[工学]CH3-2 数据链路层.ppt

3.3 使用广播信道的数据链路层3.4 传统以太网3.5 局域网扩展3.6 高速以太网 局域网中的介质访问控制方法 (1). CSMA/CD 多个站点如何安全地使用共享信道？ 最简单的思路：发送前先检测一下其它站点是否正在发送（即信道忙否）。 若信道空闲，是否可以立即发送？ 若有多个站点都在等待发送，必然冲突！ 解决：等待一段随机时间后再发（降低了冲突概率） 若信道忙，如何处理？ 继续监听： 等到信道空闲后立即发送 等到信道空闲后等待随机时间后再发送 等待一段随机时间后再重新检测信道 一旦出现两个站点同时发送的情况，如何处理？ 以上方法均无法处理！ Ethernet signals are transmitted to every host connected to the LAN using a special set of rules to determine which station can access the network. Carrier Sense Multiple Access Collision Detection (CSMA/CD) In the CSMA/CD access method, all network devices that have messages to send must listen before transmitting. If a device detects a signal from another device, it waits for a specified amount of time before attempting to transmit. When there is no traffic detected, a device transmits its message. While this transmission is occurring, the device continues to listen for traffic or collisions on the LAN. If the distance between devices is such that the latency of the signals of one device means that signals are not detected by a second device, the second device may also start to transmit. The media now has two devices transmitting signals at the same time. The messages propagate across the media until they encounter each other. At that point, the signals mix and the messages are destroyed, a collision. When a collision is detected, the transmitting devices send out a jamming signal. The jamming signal notifies the other devices of a collision, so that they invoke a backoff algorithm. This backoff algorithm causes all devices to stop transmitting for a random amount of time, which allows the collision signals to subside. CSMA/CD操作的流程图 CSMA/CD协议的争用期（时间槽） 时间槽——能够检测到冲突的时间区间（也称为争用时隙或碰撞窗口） 若两站点之间传播时延为a，则时间槽＝2a。如下图所示： 时间槽的意义： 一个站点开始发送后，若在时间槽内没有检测到冲突，则本次发送不会再发生冲突 时间槽与网络跨距、传输速率、最小帧长有密切的关系 以太网中，时间槽＝51.2μs 传输速率为10Mb/s时，一个时间槽内可发送512bits，即64字节（所以也称一个时间槽长度为64字节） 。 由此可知： 1. 冲突只可能在一帧的前64字节内发生； 2. 帧长度小于64字节时，将无法检测出冲突； 所以，以太网规定的最小帧长度为64字节 3. 长度小于64字节的帧（碎片帧）都是无效帧。 与时间槽相关的几个网络参数 采用CSMA/CD的局域