计算机网络第一次作业

1.What are two reasons for using layered protocols?

答：通过协议分层可以把设计问题划分较小的易处理的片段；其次就是分层协议的改变不会影响高层或是底层的协议。

2. In some networks, the data link layer handles transmission errors by requestint damaged frames to be retransmitted. If the probability of a frame’s being damaged is p, what is the mean number of transmissions required to send a frame? Assume that acknowledgements are never lost.

答：帧请求正好是K次的概率为Pk, 就是起初的k-1次尝试都失败的的概率为P(k-1),乘以第k次传输的概率。平均传输的的次数就为

∑kPk=∑k(1-p)P^(k-1)=1/(1-p)

3.Which of the OSI layers handles each of the following:

(a) Dividing the transmitted bit stream into frames

(b) Determining which route through the subnet to use

答：(1)数据链路层(2)网络层

4.If the unit of exchanged at the data link level is called a frame and the unit exchanged at network level is called a packet, do frame encapsulate packets or do packets encapsulate frame? Explain your answer.

答：帧封装包，当一个包达到数据链路层时，整个数据包，包括包头、数据机全部的内容都用做帧的数据区。或者说将整个包放进一个帧里。

5. A system has n-layer protocol hierarchy. Applications generate messages of length M bytes. At each of the layers, an h-byte header is added. What fraction of the network bandwidth is filled with headers?

答：再出去顶层之外的其他n-1层各加上n字节报头。报头总长：h*(n-1) 所以，最终结果为hn/(M+hn)。

6. List two ways in which the OSI reference model and the TCP/IP reference model are the same. Now list two ways in which they differ？

答：相同点：（1）都是基于独立的协议栈概念

（2）两者都有功能相似的应用层、传输层、网络层。

不同点：（1）在OSI模型中，严格定义了服务、接口、协议；在TCP/IP模型中，并没有严格区分服务、接口、协议。

（2）OSI模型中支持非连接和面向连接的网络层通信，但在传输层只支持面向连接的通信，但在传输层有支持非连接和面向连接的两种协议可供选择。