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1、Link Layerr5.1 Introduction and servicesr5.2 Error detection and correction r5.3 Multiple access protocols 多址访问协议r5.4 Link-Layer Addressingr5.5 Ethernetr5.6 Hubs and switchesr5.7 PPP15: DataLink LayerMultiple Access Links and ProtocolsTwo types of “links”:rpoint-to-point e.g.mPPP for dial-up accessm
2、DSLmpoint-to-point link between Ethernet switch and hostrbroadcast (shared wire or medium)mtraditional EthernetmCable modemm802.11 wireless LAN25: DataLink Layer多点访问链路和协议有两种类型的 “链路”:r点对点 ( e.g. PPP, SLIP)r广播式 (共享线路或介质; e.g, 以太网, 无线网, etc.)35: DataLink LayerMultiple Access protocolsrsingle shared bro
3、adcast channel rtwo or more simultaneous transmissions by nodes: interference mcollision if node receives two or more signals at the same time multiple access protocolrdistributed algorithm that determines how nodes share channel, i.e., determine when node can transmitrcommunication about channel sh
4、aring must use channel itself! mno out-of-band channel for coordinationmQ: why?45: DataLink Layer多点访问协议r一条共享的通信信道 r两个或多个结点可同时发送信号: 相互干扰 m在某一时刻只有一个结点可以成功地发送信号 r多点访问协议:m分布式的算法来决定如何共享信道, i.e., 决定工作站何时可以发送m注意：有关共享通道的通信（协商）也必须在该通道自身上解决! m我们希望多点访问协议能够解决什么问题: 同步还是异步 了解其他站点的信息 健壮性 (e.g.如何对待信道错误) 性能55: DataL
5、ink LayerIdeal Multiple Access ProtocolBroadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized:mno special node to coordinate transmissionsmno synchronization of clocks, slot
6、s 4. Simple65: DataLink LayerMultiple Access （MA）Protocols: a taxonomyThree broad classes:rChannel Partitioning 信道划分mdivide channel into smaller “pieces” (time slots, frequency, code)mallocate piece to node for exclusive userRandom Access 随机访问mchannel not divided, allow collisionsm“recover” from col
7、lisionsr“Taking turns” 轮流协议mNodes take turns, but nodes with more to send can take longer turns75: DataLink Layer多址访问协议: 分类r信道分割m将信道分割成较小的 “片” (时隙, 频率, 码分)m将小片分给各站点使用r随机访问m允许冲突m从冲突中“恢复”r“轮流”m严格协调访问来避免冲突目标: 高效, 公平, 简单, 分散控制85: DataLink LayerChannel Partitioning MA protocols: TDMArTDM (Time Division M
8、ultiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.rFDM (Frequency Division Multiplexing): frequency subdivided. TDMA: time division multiple access raccess to channel in “rounds“ reach station gets fixed length slot (length = pkt t
9、rans time) in each round runused slots go idle rexample: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 95: DataLink Layer信道分割的 MA协议: TDMATDMA: time division multiple access （时分多路）r“依次” 访问信道r每次每个站点分得固定长度的时隙 (时长 = 分组的单位 传输时间)r未用的时隙被闲置和浪费 r例如: 6个站点的LAN, 1,3,4 有分组发送, 而 2,5,6的时 隙则被闲置 105: DataLink Laye
10、rChannel Partitioning MA protocols: FDMAFDMA: frequency division multiple access rchannel spectrum divided into frequency bandsreach station assigned fixed frequency bandrunused transmission time in frequency bands go idle rexample: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle frequency
11、 bandstime115: DataLink Layer信道分割的 MA协议: FDMAFDMA: frequency division multiple access（频分复用） r信道按频谱分成若干频段r每个站点分得固定的频段r在频段不用时该部分信道被闲置和浪费 r例如: 6各站点的 LAN, 1,3,4 发送分组, 而 2,5,6 的频段被 闲置 frequency bandstime125: DataLink Layer信道分割 (CDMA)CDMA：Code Division Multiple Access（码分 多路） r每个用户分得具有唯一性的 “code”; ie, 代码集合
12、的分 割 r大部分应用在无线通信的场合 (cellular, satellite,etc)r所有用户共享相同的频道, 但每个用户用自己的“码片” 序列 (ie, code) 对数据编码r编码信号 = (原始数据) X (码片序列)r解码: 编码信号的内积和码片序列r允许多个用户共存和发送信号,且相互的干扰极小 (假 设信号码呈 “正交”状)135: DataLink LayerCDMA 编码/解码145: DataLink LayerCDMA: 两个用户互扰的情况155: DataLink LayerRandom Access ProtocolsrWhen node has packet to
13、 sendmtransmit at full channel data rate R.mno a priori coordination among nodesrtwo or more transmitting nodes : “collision”,rrandom access MA protocol specifies: mhow to detect collisionsmhow to recover from collisions (e.g., via delayed retransmissions)rExamples of random access MA protocols:mALO
14、HAmslotted ALOHAmCSMA, CSMA/CD, CSMA/CA165: DataLink Layer随机访问协议r当结点有数据要发送时：m使用信道全部的传输速率 R.m在诸多结点中不存在“预先”协商的机制r可能发生两个以上结点同时传输 - “冲突”r随机访问的 MA协议定义了: m如何检测冲突m如何从冲突中恢复 (e.g., 通过延迟重发)r随机访问 MA协议的实例:mALOHAm时隙ALOHAmCSMA and CSMA/CD175: DataLink LayerSlotted ALOHAAssumptionsrall frames same sizertime is div
15、ided into equal size slots, time to transmit 1 framernodes start to transmit frames only at beginning of slotsrnodes are synchronizedrif 2 or more nodes transmit in slot, all nodes detect collisionOperationrwhen node obtains fresh frame, it transmits in next slotrno collision, node can send new fram
16、e in next slotrif collision, node retransmits frame in each subsequent slot with probability p until success185: DataLink Layer时隙 Alohar时间划分成等长的时隙 (=分组单元传送的时长)r对待新近到达的分组结点要: 在下一个时隙开始时传输 r如果冲突: 以p的概率在后继的时隙重发分组，直到成功 为止.(S)成功, (C)冲突, (E) 空闲的时隙195: DataLink LayerSlotted ALOHAProsrsingle active node can continuously transmit at full rate of channelrhighly decentralized: only slots in nodes need to be in syncrsimpleConsrcollisions, wasting slotsridle slots
