计算机网络第四版课后习题

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Chap.1

1. Imagine that you have trained your St. Bernard, Bernie, to carry a box of

three 8mm tapes instead of a flask of brandy. (When your disk fills up, you

consider that an emergency.) These tapes each contain 7 gigabytes. The dog

can travel to your side, wherever you may be, at 18 km/hour. For what range

of distances does Bernie have a higher data rate than a transmission line

client to the server). Give an example of a network that exhibits high

bandwidth and high latency. Then give an example of one with low bandwidth

and low latency.

4. Besides bandwidth and latency, what other parameter is needed to give a good

characterization of the quality of service offered by a network used for

digitized voice traffic?

5. A factor in the delay of a store-and-forward packet-switching system is how

long it takes to store and forward a packet through a switch. If switching

time is 10 µsec, is this likely to be a major factor in the response of a

client-server system where the client is in New York and the server is in

become possible. Ultimately, existing legislatures could be eliminated, to

let the will of the people be expressed directly. The positive aspects of

such a direct democracy are fairly obvious; discuss some of the negative

aspects.

8. A collection of five routers is to be connected in a point-to-point subnet.

Between each pair of routers, the designers may put a high-speed line, a

medium-speed line, a low-speed line, or no line. If it takes 100 ms of computer

time to generate and inspect each topology, how long will it take to inspect

all of them?

9. A group of 2

- 1 routers are interconnected in a centralized binary tree,

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hosts attempt to access the channel at the same time. As a simplistic example,

suppose that time is divided into discrete slots, with each of the n hosts

attempting to use the channel with probability p during each slot. What

fraction of the slots are wasted due to collisions?

11. What are two reasons for using layered protocols?

12. The president of the Specialty Paint Corp. gets the idea to work with a local

beer brewer to produce an invisible beer can (as an anti-litter measure).

The president tells her legal department to look into it, and they in turn

ask engineering for help. As a result, the chief engineer calls his

counterpart at the other company to discuss the technical aspects of the

14. Two networks each provide reliable connection-oriented service. One of them

offers a reliable byte stream and the other offers a reliable message stream.

Are these identical? If so, why is the distinction made? If not, give an

example of how they differ.

15. What does ''negotiation'' mean when discussing network protocols? Give an

example.

16. In Fig. 1-19, a service is shown. Are any other services implicit in this

figure? If so, where? If not, why not?

17. In some networks, the data link layer handles transmission errors by

requesting damaged frames to be retransmitted. If the probability of a

exchanged at the network level is called a packet, do frames encapsulate

packets or do packets encapsulate frames? Explain your answer.

20. A system has an 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?

21. 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.

22. What is the main difference between TCP and UDP?

23. The subnet of Fig. 1-25(b) was designed to withstand a nuclear war. How many

bombs would it take to partition the nodes into two disconnected sets? Assume

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really knows for sure, one estimate put the number of hosts on it at 100

million in 2001. Use these data to compute the expected number of Internet

hosts in the year 2010. Do you believe this? Explain why or why not.

25. When a file is transferred between two computers, two acknowledgement

strategies are possible. In the first one, the file is chopped up into packets,

which are individually acknowledged by the receiver, but the file transfer

as a whole is not acknowledged. In the second one, the packets are not

acknowledged individually, but the entire file is acknowledged when it

arrives. Discuss these two approaches.

26. Why does ATM use small, fixed-length cells?

Ethernet?

29. Ethernet and wireless networks have some similarities and some differences.

One property of Ethernet is that only one frame at a time can be transmitted

on an Ethernet. Does 802.11 share this property with Ethernet? Discuss your

answer.

30. Wireless networks are easy to install, which makes them inexpensive since

installation costs usually far overshadow equipment costs. Nevertheless,

they also have some disadvantages. Name two of them.

31. List two advantages and two disadvantages of having international standards

for network protocols.

33. Make a list of activities that you do every day in which computer networks

are used. How would your life be altered if these networks were suddenly

switched off?

34. Find out what networks are used at your school or place of work. Describe

the network types, topologies, and switching methods used there.

35. The ping program allows you to send a test packet to a given location and

see how long it takes to get there and back. Try using ping to see how long

it takes to get from your location to several known locations. From thes data,

plot the one-way transit time over the Internet as a function of distance.

It is best to use universities since the location of their servers is known

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Netherlands, www.usyd.edu.au is in Sydney, Australia, and www.uct.ac.za is

in Cape Town, South Africa.

36. Go to IETF's Web site, www.ietf.org, to see what they are doing. Pick a project

you like and write a half-page report on the problem and the proposed

solution.

37. Standardization is very important in the network world. ITU and ISO are the

main official standardization organizations. Go to their Web sites,

www.itu.org and www.iso.org, respectively, and learn about their

standardization work. Write a short report about the kinds of things they

have standardized.

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Chap.2

1. Compute the Fourier coefficients for the function f(t) = t (0 ≦ t ≦ 1).

2. A noiseless 4-kHz channel is sampled every 1 msec. What is the maximum data

rate?

3. Television channels are 6 MHz wide. How many bits/sec can be sent if

four-level digital signals are used? Assume a noiseless channel.

micron?

8. It is desired to send a sequence of computer screen images over an optical

fiber. The screen is 480 x 640 pixels, each pixel being 24 bits. There are

60 screen images per second. How much bandwidth is needed, and how many

microns of wavelength are needed for this band at 1.30 microns?

9. Is the Nyquist theorem true for optical fiber or only for copper wire?

10. In Fig. 2-6 the lefthand band is narrower than the others. Why?

11. Radio antennas often work best when the diameter of the antenna is equal to

the wavelength of the radio wave. Reasonable antennas range from 1 cm to 5

meters in diameter. What frequency range does this cover?

14. The 66 low-orbit satellites in the Iridium project are divided into six

necklaces around the earth. At the altitude they are using, the period is

90 minutes. What is the average interval for handoffs for a stationary

transmitter?

15. Consider a satellite at the altitude of geostationary satellites but whose

orbital plane is inclined to the equatorial plane by an angleφ. To a

stationary user on the earth's surface at north latitudeφ, does this

satellite appear motionless in the sky? If not, describe its motion.

16. How many end office codes were there pre-1984, when each end office was named

by its three-digit area code and the first three digits of the local number?