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1、Computer Networks and Internets计算机网络与因特网课件 林坤辉 2004-02-6,PART II Packet Transmission,Chapter 11 Extending LANs: Fiber Modems, Repeaters, Bridges, and Switches局域网扩展：光纤调制解调器、中继器、网桥和交换机,11.1 Introduction,Each LAN technology is designed for a specific combination of speed, distance, and cost.The designe
2、r specifies a maximum distance that the LAN can span,with typical LANs designed to span a few hundred meters.,网络互联设备,11.2 Distance Limitation and LAN Design,Distance limitation is a fundamental part of LAN designs.Engineers choose a combination of capacity, maximum delay, and distance that can be ac
3、hieved at a given cost.To help save expense,LAN technologies usually use a shared communication medium.,A LAN design must include a mechanism that guarantees each station fair access to the shared medium.Hardware is engineered to emit a fixed amount of electrical power,the signal cannot reach arbitr
4、arily far.LAN hardware is engineered for a fixed maximum length cable, The hardware will not work correctly over wire that exceed the bound.,11.3 Fiber Optic Extensions,A pair of fiber modems and optical fibers can be used to provide a connection a computer and a remote LAN.Because delays across fib
5、er are low and bandwidth is high, the mechanism will operate correctly across distances of several kilometers.,11.4 Repeaters中继器,A repeater is usually an analog electronic device that continuously monitors electrical signals on each cable.When it senses a signal on one cable, the repeater transmits
6、an amplified copy on the other cable.,中继器（1）,中继器（2）,A repeater connects two Ethernet cables called segments,each of which has the usual termination.Repeaters do not understand the frame format,nor do they have physical addresses.Any pair of computers on the extended LAN can communication,the compute
7、rs do not know whether a repeater separates them.,The repeater attaches directly to the Ethernet cables and sends copies of electrical signals from one to other without waiting for a complete frame.A repeater does not distinguish between the signals that correspond to a valid frame and other electri
8、cal signals.If a collision or electrical interference occurs on one segment, repeaters cause the same problem to occur on all other segments.,If more than four repeaters separate any pair of stations,the network will not operate correctly.,11.5 Bridges网桥,A bridge is an electrical device that connect
9、s two LAN segments.A bridge handles complete frame and uses the same network interface as a conventional computer.The bridge listens to traffic on each segment in promiscuous mode(混合模式).When it receives a frame from one segment,the bridge verifies that frame arrived intact,and then forwards a copy o
10、f the frame to the other segment if necessary.,网桥（1）,网桥（2）,定义：网桥(bridge) 用于连接两个局域网的一种互联设备。功能：在不同的LAN之间进行互联；用于网段微化，有利于调节负载；对于802.3，可以扩展网络的物理距离；不同的网段之间进行隔离，有助于保密。,Any pair of computers on the extended LAN can communication,the computers do not know whether a bridge separates them.A bridge does not for
11、ward interference or other problems.,工作原理网桥接收一帧后的处理过程为：(1)如果目的站点与源站点在同一个LAN中，则扔掉此帧；(2)如果目的站点与源站点不在同一个LAN中，则通过某端 口转发此帧；(3)否则，将该帧扩散到除接收端口外的所有其他端口。反向学习算法网桥根据反向传送的帧来填写转发表的地址记录学习(填表)忘记重新学习Spanning Tree(802.1d)算法用生成树来解决在帧扩散中所产生的广播风暴问题,4个局域网和2个网桥的配置,两个并行的透明桥,11.6 Frame Filtering 帧过滤,A typical bridge consis
12、ts of a conventional computer with a CPU, memory, and two network interfaces.A bridge does not run application software, the CPU executes code from ROM.The most valuable function a bridge performs is frame filtering.When a frame arrives on a segment,the bridge extracts and checks the source address
13、and the destination address.,Most bridges are called adaptive or learning bridges because they learn the locations of computers automatically.,11.7 Startup and Steady State Behavior of Bridged Networks桥接网络的启动与稳态特征,When it first boots,a bridge does not know which computers attach to which LAN segment
14、. If a computer did not send any frames,a bridge could not detect its location.In the steady state,a bridge forward each frame only as far as necessary,11.8 Planning A Bridged Network 规划一个桥接网络,Bridge hardware is engineered to permit communication on separate segments at the same time.The design of a
15、 bridged network is parallelism: computers on one segment can communication at the same time as computers on another segment.A set of computers that interact frequently should be attached to the same segment.,4个源路径桥和4个局域网的配置,11.9 Bridging Between Buildings 大楼间桥接,11.10 Bridging Across Longer Distance
16、s 远程桥接,In addition to filtering, bridge hardware used with long-distance connections must perform buffering,11.11 A Cycle of Bridges 网桥环,A bridge network can span many segments.One bridge is needed to connect each segment to the rest of the bridge network.A bridge always forwards a copy of a frame sent to broadcast address.Not all bridge can be allowed to forward broadcast frames,or a cycle of bridges introduces a problem.,
