第一章
1.(Q1) W吉林雾凇图片hat is the difference between a host and an end system? List the types of end systems. Is a Web rver an end system?
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Answer: There is no difference. Throughout this text, the words “host” and “end system” are ud interchangeably. End systems include PCs, workstations, Web rvers, mail rvers, Internet-connected PDAs, WebTVs, etc.
2.(Q2) The word protocol鲁智深拳打镇关西 is often ud to describe diplomatic relations. Give an example of a diplomatic protocol.
Answer: Suppo Alice, an ambassador of country A wants to invite Bob, an ambassador of country B, over for dinner. Alice doesn’t simply just 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 he’s not available that particular date, but he is available another date. Alice and Bob continue to nd “messages” back and forth until they agree on a date and ti
me. 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 excus.
3.(Q3) What is a client program? What is a rver program? Does a rver program request and receive rvices from a client program?
Answer: 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 and receives rvices from the rver 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: residential; 4. 100 Mbps switched Etherent: c
ompany; 5. Wireless LAN: mobile; 6. Cellular mobile access (for example, 3G/4G): mobile
一千米等于多少厘米5.(Q5) List the available residential access technologies in your city. For each type of access, provide the advertid downstream rate, upstream rate, and monthly price.
Answer: 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 over?
Answer: Ethernet most commonly runs over twisted-pair copper wire and “thin” coaxial cable. It also can run over fibers optic links and thick coaxial cable.
7.(Q8)交尾群 Dial-up modems, HFC, and DSL are all ud for residential access. For each of the access technologies, provide a range of transmission rates and comment
on whether the transmission rate is shared or 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 few 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 fixed, pre-defined pattern. In TDM circuit switching, each host gets the same slot in a revolving TDM frame.
9.(Q14) Suppo urs share a 2Mbps link. Also suppo each ur requires 1Mbps when transmitting, but each ur transmits only 20 percent of the time. (See the discussion of statistical multiplexing in Section 1.3.)
a.When circuit switching is ud, how many urs can be supported?
b.植物传播种子的方法For the remainder of this problem, suppo packet switching is ud. Why will there be esntially no queuing delay before the link if two or fewer urs transmit at the same time? Why will there be a queuing delay if three urs transmit at the same time?
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c.Find the probability that a given ur is transmitting.
d.Suppo now there are three urs. Find the probability that at any given time, all three urs are transmitting simultaneously. Find the fraction of time during which the queue grows.
Answer:
a.2 urs can be supported becau each ur requires half of the link bandwidth.
b.Since each ur requires 1Mbps when transmitting, if two or fewer urs transmit simult
aneously, a maximum of 2Mbps will be required. Since the available bandwidth of the shared link is 2Mbps, there will be no queuing delay before the link. Whereas, if three urs transmit simultaneously, the bandwidth required will be 3Mbps which is more than the available bandwidth of the shared link. In this ca, there will be queuing delay before the link.
右的组词
c.Probability that a given ur is transmitting = 0.2
