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1、第一章1. (Q1) What is the difference between a host and an end system List the types of end systems. Is a Web server an end systemAnswer: There is no difference. Throughout this text, the words “host” and “end system” are used interchangeably. End systems include PCs, workstations, Web servers, mail se
2、rvers, Internet-connected PDAs, WebTVs, etc.2. (Q2) The word protocol is often used to describe diplomatic relations. Give an example of a diplomatic protocol.Answer: Suppose Alice, an ambassador of country A wants to invite Bob, an ambassador of country B, over for dinner. Alice doesnt simply just
3、call Bob on the phone and say, come to our dinner table now”. Instead, she calls Bob and suggests a date and time. Bob may respond by saying hes not available that particular date, but he is available another date. Alice and Bob continue to send “messages” back and forth until they agree on a date a
4、nd time. Bob then shows up at the embassy on the agreed date, hopefully not more than 15 minutes before or after the agreed time. Diplomatic protocols also allow for either Alice or Bob to politely cancel the engagement if they have reasonable excuses.3. (Q3) What is a client program What is a serve
5、r program Does a server program request and receive services from a client programAnswer: A networking program usually has two programs, each running on a different host, communicating with each other. The program that initiates the communication is the client. Typically, the client program requests
6、 and receives services from the server program.4. (Q4) List six access technologies. Classify each one as residential access, company access, or mobile access.Answer: 1. Dial-up modem over telephone line: residential; 2. DSL over telephone line: residential or small office; 3. Cable to HFC: resident
7、ial; 4. 100 Mbps switched Etherent: company; 5. Wireless LAN: mobile; 6. Cellular mobile access (for example, 3G/4G): mobile5. (Q5) List the available residential access technologies in your city. For each type of access, provide the advertised downstream rate, upstream rate, and monthly price.Answe
8、r: Current possibilities include: dial-up (up to 56kbps); DSL (up to 1 Mbps upstream, up to 8 Mbps downstream); cable modem (up to 30Mbps downstream, 2 Mbps upstream.6. (Q7) What are some of the physical media that Ethernet can run overAnswer: Ethernet most commonly runs over twisted-pair copper wir
9、e and “thin” coaxial cable. It also can run over fibers optic links and thick coaxial cable.7. (Q8) Dial-up modems, HFC, and DSL are all used for residential access. For each of these access technologies, provide a range of transmission rates and comment on whether the transmission rate is shared or
10、 dedicated.Answer: Dial up modems: up to 56 Kbps, bandwidth is dedicated; ISDN: up to 128 kbps, bandwidth is dedicated; ADSL: downstream channel is .5-8 Mbps, upstream channel is up to 1 Mbps, bandwidth is dedicated; HFC, downstream channel is 10-30 Mbps and upstream channel is usually less than a f
11、ew Mbps, bandwidth is shared.8. (Q13) Why is it said that packet switching employs statistical multiplexing Contrast statistical multiplexing with the multiplexing that takes place in TDM.Answer: In a packet switched network, the packets from different sources flowing on a link do not follow any fix
12、ed, pre-defined pattern. In TDM circuit switching, each host gets the same slot in a revolving TDM frame.9. (Q14) Suppose users share a 2Mbps link. Also suppose each user requires 1Mbps when transmitting, but each user transmits only 20 percent of the time. (See the discussion of statistical multipl
13、exing in Section .)a. When circuit switching is used, how many users can be supportedb. For the remainder of this problem, suppose packet switching is used. Why will there be essentially no queuing delay before the link if two or fewer users transmit at the same time Why will there be a queuing dela
14、y if three users transmit at the same timec. Find the probability that a given user is transmitting.d. Suppose now there are three users. Find the probability that at any given time, all three users are transmitting simultaneously. Find the fraction of time during which the queue grows.Answer: a. 2
15、users can be supported because each user requires half of the link bandwidth.b. Since each user requires 1Mbps when transmitting, if two or fewer users transmit simultaneously, a maximum of 2Mbps will be required. Since the available bandwidth of the shared link is 2Mbps, there will be no queuing de
16、lay before the link. Whereas, if three users transmit simultaneously, the bandwidth required will be 3Mbps which is more than the available bandwidth of the shared link. In this case, there will be queuing delay before the link.c. Probability that a given user is transmitting = d. Probability that all three users are transmitting simultaneously. Since the queue grows when all the users are transmitting, the
