chapter 4 data link layer

《chapter 4 data link layer》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

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