Data and Computer Data and Computer CommunicationsCommunications

Eighth EditionEighth Edition

by William Stallingsby William Stallings

Lecture slides by Lawrie BrownLecture slides by Lawrie Brown

Chapter 10 – Circuit SwitchingChapter 10 – Circuit Switching and and Packet SwitchingPacket Switching

Circuit SwitchingCircuit Switching and Packet and Packet SwitchingSwitching

He got into a District Line train at Wimbledon He got into a District Line train at Wimbledon Park, changed on to the Victoria Line at Park, changed on to the Victoria Line at Victoria and on to the Jubilee Line at Green Victoria and on to the Jubilee Line at Green Park for West Hampstead. It was a long and Park for West Hampstead. It was a long and awkward journey but he enjoyed it.awkward journey but he enjoyed it.

—King Solomon's Carpet, Barbara Vine (Ruth Rendell)

Switched NetworkSwitched Network

NodesNodes

a collection of nodes and connections is a a collection of nodes and connections is a communications network communications network

nodes may connect to other nodes only, or to nodes may connect to other nodes only, or to stations and other nodesstations and other nodes

network is usually partially connectednetwork is usually partially connected some redundant connections are desirable some redundant connections are desirable

have two different switching technologieshave two different switching technologies circuit switchingcircuit switching packet switchingpacket switching

Circuit SwitchingCircuit Switching uses a dedicated path between two stationsuses a dedicated path between two stations has three phaseshas three phases

establishestablish transfertransfer disconnectdisconnect

inefficientinefficient channel capacity dedicated for duration of connectionchannel capacity dedicated for duration of connection if no data, capacity wastedif no data, capacity wasted

set up (connection) takes timeset up (connection) takes time once connected, transfer is transparentonce connected, transfer is transparent

Public Circuit Switched Public Circuit Switched NetworkNetwork

Circuit EstablishmentCircuit Establishment

Circuit Circuit Switch Switch

ElementsElements

Blocking or Non-blockingBlocking or Non-blocking

blocking networkblocking network may be unable to connect stations because may be unable to connect stations because

all paths are in useall paths are in use used on voice systemsused on voice systems

non-blocking networknon-blocking network permits all stations to connect at oncepermits all stations to connect at once used for some data connectionsused for some data connections

Space Division SwitchSpace Division Switch

3 Stage 3 Stage Space Division Space Division SwitchSwitch

Time Division SwitchingTime Division Switching

modern digital systems use intelligent modern digital systems use intelligent control of space & time division elementscontrol of space & time division elements

use digital time division techniques to set use digital time division techniques to set up and maintain virtual circuitsup and maintain virtual circuits

partition low speed bit stream into pieces partition low speed bit stream into pieces that share higher speed streamthat share higher speed stream

individual pieces manipulated by control individual pieces manipulated by control logic to flow from input to outputlogic to flow from input to output

SoftswitchSoftswitch

Traditional Circuit SwitchingTraditional Circuit Switching

Packet SwitchingPacket Switching

circuit switching was designed for voicecircuit switching was designed for voice packet switching was designed for datapacket switching was designed for data transmitted in small packetstransmitted in small packets packets contains user data and control infopackets contains user data and control info

user data may be part of a larger messageuser data may be part of a larger message control info includes routing (addressing) infocontrol info includes routing (addressing) info

packets are received, stored briefly (buffered) packets are received, stored briefly (buffered) and past on to the next nodeand past on to the next node

Packet SwitchingPacket Switching

AdvantagesAdvantages

line efficiencyline efficiency single link shared by many packets over timesingle link shared by many packets over time packets queued and transmitted as fast as possiblepackets queued and transmitted as fast as possible

data rate conversiondata rate conversion stations connects to local node at own speedstations connects to local node at own speed nodes buffer data if required to equalize ratesnodes buffer data if required to equalize rates

packets accepted even when network is busypackets accepted even when network is busy priorities can be usedpriorities can be used

Switching TechniquesSwitching Techniques

station breaks long message into packetsstation breaks long message into packets packets sent one at a time to the networkpackets sent one at a time to the network packets can be handled in two wayspackets can be handled in two ways

datagramdatagram virtual circuitvirtual circuit

DatagramDatagramDiagramDiagram

VirtualVirtualCircuitCircuit

DiagramDiagram

Virtual Circuits v DatagramVirtual Circuits v Datagram

virtual circuitsvirtual circuits network can provide sequencing and error network can provide sequencing and error

controlcontrol packets are forwarded more quicklypackets are forwarded more quickly less reliableless reliable

datagramdatagram no call setup phaseno call setup phase more flexiblemore flexible more reliablemore reliable

Packet Packet SizeSize

Circuit v Packet SwitchingCircuit v Packet Switching

performance depends on various delaysperformance depends on various delays propagation delaypropagation delay transmission timetransmission time node delaynode delay

range of other characteristics, including:range of other characteristics, including: transparencytransparency amount of overheadamount of overhead

Event TimingEvent Timing

X.25X.25

ITU-T standard for interface between host ITU-T standard for interface between host and packet switched networkand packet switched network

almost universal on packet switched almost universal on packet switched networks and packet switching in ISDNnetworks and packet switching in ISDN

defines three layersdefines three layers PhysicalPhysical LinkLink PacketPacket

X.25 - PhysicalX.25 - Physical

interface between station node linkinterface between station node link two ends are distinct two ends are distinct

Data Terminal Equipment DTE (user Data Terminal Equipment DTE (user equipment)equipment)

Data Circuit-terminating Equipment DCE Data Circuit-terminating Equipment DCE (node)(node)

physical layer specification is X.21physical layer specification is X.21 can substitute alternative such as EIA-232can substitute alternative such as EIA-232

X.25 - LinkX.25 - Link

Link Access Protocol Balanced (LAPB)Link Access Protocol Balanced (LAPB) Subset of HDLCSubset of HDLC see chapter 7see chapter 7

provides reliable transfer of data over linkprovides reliable transfer of data over link sending as a sequence of framessending as a sequence of frames

X.25 - PacketX.25 - Packet

provides a logical connections (virtual provides a logical connections (virtual circuit) between subscriberscircuit) between subscribers

all data in this connection form a single all data in this connection form a single stream between the end stationsstream between the end stations

established on demandestablished on demand termed external virtual circuitstermed external virtual circuits

X.25 Use of Virtual CircuitsX.25 Use of Virtual Circuits

User Data and X.25 Protocol User Data and X.25 Protocol Control InformationControl Information

Issues with X.25Issues with X.25

key features include:key features include: call control packets, in band signalingcall control packets, in band signaling multiplexing of virtual circuits at layer 3multiplexing of virtual circuits at layer 3 layers 2 and 3 include flow and error controllayers 2 and 3 include flow and error control

hence have considerable overheadhence have considerable overhead not appropriate for modern digital systems not appropriate for modern digital systems

with high reliabilitywith high reliability

Frame RelayFrame Relay

designed to eliminate most X.25 overheaddesigned to eliminate most X.25 overhead has large installed basehas large installed base key differences:key differences:

call control carried in separate logical connectioncall control carried in separate logical connection multiplexing and switching at layer 2multiplexing and switching at layer 2 no hop by hop error or flow controlno hop by hop error or flow control hence end to end flow and error control (if used) are hence end to end flow and error control (if used) are

done by higher layerdone by higher layer

a single user data frame is sent from source to a single user data frame is sent from source to destination and higher layer ACK sent backdestination and higher layer ACK sent back

Advantages and Advantages and DisadvantagesDisadvantages

lost link by link error and flow controllost link by link error and flow control increased reliability means less an issueincreased reliability means less an issue streamlined communications processstreamlined communications process

lower delaylower delay higher throughputhigher throughput

frame relay can be used for access frame relay can be used for access speeds up to and over 2Mbpsspeeds up to and over 2Mbps

Protocol ArchitectureProtocol Architecture

LAPF FunctionalityLAPF Functionality LAPF (Link Access Procedure for Frame Mode LAPF (Link Access Procedure for Frame Mode

Bearer Services) defined in Q.922Bearer Services) defined in Q.922 only core functionality used:only core functionality used:

frame delimiting, alignment and transparencyframe delimiting, alignment and transparency frame mux and demux using addressing fieldframe mux and demux using addressing field ensure frame is integral number of octets ensure frame is integral number of octets ensure frame is neither too long nor shortensure frame is neither too long nor short detection of transmission errorsdetection of transmission errors congestion control functionscongestion control functions

form sub-layer of data link layerform sub-layer of data link layer data transfer between subscribers onlydata transfer between subscribers only

Frame Relay Data Link Frame Relay Data Link ConnectionsConnections

logical connection between subscriberslogical connection between subscribers data transferred over themdata transferred over them not protected by flow or error controlnot protected by flow or error control uses separate connection for call controluses separate connection for call control overall results in significantly less work in overall results in significantly less work in

networknetwork

User Data TransferUser Data Transfer

only have one frame type whichonly have one frame type which carries user datacarries user data

no control frames meansno control frames means no inband signalingno inband signaling no sequence numbersno sequence numbers

flag and FCS function as in HDLCflag and FCS function as in HDLC address field carries DLCIaddress field carries DLCI DLCI (Data Link Connection Identifier) has DLCI (Data Link Connection Identifier) has

local significance onlylocal significance only

SummarySummary

circuit verses packet switching network circuit verses packet switching network approachesapproaches

X.25X.25 frame relayframe relay