《计算机网络——自顶向下方法及Internet特色》幻灯片Lecture52.ppt

Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3 Multiple access protocols 多址访问协议 5.4 Link-Layer Addressing 5.5 Ethernet 5.6 Hubs and switches 5.7 PPP Multiple Access Links and Protocols Two types of “links”: point-to-point – e.g. PPP for dial-up access DSL point-to-point link between Ethernet switch and host broadcast (shared wire or medium) traditional Ethernet Cable modem 802.11 wireless LAN 多点访问链路和协议 有两种类型的 “链路”: 点对点 ( e.g. PPP, SLIP) 广播式 (共享线路或介质; e.g, 以太网, 无线网, etc.) Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes: interference collision if node receives two or more signals at the same time multiple access protocol distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself! no out-of-band channel for coordination Q: why? 多点访问协议 一条共享的通信信道 两个或多个结点可同时发送信号: 相互干扰 在某一时刻只有一个结点可以成功地发送信号 多点访问协议: 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 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! 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 Ideal Multiple Access Protocol Broadcast 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: no special node to coordinate transmissions no synchronization of clocks, slots 4. Simple Multiple Access （MA）Protocols: a taxonomy Three broad classes: Channel Partitioning 信道划分 divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use Random Access 随机访问 channel not divided, allow collisions “recover” from collisions “Taking turns” 轮流协议 Nodes take turns, but nodes with more to send can take longer turns 多址访问协议: 分类 信道分割 将信道分割成较小的 “片” (时隙, 频率, 码分) 将小片分给各站点使用 随机访问 允许冲突 从冲突中“恢复” “轮流” 严格协调访问来避免冲突 Channel Partitioning MA protocols: TDMA TDM (Time Division Multiplexing): channel divided into N tim