1 Computer Communications - CSCI 551 Copyright © William C. ChengCS551Link Layer Issues for Wireless LANs[Bharghavan94a]Bill Chenghttp://merlot.usc.edu/cs551-f12force us to rethink many of our assumptionsWireless access and mobility 2Overview Computer Communications - CSCI 551 Copyright © William C. Chenglink layer issuesFocus of this paper:... in ad-hoc mobile wireless networks... in combined wired-wireless networkspacket delivery and routingtransport layer issueswhy contention? because moving nodes could causefrequent token lossContention-based vs. token-based3Wireless MAC Options Computer Communications - CSCI 551 Copyright © William C. Chengwhy base-station? simpler if you have a leader thatcan assign things (esp. if non-mobile)Base-station vs. ad-hocwhy ad-hoc? don’t always have leaderMACAW and 802.11 do bothpropagation can ber -3 or r -1 (near or far)Simple model: fixed tx range4Radio Propagation Computer Communications - CSCI 551 Copyright © William C. Chengissues: collisions,capture, interferencegood simple model,but only anapproximationMulti-path fadingReality is much worsetime-varying effectsTime since start (in hours)connectivity from one node to others[data from Jerry Zhao, ISI, 2002]% pkts received to 5 destsradio transmission range defined by cellToken-based or multiple access or spread spectrum5The Physical Layer Computer Communications - CSCI 551 Copyright © William C. ChengFirst study a simple modela receiver within range can hear transmissioninteraction of multiple transmitters at receiverOther, more complex environmental interactionsmultipath: reflected signals interfere with originalcollision: if receiver is within range of two transmitters,but can’t extract eithercapture: one signal stronger than otherinterference: in-range of one transmitter, out of range ofanother, but can’t extract signalwhy not for wireless?because receiver andsender "sense"different "carrier"works in EthernetCarrier Sense: beforetransmitting, check ifcarrier present6Carrier Sense in Wireless Computer Communications - CSCI 551 Copyright © William C. ChengIssuesABCABCABChiddenterminalexposedterminalhidden terminal:A and C do not know that B cannot hear eitherexposed terminal: B is busy sending to A, when does C getto talk to B?overhearers deferSrc sends Ready-to-Send(RTS) before data7Karn/MACA RTS-CTS Computer Communications - CSCI 551 Copyright © William C. ChengABChiddenterminalscenariooverhearers deferDst replies with Clear-to-Send (CTS)RTS around src, CTSaround dest, so everyoneshould be quietMust also deal with collisions, etc.A sends RTS⇒ B gets RTSand sends CTS⇒ C hears CTS andis quiet (no hiddenterminal)link-layer protocolsGeneral solutionsC doesn’t know ifRTS/CTS was successful,An exposed terminal may notbe able to compete effectively... so reduced to trying atrandom timestends to back-off moreand morecarrier senseFix:...or a DS (Data Sending)packet (include data length)Doesn’t solve all fairness issuesABCB sends RTS⇒ A gets RTSand sends CTS;but C missesCTS⇒ B sends DS⇒ C hears DS (anddata length) andso knows whento try RTS again⇒ B sends DATA⇒ C knows to RTSafter data8 Computer Communications - CSCI 551 Copyright © William C. Cheng Continuing Fairness ProblemsBack-off counter BO estimates populationBO = 0 after successBack-off algorithm:9Back-off Issues Computer Communications - CSCI 551 Copyright © William C. Chengrandomly wait [0,BO] before sendingoriginal: binary exponential:BO *= 2 after collisionif I succeed, my BO = 0, so I am likely to win againProblem: channel captureothers who fail get slower and slowerAB Cshare BO (send in each packet)Fixes:increase multiplicatively, decrease additively ("MILD")per-destination back-offABCnoise or collisionsWireless losses possible10Adding Link-level ACKs Computer Communications - CSCI 551 Copyright © William C. Chengend-to-end argument?lost DATA => no ACK =>retransmissionAdd link-level ACK ofDATAlost ACK => sender retxRTS, receiver sends ACKinstead of CTSA sends RTS⇒ B gets RTSand sends CTS⇒ A sends DATA⇒ B sends ACK⇒ if no ACK, Aresends RTSThis approach is also used in 802.11Basic MAC is a CSMA/CACarrier-sense andtransmit, ACKStandard for wireless communication11Commercializing MACAW: IEEE 802.11 Computer Communications - CSCI 551 Copyright © William C. ChengMAC-layer uses many of theideas discussedRTS/CTS exchange is optionalad-hoc (DCF: DistributedCoordination Function)Allows two modesbase-station (PCF: PointCoordination Function)DCFAP APPCFBSSBSSBSS12 Computer Communications - CSCI 551 Copyright © William C. Cheng 802.11 Detailsbecause it’s realMuch more complex than MACAWbecause it’s designed by committeeDoes not include all MACAWIn PCF (base station mode), quite differentless emphasis on fairness (e.g., no shared backoff)base station polls nodes to see if they have traffic to sendcan arbitrrate transmissionsIn DCF (ad-hoc mode)CSMA/CA with ACKoptional RTS/CTSMILD backoffno DS, RRTS, etc.nice exposition of various fairness issues with wirelessMACsImpact13 Computer Communications - CSCI 551 Copyright © William C. Cheng Discussionmost currently used ideas had already been developedwith MACAContextGood use of simple examples to understand various problemsin wireless communicationNo implmentation,
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