CHAPTER 3:PACKET SWITCHING

ObjectivesIntroduction on Packet Switching4 methods of accessing the

serviceDATAPLUS packet switchingDATAPLUS overseas destinationsMODEMRates and packet traffic charges

of DATAPLUS

Introduction on Packet SwitchingIn message switching, each

switching centre stores incoming messages until required outgoing circuit becomes free and then re-transmits them.

Result in delays.Not suitable for data.That’s why data networks use a

modified form of message switching called PACKET SWITCHING

How packets are transferred?

Data sent out of sequenceLong messages are split into

number of PACKETSSmall chunks (packets) of data sent

at a timePackets passed from node to node

between source and destination

Data must be preceded by a HEADER.

Then there is data.Packet ends with bits added for error

detection and correction.The most known is CRC-Cyclic

Redundancy Check code.

HEADER TEXT DATA CRC

A format of a typical packet shown below:

The header should consists of:Destination address.Sequence number for reassembling in correct

order at destination.Sending address, for request retransmission.The length of the packet, so that the receiver

knows it has completely received the data. Some systems use a FLAG, to predetermine unique combination of digits to indicate the end of the packet.

HEADER TEXT DATA CRC

Destination

AddressOp

CodeSourceaddres

sSeq Byte

count

The data network and terminals, DTE (Data Terminal Equipment) handle the packets by procedures known as PROTOCOL.

This PROTOCOL is defined in terms of the OSI 7-Layers Model.

Packet switching was developed firstly for private data networks.

It is widely used in LAN – Local Area Network.

Now is also used in WAN – Wide Area Network.

Linking two networks that uses same protocol, a simple equipment is used called BRIDGE.

If they uses different protocol, a complex equipment is use called GATEWAY.

4.1 – Packet Switching ProtocolsPacket switching offers users an open

structure communications environment, which allows flexibility in the choice of terminal and computing equipment combined with growth capability.

The network makes cost effective use of line bandwidth for data transmission in computer to computer transactions, maximising data integrity and network availability while minimising the cost.

Packet switch uses the multi layer CCITT X.25 Protocol to establish switched virtual circuits between any points in the data networks.

Packet switching is becoming more established around the world and this is possible by the use of CCITT X.25.

One greatest attraction of CCITT X.25 is FLEXIBLE ROUTING.

The network can be left to decide its path to reach its destination, hence it prevent calls from being blocked, known as BOTTLENECKS.

Advantages of Packet SwitchingHandling large data volumesSecurity, reliability & speed which is not

possible on PSTNWorld wide accessOn-line information retrieval to get most

up-to-date information quickly and efficiently

Access to public data to have news, publications, technical research data, portfolios, statistics etc

And lots more…

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Disadvantages of Packet SwitchingEach packet switching node introduces a delayOverall packet delay can vary substantially

◦ This is referred to as jitter◦ Caused by differing packet sizes, routes taken and

varying delay in the switchesEach packet requires overhead information

◦ Includes destination and sequencing information◦ Reduces communication capacity

More processing required at each node

DATA LINK

SESSION

APPLICATION

NETWORK

PHYSICAL

PRESENTATION

TRANSPORT

Electrical/optical & mechanical requirements to permit a continuous path for data transfer to be provided over physical medium.Data frame composition including header, control & error detection/correction to enable data transfer over the path provided by layer 1.Interconnection of paths provided by layer 1 carrying the data composed by layer 2, including the requirements to deliver the data in correct sequence.Matching between data users of any multiplexing requirements as predetermined by, for e.g. packet-switched methods of communications.Matching between data users of data-transfer modes(2 way simultaneous/alternate or one way only), sync requirements etc.Matching data structures and coding method between data users.It concerns the relationship between a data user’s computer program and the communication environments.

4.2 - The OSI 7-Layers

LAYER NAME7 APPLICATION All6 PRESENTATION People5 SESSION Seem4 TRANSPORT To3 NETWORK Need2 DATA LINK Data1 PHYSICAL Processing

4.3 CITT/ITU ProtocolStands for Consultative

Committee for International Telephone & Telegraphy

An agency of UN that develops recommended world-wide standards & protocol for telecommunications industry.

Now known as ITU-TS (stands for International Telecommunication Union-Telecommunication Standardization.

X.3X.28X.29

Asynchronous & telex terminal to the Packet Switched Data Network (PSDN), Packet Assembler/Disassembler (PAD)

X.25 Computer/Intelligent terminals (users) to the PSDN.

X.75 PSDN to PSDN

4.3 - Some of the CITT/ITU Protocol

4.4 - CITT X.25 three layer protocol

Layer 1 SynchronizationLayer 2 Error detection/correction by

retransmissionNetwork Transparency

Layer 3 SequencingFlow controlMultiplexingCall setup & ClearingNetwork Inter-workingLogical In/Out Band Signalling

4.5 - Four ways of accessing service in Brunei

By using a dedicated/leased line circuit

By using the PSTN through dial-up circuit

By using an electronic telex/teletax access

By using paging circuit

What is DATAPLUS?Stands for Data Packet Link Usage Services

Designed to provide common public network simultaneous use by number of varies data terminals and computer users

To facilitate the interconnection equipment of various origin

4.6 - MODEMIt is a MOdulator &

DEModulatorUsed to convert digital signal

into analogue signal suitable for telephone lines and vice versa.

Do we need modem for DATAPLUS?Yes. Even two modem is needed.One at exchange and another at

subscriber premises.

Dedicated subscriber uses 4 wires connection and require dedicated modem

Dial-up subscriber uses 2 wires connection and require dial-up modem

4.8 – DATAPLUS National ServicesTwo types of circuit:

◦Dedicated circuit (4 wires) BSP Great Eastern Life Tractor Malaysia Police

◦Dial up circuit (2 wires) Brunei Investment Agency Slumberger

4.10 – EWSP system in BruneiEWSP is not used for telephony but

for packet switching.This system is a packet switching

with modular system architecture based on a multiprocessor concept which enables optimum network configurations to be set up to suit different network requirements

It is used for transmitting large quantities of data, such as for telex, teletext, video text and data processing, by packet switching.

EWSP permits interworking with existing switching and transmission facilities.

Network interworking is possible from packet switching network to:◦PSTN◦Circuit switched public data network◦Another Packet Switched Public Data

Network (PSPDN)◦TELEX network◦ISDN via a Packet Server Module (PSM)

PSPDN+

EWSP

TELEXNETWORK

PSPDN

ISDNPSTN

PSMX.32

Packet DTE

Async

DTEX.28

X.75 X.28

X.75