課程名稱：計算機網路 課程性質：系必修 課程教師：逄愛君 開課學院：電資學院 開課系所：資工系 考試日期(年月日)：2011.4.20 考試時限(分鐘)：180 是否需發放獎勵金：是 （如未明確表示，則不予發放） 試題 : 1.(5%) Suppose there iis exactly one packet switch between a sending host and a receiving host. The transmission rates between the sending host and the switch, and between the switch and the receiving host are R2 and R2, respectively. Assuming that the switch uses store-and-forward packet switching, what is the total end-to-end delay to send a packet of length L? (Ignore queuing, propagaation, and processing delay.) 2.(15%) Circuit switching vs. packet switching (a) (5%) What advantage does a circuit-switched network have over a packet-switched network? (b) (10%) Given a 1Gbps links, users are generating data at a rate of 100kbps when busy, but are busy generating data only with probability p = 0.1. 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.(Alteration of P8, from pp.97 of the 4th edition textbook) 3.(5%) Consider the throughput example corresponding to the following figure. Suppose that there are M client-server pairs. Denote Rs, Rc, and R for the rates of the server links, client links, and network link. Assume all other links have abundant capacity and that there is no other traffic in the network besides the traffic generated by the M client -server pairs. Derive a general expression for throughput in terms of Rs, Rc, R, and M. (P17, from pp.99 of the 4th edition textbook) 4.(5%) If you would like to access the Internet at home, what factors do you consider for your access network and physical media? 5.(5%) Please give an example(with host, switch, and router) to describe the concept of encapsulation. 6.(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 message 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. 7.(10%) By using the following example (Figure 2.13 on pp.138 of the 4th edition textbook), please describe how Web caching can significantly reduce the delay in receiving a requested object with low-cost installation. 8.(10%) Consider distributing a file of F = 20 Gbits to N peers. The server has an upload rate of us = 50 Mpbs, and each peer has download rate of di = 1 Mbps and an upload rate of u. For N = 10, 100, and 1,000 and u = 100Kbps, 500Kbps, and 1 Mbps, prepare a chart giving the minimum distribution time for each of the combinations of N annd u for both client-server distribution and P2P distribution. 9.(10%) Is it possible for an organization's Web server and mail server to have exactly the same alias for a hostname(for example, foo.com)? Why or why not? What would be the type for the RR that contaions the hostname of the mail server?(R18, from pp.2995 of the 4th edition text -book) 10.(25%) Reliable Data Transfer (a)(5%) Why the rdt protocols need to introduce sequence numbers and t -imers? (b)(20%) Consider a scenario in which Host A and Host B want to send m -essages to Host C. Host A and C are connected by a channell that can lose and corrupt(but not reorder) messages. Host B and C are connected by another channel(independent of the channel connecting A and C) with the same properties. The transport layer at Host C should alternate in delivering missages from A and B to the layer above(that is, it should first deliver the data from packet from A, then the data from a packet from B, and so on)/ Design a stop-and-wait-link error-control protoco -l for reliably transferring packets from A and B to C, with alternati -ng delivery at C as described above. Give FSM descriptions of A and C . (Hint：The FSM for B should be essentially the same as for A). Also, give a description of the packet format(s) used -- 解方塊不需要思考 但是思考會讓解方塊更有意義 by aegius1r --

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