COMPUTER AND COMMUNICATION

NETWORKS

G.KEERTHI3142065230031/2M.TECH R&M

Switching TechniqueStation breaks long message into

packetsPackets sent one at a time to the

networkPackets handled in two ways

◦Datagram◦Virtual circuit

DatagramEach packet treated

independentlyPackets can take any practical

routePackets may arrive out of orderPackets may go missingUp to receiver to re-order packets

and recover from missing packets

DatagramDiagram

Virtual CircuitPreplanned route established before any

packets sentCall request and call accept packets

establish connection (handshake)Each packet contains a virtual circuit

identifier instead of destination addressNo routing decisions required for each

packetClear request to drop circuitNot a dedicated path

VirtualCircuitDiagram

Virtual Circuits v DatagramVirtual circuits

◦Network can provide sequencing and error control

◦Packets are forwarded more quickly No routing decisions to make

◦Less reliable Loss of a node looses all circuits through that

nodeDatagram

◦No call setup phase Better if few packets

◦More flexible Routing can be used to avoid congested parts of

the network

Packet Size

Circuit v Packet SwitchingPerformance

◦Propagation delay: The time it takes a signal to propagate from one node to the next.

◦Transmission time: The time it takes for a transmitter to send out a block of data.

◦Node delay: The time it takes for a node to perform the necessary processing as it switches data.

Event Timing

X.251976Interface between host and

packet switched networkAlmost universal on packet

switched networks and packet switching in ISDN

Defines three layers◦Physical◦Link◦Packet

X.25 - PhysicalInterface between attached station

and link to nodeData terminal equipment DTE (user

equipment)Data circuit terminating equipment

DCE (node)Uses physical layer specification

X.21Reliable transfer across physical linkSequence of frames

X.25 - LinkIt provides the reliable transfer of

data across the physical link by transmitting the data as sequence of frames.

Link level standard is Link Access Protocol Balanced (LAPB)◦Subset of HDLC

X.25 - Packet

It provides a virtual circuit service.

External virtual circuitsEnables Logical connections

called virtual circuits between subscribers

X.25 Use of Virtual Circuits

Virtual Circuit ServiceLogical connection between two stations

◦External virtual circuitSpecific preplanned route through network

◦ Internal virtual circuitTypically one to one relationship between

external and internal virtual circuitsCan employ X.25 with datagram style

networkExternal virtual circuits require logical

channel ◦All data considered part of stream

X.25 LevelsUser data passes to X.25 level 3X.25 appends control information

◦Header◦Identifies virtual circuit◦Provides sequence numbers for flow

and error controlX.25 packet passed down to

LAPB entityLAPB appends further control

information

User Data and X.25 Protocol Control Information

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