數(shù)據(jù)與計算機(jī)通信

mes

Buildings

People

PublicInfrastructure

DataandComputer

Communications

PartIIIWAN

電路交換與分組交換

CircuitSwitchingandPacketSwitching

交換式通信網(wǎng)電路交換網(wǎng)

電路交換的概念控制信令

軟交換體系結(jié)構(gòu)分組交換原理

X.25幀中繼

OVERVIEW

/SwitchingNetworks交換式通信網(wǎng)

/CircuitSwitching電路交換網(wǎng)

/CircuitSwitchingConcepts電路交換的概念

/ControlSignaling控制信令難點！

/SoftswitchArchitecture軟交換體系結(jié)構(gòu)

/PacketSwitchingPrinciples分組交換原理

/X.25(Optional)

/FrameRelay幀中繼(Optional)

3

10.1SwitchingNetworks

vLongdistancetransmissionistypically

doneoveranetworkofswitchednodes

/Nodesnotconcernedwithcontentofdata

/Enddevicesarestations

>Computer,terminal,phone,etc.

，Acollectionofnodesandconnectionsisa

communicationsnetwork

'Dataroutedbybeingswitchedfromnodeto

node

4

Nodes

/Nodesmayconnecttoothernodesonly,orto

stationsandothernodes僅與其他節(jié)點相連5-7

/Nodetonodelinksusuallymultiplexed

/Networkisusuallypartiallyconnected

>Someredundantconnectionsaredesirablefor

reliability

/Twodifferentswitchingtechnologies

>Circuitswitching

>Packetswitching

5

OVERVIEW

/SwitchingNetworks交換式通信網(wǎng)

,CircuitSwitching電路交換網(wǎng).

/CircuitSwitchingConcepts電路交換的概念

/ControlSignaling控制信令難點！

/SoftswitchArchitecture軟交換體系結(jié)構(gòu)

/PacketSwitchingPrinciples分組交換原理

/X.25(Optional)

/FrameRelay幀中繼(Optional)

7

10.2CircuitSwitching

vDedicatedcommunicationpathbetween

twostations

vThreephases

>Establish

>Transfer

>Disconnect

'Musthaveswitchingcapacityandchannel

capacitytoestablishconnection

'Musthaveintelligencetoworkoutrouting

CircuitSwitching-Applications

vInefficient

>Channelcapacitydedicatedfordurationof

connection連接期間信道的容量是專用的

>Ifnodata,capacitywasted

/Setup(connection)takestime

，Onceconnected,transferistransparent

'Developedforvoicetraffic(phone)

PublicCircuitSwitched

Network

o0

o0Long-distanceLong-distance

o0officeoffice

c^

o0o

o0

EndOffice

EndOffice

。o

。o

*o

SubscriberLoopo

。q

。

ConnectingTrunkIntercityTrunk

ConnectingTrunk

DigitalPBX

10

TelecommsComponents

，Subscriber

>Devicesattachedtonetwork

/Subscriberline用方線路

>LocalLoop

>Subscriberloop

>Connectiontonetwork

>Fewkmuptofewtensofkm

/Exchange交換局

>Switchingcenters

>Endoffice-supportssubscribers

/Trunks中繼線

>Branchesbetweenexchanges

CircuitEstablishment

End

office

Trunk

■Intermediate

exchange

Trunk

End

office

OVERVIEW

/SwitchingNetworks交換式通信網(wǎng)

/CircuitSwitching電路交換網(wǎng)

/CircuitSwitchingConcepts電路交換的概念

/ControlSignaling控制信令難點！

/SoftswitchArchitecture軟交換體系結(jié)構(gòu)

/PacketSwitchingPrinciples分組交換原理

/X.25(Optional)

/FrameRelay幀中繼(Optional)

13

10.3CircuitSwitchElements

ControlUnit

-O……

t

Q……

-O……

\

%

SQ*

8a

W>O

-9

xP

①-o

-p：

d①DigitalSwitch

n、1

O……?■

p1a

=—■

n

-O,?

??

LL0?B

1-O……?.

■O

9

■

■o-…-

-O……

t

1

■o*

■

K>…

-O

14

Network

Interface

CircuitSwitchingConcepts

/DigitalSwitch

>Providetransparentsignalpathbetweendevices

/NetworkInterface

'ControlUnit

>Establishconnections

?Generallyondemand

?Handleandacknowledgerequests

?Determineifdestinationisfree

?constructpath

>Maintainconnection

>Disconnect15

BlockingorNon-blocking

vBlocking

AAnetworkisunabletoconnectstationsbecause

allpathsareinuse

AAblockingnetworkallowsthis

>Usedonvoicesystems

?Shortdurationcalls

/Non-blocking

>Permitsallstationstoconnect(inpairs)atonce

>Usedforsomedataconnections

10.3.1SpaceDivisionSwitching

空分交換

/Developedforanalogenvironment

VSeparatephysicalpaths

/Crossbarswitch縱橫交換機(jī)

>Numberofcrosspoints（交叉點）growsas

squareofnumberofstations

>Lossofcrosspointpreventsconnection

>Inefficientuseofcrosspoints

*Allstationsconnected,onlyafewcrosspointsinuse

>Non-blocking

17

SpaceDivisionSwitch

空分交換矩陣

TN_TN~ZN-ZS-ZS-~~~7

3d㈠㈠㈠㈠㈠㈠㈠

8㈠㈠㈠㈠㈠㈠㈠㈠㈠

s

①8㈠㈠㈠U㈠㈠㈠㈠

u

_

_lJ8㈠㈠㈠㈠㈠㈠㈠㈠㈠

dn

_u8㈠㈠㈠㈠㈠㈠㈠㈠㈠

8㈠㈠㈠㈠㈠㈠㈠d

3d㈠㈠㈠㈠㈠㈠㈠

3㈠㈠㈠㈠㈠㈠一㈠㈠

"TTTTTTTTTt18

OutputLines

MultistageSwitch多級交換

/Reducednumberofcrosspoints

/Morethanonepaththroughnetwork

>Increasedreliability

，Morecomplexcontrol

“Maybeblocking

ThreeStageSpaceDivision

10.3.2TimeDivisionSwitching

時分交換

/Modemdigitalsystemsrelyonintelligent

controlofspaceandtimedivisionelements

，Usedigitaltimedivisiontechniquestoset

upandmaintainvirtualcircuits

/Partitionlowspeedbitstreamintopieces

thatsharehigherspeedstream

21

OVERVIEW

/SwitchingNetworks交換式通信網(wǎng)

/CircuitSwitching電路交換網(wǎng)

/CircuitSwitchingConcepts電路交換的概念

/ControlSignaling控制信令難點！

/SoftswitchArchitecture軟交換體系結(jié)構(gòu)

/PacketSwitchingPrinciples分組交換原理

/X.25(Optional)

/FrameRelay幀中繼(Optional)

22

10.4.1ControlSignalingFunctions

信令功能

/Audiblecommunicationwithsubscriber

>Dialtone,ringingtone,busysignal,ColorRingBackTone

/Transmissionofdialednumber

/Callcannotbecompletedindication

/Callendedindication

/Signaltoringphone

/Billinginfo

/Equipmentandtrunkstatusinfo

/Diagnosticinfo

/Controlofspecialistequipment

23

ControlSignalSequence

/Bothphonesonhook(notinuse)

/Subscriberliftsreceiver(offhook)

/Endofficeswitchsignaled

/Switchrespondswithdialtone

/Callerdialsnumber

/Iftargetnotbusy,sendringersignaltotargetsubscriber

/Feedbacktocaller

>Ringingtone,engagedtone(busy),unobtainable

/Targetacceptscallbyliftingreceiver

/Switchterminatesringingsignalandringingtone

/Switchestablishesconnection

/ConnectionreleasewhenSourcesubscriberhang^2j^_

SwitchtoSwitchSignaling

vSubscribersconnectedtodifferentswitches

/Originatingswitchseizesinterswitchtrunk（空

閑的）

/Sendoffhooksignalontrunk,requestingdigit

register（計發(fā)器）attargetswitch（foraddress）

/Terminatingswitchsendsoffhookfollowedby

onhook（wink閃掛）toshowregisterready

vOriginatingswitchsendsaddress

25

10.4.2LocationofSignaling

信令的位置

vSubscribertonetwork

>Dependsonsubscriberdeviceandswitch

/Withinnetwork

>Managementofsubscribercallsandnetwork

>Morecomplex

10.4.3InChannelSignaling

隨路信令

Usesamechannelforsignalingandcall

>Requiresnoadditionaltransmissionfacilities

/Inband（帶內(nèi)信令）

>Usessamefrequenciesasvoicesignal

>Cangoanywhereavoicesignalcan

>Impossibletosetupacallonafaultyspeechpath

/Outofband

>Voicesignalsdonotusefull4kHzbandwidth

>Narrowsignalbandwithin4kHzusedforcontrol

>Canbesentwhetherornotvoicesignalsarepresent

>Needextraelectronics27

DrawbacksofInChannelSignaling

VLimitedtransferrate

/Delaybetweenenteringaddress(dialing)and

connection

/Overcomebyuseofcommonchannel

signaling(共路信令)

28

CommonChannelSignaling

共路信令

/Controlsignalscarriedoverpathsindependentof

voicechannel

/Onecontrolsignalchannelcancarrysignalsfora

numberofsubscriberchannels

/Commoncontrolchannelforthesesubscriberlines

/AssociatedMode（相關(guān)方式）

>Commonchannelcloselytracksinterswitchtrunks

/DisassociatedMode

>Additionalnodes（signaltransferpoints信號傳送點）

>Effectivelytwoseparatenetworks

29

CommonvsInChannelSignaling

共路和隨路信令

(a)Inchannel

(b)Commonchannel

CCISSIG:Common-channelinterofficesignalingequipment

SIG:Per-trunksianalinaeauioment

Common

Channel

Signaling

Modes

共路信令方式

/\

□,??\

，*

O'

(b)Disassociated

10.4.4SignalingSystemNumber7

7號信令系統(tǒng)

/SS7

/Commonchannelsignalingscheme

/ISDN

/Optimizedfor64kdigitalchannelnetwork

/Callcontrol,remotecontrol,managementand

maintenance

/Reliablemeansoftransferofinfoinsequence

vWilloperateoveranalogandbelow64k

/Pointtopointterrestrialandsatellitelink^2

SS7SignalingNetwork

Elements信令網(wǎng)絡(luò)元素

/Signalingpoint（S。信令點）

>AnypointinthenetworkcapableofhandlingSS7

controlmessage

/Signaltransferpoint（STP信號傳送點）

AAsignalingpointcapableofroutingcontrol

messages

/Controlplane控制平面

>Responsibleforestablishingandmanaging

connections

，Informationplane信息平面

>Onceaconnectionissetup,infoistransferred填the

informationplane

Transfer

Points

信息傳送點

STP=Signalingtransferpoint

SP=Signalingpoint

TC=Transitcenter

LE=LocalExchange

SignalingNetworkStructures

/STPcapacities

>Numberofsignalinglinksthatcanbehandled

>Messagetransfertime

>Throughputcapacity報文吞吐容量

，Networkperformance

>NumberofSPs

>Signalingdelays

vAvailabilityandreliability

>Abilityofnetworktoprovideservicesinthe

faceofSTPfailures■

OVERVIEW

/SwitchingNetworks交換式通信網(wǎng)

/CircuitSwitching電路交換網(wǎng)

/CircuitSwitchingConcepts電路交換的概念

/ControlSignaling控制信令難點！

/SoftswitchArchitecture軟交換體系結(jié)構(gòu)

/PacketSwitchingPrinciples分組交換原理

/X.25(Optional)

/FrameRelay幀中繼(Optional)

36

10.5SoftswitchArchitecture

軟交換體系結(jié)構(gòu)

/Generalpurposecomputerrunningsoftwaretomakeita

smartphoneswitch

/Lowercosts

/Greaterfunctionality

>Packetizingofdigitizedvoicedata

>AllowingvoiceoverIP

/Mostcomplexpartoftelephonenetworkswitchissoftware

controllingcallprocess（控制呼叫過程的軟件）

>Callrouting

>Callprocessinglogic

>Typicallyrunningonproprietaryprocessor

vSeparatecallprocessingfromhardwarefunctionofswitch

/Physicalswitchingdonebymediagateway（媒體網(wǎng)關(guān)）

/Callprocessingdonebymediagatewaycontroller^2

TraditionalCircuitSwitching

Softswitch

39

OVERVIEW

/SwitchingNetworks交換式通信網(wǎng)

/CircuitSwitching電路交換網(wǎng)

/CircuitSwitchingConcepts電路交換的概念

/ControlSignaling控制信令難點！

/Softswitch軟交換體系結(jié)杓

/PacketSwitchingPrinciples分組交換原理

/X.25(Optional)

/FrameRelay幀中繼(Optional)

40

10.6PacketSwitchingPrinciples

分組交換原理

/Circuitswitchingdesignedfbrvoice

>Resourcesdedicatedtoaparticularcall

>Muchofthetimeadataconnectionisidle

>Datarateisfixed

?Bothendsmustoperateatthesamerate

/電路交換為話音而設(shè)計■■

>資源被-次特定的呼叫所獨(dú)占

>數(shù)據(jù)連接的多數(shù)時間是空閑的

?數(shù)據(jù)率固定

?兩端以相同速率工作——對C/S模式數(shù)據(jù)傳輸有何影響?

41

BasicOperation分組交換的基本操作

/Datatransmittedinsmallpackets以小的分組傳輸

數(shù)據(jù)

>Typically1000octets典型的有1000八比特組

>Longermessagessplitintoseriesofpackets長寸艮文分

成一連串分戒

>Eachpacketcontainsaportionofuserdataplussome

controlinfo每個分組包含部分?jǐn)?shù)據(jù),卜加一些控制彳言

息

/Controlinfd控制信息

>懿9端,ddressing)info路由(尋址)信息或者虛電路

/Packetsarereceived,storedbriefly(buffered)and

pastontothenextnode接收到分組后短暫存儲，

然后傳遞給下一節(jié)點42

>Storeandforward存儲轉(zhuǎn)發(fā)

UseofPackets

43

Advantages

/Lineefficiency

>Singlenodetonodelinkcanbesharedbymanypackets

overtime

>Packetsqueuedandtransmittedasfastaspossible

/Datarateconversion

>Eachstationconnectstothelocalnodeatitsownspeed

>Nodesbufferdataifrequiredtoequalizerates

/Packetsareacceptedevenwhennetworkisbusy

>Deliverymayslowdown

/Prioritiescanbeused

優(yōu)點

/線路高效率

A單條節(jié)點到節(jié)點鏈路可在一段時間內(nèi)由許多分

組共享

A分組排隊并盡可能快地發(fā)送..

/實現(xiàn)不同數(shù)據(jù)率轉(zhuǎn)換■

?每個站點可以自己的速率連接到本地節(jié)點H

?需要相等速率時由節(jié)點對數(shù)據(jù)給予緩沖H

，即使網(wǎng)絡(luò)繁忙也能接受分組

A交付可能慢一些

,可以使用優(yōu)先級

45

10.6.1SwitchingTechnique交換技術(shù)

/Stationbreakslongmessageintopackets

/Packetssentoneatatimetothenetwork

/Packetshandledintwoways

>Datagram

>Virtualcircuit

/站點負(fù)責(zé)將長報文分割成分組■

/-次送一個分組到網(wǎng)絡(luò)

，分組以兩種方式處理

A數(shù)據(jù)報Datagram

?虛電路Virtualcircuit46

Datagram數(shù)據(jù)報

/Eachpackettreatedindependently

/Packetscantakeanypracticalroute

/Packetsmayarriveoutoforder

/Packetsmaygomissing

/Uptoreceivertore-orderpacketsandrecoverfrom

missingpackets

,每個分組獨(dú)立地處理

,分組可以選擇任何存在的路由

/分組到達(dá)可能失序

/分組可能丟失

/接收方對分組重新排序，并對丟失分組給予恢復(fù)

Datagram

Diagram

數(shù)據(jù)報方式

(e)

VirtualCircuit

/Preplannedrouteestablishedbeforeany

packetssent

/Callrequestandcallacceptpacketsestablish

connection(handshake)

VEachpacketcontainsavirtualcircuit

identifierinsteadofdestinationaddress

/Noroutingdecisionsrequiredforeachpacket

/Clearrequesttodropcircuit

/Notadedicatedpath

49

虛電路

/建立預(yù)定的路由之后再發(fā)送所有分組

,由呼叫callrequest分組和callaccept分組建立連接

A握手(handshake)

,每個分組含有一個虛電路標(biāo)識碼，而不是使用目

的地址

,對每個分組不需要路由抉擇

,通過一個dearrequest分組終止該電路

,不是-條像電路交換那樣的獨(dú)占通路

A不保留交換和鏈路容量

＞其它虛電路上的分組也可共享同一鏈路

?專用路由，而非專用鏈路

Virtual

Circuit

Diagram

虛電路方式

(e)

智

BOMPUoqjjosgdPB窩uo。POAP2posnD(qup。JnoM*

sgaA

qq-g」on

怎

SJodopd蕓出J-JB8*

DSEqddnps一一E。ONA

UIRIMWa

opoujpqjqnonojqjSESJO=pSOSOOIDtpoupJoSS03*

A

9qEWssgq

gapul2SU2.SOOP柿￡JnoJON*

A

AsOInbgjoiupgpJdAuqJgjpSJgaopd

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上

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luuelMElEasasl-nj上§b.A

VirtualCircuitsvsDatagram

vVirtualcircuits

?網(wǎng)絡(luò)能夠提供按序傳輸和差錯控制

A分組可以更快地轉(zhuǎn)發(fā)

集不必作出選路的決策

A可靠性差

率單點故障：一個節(jié)點失效將使通過該節(jié)點的全部電

—效

'Datagram

A不需呼叫建立階段

率尤其少量分組時優(yōu)勢明顯

?更靈活

(a)1-packetmessage(b)2-packetmessage(c)5-packetmessage(d)10-packetmessage

10.6.2

PacketSize

Message

switching

10.6.3CircuitvsPacketSwitching

電路交換與分組交換的比較

/Performance性能

>Propagationdelay傳播時延

>Transmissiontime傳輸時間

>Nodedelay節(jié)點時延

?Processingdelay

EventTiming

laiCircuitswitching<h)Vinuulchvuniwkclswitching(e>Datugrampacketsnitching

56

???0

CircuitswitchingDatagrampyckrt?witchin?Virtiuil-circuifpseckrt

switchingTable10.2Other

DedicatedtransmissionpathNodedicatedpathNodedicatedpath

ContinuoustransmissionofTransmissionnipu<kctsTransmissionofpacketsCharacteristics

data

FastenoughQrinteractiveFastenoughforinteractiveFastenouLjliforintdiactive

MessagesarenotstnirdPacketsmaybestoneduntilPacketsstoreduntildchvered

dchvered

ThepathisestablishedfbcRouleestablishedforeachRouteestablishedforentire

entireconversationpacketconversation

Call沁lupdelay;negligiblePackettransmissiondelayCallsetupdelay:packet

transmissiondelaytransmissiondelay

Bus>signalifcalledpartySendermaybenotifiedifSendernuliGedofconnection

busypacketnotdelivereddenial

OverloadinayblockcallOverheadincreasespacketOverloadmayblockcall

setup,no<leluyforestablisheddehssetup;increasespacketdelay

calls

EleutromecluinicalorSmallsuitchinv*nmlcSmallswitchingnode>

computerizeditchingrnnkb

UsertespcnsiblclormessageNetworkmayberesponsibleNetworknuyberespunsibk

lossproteclionforindividualpuck山forpuckelsequence>

Usuallyno*edorcodeSpeedandcodecon\CIM-JIISpeedandcodeconversion

conversion

FixedbunduidthDynamicuuubandwidihDynamicuseorbamiwidth

Nooverheadbitsuitercall?1.'11uchp；K>.<)xerheadbitsineachpackci57

setup

OVERVIEW

/SwitchingNetworks交換式通信網(wǎng)

/CircuitSwitching電路交換網(wǎng)

/CircuitSwitchingConcepts電路交換的概念

/ControlSignaling控制信令難點！

/SoftswitchArchitecture軟交換體系結(jié)構(gòu)

/PacketSwitchingPrinciples分組交換原理

/X.25(Optional)

7FrameRelay幀中繼(Optional)

58

10.7X.25

?1976

VInterfacebetweenhostandpacketswitched

network

/Almostuniversalonpacketswitched

networksandpacketswitchinginISDN

VDefinesthreelayers

>Physical

>Link

>Packet

59

10.7X.25

?1976

/主機(jī)與分組交換網(wǎng)絡(luò)間的接口■

,幾乎通用于全球的分組交換網(wǎng)絡(luò)I

,甚至可用于ISDN中的分組交換I

,定義了三個layers

A物理級Physicallevel/Physicallayer

A鏈路級Linklevel/Framelayer/LAPBlayer

A分組級Packetlevel/Packetlayer

X.25-Physical

vInterfacebetweenattachedstationandlinkto

node

vDataterminalequipmentDTE(userequipment)

/DatacircuitterminatingequipmentDCE(node)

/UsesphysicallayerspecificationX.21

/Reliabletransferacrossphysicallink

/Sequenceofframes

61

X.25的物理層

/接入的站點與通往端節(jié)點鏈路的接口■

＞建立DTE-DCE物理連接

'DataterminalequipmentDTE(userequipment)

'DatacircuitterminatingequipmentDCE(node)

/使用X.21物理層規(guī)約

,有時也用EIA-232替代

62

X.25-LinkX.25的鏈路層

/在DTE-DCE物理連接上的可靠數(shù)據(jù)傳送

,以幀序列的形式傳送數(shù)據(jù)

/鏈路層標(biāo)準(zhǔn)協(xié)議―

ALinkAccessProtocol,Balanced(LAPB)

琳HDLC子集SubsetofHDLC(見教材第7章)

，點到點鏈路差錯與流量的控制

FormatofaLAPBFrame

FlagAddressControlInformationFCSFlag

I-frame:userdata

S-frame:empty

U-frame:controldata

SpecialAddressinginLAPB

□

Address:00000001Address:00000011

CommandCommand

Address:00000001Address:00000011

ResponseResponse

Three

Phases

ofthe

FrameDTE

LayerSABM_____________a

LinkU-frames

setup

/_________UA

Sendingandreceivingnetworklayer

packets(encapsulatedinI-frames)I-framesand

DataandcontrolandusingS-framestocontrolS-frames

transfer

theflowanderrorofthoseI-frames

DISC_____________K,

Link

disconnectU-frames

/_________UA

X.25-PacketX.25的分組層

/Externalvirtualcircuits外部虛電路

VLogicalconnections(virtualcircuits)

betweensubscribersDTE設(shè)備之間端到端

的邏輯連接(virtualcircuits)

X>25UseofVirtualCircuits

Solidline=physicallink68

Dashedline=virtualcircuit

FrameLayerandPacketLayerDomains

Packetlayerdomain

FramelayerdomainFramelayerdomain

X.25network

DTEDCEDCEDTE

VirtualCircuitService

vLogicalconnectionbetweentwostations

>Externalvirtualcircuit

'Specificpreplannedroutethroughnetwork

>Internalvirtualcircuit

/Typicallyonetoonerelationshipbetween

externalandinternalvirtualcircuits

/CanemployX.25withdatagramstylenetwork

有可能在數(shù)據(jù)報類型的網(wǎng)路上采用X.25

vExternalvirtualcircuitsrequirelogicalchannel

>Alldataconsiderednartofstream

X.25Levels層

/UserdatapassestoX.25level3

VX.25appendscontrolinformation

>Header

>Identifiesvirtualcircuit

>Providessequencenumbersfbrflowanderror

control

/X.25packetpasseddowntoLAPBentity

vLAPBappendsfurthercontrolinformation

UserDataandX.25Protocol

ControlInformation

X.25packe

LAPBframe

72

EncapsulationofPLPPacket

PLPPacketNetwor

Data

FlagAddr.ControlInformationlink

I-Frame

Physical

OVERVIEW

/SwitchingNetworks交換式通信網(wǎng)

/CircuitSwitching電路交換網(wǎng)

/CircuitSwitchingConcepts電路交換的概念

/ControlSignaling控制信令難點！

/SoftswitchArchitecture軟交換體系結(jié)構(gòu)

/PacketSwitchingPrinciples分組交換原理

/X.25(Optional)

/FrameRelay幀中繼(Optional)

74

10.8FrameRelay幀中繼

/DesignedtobemoreefficientthanX.25

/DevelopedbeforeATM

/LargerinstalledbasethanATM安裝的基礎(chǔ)比

ATM大

/ATMnowofmoreinterestonhighspeed

networks

75

FrameRelayBackground幀中

繼背景?X.25

/Callcontrolpackets,inbandsignaling帶內(nèi)信令

警呼叫控制分組與數(shù)據(jù)分組在相同信道和虛電路上運(yùn)載

/Multiplexingofvirtualcircuitsatlayer3

vLayer2and3includeflowanderrorcontrol

/Considerableoverhead相當(dāng)可觀的額外開銷

/Notappropriatefbrmodemdigitalsystemswith

highreliability與高可靠性的現(xiàn)代募字系統(tǒng)不

相適應(yīng)

76

FrameRelay-Differences

/Callcontrolcarriedinseparatelogicalconnection

/Multiplexingandswitchingatlayer2

>Eliminatesonelayerofprocessing

/Nohopbyhoperrororflowcontrol不采用逐跳的差

錯控制與流量控制

>Endtoendflowanderrorcontrol(ifused)aredonebyhigher

layer

>Singleuserdataframesentfromsourcetodestinationand

ACK(fromhigherlayer)sentback一個用戶數(shù)據(jù)幀從源站

發(fā)往目的站，來自高層的確認(rèn)幀則由反向的一個幀返回