1.第一章课后习题及答案

第一章1.(Q1) What is the difference between a host and an end system List the types ofend 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 inc lude PCs, workstations, Web servers, mail servers, Internet-connected PDAs, WebTVs, etc.2.(Q2) The word protocol is often used to describe diplomatic relations. Give anexample 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 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 send “messages” back and forth until they agree on a date and 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 server program Does a server programrequest 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 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: residential; 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 eachtype of access, provide the advertised 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 overAnswer: 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) Dial-up modems, HFC, and DSL are all used for residential access. For eachof these 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 multiplexingContrast 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) Suppose users share a 2Mbps link. Also suppose each user requires 1Mbpswhen transmitting, but each user transmits only 20 percent of the time. (See the discussion of statistical multiplexing 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. Whywill 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 delay 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 giventime, all three users are transmitting simultaneously. Find the fraction of time during which the queue grows.Answer:a. 2 users can be supported because each user requires half of the linkbandwidth.b.Since each user requires 1Mbps when transmitting, if two or fewer userstransmit simultaneously, 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 users transmit simultaneously, thebandwidth 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 transmittingsimultaneously. Since the queue grows when all the users are transmitting, the fraction of time during which the queue grows (which is equal to the probability that all three users are transmitting simultaneously) is .10.(Q16) Consider sending a packet from a source host to a destination host overa fixed route. List the delay components in the end-to-end delay. Which of thesedelays are constant and which are variableAnswer:The delay components are processing delays, transmission delays, propagation delays, and queuing delays. All of these delays are fixed, except for the queuing delays, which are variable.11.(Q19) Suppose Host A wants to send a large file to Host B. The path from HostA to HostB has three links, of rates R1 = 250 kbps, R2 = 500 kbps, and R3 = 1Mbps.a.Assuming no other traffic in the network, what is the throughput for thefile transfer.b.Suppose the file is 2 million bytes. Roughly, how long will it take totransfer the file to Host Bc.Repeat (a) and (b), but now with R2 reduced to 200 kbps.Answer:a.250 kbpsb.64 secondsc.200 kbps; 80 seconds12.(P2) Consider the circuit-switched network in Figure . Recall that there aren circuits on each link.a.What is the maximum number of simultaneous connections that can be inprogress at any one time in this networkb.Suppose that all connections are between the switch in the upper-left-handcorner and the switch in the lower-right-hand corner. What is the maximumnumber of simultaneous connections that can be in progressAnswer:a.We can n connections between each of the four pairs of adjacent switches.This gives a maximum of 4n connections.b.We can n connections passing through the switch in the upper-right-handcorner and another n connections passing through the switch in the lower-left-hand corner, giving a total of 2n connections.13.(P4) Review the car-caravan analogy in Section . Assume a propagation speed of50 km/hour.a.Suppose the caravan travels 150 km, beginning in front of one tollbooth,passing through a second tollbooth, and finishing just before a thirdtollbooth. What is the end-to-end delayb.Repeat (a), now assuming that there are five cars in the caravan insteadof ten.Answer: Tollbooths are 150 km apart, and the cars propagate at 50 km/hr, Atollbooth services a car at a rate of one car every 12 seconds.a.There are ten cars. It takes 120 seconds, or two minutes, for the firsttollbooth to service the 10 cars. Each of these cars has a propagation delay of 180 minutes before arriving at the second tollbooth. Thus, all the cars are lined up before the second tollbooth after 182 minutes. The whole process repeats itself for traveling between the second and third tollbooths. Thus the total delay is 364 minutes.b.Delay between tollbooths is 5*12 seconds plus 180 minutes, ., 181minutes.The total delay is twice this amount, ., 362 minutes.14.(P5) This elementary problem begins to explore propagation delay andtransmission delay, two central concepts in data networking. Consider two hosts,A and B, connected by a single link of rate R bps. Suppose that the two hostsare separated by m meters, and suppose the propagation speed along the link is s meters/sec. Host A is to send a packet of size L bits to Host B.a.Express the propagation delay, d , in terms of m and s.b.Determine the transmission time of the packet, d , in terms of L and R.c.Ignoring processing and queuing delays, obtain an expression for theend-to-end delay.d.Suppose Host A begins to transmit the packet at time t = 0. At time t =d , where is the last bit of the packete.Suppose d is greater than d . At time t = d , where is the first bit ofthe packetf.Suppose d is less than d . At time t = d , where is the first bit of thepacketg.Suppose s = *108, L = 100bits, and R = 28kbps. Find the distance m so thatd equals d .Answer:a. d = m/s seconds.b. d = L/R seconds.c. d = (m/s + L/R) seconds.d.The bit is just leaving Host A.e.The first bit is in the link and has not reached Host B.f.The first bit has reached Host B.g.Want15.(P6) In this problem we consider sending real-time voice from Host A to HostB over a packet-switched network (VoIP). Host A converts analog voice to adigital 64 kbps bit stream on the fly. Host A then groups the bits into 56-Byte packets. There is one link between Host A and B; its transmission rate is 500 kbps and its propagation delay is 2 msec. As soon as Host A gathers a packet, it sends it to Host B. As soon as Host B receives an entire packet, it converts the packet’s bits to an analog signal. How much time elapses from the time a bit is created (from the original analog signal at Host A) until the bit is decoded (as part of the analog signal at Host B)Answer: Consider the first bit in a packet. Before this bit can be transmitted, all of the bits in the packet must be generated. This requiresThe time required to transmit the packet isPropagation delay = 2 msec.The delay until decoding is7msec + 896μsec + 2msec = msecA similar analysis shows that all bits experience a delay of msec.16.(P9) Consider a packet of length L which begins at end system A, travels overone link to a packet switch, and travels from the packet switch over a second link to a destination end system. Let d i, s i, and R i denote the length, propagation speed, and the transmission rate of link i, for i= 1, 2. The packet switch delays each packet by d proc. Assuming no queuing delays, in terms of d i, s i, R i, (i = 1,2), and L, what is the total end-to-end delay for the packet Suppose now thepacket Length is 1,000 bytes, the propagation speed on both links is * 108m/s, the transmission rates of both links is 1 Mbps, the packet switch processing delay is 2 msec, the length of the first link is 6,000 km, and the length of the last link is 3,000 km. For these values, what is the end-to-end delayAnswer: The first end system requires L/R1to transmit the packet onto the first link; the packet propagates over the first link in d1/s1; the packet switch addsa processing delay of d proc; after receiving the entire packet, the packet switchrequires L/R2to transmit the packet onto the second link; the packet propagates over the second link in d2/s2. Adding these five delays givesd end-end = L/R1 + L/R2 + d1/s1 + d2/s2 + d procTo answer the second question, we simply plug the values into the equation to get 8 + 8 + 24 + 12 + 2 = 54 msec.17.(P10) In the above problem, suppose R1 = R2 = R and d proc= 0. Further suppose thepacket switch does not store-and-forward packets but instead immediately transmits each bit it receivers before waiting for the packet to arrive. What is the end-to-end delayAnswer: Because bits are immediately transmitted, the packet switch does not introduce any delay; in particular, it does not introduce a transmission delay.Thus,d end-end = L/R + d1/s1 + d2/s2For the values in Problem 9, we get 8 + 24 + 12 = 44 msec.18.(P11) Suppose N packets arrive simultaneously to a link at which no packets arecurrently being transmitted or queued. Each packet is of length L and the link has transmission rate R. What is the average queuing delay for the N packets Answer: The queuing delay is 0 for the first transmitted packet, L/R for the second transmitted packet, and generally, (n-1)L/R for the nth transmitted packet. Thus, the average delay for the N packets is(L/R + 2L/R + ....... + (N-1)L/R)/N = L/RN(1 + 2 + ..... + (N-1)) = LN(N-1)/(2RN) = (N-1)L/(2R)Note that here we used the well-known fact that1 +2 + ....... + N = N(N+1)/219.(P14) Consider the queuing delay in a router buffer. Let I denote trafficintensity; that is, I = La/R. Suppose that the queuing delay takes the form IL/R (1-I) for I<1.a.Provide a formula for the total delay, that is, the queuing delay plus thetransmission delay.b.Plot the total delay as a function of L/R.Answer:a.The transmission delay is L / R . The total delay isb.Let x = L / R.20.(P16) Perform a Traceroute between source and destination on the same continentat three different hours of the day.a.Find the average and standard deviation of the round-trip delays at eachof the three hours.b.Find the number of routers in the path at each of the three hours. Did thepaths change during any of the hoursc.Try to identify the number of ISP networks that the Traceroute packets passthrough from source to destination. Routers with similar names and/or similar IP addresses should be considered as part of the same ISP. In your experiments, do the largest delays occur at the peering interfaces between adjacent ISPsd.Repeat the above for a source and destination on different continents.Compare the intra-continent and inter-continent results.Answer: Experiments.21.(P18) Suppose two hosts, A and B, are separated by 10,000 kilometers and areconnected by a direct link of R = 2 Mbps. Suppose the propagation speed over the link is •108 meters/sec.a.Calculate the bandwidth-delay product, R •d prop.b.Consider sending a file of 400,000 bits from Host A to Host B. Suppose thefile is sent continuously as one large message. What is the maximum number of bits that will be in the link at any given timec.Provide an interpretation of the bandwidth-delay product.d.What is the width (in meters) of a bit in the link Is it longer than a footballfielde.Derive a general expression for the width of a bit in terms of the propagationspeed s, the transmission rate R, and the length of the link m.Answer:a.d prop = 107 / •108 = sec; so R •d prop = 80,000bitsb.80,000bitsc.The bandwidth-delay product of a link is the maximum number of bits thatcan be in the link.d. 1 bit is 125 meters long, which is longer than a football fielde.m / (R • d prop ) = m / (R * m / s) = s/R22.(P20) Consider problem P18 but now with a link of R = 1 Gbps.a.Calculate the bandwidth-delay product, R·d .b.Consider sending a file of 400,000 bits from Host A to Host B. Suppose thefile is sent continuously as one big message. What is the maximum numberof bits that will be in the link at any given timec.What is the width (in meters) of a bit in the linkAnswer:a.40,000,000 bits.b.400,000 bits.c. meters.23.(P21) Refer again to problem P18.a.How long does it take to send the file, assuming it is sent continuouslyb.Suppose now the file is broken up into 10 packet is acknowledged by thereceiver and the transmission time of an acknowledgment packet isnegligible. Finally, assume that the sender cannot send a packet until thepreceding one is acknowledged. How long does it take to send the filepare the results from (a) and (b).Answer:a. d + d = 200 msec + 40 msec = 240 msecb.10 * (t + 2 t ) = 10 * (20 msec + 80 msec) = sec。