课程名称︰计算机网路 课程性质︰系必修 课程教师︰逄爱君 开课学院：电资学院 开课系所︰资工系 考试日期（年月日）︰2010.4.21 考试时限（分钟）：180 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : 1.(6%)Why is it said that pachet switching emplots statistical multiplexing? Constrast statistical multiplexing with the multiplexing that takes places in TDM. Ans: (a)In a packet switched network, the packets from different sources flowing on a link do not follow any fixed, pre-defined pattern. (b)In TDM circuit seitching, each host gets the same slot in a revolving TDM frame. Statistical multiplexing ensures that slots will not be wasted (whereas TDM can waste slots). 2.(8%)Consider a 1Gbps link. Users are generationg data at a rate of 100kbps when busy, but are busy generation data only with probability p=0.1. a.What is N, the maximum number of users that can be supported simultaneously under circuit switching? b.Now consider packet switching and a user population of M users. Give a formula (in terms of p, M, N) for the probability that more than N users are sending data. 3.(8%)Consider the car-caravan analogy shown in Figure 1. Assume a propagation speed of 50 km/hour, and a toll booth takes 12 seconds to service a car. a.Suppose the caravan travels 200km, beginning in front of one tollbooth, passing through a second tollbooth, and finishing just before a third tollbooth. What is the end-to end delay? b.Repeat (a), now assuming that there are five cars in the caravan instead of ten 4.(12%)In modern packet-switched networks, the source host segments long, application-layer messages (for example, an image or a music file) into smaller packets and sends the packets into the network. The receiver then reassembles the packets back into the original message. We refer to this process as message segmentation. Figure 2 illustrates the end-to-end transport of a message with and without message segmentation. Consider a message that is 8*10^6 bits long that is to be sent form source to destination. Suppose each link in the figure is 2Mbps. Ignore propagation, queuing, and processing delays. a.Consider sending the message from source to destination without message segmentation. How long does it take to move the message from the source host to destination host? b.Now suppose that the message is segmented into 4,000 packets, with each packet being 2,000 bits long. Compare this result with your answer in part (a). c.Discuss the drawbacks of message segmentation. i.Pachets have to be put in sequence at the destination. ii.Message segmentation results in many smaller packets. Since header size is usually the same for all packets regardless of their size, with message segmentation the total amount of header bytes is more. 5.(6%)Layering of Internet a.Why do we need the layering concept for Internet? b.Please claborate on the pros and cons of the Layering. 6.(6%)Why do HTTP, FTP, SMTP, and POP3 run on top of TCP rather than on UDP? Ans: The applications associated with those protocols require that all application data be received in the correct order and without gaps. TCP provides this service whereas UDP does not. 7.(10%)Suppose Alice, with a Web-based e-mail account (such as Hotmail or gmail), sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the nessage gets from Alice's host to Bob's host. Be sure to list the series of application-layer protocols that are used to move the message between the two hosts. 8.(16%)HTTP and DNS a.What is web caching? b.Why HTTP/1.1 uses persistent TCP connection for web accessing? c.What are Top-level domain(TLD) servers and Authoritative DNS servers? d.Bob access the Internet at home via a dynamic IP maintained by HiNet. If Bob would like to install a web server at home with the demain name "babhome.idv.tw". Please elaborate on how Bob register and activates the domain name. Note that the provider of "idv.tw" is "TWNIC", and the authoritative DNS server is "ns1.twnic.net.tw" 9.(8%)Consider a new peer Alice who joins BitTorrent without possessing any chunks. Without any chunks, she cannot become a top-four uploader for any of the other peers, since she has nothing to upload. How then will Alice get her first chunk? 10.(20%)Realiable Data Transfer a.Protocol rdt 2.1 provides reliable data transfer by using ACK/NAK and putting a sequence number into the transmitted data. Instead of using rdt 2.1, some of the students in the class proposed a new protocol to let the sequence numver "1" represent "New Packet" and the wequence number "0" represent "Retransmitted Packet". Please re-draw the state diagrams of senders and receivers. b.Is there any problem for the new protocol? Please elaborate on ti with a message flow. If so, does the situation also occur for rdy 2.1? Why of Why not? --

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