计算机网络系统方法（英文版）课后习题及解答

第一章

(1.2 1.3节)

5.Calculate the total time required to transfer a 1,000-KB ?le in the following cases, assuming an RTT of 100 ms, a packet size of 1-KB data, and an initial 2 × RTT of “handshaking” before data is sent.

(a) The bandwidth is 1.5 Mbps, and data packets can be sent continuously.

(b) The bandwidth is 1.5 Mbps, but after we ?nish sending each data packet we must wait one RTT before sending the next.

(c) The bandwidth is “in?nite,”meaning that we take transmit time to be zero, and up to 20 packets can be sent per RTT.

(d) The bandwidth is in?nite, and during the ?rst RTT we can send one packet (21?1), during the second RTT we can send two packets (22?1), during the third we can send four (23?1), and so on. (A justi?cation for such an exponential increase will be given in Chapter 6.)

7. Consider a point-to-point link 2 km in length. At what bandwidth would propagation delay (at a speed of 2 × 108m/sec) equal transmit delay for 100-byte packets? What about 512-byte packets?

13.How “wide” is a bit on a 1-Gbps link? How long is a bit in copper wire, where the speed of propagation is 2.3 × 108 m/s?

15.Suppose a 100-Mbps point-to-point link is being set up between Earth and a new lunar colony. The distance from the moon to Earth is approximately 385,000 km, and data travels over the link at the speed of light—3 × 108 m/s.

(a) Calculate the minimum RTT for the link.

(b) Using the RTT as the delay, calculate the delay × bandwidth product for the link.

(c) What is the signi?cance of he delay × bandwidth product computed in (b)?

(d) A camera on the lunar base takes pictures of Earth and saves them in digital format to disk. Suppose Mission Control on Earth wishes to download the most current image, which is 25 MB. What is the minimum amount of time that will elapse between when the request for the data goes out and the transfer is ?nished?

18. Calculate the latency (from ?rst bit sent to last bit received) for the following:

(a) A 10-Mbps Ethernet with a single store-and-forward switch in the path, and a packet size of 5,000 bits. Assume that each link introduces a propaga- tion delay of 10 μs, and that the switch begins retransmitting immediately after it has ?nished receiving the packet.

(b) Same as (a) but with three switches.

(c) Same as (a) but assume the switch implements “cut-through” switching: it is able to begin retransmitting the packet after the ?rst 200 bits have been received.

第二章

（除2.7 2.9 节）

1.Show the NRZ, Manchester, and NRZI encodings for the bit pattern shown in Figure 2.46. Assume that the NRZI signal starts out low.

23.Consider an ARQ algorithm running over a 20-km point-to-point ?ber link.

(a) Compute the propagation delay for this link, assuming that the speed of light is 2 × 108 m/s in the ?ber.

(b) Suggest a suitable timeout value for the ARQ algorithm to use.

(c) Why might it still be possible for the ARQ algorithm to time out and

retransmit a frame, given this timeout value?

26.The text suggests that the sliding window protocol can be used to implement ?ow control. We can imagine doing this by having the receiver delay ACKs, that is, not send the ACK until there is free buffer space to hold the next frame. In doing so, each ACK would simultaneously acknowledge the receipt of the last frame and tell the source that there is now free buffer space available to hold the next frame. Explain why implementing ?ow control in this way is not a good idea.

44.Let A and B be two stations attempting to transmit on an Ethernet. Each has steady queue of frames ready to send; A’s frames will be numbered A 1, A2 , and so on, and B’s similarly. Let T = 51.2 μs be the exponential backoff base unit. Suppose A and B simultaneously attempt to send frame 1, collide, and happen to choose backoff times of 0 × T and 1 × T, respectively, meaning A wins the race and transmits A 1 while B waits. At the end of this transmission, B will attempt to retransmit B1 while A will attempt to transmit A2 . These ?rst attempts will collide, but now A backs off for either 0 × T or 1 × T, while B backs off for time equal to one of 0 × T, . . . , 3 × T.

(a) Give the probability that A wins this second backoff race immediately after this ?rst collision , that is, A’s ?rst choice of backoff time k × 51.2 is less than B’s. (b) Suppose A wins this second backoff race. A transmits A 3 , and when it is ?nished, A and B collide again as A tries to transmit A4 and B tries once more to transmit B1. Give the probability that A wins this third backoff race immediately after the ?rst collision.

(c) Give a reasonable lower bound for the probability that A wins all the re- maining backoff races.

(d) What then happens to the frame B1?

This scenario is known as the Ethernet capture effect.

48. Repeat the previous exercise, now with the assumption that Ethernet is p -persistent with p = 0.33 (that is, a waiting station transmits immediately with probability p when the line goes idle, and otherwise defers one 51.2-μs slot time and repeats the process). Your timeline should meet criterion (1) of the previous problem, but in lieu of criterion (2), you should show at least one collision and at least one run of four deferrals on an idle line. Again, note that many solutions are possible.

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