计算机网络复习重点重点终结版.docx

计算机网络复习重点重点终结版.docx

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计算机网络复习重点重点终结版

物理拓扑结构（physicaltopology）

网状拓扑结构（meshtopology）

各台设备之间都有一条专用的点到点的链路，每台设备必须有n-1个I/O端口

星型拓扑结构（startopology）

每台设备拥有一条仅与中央控制器连接的点到点专用链路

总线拓扑结构（bustopology）

环状拓扑结构（ringtopology）

协议和标准:

♦协议是用来管理数据通信的一组规则。

协议规定了通信的内容、通信的方式和通信的时间。

♦标准是由标准化组织协商达成一致的规则。

因特网模型（Internetmodel）

物理层;数据链路层;网络层;传输层;应用层

AnexchangeusingtheInternetmodel

Physicallayer

Note:

Thephysicallayerisresponsiblefortransmittingindividualbitsfromonenodetothenext.

Datalinklayer

Note:

Thedatalinklayerisresponsiblefortransmittingframesfromonenodetothenext.

Networklayer

Note:

Thenetworklayerisresponsibleforthedeliveryofpacketsfromtheoriginalsourcetothefinaldestination.

Transportlayer

Note:

Thetransportlayerisresponsiblefordeliveryofamessagefromoneprocesstoanother.

Applicationlayer

Note:

Theapplicationlayerisresponsibleforprovidingservicestotheuser.

Summaryofduties

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Positionofthephysicallayer

Note:

Frequencyandperiodareinversesofeachother.

Note:

●Adigitalsignalisacompositesignalwithaninfinitebandwidth.

●Thebitrateandthebandwidthareproportionaltoeachother.

●Theanalogbandwidthofamediumisexpressedinhertz;thedigitalbandwidth,inbitspersecond.

●Digitaltransmissionneedsalow-passchannel.

●Analogtransmissioncanuseaband-passchannel.

noiselesschannel

noisychannel

Attenuation

Distortion

Noise

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Unipolar:

Unipolarencodingusesonlyonevoltagelevel.

Polar:

Polarencodingusestwovoltagelevels（positiveandnegative）.

NRZ-L:

InNRZ-Lthelevelofthesignalisdependentuponthestateofthebit.

NRZ-I:

InNRZ-Ithesignalisinvertedifa1isencountered.

RZencoding

Agoodencodeddigitalsignalmustcontainaprovisionforsynchronization.

Manchesterencoding

InManchesterencoding,thetransitionatthemiddleofthebitisusedforbothsynchronizationandbitrepresentation.

DifferentialManchesterencoding

IndifferentialManchesterencoding,thetransitionatthemiddleofthebitisusedonlyforsynchronization.Thebitrepresentationisdefinedbytheinversionornoninversionatthebeginningofthebit.

Bipolar

Inbipolarencoding,weusethreelevels:

positive,zero,andnegative.

BipolarAMIencoding

FromanalogsignaltoPCMdigitalcode

AccordingtotheNyquisttheorem,thesamplingratemustbeatleast2timesthehighestfrequency

Howmanybitsshouldbesentforeachsample?

1bitforthesignandXbitsforthevalue.

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Bitrateisthenumberofbitspersecond.Baudrateisthenumberofsignalunitspersecond.Baudrateislessthanorequaltothebitrate.

ASK

RelationshipbetweenbaudrateandbandwidthinASK

InASKthebaudrateandbitratearethesame.

AnASKsignalrequiresaminimumbandwidthequaltoitsbaudrate

FSK

InFSKthebaudrateandbitratearethesame.

BW=fc1-fc0+Nbaud

PSK

The4-PSKmethod

The8-PSKcharacteristics

RelationshipbetweenbaudrateandbandwidthinPSK

Atelephonelinehasabandwidthofalmost2400Hzfordatatransmission.

ThetotalbandwidthrequiredforAMcanbedeterminedfromthebandwidthoftheaudiosignal:

BWt=2xBWm.

ThetotalbandwidthrequiredforFMcanbedeterminedfromthebandwidthoftheaudiosignal:

BWt=10xBWm.

Thebandwidthofastereoaudiosignalisusually15KHz.Therefore,anFMstationneedsatleastabandwidthof150KHz.TheFCCrequirestheminimumbandwidthtobeatleast200KHz（0.2MHz）.

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multiplexing

FDMisananalogmultiplexingtechniquethatcombinessignals.

FDMprocess&&FDMdemultiplexingexample

WDM

WDMisananalogmultiplexingtechniquetocombineopticalsignals（光信号）.

TDM

TDMisadigitalmultiplexingtechniquetocombinedata.

InaTDM,thedatarateofthelinkisntimesfaster,andtheunitdurationisntimesshorter.

Radiowavesareusedformulticastcommunications,suchasradioandtelevision,andpagingsystems.

Microwavesareusedforunicastcommunicationsuchascellulartelephones,satellitenetworks,andwirelessLANs.

Infraredsignalscanbeusedforshort-rangecommunicationinaclosedareausingline-of-sightpropagation.

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ADSLisanasymmetriccommunicationtechnologydesignedforresidentialusers;itisnotsuitableforbusinesses.

Theexistinglocalloopscanhandlebandwidthsupto1.1MHz.语音通信带宽为4k

DataLinkLayer

奇偶性校验CRC校验（循环冗余码校验）校验和校验

Simpleparitycheckcandetectallsingle-biterrors.Itcandetectbursterrorsonlyifthetotalnumberoferrorsineachdataunitisodd.

Two-dimensionalparity

BinarydivisioninaCRCgeneratorChecksum

PositionsofredundancybitsinHammingcode

Stop-and-WaitARQPiggybacking

Go-Back-NARQ

Go-Back-NARQ:

senderwindowsize

InGo-Back-NARQ,thesizeofthesenderwindowmustbelessthan2m;thesizeofthereceiverwindowisalways1.

SelectiveRepeatARQ

InSelectiveRepeatARQ,thesizeofthesenderandreceiverwindowmustbeatmostone-halfof2m.

HDLC高级数据链路控制

HDLCframe

Bitstuffing（比特填充）istheprocessofaddingoneextra0whenevertherearefiveconsecutive1sinthedatasothatthereceiverdoesnotmistakethedataforaflag.

Point-to-Point

12.1PPPframe

TransitionstatesMultiple-accessprotocols

Figure13.5CSMApersistentstrateg

13.3Channelization

FDMA:

InFDMA,thebandwidthisdividedintochannels.

TDMA:

InTDMA,thebandwidthisjustonechannelthatistimeshared.

CDMA:

InCDMA,onechannelcarriesalltransmissionssimultaneously.

Figure13.16CDMAmultiplexer

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=*****************************************************************=

ArepeaterconnectssegmentsofaLAN.

Arepeaterforwardseveryframe;ithasnofilteringcapability.

Arepeaterisaregenerator,notanamplifier.

Hubs

Abridgehasatableusedinfilteringdecisions.

Abridgedoesnotchangethephysical（MAC）addressesinaframe.

AMPS（高级移动电话系统,北美）isananalogcellularphonesystemusingFDMA.

D-AMPS,orIS-136,isadigitalcellularphonesystemusingTDMAandFDMA.

GSMisadigitalcellularphonesystemusingTDMAandFDMA.

IS-95isadigitalcellularphonesystemusingCDMA/DSSSandFDMA.

GSM

TheIridiumsystemhas66satellitesinsixLEOorbits,eachatanaltitudeof750km.

Iridiumisdesignedtoprovidedirectworldwidevoiceanddatacommunicationusinghandheldterminals,aservicesimilartocellulartelephonybutonaglobalscale.

Teledesichas288satellitesin12LEOorbits,eachatanaltitudeof1350km.

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FrameRelayoperatesonlyatthephysicalanddatalinklayers.

FrameRelaydoesnotprovidefloworerrorcontrol;theymustbeprovidedbytheupper-layerprotocols.

VirtualconnectionidentifiersinUNIsandNNIs

18.21AnATMcell

NetworkLayer

SwitchingatthenetworklayerintheInternetisdoneusingthedatagramapproachtopacketswitching.

CommunicationatthenetworklayerintheInternetisconnectionless.

分类

Figure19.17Networkaddress

Inclassfuladdressing,thenetworkaddressistheonethatisassignedtotheorganization.

Anetworkaddressisdifferentfromanetid.Anetworkaddresshasbothnetidandhostid,with0sforthehostid.

NetworkLayerProtocols:

ARP,IPv4,ICMPv4,IPv6,andICMPv6

AnARPrequestisbroadcast;anARPreplyisunicast.

Figure20.7IPdatagram

ThereisnoflowcontrolorcongestioncontrolmechanisminIP.

IPv6address

IPv4-IPv6Threetransitionstrategies

双协议栈隧道技术数据包头转换

RIPUpdatingAlgorithm

Receive:

aresponseRIPmessage

1.Addonehoptothehopcountforeachadvertiseddestination.

2.Repeatthefollowingstepsforeachadvertiseddestination:

1.If（destinationnotintheroutingtable）

1.Addtheadvertisedinformationtothetable.

2.Else

1.If（next-hopfieldisthesame）

1.Replaceentryinthetablewiththeadvertisedone.

2.Else

1.If（advertisedhopcountsmallerthanoneinthetable）

1.Replaceentryintheroutingtable.

3.Return.

Figure21.20Shortest-pathcalculation

DijkstraAlgorithm

1.Startwiththelocalnode（router）:

therootofthetree.

2.Assignacostof0tothisnodeandmakeitthefirstpermanentnode.

3.Examineeachneighbornodeofthenodethatwasthelastpermanentnode.

4.Assignacumulativecosttoeachnodeandmakeittentative.

5.Amongthelistoftentativenodes

1.Findthenodewiththesmallestcumulativecostandmakeitpermanent.

2.Ifanodecanbereachedfrommorethanonedirection

1.Selectthedirectionwiththeshortestcumulativecost.

6.Repeatsteps3to5untileverynodebecomespermanent.

TransportLayer

UDPisaconnectionless,unreliableprotocolthathasnoflowanderrorcontrol.Itusesportnumberstomultiplexdatafromtheapplicationlayer.

ApplicationLayer

DNScanusetheservicesofUDPorTCP,usingthewell-knownport53.

FTPusestheservicesofTCP.ItneedstwoTCPconnections.Thewell-knownport21isusedforthecontrolconnection,andthewell-knownport20isusedforthedataconnection.

HTTPusestheservicesofTCPonwell-knownport80.

RTPusesatemporaryeven-numberedUDPport.

RTCP（实时传输控制协议）messagetypes

RTCPusesanodd-numberedUDPportnumberthatfollowstheportnumberselectedfor