数据通讯与计算机网络讲义 11章 a.ppt

第 11章;11.1 成帧; 面向字符的成帧协议;字节填充示例;比特填充——在帧的数据、首部、尾部中出现的连续5个“1”之后添加一个额外的“0”比特。;比特填充示例;11.2 流控和差控; 差控 差错检测 损坏（damaged） (由CRC、校验和等检测) 错序（out of order） (由序号检测) 重复（duplicated） (由信号检测) 丢失（lost） (由发送方的定时器检测) 差错纠正：重发 Automatic Repeat reQuest (三种ARQ技术);11.3 链路层控制协议;11.4 无噪声信道; 最简单协议;发送方;例 11.1; 停-等协议;停-等协议：发送方算法;停-等协议：接收方算法;例 11.2;11.5 噪声信道; 停-等ARQ;对帧标注序号。序号按模-2运算，即依次取“0, 1”。; 停-等ARQ;停-等ARQ：发送方算法;(continued);Escape();例 11.3;终端A和终端B采用停-等ARQ协议进行通信，链路的数据率为1 Mbps， 往返时延为20 ms。数据帧长度是1000 bits。链路的利用率是多少?;例 11.5; 在“Go-Back-N”ARQ协议中，序号为模-2m，m是序号字段的比特位数。;After received ‘ACK 3’;接收方滑动窗口; 定时器 仅对第1个已发出的帧设置定时器。 确认 对收到的正确帧回发ACK；ACK具有累积作用。 对差错帧不回发ACK（Silent）。 重发 (N帧) 发送方的定时器超时后，发送方重发定时器对应的帧 及其该帧之后的所有已发帧。;“Go-Back-N”ARQ;“Go-Back-N” ARQ的窗口尺寸;“Go-Back-N”ARQ 的发送方算法;Escape();Escape();例 11.6;例 11.6;Example 11.17 shows what happens when a frame is lost. Frames 0, 1, 2, and 3 are sent. However, frame 1 is lost. The receiver receives frames 2 and 3, but they are discarded because they are received out of order. The sender receives no acknowledgment about frames 1, 2, or 3. Its timer finally expires. The sender sends all outstanding frames (1, 2, and 3) because it does not know what is wrong. Note that the resending of frames 1, 2, and 3 is the response to one single event. When the sender is responding to this event, it cannot accept the triggering of other events. This means that when ACK 2 arrives, the sender is still busy with sending frame 3. The physical layer must wait until this event is completed and the data link layer goes back to its sleeping state. We have shown a vertical line to indicate the delay. It is the same story with ACK 3; but when ACK 3 arrives, the sender is busy responding to ACK 2. It happens again when ACK 4 arrives. Note that before the second timer expires, all outstanding frames have been sent and the timer is stopped.;例 11.7