计算机网络第四版习题答案(中文版)

2-44 In a typical mobile phone system with hexagonal cells, it is forbidden to reuse a frequency band in an adjacent cell. If 840 frequencies are available, how many can be used in a given cell? 在一个典型的移动电话系统中,蜂窝单元为六角形,在相邻的单元内禁止重新使用频段。如果总共有840个频率可以使用的话,则任何一个给定的单元内可以使用多少个频率? 每个单元有6个邻居。如果中间的单元使用频段组合A,它的六个邻居可以分别使用的频段组合B, C, B, C, B, C。换句话说,只需要3个单一的单元。因此,每个单元可以使用280个频率。

2-50 Suppose that A, B, and C are simultaneously transmitting 0 bits, using a CDMA system with the chip sequences of Fig. 2-45(b). What is the resulting chip sequence?

FIG 2-45(b)

结果是通过对A、B、C求反再将这三个码片序列相加得到的。 结果是(+3 +1 +1 ?1 ?3 ?1 ?1 +1).

2-53 A CDMA receiver gets the following chips: (-1 +1 -3 +1 -1 -3 +1 +1). Assuming the chip sequences defined in Fig. 2-45(b), which stations transmitted, and which bits did each one send? 一个CDMA接收器得到了下面的时间片(-1+1-3+1-1-3+1+1)。假设时间片序列如图2.45b中所定义,请问那些移动站传输了数据?每个站发送了什么位? Just compute the four normalized inner products:此处答案中的~疑为-号之误?

(?1 +1 ?3 +1 ?1 ?3 +1 +1) d (?1 ?1 ?1 +1 +1 ?1 +1 +1)/8 = 1 (?1 +1 ?3 +1 ?1 ?3 +1 +1) d (?1 ?1 +1 ?1 +1 +1 +1 ?1)/8 = ?1 (?1 +1 ?3 +1 ?1 ?3 +1 +1) d (?1 +1 ?1 +1 +1 +1 ?1 ?1)/8 = 0 (?1 +1 ?3 +1 ?1 ?3 +1 +1) d (?1 +1 ?1 ?1 ?1 ?1 +1 ?1)/8 = 1

结果是 A 和 D 发送了1位, B发送了0位, C 没有发送。

第 9 页 共 40 页

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:

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