of open systems interconnection

model.

2.4 TCP/IP Protocol Suite

Dominant commercial protocol architecture Specified and extensively used before OSI Developed by research funded US Department of Defense Used by the Internet

TCP/IP Protocol Architecture (1)

Application Layer - Communication between processes or applications End to end or transport layer (TCP/UDP/)

End to end transfer of data May include reliability mechanism (TCP) Hides detail of underlying network

Internet Layer (IP) - Routing of dataTCP/IP Protocol Architecture (2)

Network Layer- Logical interface between end system and network Physical Layer Transmission medium & Signal rate and encodingTextbook: Pages 54 60

This section gives you brief description of transmission control protocol

and Internet protocol.

Review Activities:

2.1 Briefly describe Layered protocol architecture

2.2 What is a communication protocol and why is it needed?

2.3 Briefly describe the similarities and differences between OSI

Reference Model and the TCP/IP architecture.

2.4 (a) The ISO OSI Reference Model and TCP/IP both use layered

network architectures. Briefly describe how the layered architectures are

implemented (i.e. how layers communicate with each other?)

(b) The protocol used at the network layer of the Internet is called IP

(Internet Protocol). Describe the main functions of the IP.


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Feedback to the Review Activities:

Review Activity 2.1

Refers to the communication architecture which consists of a

number of layers and each layer has its own protocols. For example, OSI

reference model consists of seven layers and the Internet architecture

consists of five layers.

Review Activity 2.2

Communication protocol is a set of rules that governs the required

communications. It is needed to ensure that the communication entities can

understand each other and can carry out meaningful communication

Review Activity 2.3

Similarity: both are layered architecture and most layers are similar

in the two architectures. Differences: the OSI reference model has seven

layers while the TCP/IP architecture has only five layers. The main

functions of session later, presentation layer and application layer of the

OSI model are combined to form the application layer in the TCP/IP

architecture

Review Activity 2.4

(a) A network is a complicated system. The layered architecture

makes the design and implementation of a network manageable. Another

advantage of the layered architecture is that it makes the network more

modular so that we can change/upgrade one layer without affecting the

functions / implementation of the other layers. Each layer implements the

well-defined function and interface between layers. It works based on the

concept of encapsulation, whereby functions/instructions of one layer are

included in the header for other layers to use.

(b) The main functions of the network layer are to inter-connect

different networks to form a large network and to route packets from the

source to destination.


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3

DATA

TRANSMISSION


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TOPIC 3

3.0 DATA TRANSMISSION

Transferring information (voice, data, image, and video) from one

device to another device is called as data transmission. The successful

transmission of data depends on two factors: the quality of the signal being

transmitted and the characteristics of the transmission medium.

3.1Concepts

Data transmission occurs between transmitter and receiver over some

transmission medium. The transmission medium can be classified as

Guided media and Unguided media. In guided media, the waves are

guided along a physical path. E.g. twisted pair, coaxial cable and optical

fiber. In unguided media it do not guide them. E.g. propagation through

air, vacuum and sea water.

Direct link - No intermediate devices

Point-to-point - Direct link, Only 2 devices share link.

Multi-point - More than two devices share the link.

The data transmission may be simplex, half-duplex or full duplex.

Simplex - One direction e.g. Television

Half-duplex -Either direction, but only one way at a time. E.g. police radio

Full duplex - Both directions at the same time. E.g. telephone

Textbook: Pages 69 78 V7: 57 - 67

This section gives you brief description of data transmission concepts and

terminology.

3.2 Analog and digital transmission

Data are entities that convey meaning. Signals are the electric or

electromagnetic representations of data. Transmission - Communication of

data by propagation and processing of signals.

The data can be classified as analog and digital data. Analog - continuous

values within some interval. E.g. sound, video. Digital - Discrete values

e.g. text, integers


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Signals - Means by which data are propagated

Analog Continuously variable

Various media- wire, fiber optic, space

Speech bandwidth 100Hz to 7kHz Telephone bandwidth 300Hz to 3400Hz Video bandwidth 4MHz Digital

- Use two DC components

Data and Signals

Usually use digital signals for digital data and analog signals foranalog data

Can use analog signal to carry digital data - Modem Can use digital signal to carry analog data - Compact Disc audio

Analog Transmission

Analog signal transmitted without regard to content May be analog or digital data Attenuated over distance Use amplifiers to boost signal Also amplifies noise

Digital Transmission

Concerned with content

Integrity endangered by noise, attenuation etc. Repeaters used Repeater receives signal Extracts bit pattern Retransmits Attenuation is overcome Noise is not amplified


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Advantages of Digital Transmission

Digital technology - Low cost LSI/VLSI technology Data integrity - Longer distances over lower quality lines Capacity utilization - High bandwidth links economical; High

degree of multiplexing easier with digital techniques

Security & Privacy - Encryption Integration - Can treat analog and digital data similarly


This section gives you brief description of analog and digital data

transmission.

3.3 Transmission Impairments

Signal received may differ from signal transmitted Analog - degradation of signal quality Digital - bit errors Caused by

Attenuation and attenuation distortion Delay distortion Noise

Attenuation

Signal strength falls off with distance Depends on medium Received signal strength: must be enough to be detected must be sufficiently higher than noise to be received without error

Attenuation is an increasing function of frequencyDelay Distortion

Only in guided media Propagation velocity varies with frequencyNoise

Additional signals inserted between transmitter and receiver


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Thermal Due to thermal agitation of electrons Uniformly distributed White noise

Intermodulation Signals that are the sum and difference of original frequencies

sharing a medium

Crosstalk A signal from one line is picked up by another

Impulse Irregular pulses or spikes e.g. External electromagnetic interference Short duration High amplitude


This section gives you brief description of different data transmission

impairments.

Review Activities:

3.1 Suppose an analog audio signal has the frequency range of 0 to 20

kHz. This analog signal is to be converted into digital signal.

(i) What should be the minimum sampling rate in order to convert the

signal correctly?

(ii) Assume that 16 bits are used to represent each sample. Calculate the

amount of storage required to store 2 hours of above digital signal

Solution:

(i) According to Nyquist theorem, the sampling rate should be at least 20

kHz.

(ii) Amount of storage = 20000 x 2 x 3600 x 16/8= 288,

000, 000 bytes = 274.66 Mbytes.


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3.2

Bandwidth

Is the maximum amount of data that can travel a communication path in a

given time period and is often measured in bits per second.

Latency

Latency is the amount of time that it takes a packet to travel from the

source to the destination.

Throughput

Is the data transfer rate that is achieved by combining the effects of

Bandwidth and Latency putting it simple bandwidth is what you pay for

and throughput is what you actually receive.

Capacity

Is the practical maximum data carrying capability.

Bottleneck

A bottleneck is a delay that occurs when part of a network is slower than

others.

Collisions

Are Frames that were not sent successfully on a shared medium because

the senders tried to send frames at the same time.


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4

TRANSMISSION

MEDIA


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TOPIC 4

4.0 TRANSMISSION MEDIA

Transmission media is used to convey the information and which

carries the information from one device to another device. The

transmission media can be classified as guided and unguided.

4.1Guided transmission media

The transmission medium is the physical path between transmitter and

receiver and the communication is in the form of electromagnetic waves.

In the guided medium, the waves are guided along a solid medium, such as

copper twisted pair, copper coaxial cable, and optical fiber.

Twisted pair wire

Twisted pair wire consists of two insulated wires arranged in a

regular spiral pattern. A wire pair acts as a single communication link.

Often bundles into cables and usually installed in buildings during

construction.

Twisted Pair Applications

Most common medium Telephone network Between house and local exchange (subscriber loop)

Within buildings - To private branch exchange (PBX) For local area networks (LAN) - 10Mbps or 100MbpsTwisted Pair - Pros and Cons

Cheap Easy to work with Low data rate Short range

Twisted Pair - Transmission Characteristics

Analog - Amplifiers every 5km to 6km Digital - Use either analog or digital signals; repeater every 2km or

3km

Limited distance


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Limited bandwidth (1MHz) Limited data rate (100MHz) Susceptible to interference and noise

Unshielded Twisted Pair (UTP)

Ordinary telephone wire Cheapest Easiest to install Suffers from external EM interference

Shielded Twisted Pair (STP)

Metal braid or sheathing that reduces interference

More expensive Harder to handle (thick, heavy)

Cat 3

up to 16MHz Voice grade found in most offices Twist length of 7.5 cm to 10 cmCat 4 - up to 20 MHzCat 5

up to 100MHz Commonly pre-installed in new office buildings Twist length 0.6 cm to 0.85 cm

Coaxial Cable

The coaxial cable consists of a hallow outer cylindrical conductor thatsurrounds a single inner wire conductor. The inner conductor is held in

place by either regularly spaced insulating rings or a solid dielectric

material. The outer conductor is covered with a jacket or shield.

Coaxial Cable Applications

Most versatile medium Television distribution - Ariel to TV, Cable TV

Long distance telephone transmission


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Can carry 10,000 voice calls simultaneously Being replaced by fiber optic

Short distance computer systems links

Local area networksCoaxial Cable - Transmission Characteristics

Analog

Amplifiers every few km Closer if higher frequency Up to 500MHz

Digital

Repeater every 1km

Closer for higher data rates

Optical Fiber

An optical fiber is a thin, flexible capable of guiding an optical ray.

Various glasses and plastics can be used to make optical fibers. An optical

fiber cable has a cylindrical shape and consists of three concentric

sections: the core, the cladding, and the jacket.

Optical Fiber Benefits

Greater capacity - Data rates of hundreds of Gbps Smaller size & weight Lower attenuation Electromagnetic isolation Greater repeater spacing - 10s of km at least

Optical Fiber Applications

Long-haul trunks Metropolitan trunks Rural exchange trunks Subscriber loops LANs


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Textbook: Pages 110 119 V7: 95 - 106

This section give you brief description of guided transmission media.

4.2 Wireless Transmission

Unguided media Transmission and reception via antenna Directional

Focused beam Careful alignment required

Omnidirectional Signal spreads in all directions Can be received by many antennae

Terrestrial Microwave

Parabolic dish Focused beam Line of sight Long haul telecommunications Higher frequencies give higher data rates

Satellite Microwave

Satellite is relay station Satellite receives on one frequency, amplifies or repeats signal and

transmits on another frequency

Requires geo-stationary orbit - Height of 35,784km Television Long distance telephone Private business networksBroadcast Radio

Omnidirectional FM radio

UHF and VHF television


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Line of sight Suffers from multipath interference

Text book: Pages 119 126 V7: 107 - 123This section gives you brief description of wireless data transmission.

Review Activities:

4.1 List three types of transmission media and describe their respective

advantages and disadvantages.

Solution:

Advantages Disadvantages

Twisted pair Cheap Low bandwidth

Optical fiber High bandwidth Expensive to install

Infrared Secure Short distance and cannot penetrate

solid objects

4.2 Name three different transmission media and describe their strengths

and weaknesses.

Solution:

Media type strength weakness

Coaxial cable Large capacity / bandwidth More expensive than

twisted par

Twisted pair Cheap Low capacity/bandwidth

Optic fiber High bandwidth Relatively expensive in

initial cost


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5

DATA ENCODING


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TOPIC 5

5.0 DATA ENCODING

Both digital and analog information can be encoded as either

analog or digital signals. The particular encoding that is chosen depends on

the specific requirements to be met and the media and communications

facilities available.

Encoding Techniques

Digital data, digital signal Analog data, digital signal Digital data, analog signal Analog data, analog signal

5.1 Digital Data, Digital Signal

Digital signal

Discrete, discontinuous voltage pulses Each pulse is a signal element Binary data encoded into signal elements

Unipolar

All signal elements have same signPolar

One logic state represented by positive voltage the other bynegative voltage

Data rate - Rate of data transmission in bits per second

Duration or length of a bit - Time taken for transmitter to emit the bit

Modulation rate

Mark and Space Rate at which the signal level changes Measured in baud = signal elements per second Binary 1 and Binary 0 respectivelyComparison of Encoding Schemes

Signal Spectrum

Lack of high frequencies reduces required bandwidth


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Lack of dc component allows ac coupling via transformer,providing isolation

Concentrate power in the middle of the bandwidthClocking

Synchronizing transmitter and receiver External clock Sync mechanism based on signal

Error detection - Can be built in to signal encoding

Signal interference and noise immunity - Some codes are better than others

Cost and complexity - Higher signal rate (& thus data rate) lead to higher

costs. Some codes require signal rate greater than data rate

Encoding Schemes

Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Bipolar -AMI Pseudoternary Manchester Differential Manchester

B8ZS HDB3Textbook: Pages 132 142 V7: 131 - 141

This section gives you brief description of digital to digital data encoding.

5.2 Digital Data, Analog Signal

Public telephone system 300Hz to 3400Hz Use modem (modulator-demodulator)

Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PK)

Text book: Pages 142 148 V7: 142 152

This section gives you brief description of digital to analog encoding.


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5.3 Analog Data, Digital Signal

Digitization

Conversion of analog data into digital data

Digital data can then be transmitted using NRZ-L Digital data can then be transmitted using code other than NRZ-L Digital data can then be converted to analog signal Analog to digital conversion done using a code. Pulse code modulation Delta modulation

Pulse Code Modulation(PCM)

If a signal is sampled at regular intervals at a rate higher than twice thehighest signal frequency, the samples contain all the information of the

original signal - (Proof - Stallings appendix 4A)

Voice data limited to below 4000Hz Require 8000 sample per second Analog samples (Pulse Amplitude Modulation, PAM) Each sample assigned digital value 4 bit system gives 16 levels Quantized

Quantizing error or noise Approximations mean it is impossible to recover original

exactly

8 bit sample gives 256 levels Quality comparable with analog transmission

8000 samples per second of 8 bits each gives 64kbpsText book: Pages 148 155 V7: 152 158

This section gives you brief description of digital to analog encoding.

5.4 Analog Data, Analog Signals

Why modulate analog signals?

Higher frequency can give more efficient transmission Permits frequency division multiplexing (chapter 8)


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Types of modulation

Amplitude Frequency Phase

Spread Spectrum

Analog or digital data Analog signal Spread data over wide bandwidth Makes jamming and interception harder Frequency hoping

Signal broadcast over seemingly random series offrequencies

Direct Sequence Each bit is represented by multiple bits in transmitted

signal

Chipping codeTextbook: Pages 155 166 V7: 159 165

This section gives you brief description of wireless data transmission.

Review Activities:

5.1 In pulse code modulation, how is the sampling rate decided?The sampling rate is determined by the Nyquist theorem, i.e., the

sampling rate should be at least being twice of the signal frequency in

order to represent the signal completely.

5.2 Briefly describe the Manchester encoding format.

In the Manchester code, there is a transition at the middle of each

bit period. The mid-bit transition serves as a clocking mechanism and as

data: a low to high transition represents a 1, and a high to low transition

represents a 0. In Differential Manchester, the mid bit transition is used

only to provide clocking. The encoding of a 0 is represented by the

presence of a transition at the beginning of a bit periods and a 1 is

represented by the absence of a transition at the beginning of a bit periods.

Differential Manchester has the added advantage of employing differential

encoding.


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6

DATACOMMUNICATION

INTERFACE


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TOPIC 6

6.0 DATA COMMUNICATION INTERFACE

The transmission of a stream of bits from one device to another

across a transmission link involves a great deal of cooperation and

agreement between the two sides. One of the most fundamental

requirements is synchronization. The receiver at regular intervals to

determine the value of each received bit. Two techniques are in common

for this purpose. They are synchronous transmission and asynchronous

transmission.

6.1 Asynchronous transmission

Data transmitted on character at a time - 5 to 8 bits Timing only needs maintaining within each character Resync with each character

Asynchronous Behavior

In a steady stream, interval between characters is uniform (lengthof stop element)

In idle state, receiver looks for transition 1 to 0 Then samples next seven intervals (char length) Then looks for next 1 to 0 for next char Simple Cheap Overhead of 2 or 3 bits per char (~20%) Good for data with large gaps (keyboard)

Synchronous - Bit Level

Block of data transmitted without start or stop bits Clocks must be synchronized Can use separate clock line

Good over short distances Subject to impairments


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Embed clock signal in data Manchester encoding Carrier frequency (analog)

Synchronous - Block Level

Need to indicate start and end of block Use preamble and postamble

e.g. series of SYN (hex 16) characters e.g. block of 11111111 patterns ending in 11111110

More efficient (lower overhead) than asyncText book: Pages 174 178 V7: 173 - 176

This section gives you brief description of asynchronous and synchronous

transmission.

6.2 Line Configuration

Topology

Physical arrangement of stations on medium Point to point Multi point - Computer and terminals, local area network

Half duplex

Only one station may transmit at a time Requires one data path

Full duplex

Simultaneous transmission and reception between two stations Requires two data paths (or echo canceling)

Text book: Pages 178 180 V7: 191 - 193

This section gives you brief description of line configurations and its

characteristics..


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6.3 Interfacing

Data processing devices (or data terminal equipment, DTE) do not(usually) include data transmission facilities

Need an interface called data circuit terminating equipment (DCE)e.g. modem, NIC

DCE transmits bits on medium DCE communicates data and control info with DTE Done over interchange circuits Clear interface standards required

Characteristics of Interface

Mechanical - Connection plugs

Electrical - Voltage, timing, encoding

Functional - Data, control, timing, grounding

Procedural - Sequence of events

Text book: Pages 180 190 V7: 193 - 203

This section gives you brief description of interfacing and its

characteristics.

Review Activities:

6.1 Briefly describe the following:

Synchronous data transmission Asynchronous data transmission

Synchronous data transmission

In this transmission mode, synchronization between the transmitter

and receiver fully depends on the clock synchronization. No start bit and

stops bit are used.

Asynchronous data transmission

It refers to data transmission where start bits and stop element are

used for each character (represented by a number of bits) so that the

transmitter and the receiver can be synchronized. This is opposed to

synchronous transmission where no start and stop bits are used and

synchronization between the transmitter and receiver fully depends on the

clock synchronization.


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7

DATA LINK

CONTROL


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TOPIC 7

7.0 DATA LINK CONTROL

Because of the possibility of transmission errors and because the

receiver of data may need to regulate the rate at which data arrive,

synchronization and interfacing are insufficient by themselves. It is

necessary to impose a layer of control in each communicating device that

provides functions such as flow control, error detection, and error control.

This layer of control is known as a data link control protocol.

7.1 Flow Control

Ensuring the sending entity does not overwhelm the receivingentity

Preventing buffer overflow Transmission time

Time taken to emit all bits into medium Propagation time

Time for a bit to traverse the link

Stop and Wait

Source transmits frame Destination receives frame and replies with acknowledgement Source waits for ACK before sending next frame Destination can stop flow by not send ACK Works well for a few large frames

Fragmentation

Large block of data may be split into small frames

Limited buffer size Errors detected sooner (when whole frame received) On error, retransmission of smaller frames is needed Prevents one station occupying medium for long periods Stop and wait becomes inadequate


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Sliding Windows Flow Control

Allow multiple frames to be in transit Receiver has buffer W long

Transmitter can send up to W frames without ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)

Frames are numbered modulo 2k

Sliding Window Enhancements

Receiver can acknowledge frames without permitting furthertransmission (Receive Not Ready)

Must send a normal acknowledge to resume If duplex, use piggybacking

If no data to send, use acknowledgement frame If data but no acknowledgement to send, send last

acknowledgement number again, or have ACK valid flag

(TCP)

Text book: Pages 194 200 V7: 208 - 215

This section gives you brief description of flow control and its techniques.

7.2 Error Detection

Additional bits added by transmitter for error detection code Parity - Value of parity bit is such that character has even (even

parity) or odd (odd parity) number of ones.

Even number of bit errors goes undetected

Cyclic Redundancy Check

For a block ofkbits transmitter generates n bit sequence Transmit k+n bits which is exactly divisible by some number


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Receive divides frame by that number - If no remainder, assume noerror.

Text book: Pages 201 207 V7: 177 - 185This section gives you brief description of error detection and parity

checking.

7.3 Error Control

Detection and correction of errors Lost frames

Damaged frames Automatic repeat request

Error detection Positive acknowledgment Retransmission after timeout Negative acknowledgement and retransmission

Automatic Repeat Request (ARQ)

Stop and wait Go back N Selective reject (selective retransmission)

Stop and Wait

Source transmits single frame Wait for ACK If received frame damaged, discard it

Transmitter has timeout If no ACK within timeout, retransmit

If ACK damaged, transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1


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Go Back N

Based on sliding window If no error, ACK as usual with next frame expected Use window to control number of outstanding frames If error, reply with rejection

Discard that frame and all future frames until error framereceived correctly

Transmitter must go back and retransmit that frame and allsubsequent frames

Selective Reject

Also called selective retransmission Only rejected frames are retransmitted Subsequent frames are accepted by the receiver and buffered Minimizes retransmission Receiver must maintain large enough buffer More complex login in transmitter

Text book: Pages 208 213 V7: 215 - 220

This section gives you brief description of error control techniques.

7.4 High Level Data Link Control (HDLC)

The HDLC is not only widely used but it is the basis for many

other important data link control protocols, which use the same or similar

formats and the same mechanisms as employed in HDLC.

HDLC Station Types

Primary station Controls operation of link Frames issued are called commands Maintains separate logical link to each secondary station

Secondary station Under control of primary station Frames issued called responses

Combined station May issue commands and responses

Text book: Pages 213 223 V7: 221 - 228

This section gives you brief description of high-level data link control and

other data link control protocols.


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Review Activities:

7.1 Describe Flow control

Flow control is a technique for assuring that a transmitting entity

does not overwhelm a receiving entity with data. The receiving entity

typically allocates a data buffer of some maximum length for a transfer.

When data are received, the receiver must do a certain amount of

processing before passing the data to the higher-level software. In the

absence of flow control, the receivers buffer may fill up and overflow

while it is processing old data.

7.2 What are the main functions of the data link layer?

Data link layer is the second layer of the OSI reference model. Itsmain function is to break the continuous bit stream of physical layer into

meaningful frames/blocks. In addition, it provides medium access control

function.

7.3 Briefly describe two flow control techniques.

Stop-and-wait: the transmitter sends a packet and waist for the

acknowledgement from the receiver before sending another packet.

Sliding-window flow control: it can be treated as an extension of stop-and-

wait. Instead of sending one packet and then waiting, a transmitter can

continuously sends a fixed number of packets (window size) without

needing acknowledgement from the receiver. When a packet is

acknowledged, the number of packets equal to the window size can be

transmitted.

7.4 Using an example, explain how parity check is used to detect data

error. OR Use an example to describe an error detection technique for data

transmission.

Parity check is a simple error detection method where a parity bit is

added to detect if a bit error occurred. For example, if we want to send

data 10101110 and even parity is used, then the data including parity bit is

101011101. If any bit is changed, the number of 1 bits will be odd and we

will be sure that one error has occurred.


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8

MULTIPLEXING


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TOPIC 8

8.0 MULTIPLEXING

To make efficient use of high-speed telecommunication lines, some

form of multiplexing is used. Multiplexing allows several transmission

sources to share a larger transmission capacity. The two common

multiplexing are frequency division multiplexing (FDM) and time division

multiplexing (TDM).

8.1 Frequency Division Multiplexing

Useful bandwidth of medium exceeds required bandwidth ofchannel

Each signal is modulated to a different carrier frequency Carrier frequencies separated so signals do not overlap (guard

bands)

e.g. broadcast radio Channel allocated even if no data

Analog Carrier Systems

Hierarchy of FDM schemes Group 12 voice channels (4kHz each) = 48kHz Range 60kHz to 108kHz

Supergroup

60 channel FDM of 5 group signals on carriers between 420kHz and 612 kHzMastergroup - 10 supergroups

Text book: Pages 237 244 V7: 243 - 250

This section gives you brief description of frequency division multiplexing.


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8.2 Synchronous Time Division Multiplexing

Data rate of medium exceeds data rate of digital signal to betransmitted

Multiple digital signals interleaved in time

May be at bit level of blocks Time slots preassigned to sources and fixed Time slots allocated even if no data Time slots do not have to be evenly distributed amongst sourcesTDM Link Control

No headers and tailers

Data link control protocols not neededFlow control

Data rate of multiplexed line is fixed If one channel receiver can not receive data, the others must carry

on

The corresponding source must be quenched This leaves empty slots

Error control - Errors are detected and handled by individual channel

systems

Framing

No flag or SYNC characters bracketing TDM frames Must provide synchronizing mechanism Added digit framing One control bit added to each TDM frame - Looks like another

channel - control channel

Identifiable bit pattern used on control channel e.g. alternating 01010101unlikely on a data channel Can compare incoming bit patterns on each channel with sync

pattern

Pulse Stuffing

Problem - Synchronizing data sources Clocks in different sources drifting


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Data rates from different sources not related by simple rational number Solution - Pulse Stuffing Outgoing data rate (excluding framing bits) higher than sum of

incoming rates Stuff extra dummy bits or pulses into each incoming signal until it

matches local clock

Stuffed pulses inserted at fixed locations in frame and removed atdemultiplexer

Text book: Pages 244 256 V7: 250 - 259

This section gives you brief description of synchronous time-division

multiplexing.

8.3 Statistical TDM

In Synchronous TDM many slots are wasted Statistical TDM allocates time slots dynamically based on demand Multiplexer scans input lines and collects data until frame full Data rate on line lower than aggregate rates of input linesPerformance

Output data rate less than aggregate input rates May cause problems during peak periods Buffer inputs Keep buffer size to minimum to reduce delay

Asymmetrical Digital Subscriber Line

Link between subscriber and network - Local loop Uses currently installed twisted pair cable - Can carry broader

spectrum; 1 MHz or more

ADSL Design

Asymmetric - Greater capacity downstream than upstream


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Frequency division multiplexing - Lowest 25kHz for voice; Plain oldtelephone service (POTS)

Use echo cancellation or FDM to give two bands

Use FDM within bands Range 5.5km

Textbook: Pages 257 268 V7: 260 - 271

This section gives you brief description of statistical time division

multiplexing.

Review Activities:

8.1 Describe Synchronous data transmission. Compare synchronous time-

division multiplexing and statistical time-division multiplexing

In this transmission mode, synchronization between the transmitter

and receiver fully depends on the clock synchronization. No start bit and

stop bit are used.In both types of multiplexing, the transmission time is divided into

fixed length time slots. In synchronous time division, a transmitter can use

certain time slots (for example, every 4th slot). But in statistical time

division, a transmitter can use any un-used slot. Statistical time division is

more efficient that synchronous time division multiplexing.


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9

SWITCHING


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TOPIC 9

9.0 SWITCHING

For Transmission of data beyond a local area, communication is

typically achieved by transmitting data from source to destination through

a network of intermediate switching nodes; this switched network design is

sometimes used to implement Local Area Networks as well.

9.1 Switching Networks

Long distance transmission is typically done over a network ofswitched nodes

Nodes not concerned with content of data End devices are stations - Computer, terminal, phone, etc. A collection of nodes and connections is a communications

network

Data routed by being switched from node to nodeNodes

Nodes may connect to other nodes only, or to stations and othernodes

Node to node links usually multiplexed Network is usually partially connected - Some redundant

connections are desirable for reliability

Two different switching technologies : Circuit switching, Packetswitching

9.2 Circuit Switching

Dedicated communication path between two stations Three phases Establish, Transfer and, Disconnect Must have switching capacity and channel capacity to establish

connection

Must have intelligence to work out routing


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Circuit Switching - Applications

Inefficient - Channel capacity dedicated for duration of connection,If no data, capacity wasted

Set up (connection) takes time

Once connected, transfer is transparent Developed for voice traffic (phone)

Telecommunication Components

Subscriber -Devices attached to network Local Loop - Subscriber loop, Connection to network Exchange - Switching centers, End office - supports subscribers

Trunks - Branches between exchanges - MultiplexedCircuit Switching Concepts

Digital Switch - Provide transparent signal path between devices Network Interface Control Unit

Establish connections Generally on demand Handle and acknowledge requests Determine if destination is free construct path

Maintain connection Disconnect

Blocking or Non-blocking

Blocking

A network is unable to connect stations because all paths are in use

A blocking network allows this Used on voice systems - Short duration calls

Non-blocking

Permits all stations to connect (in pairs) at once Used for some data connections

Space Division Switching

Developed for analog environment


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Separate physical paths Crossbar switch

Number of crosspoints grows as square of number ofstations

Loss of crosspoint prevents connection Inefficient use of crosspoints - All stations connected, only

a few crosspoints in use

Non-blockingTime Division Switching

Partition low speed bit stream into pieces that share higher speedstream

e.g. TDM bus switching based on synchronous time division multiplexing Each station connects through controlled gates to high

speed bus

Time slot allows small amount of data onto bus Another lines gate is enabled for output at the same time

Routing

Many connections will need paths through more than one switch Need to find a route Efficiency, Resilience Public telephone switches are a tree structure - Static routing uses

the same approach all the time

Dynamic routing allows for changes in routing depending on traffic- Uses a peer structure for nodes

Text book: Pages 276 288 v7: 299 - 307

This section gives you brief description of circuit switching networks and

circuit switching concept..

9.3 Control Signaling Functions

Audible communication with subscriber Transmission of dialed number


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Call can not be completed indication Call ended indication Signal to ring phone

Billing info Equipment and trunk status info Diagnostic info Control of specialist equipment

Text book: Pages 289 300 V7: 307 - 316

This section gives you brief description of control signaling.

9.4 Packet SwitchingPacket switching was designed to provide a more efficient facility

than circuit switching for bursty data traffic. With packet switching, a

station transmits data in small blocks, called packets. Each packet contains

some portion of the user data plus control information needed for proper

functioning of the network.

Basic Operation Data transmitted in small packets

Typically 1000 octets Longer messages split into series of packets Each packet contains a portion of user data plus some

control info

Control info - Routing (addressing) info Packets are received, stored briefly (buffered) and past on to the

next node - Store and forward

Advantages

Line efficiency

Single node to node link can be shared by many packets over time Packets queued and transmitted as fast as possible

Data rate conversion

Each station connects to the local node at its own speed


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Nodes buffer data if required to equalize ratesPackets are accepted even when network is busy

Delivery may slow downPriorities can be usedSwitching Technique

Station breaks long message into packets Packets sent one at a time to the network Packets handled in two ways : Datagram, Virtual circuit

Datagram

Each packet treated independently

Packets can take any practical route Packets may arrive out of order Packets may go missing Up to receiver to re-order packets and recover from missing

packets

Virtual Circuit

Preplanned route established before any packets sent

Call request and call accept packets establish connection(handshake)

Each packet contains a virtual circuit identifier instead ofdestination address

No routing decisions required for each packet Clear request to drop circuit Not a dedicated path

Routing - Complex, crucial aspect of packet switched networksCharacteristics required : Correctness, Simplicity, Robustness, Stability,

Fairness, Optimality and Efficiency,

Text book: Pages 304 332 V7: 316 - 332

This section gives you brief description of packet switching network,

routing and X.25.


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Review Activities:

9.1 What is circuit switching? What is packet switching? What are their

respective advantages and disadvantages?

This is a switching technique in which a physical path is set up between

the transmitter and receiver for the duration of data transmission.

definition advantages disadvantages

Circuit

switching

A dedicated physical

connection between

transmitter and receiver is

established for the duration of

Communication

Low delay and

delay jitter. Can

Guarantee

quality.

May be expensive and

less efficient.

Packet

switching

Routers are used to send

packets to different

destinations.

Cheap and

efficient.

Difficult to guarantee

quality.

9.2 In a network that has a maximum packet size of 100 bytes, a maximum

packet lifetime of 10 seconds, and an 10-bit packet sequence number, what

is the maximum data rate per connection (assuming that each packet over a

connection must be uniquely identified using a sequence number)?

Within the lifetime of 10 second, each packet must be uniquely

identified with 10 bits. That is, within 10 seconds, 210

=1024 packets can be

sent. The number of packet per second = 1024/10 packets. So the

maximum data rate = the number of packets per second x packet size =

(1024/10) x 100 x 8 =81920 bps (bits per second).

9.3 In a network that has a maximum packet size of 128 bytes, a maximum

packet lifetime of 20 seconds, and an 8-bit packet sequence number, what

is the maximum data rate per connection (assuming that each packet over a

connection must be uniquely identified using a sequence number)?

Within the lifetime of 20 second, each packet must be uniquely

identified with 8bits. That is within 20 seconds, 28=256 packets can be

sent. The number of packet per second = 256/20 packets. So the maximum

data rate = the number of packets per second x packet size = (256/20) x

128 x 8 =13107.2 bps (bits per second).


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10

CONGESTION

CONTROL


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TOPIC 10

10.0 CONGESTION CONTROL

Congestion occurs when the number of packets being transmitted

through the network approaches the packet handling capacity of the

network. Congestion control aims to keep number of packets below level

at which performance falls off dramatically. Data network is a network of

queues, Generally 80% utilization is critical. Finite queues mean data may

be lost.

10.1 Effects of Congestion

Packets arriving are stored at input buffers Routing decision made Packet moves to output buffer Packets queued for output transmitted as fast as possible

Statistical time division multiplexing If packets arrive to fast to be routed, or to be output, buffers will

fill

Can discard packets Can use flow control

Can propagate congestion through network

Text book: Pages 385 389 V7: 397 - 401

This section gives you brief description of the effect of the congestion.

10.2 Congestion Control

Backpressure

If node becomes congested it can slow down or halt flow ofpackets from other nodes

May mean that other nodes have to apply control on incomingpacket rates

Propagates back to source


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Can restrict to logical connections generating most traffic Used in connection oriented that allow hop by hop congestion

control (e.g. X.25)

Not used in ATM nor frame relay

Only recently developed for IP

Choke Packet

Control packet Generated at congested node Sent to source node e.g. ICMP source quench

From router or destination Source cuts back until no more source quench

message

Sent for every discarded packet, or anticipated Rather crude mechanism

Implicit Congestion Signaling

Transmission delay may increase with congestion Packet may be discarded Source can detect these as implicit indications of congestion Useful on connectionless (datagram) networks. E.g. IP based (TCP

includes congestion and flow control )

Used in frame relay LAPF

Explicit Congestion Signaling

Network alerts end systems of increasing congestion End systems take steps to reduce offered load Backwards

o Congestion avoidance in opposite direction to packetrequired


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Forwards -Congestion avoidance in same direction aspacket required

Text book: Pages 390 393 V7: 401 - 404This section gives you brief description of various congestion control

techniques.

Review Activities:

10.1 What is the main difference between flow control and congestion

control?

Flow control is concerned with if the receiver can cope with the

data sent by the transmitter and congestion control is concerned with if the

network is overload to the extent that performance is reduced dramatically.

Flow control is an issue specific to a pair of hosts while congestion control

is a global issue.

(b) In a network that has a maximum packet size of 1000 bytes, a

maximum packet lifetime of 10 seconds, and an 8-bit packet sequence

number, what is the maximum data rate per connection (assuming that

each packet over a connection must be uniquely identified using a

sequence number)?

Within 10 second, only 256 packets can be sent, so the data rate =

256 x 1000 x 8/10=204.8 kbps

10.2 A 8920-octet IP datagram is to be transmitted and needs to be

fragmented because it has to pass through an Ethernet with a maximum

payload of 1500 octets. (i) How many fragments are needed? (ii) Explain

how these fragments can be reassembled into the original datagram at

the destination.

(i) Since the IP header has 20 octets, the 8920-octets

Datagram contains 8900 octets of actual data.


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A payload of 1500 octets can carry a maximum 1480

octets of data (the remaining are IP header and other

Headers).

8900/1480= 6.014.

So at least 7 fragments are needed.

(ii) Each fragment contains the fields of length of the

fragment and the offset from the start of the datagram and

a flag to indicate if it is the last fragment of the datagram.

The destination can assemble the original datagram based

on these three pieces of information.

10.3 Data transfer rate

The speed with which data can be transmitted from one device to another.

Data rates are often measured in megabits (million bits) or megabytes

(million bytes) per second. These are usually abbreviated as Mbps and

MBps, respectively.
http://www.webopedia.com/TERM/D/data.htmlhttp://www.webopedia.com/TERM/D/device.htmlhttp://www.webopedia.com/TERM/D/data_transfer_rate.html##http://www.webopedia.com/TERM/D/megabit.htmlhttp://www.webopedia.com/TERM/D/bit.htmlhttp://www.webopedia.com/TERM/D/megabyte.htmlhttp://www.webopedia.com/TERM/D/byte.htmlhttp://www.webopedia.com/TERM/D/byte.htmlhttp://www.webopedia.com/TERM/D/megabyte.htmlhttp://www.webopedia.com/TERM/D/bit.htmlhttp://www.webopedia.com/TERM/D/megabit.htmlhttp://www.webopedia.com/TERM/D/data_transfer_rate.html##http://www.webopedia.com/TERM/D/device.htmlhttp://www.webopedia.com/TERM/D/data.html


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11

LOCAL AREA

NETWORK


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TOPIC 11

11.0 LOCAL AREA NETWORK

A Local Area Network (LAN) consists of a shared transmission

medium and a set of hardware and software for interfacing devices to the

medium and regulating the orderly access to the medium. The topologies

that have been used for LANs are ring, bus, tree and star. Traditional LAN

provides data rate in a range from about 1 to 20 Mbps and high-speed

LANs provides data rate of 100 Mbps to 1 Gbps.

11. LAN Applications

Personal computer LANs Low cost Limited data rate

Back end networks and storage area networks Interconnecting large systems (mainframes and large

storage devices)

High data rate High speed interface Distributed access Limited distance Limited number of devices

High speed office networks Desktop image processing High capacity local storage

Backbone LANs

Interconnect low speed local LANs Reliability Capacity & Cost

Text book: Pages 425 427 V7: 466 - 475

This section gives you brief description of Local area network

applications.


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11.2 LAN Architecture

Protocol Architecture

Lower layers of OSI model

IEEE 802 reference model Physical Logical link control (LLC) Media access control (MAC)

Media Access Control

Assembly of data into frame with address and error detection fields Disassembly of frame

Address recognition

Error detection Govern access to transmission medium

Not found in traditional layer 2 data link control For the same LLC, several MAC options may be available

LAN Topologies

Bus and Tree

Multipoint medium Transmission propagates throughout medium Heard by all stations

Need to identify target station - Each station has uniqueaddress

Full duplex connection between station and tap Allows for transmission and reception

Need to regulate transmission To avoid collisions To avoid hogging - Data in small blocks - frames

Terminator absorbs frames at end of medium

Ring Topology

Repeaters joined by point to point links in closed loop


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Receive data on one link and retransmit on another Links unidirectional Stations attach to repeaters

Data in frames Circulate past all stations Destination recognizes address and copies frame Frame circulates back to source where it is removed

Media access control determines when station can insert frame

Star Topology

Each station connected directly to central node Usually via two point to point links

Central node can broadcast Physical star, logical bus Only one station can transmit at a time

Central node can act as frame switch

Text book: Pages 428 439 V7: 475 - 483

This section gives you brief description of protocol architecture LAN

topologies and media access control.

11.3 Ethernet

The most commonly used medium access control technique for bus

and star topologies is carrier sense multiple access with collision detection

(CSMA/CD). CSMA/CD and its precursors can be termed random

(stations access medium randomly) or contention (station content for time

on medium) techniques.

ALOHA

Packet Radio When station has frame, it sends Station listens (for max round trip time)plus small increment If ACK, fine. If not, retransmit


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If no ACK after repeated transmissions, give up Frame check sequence (as in HDLC) If frame OK and address matches receiver, send ACK

Frame may be damaged by noise or by another station transmittingat the same time (collision)

Any overlap of frames causes collision Max utilization 18%

Slotted ALOHA

Time in uniform slots equal to frame transmission time Need central clock (or other sync mechanism) Transmission begins at slot boundary Frames either miss or overlap totally Max utilization 37%

CSMA

Propagation time is much less than transmission time All stations know that a transmission has started almost

immediately

First listen for clear medium (carrier sense) If medium idle, transmit If two stations start at the same instant, collision Wait reasonable time (round trip plus ACK contention) No ACK then retransmit Max utilization depends on propagation time (medium length) and

frame length

Longer frame and shorter propagation gives betterutilization

CSMA/CD

With CSMA, collision occupies medium for duration oftransmission

Stations listen whilst transmitting If medium idle, transmit If busy, listen for idle, then transmit


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If collision detected, jam then cease transmission After jam, wait random time then start again

Binary exponential back offCollision Detection

On baseband bus, collision produces much higher signal voltagethan signal

Collision detected if cable signal greater than single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m (10Base2) For twisted pair (star-topology) activity on more than one port is

collision Special collision presence signal

Text book: Pages 470 481 V7: 502 - 515

This section gives you brief description of carrier sense multiple access

with collision detection technique.

Review Activities:

11.1 Describe three medium access control techniques commonly used in

local area networks.

(i) Carrier sense multiple access (CSMA): It is a contention media access

control mechanism. The access rules are:

(1) Check if the transmission medium is busy. If the medium is

idle, transmit; otherwise, go to step (2).

(2) If the medium is busy, continue checking until it is idle, then

transmit data immediately.

(ii) Carrier sense multiple access with collision detection (CSMA/CD): It

is a contention media access control mechanism. The access rules are:

(1) Check if the transmission medium is busy. If the medium is

idle, transmit; otherwise, go to step (2).

(2) If the medium is busy, continue checking until it is idle, then

transmit data immediately.


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(3) If a collision is detected during transmission, send a signal to

ensure all stations know the collision and then stop transmission.

(4) After sending the collision signal, wait a random amount of

time and then transmit again. (repeat from step (1)).

(iii) Token passing: A token circulates around the network. A station

captures a free token can send certain amount of data. Once completing

data transmission, the station set the token free and another station can

capture it and send data.

11.2 Describe the operating principle of the token ring network.

The following are layers and corresponding common protocols:

Network access: CSMA/CD, token bus. Internet layer: IP Transport layer: TCP, UDP Application layer: SMTP, FTP, MIME, and HTTP.

11.3 What is a medium access control (MAC) layer? Describe two MAC

protocols?

11.4 WANConnection Types (Public Carriers):

When designing a wide area network (WAN), one of the most challenging

issues is choosing the correct connection type. You need to consider

several factors before implementation can beginand a solid

understanding of all the connection types is critical to making the right

choice. Most carriers offer three connection types:

1. Circuit-switched connections2. Packet-switched or cell-switched connections3. Dedicated connections

Each type of connection has its advantages and disadvantages. This article

will summarize what each connection type has to offer, with consideration

given to bandwidth, availability, cost, and ease of management.


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Circuit-switched connections

Circuit-switched connections are currently the most popular type of WAN

connection. Circuit switching transmits data streams and datagrams across

dedicated physical circuits. To provide asynchronous dial-in and ISDN

services, the telephone companies use circuit switching.

Asynchronous dial-in

The public switched telephone network (PSTN) uses circuit-switched

technology to provide asynchronous services (otherwise known as normal

telephone service). Asynchronous dial-in connections offer a low-

bandwidth, easily managed, cost-effective solution that is available almost

anywhere in the world.

ISDN

Integrated Services Digital Network(ISDN) is a digital circuit switching

technology used to transport voice, data, and video. With speeds up to 1.54

Mbps (T1) in North America and 2.048 Mbps (E1) in Europe, ISDN is a

low- to medium-bandwidth solution with relatively low cost. However,

availability is somewhat limited and configuration and support are more

difficult than for asynchronous communication.

Packet-switched or cell-switched connections

Packet-switched and cell-switched networks are point-to-point connections

that travel across a public carriers n

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