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ID:19479098
大小:688.00 KB
页数:75页
时间:2018-10-02
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1、Chapter4DataLinkLayerFramingErrorcontrolFlowcontrolMultiplexingLinkMaintenanceSecurity1ServicesTransfersframesacrossdirectconnectionsDirectlyconnected(canbewireless),wire-likeLosses&errors,butnoout-of-sequenceframesMoredetailedservicesFraming(bits↔frames)Err
2、orcontrol(protectionfromimpairment)FlowcontrolMultiplexingLinkMaintenanceSecurity:Authentication&Encryption2DataLinkProtocolsExamplesPPPHDLCEthernetLANIEEE802.11(WiFi)LANDatalinklayerPhysicallayerPhysicallayerDatalinklayerABPacketsPacketsFrames3Framing(Chapte
3、r5.4inLeon-Garcia)4FramingBitstream-framesFrameboundariescanbedeterminedusing:CharacterCountsControlCharactersFramingBitsFramingbyillegalcode0110110111Framingreceivedframes0111110101transmittedframes5DatatobesentADLEBETXDLESTXEAfterstuffingandframingDLEDLEBET
4、XDLEDLESTXDLESTXAEDLEETXControlCharactersWhatabouttransmissionofdata(includingnon-printablecharacters)?IntroduceDLE(datalinkescape)=0x10DLESTX(DLEETX)usedtoindicatebeginning(end)offrameInsertextraDLEinfrontofoccurrenceofDLESTX(DLEETX)inframeAllDLEsoccurinpair
5、sexceptatframeboundaries.TransmissionofprintablecharactersusingASCIIOctetswithHEXvalue<0x20arenonprintableUsecontrolcharacters:STX(startoftext)=0x02;ETX(endoftext)=0x03.6BitStuffingFramedelineatedbyflagcharacterHDLCusesbitstuffingtopreventoccurrenceofflag0111
6、1110insidetheframeTransmitterinsertsextra0aftereachconsecutivefive1sinsidetheframeReceiverchecksforfiveconsecutive1sifnextbit=0,itisremovedifnexttwobitsare10,thenflagisdetectedIfnexttwobitsare11,thenframehaserrorsFlagFlagAddressControlInformationFCSHDLCframea
7、nynumberofbits70110111111111100DatatobesentAfterstuffingandframing0111111001101111101111100001111110(a)*000111011111-11111-110*DatareceivedAfterdestuffinganddeframing01111110000111011111011111011001111110(b)Example:Bitstuffing8Example:FraminginEthernetEtherne
8、tcompliestostandardIEEE802.3Anillegalmanchestercodingisusedforframing.Acharactercountisalsoincludedintheheader.Allframeshaveanintegralnumberofbytes.Ifnot,theframeisconsideredtobereceivedi
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