计算机网络第四版答案潘爱民译

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1、第 1 章 概述1. 答：狗能携带21千兆字节或者168千兆位的数据。18 公里/小时的速度等于0.005 公里/秒，走过x公里的时间为x / 0.005 = 200x秒， 产生的数据传输速度为168/200x Gbps或者840 /x Mbps。因此，与通信线路相比较，若x 32/x，或f max =x/32。36. 答：10-9 的漂移意味着109 秒中的1 秒，或1 秒中的10-9 秒。对于OC-1 速率，即51.840Mbps，取近似值50Mbps，大约一位持续20ns。这就说明每隔20 秒，时钟就要偏离1位。这就说明，时钟必须每隔10 秒或更频繁地进行同步，才能保持不会偏离太大。37

2、. 答：基本的SONET 帧是美125产生810 字节。由于SONET 是同步的，因此不论是否有数据，帧都被发送出去。每秒8000 帧与数字电话系统中使用的PCM 信道的采样频率完全一样。810字节的SONET 帧通常用90列乘以9行的矩形来描述，每秒传送51.84Mbps，即8810800051840000bps。这就是基本的SONET 信道，它被称作同步传输信号STS-1，所有的SONET 干线都是由多条STS-1构成。每一帧的前3 列被留作系统管理信息使用，前3 行包含段开销，后6 行包含线路开销。剩下的87 列包含8798800050112000bps。被称作同步载荷信封的数据可以在任

3、何位置开始。线路开销的第一行包含指向第一字节的指针。同步载荷信封（SPE）的第一列是通路开销。通路开销不是严格的SONET 结构，它在嵌入在载荷信封中。通路开销端到端的流过网络，因此把它与端到端的运载用户信息的SPE 相关联是有意义的。然而，它确实从可提供给端点用户的50.112Mbps 中又减去1988000576000bps，即0.576Mbps，使之变成49.536Mbps 。OC-3相当于3个OC-1复用在一起，因此其用户数据传输速率是49.546 3 148.608 Mbps。38. VT1.5 can accommodate 8000 frames/sec 3 columns 9

4、rows 8 bits =1.728 Mbps. It can be used to accommodate DS-1. VT2 can accommodate 8000 frames/sec 4 columns 9 rows 8 bits = 2.304 Mbps. It can be used to accommodate European CEPT-1 service. VT6 can accommodate 8000 frames/sec 12 columns 9 rows 8 bits = 6.912 Mbps. It can be used to accommodate DS-2

5、service.39. Message switching sends data units that can be arbitrarily long. Packet switching has a maximum packet size. Any message longer than that is split up into multiple packets.40. 答：当一条线路（例如OC-3）没有被多路复用，而仅从一个源输入数据时，字母c（表示conactenation，即串联）被加到名字标识的后面，因此，OC-3 表示由3 条单独的OC-1 线路复用成155.52Mbps，而OC-

6、3c 表示来自单个源的155.52Mbps 的数据流。OC-3c 流中所包含的3 个OC-1 流按列交织编排，首先是流1 的第1 列，流2 的第1 列，流3 的第1 列，随后是流1 的第2 列，流2 的第2 列，以此类推，最后形成270 列宽9 行高的帧。OC-3c 流中的用户实际数据传输速率比OC-3 流的速率略高（149.760Mbps 和148.608Mbps），因为通路开销仅在SPE 中出现一次，而不是当使用3 条单独OC-1 流时出现的3 次。换句话说，OC-3c 中270 列中的260 列可用于用户数据，而在OC-3 中仅能使用258列。更高层次的串联帧（如OC-12c）也存在。O

7、C-12c 帧有12*90=1080 列和9 行。其中段开销和线路开销占12*3=36 列，这样同步载荷信封就有1080-36=1044 列。SPE 中仅1 列用于通路开销，结果就是1043 列用于用户数据。由于每列9 个字节，因此一个OC-12c 帧中用户数据比特数是8 9104375096。每秒8000 帧，得到用户数据速率750968000 =600768000bps，即600.768Mbps。 所以，在一条OC-12c 连接中可提供的用户带宽是600.768Mbps。41. 答：The three networks have the following properties: 星型：最

8、好为2，最差为2，平均为2；环型：最好为1，最差为n/2，平均为n/4如果考虑n 为奇偶数，则n 为奇数时，最坏为（n-1）/2，平均为（n+1）/4n 为偶数时，最坏为 n/2，平均为n2/4(n-1) 全连接：最好为1，最差为1，平均为1。42. 对于电路交换， t= s时电路建立起来；t s+ + x /d 时报文的最后一位发送完毕；t = s+ x/b+kd时报文到达目的地。而对于分组交换，最后一位在t=x/b 时发送完毕。为到达最终目的地，最后一个分组必须被中间的路由器重发k-1次，每次重发花时间p/ b，所以总的延迟为为了使分组交换比电路交换快，必须：所以：43. 答：所需要的分组

9、总数是x /p ，因此总的数据加上头信息交通量为(p+h)x/p位。源端发送这些位需要时间为(p+h )x / /pb中间的路由器重传最后一个分组所花的总时间为(k-1)(p +h )/ b 因此我们得到的总的延迟为对该函数求p 的导数，得到令得到因为 p0，所以故时能使总的延迟最小。44. Each cell has six neighbors. If the central cell uses frequency group A, its six neighbors can use B, C, B, C, B, and C respectively. In other words, onl

10、y 3 unique cells are needed. Consequently, each cell can have 280 frequencies.45. First, initial deployment simply placed cells in regions where there was high density of human or vehicle population. Once they were there, the operator often did not want to go to the trouble of moving them. Second, a

11、ntennas are typically placed on tall buildings or mountains. Depending on the exact location of such structures, the area covered by a cell may be irregular due to obstacles near the transmitter. Third, some communities or property owners do not allow building a tower at a location where the center

12、of a cell falls. In such cases, directional antennas are placed at a location not at the cell center.46. If we assume that each microcell is a circle 100 m in diameter, then each cell has an area of 2500. If we take the area of San Francisco, 1.2 108 m2 and divide it by the area of 1 microcell, we g

13、et 15,279 microcells. Of course, it is impossible to tile the plane with circles (and San Francisco is decidedly three-dimensional), but with 20,000 microcells we could probably do the job.47. Frequencies cannot be reused in adjacent cells, so when a user moves from one cell to another, a new freque

14、ncy must be allocated for the call. If a user moves into a cell, all of whose frequencies are currently in use, the users call must be terminated.48. It is not caused directly by the need for backward compatibility. The 30 kHz channel was indeed a requirement, but the designers of D-AMPS did not hav

15、e to stuff three users into it. They could have put two users in each channel, increasing the payload before error correction from 260 50= 13 kbps to 260 75 = 19.5 kbps. Thus, the quality loss was an intentional trade-off to put more users per cell and thus get away with bigger cells.49. D-AMPS uses

16、 832 channels (in each direction) with three users sharing a single channel. This allows D-AMPS to support up to 2496 users simultaneously per cell. GSM uses 124 channels with eight users sharing a single channel. This allows GSM to support up to 992 users simultaneously. Both systems use about the

17、same amount of spectrum (25 MHz in each direction).D-AMPS uses 30 KHz 892 = 26.76 MHz. GSM uses 200 KHz 124 =24.80 MHz. The difference can be mainly attributed to the better speech quality provided by GSM (13 Kbps per user) over D-AMPS (8 Kbps per user).50. The result is obtained by negating each of

18、 A, B, and C and then adding the three chip sequences. Alternatively the three can be added and then negated. The result is (+3 +1 +1 .1 .3 .1 .1 +1).51. By definitionIf T sends a 0 bit instead of 1 bit, its chip sequence is negated, with the i-th element becoming .Ti . Thus,52. When two elements ma

19、tch, their product is +1. When they do not match, their product is .1. To make the sum 0, there must be as many matches as mismatches. Thus, two chip sequences are orthogonal if exactly half of the corresponding elements match and exactly half do not match.53. Just compute the four normalized inner

20、products:(.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 = 1The result is that A and D sent 1 bits, B sent a 0 bi

21、t, and C was silent.54. 答：可以，每部电话都能够有自己到达端局的线路，但每路光纤都可以连接许多部电话。忽略语音压缩，一部数字PCM电话需要64kbps 的带宽。如果以64kbps 为单元来分割10Gbps，我们得到每路光缆串行156250 家。现今的有线电视系统每根电缆串行数百家。55. 答：它既像TDM，也像FDM。100 个频道中的每一个都分配有自己的频带（FDM），在每个频道上又都有两个逻辑流通过TDM 交织播放（节目和广告交替使用频道）。This example is the same as the AM radio example given in the t

22、ext, but neither is a fantastic example of TDM because the alternation is irregular. 56. A 2-Mbps downstream bandwidth guarantee to each house implies at most 50 houses per coaxial cable. Thus, the cable company will need to split up the existing cable into 100 coaxial cables and connect each of the

23、m directly to a fiber node.57. The upstream bandwidth is 37 MHz. Using QPSK with 2 bits/Hz, we get 74 Mbps upstream. Downstream we have 200 MHz. Using QAM-64, this is 1200 Mbps. Using QAM-256, this is 1600 Mbps.58. Even if the downstream channel works at 27 Mbps, the user interface is nearly always

24、10-Mbps Ethernet. There is no way to get bits to the computer any faster than 10-Mbps under these circumstances. If the connection between the PC and cable modem is fast Ethernet, then the full 27 Mbps may be available. Usually, cable operators specify 10 Mbps Ethernet because they do not want one u

25、ser sucking up the entire bandwidth.第 3 章 数据链路层1. 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：2. The solution is(a) 00000100 01000111 11100011 11100000 01111110(b) 01111110 01000111 11100011 11100000 111000

26、00 11100000 0111111001111110(c) 01111110 01000111 110100011 111000000 011111010 011111103. After stuffing, we get A B ESC ESC C ESC ESC ESC FLAG ESC FLAG D. 4. If you could always count on an endless stream of frames, one flag byte might be enough. But what if a frame ends (with a flag byte) and the

27、re are no new frames for 15 minutes. How will the receiver know that the next byte is actually the start of a new frame and not just noise on the line? The protocol is much simpler with starting and ending flag bytes.5. 6. 答：可能。假定原来的正文包含位序列01111110 作为数据。位填充之后，这个序列将变成01111010。如果由于传输错误第二个0 丢失了，收到的位串又变

28、成01111110，被接收方看成是帧尾。然后接收方在该串的前面寻找检验和，并对它进行验证。如果检验和是16 位，那么被错误的看成是检验和的16 位的内容碰巧经验证后仍然正确的概率是1/216。如果这种概率的条件成立了，就会导致不正确的帧被接收。显然，检验和段越长，传输错误不被发现的概率会越低，但该概率永远不等于零。7. 答：如果传播延迟很长，例如在探测火星或金星的情况下，需要采用前向纠错的方法。还有在某些军事环境中，接收方不想暴露自己的地理位置，所以不宜发送反馈信号。如果错误率足够的低，纠错码的冗余位串不是很长，又能够纠正所有的错误，前向纠错协议也可能是比较合理和简单的。8. Making o

29、ne change to any valid character cannot generate another valid character due to the nature of parity bits. Making two changes to even bits or two changes to odd bits will give another valid character, 所以Hamming 距离为29. Parity bits are needed at positions 1, 2, 4, 8, and 16, so messages that do not ex

30、tend beyond bit 31 (including the parity bits) fit. Thus, five parity bits are 10. 11. If we number the bits from left to right starting at bit 1, in this example, bit 2 (a parity bit) is incorrect. The 12-bit value transmitted (after Hamming encoding) was 0xA4F. The original 8-bit data value was 0x

31、AF.12. 答：单个错误将引起水平和垂直奇偶检查都出错。两个错误，无论是否同行或者同列，也容易被检测到。对于有三位错误的情况，就有可能无法检测了。for example, if some bit is inverted along with its row and column parity bits. Even the corner bit will not catch this.13. 答：用n 行k 列的矩阵来描述错误图案，在该矩阵中，正确的位用0 表示，不正确的位用1 表示。由于总共有4 位传输错误，每个可能的错误矩阵中都恰有4 个1。则错误矩阵的个数总共有C nk4个。而在错误矩阵中，当4 个1 正好构成一个矩形的4 个顶点的时候，这样的错误是检测不出来的。则检测不出来的错误矩阵