计算机网络第四版课后英文题目(7)

30.In Fig. 7-25, www.aportal.com keeps track of user preferences in a cookie. A disadvantage of this scheme is that cookies are limited to 4 KB, so if the preferences are extensive, for example, many stocks, sports teams, types of news stories, weather for multiple cities, specials in numerous product categories, and more, the 4-KB limit may be reached. Design an alternative way to keep track of preferences that does not have this problem. 31.Sloth Bank wants to make on-line banking easy for its lazy customers, so after a customer signs up and is authenticated by a password, the bank returns a cookie containing a customer ID number. In this way, the customer does not have to identify himself or type a password on future visits to the on-line bank. What do you think of this idea? Will it work? Is it a good idea?

32.In Fig. 7-26, the ALT parameter is set in the tag. Under what conditions does the browser use it, and how?

33.How do you make an image clickable in HTML? Give an example. 34.Show the tag that is needed to make the string ''ACM'' be a hyperlink to http://www.acm.org. 35.Design a form for a new company, Interburger, that allows hamburgers to be ordered via the Internet. The form should include the customer's name, address, and city, as well as a choice of size (either gigantic or immense) and a cheese option. The burgers are to be paid for in cash upon delivery, so no credit card information is needed.

36.Design a form that requests the user to type in two numbers. When the user clicks on the submit button, the server returns their sum. Write the server side as a PHP script.

37.For each of the following applications, tell whether it would be (1) possible and (2) better to use a PHP script or JavaScript and why.

a. (a) Displaying a calendar for any requested month since

September 1752.

b. (b) Displaying the schedule of flights from Amsterdam to New

York.

c. (c) Graphing a polynomial from user-supplied coefficients

38.Write a program in JavaScript that accepts an integer greater than 2 and tells whether it is a prime number. Note that JavaScript has if and while statements with the same syntax as C and Java. The modulo operator is %. If you need the square root of x, use Math.sqrt (x).

39.An HTML page is as follows:

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If the user clicks on the hyperlink, a TCP connection is opened and a series of lines is sent to the server. List all the lines sent. 40.The If-Modified-Since header can be used to check whether a cached page is still valid. Requests can be made for pages containing images, sound, video, and so on, as well as HTML. Do you think the effectiveness of this technique is better or worse for JPEG images as compared to HTML? Think carefully about what ''effectiveness'' means and explain your answer.

41.On the day of a major sporting event, such as the championship game in some popular sport, many people go to the official Web site. Is this a flash crowd in the same sense as the Florida election in 2000? Why or why not?

42.Does it make sense for a single ISP to function as a CDN? If so, how would that work? If not, what is wrong with the idea? 43.Under what conditions is using a CDN a bad idea?

44.Wireless Web terminals have low bandwidth, which makes efficient coding important. Devise a scheme for transmitting English text efficiently over a wireless link to a WAP device. You may assume that the terminal has several megabytes of ROM and a moderately powerful CPU. Hint: think about how you transmit Japanese, in which each symbol is a word.

45.A compact disc holds 650 MB of data. Is compression used for audio CDs? Explain your reasoning.

46.In Fig. 7-57(c) quantization noise occurs due to the use of 4-bit samples to represent nine signal values. The first sample, at 0, is exact, but the next few are not. What is the percent error for the samples at 1/32, 2/32, and 3/32 of the period?

47.Could a psychoacoustic model be used to reduce the bandwidth needed for Internet telephony? If so, what conditions, if any, would have to be met to make it work? If not, why not?

48.An audio streaming server has a one-way distance of 50 msec with a media player. It outputs at 1 Mbps. If the media player has a 1-MB buffer, what can you say about the position of the low-water mark and the high-water mark?

49.The interleaving algorithm of Fig. 7-60 has the advantage of being able to survive an occasional lost packet without introducing a gap in the playback. However, when used for Internet telephony, it also has a small disadvantage. What is it?

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50.Does voice over IP have the same problems with firewalls that streaming audio does? Discuss your answer.

51.What is the bit rate for transmitting uncompressed 800 x 600 pixel color frames with 8 bits/pixel at 40 frames/sec?

52.Can a 1-bit error in an MPEG frame affect more than the frame in which the error occurs? Explain your answer.

53.Consider a 100,000-customer video server, where each customer watches two movies per month. Half the movies are served at 8 P.M. How many movies does the server have to transmit at once during this time period? If each movie requires 4 Mbps, how many OC-12 connections does the server need to the network?

54.Suppose that Zipf's law holds for accesses to a 10,000-movie video server. If the server holds the most popular 1000 movies on magnetic disk and the remaining 9000 on optical disk, give an expression for the fraction of all references that will be to magnetic disk. Write a little program to evaluate this expression numerically.

第八章

Problems

1. Break the following monoalphabetic cipher. The plaintext,

consisting of letters only, is a well-known excerpt from a poem by Lewis Carroll.

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