南海凝心计算机网络第四版习题答案(中文版)
第 3 章 数据链路层
3-1 An upper-layer packet is split into 10 frames, each of which has an 80 percent chance of arriving undamaged. If no error control is done by the data link protocol, how many times must the message be sent on average to get the entire thing through?
一个上层的分组被切分成10帧，每一帧有80％的机会可以无损地到达。如果数据链路协议没有提供错误控制的话，请问，该报文平均需要发送多少次才能完整地到达接收方？
答：由于每一帧有0.8 的概率正确到达，整个信息正确到达的概率为 p=0.810=0.107。
为使信息完整的到达接收方，发送一次成功的概率是p ，二次成功的概率是(1-p)p，三次成功的概率为(1-p)2 p，i 次成功的概率为(1-p)i-1 p，因此平均的发送次数等于：
3-2The following character encoding is used in a data link protocol: A: 01000111; B: 11100011; FLAG: 01111110; ESC: 11100000 Show the bit sequence transmitted (in binary) for the four-character frame: A B ESC FLAG when each of the following framing methods are used:
