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