program程序语言写作、辅导Python设计编程

” program程序语言写作、辅导Python设计编程Friday 20 December 201913.00 – 14.30(90 minutes)DEGREES of MSci, MEng, BEng, BSc, MA and MA (Social Sciences)NETWORKS AND OPERATING SYSTEMSESSENTIALS 2Answer all 2 questionsThis examination paper is worth a total of 60 marks.The use of calculators is not permitted in this examination.INSTRUCTIONS TO INVIGILATORS: Please collect all examquestion papers and exam answer scripts and retain for schoolto collect. Candidates must not remove exam question papers.1. Networked Systems(a) Describe What is meant by a best-effort service and explain why the Internet offers such aservice. [4](b) The Open Systems Interconnection (OSI) reference model provides a standard layeredsystem model. Name the seven layers used in the OSI reference model, in order from lowest(closest to hardware) to highest (closest to the user), and explain what this layering meansfor the implementers of each layer. [6](c) Consider the network graph pictured below. Nodes represent devices on the network, edgesrepresent links and the numbers by the edges represent the cost of forwarding a messageacross that link. Assume that the network is using a Distance Vector protocol, that allmessage exchanges happen at the beginning of every round of the protocol, and that hostsupdate their state after theyve received all messages destined to them in the current round.Show the initial routing state of all nodes in the network, and their routing state after everyround of the protocol. In your answer please show the routing table entries (distance/nexthop) of all nodes in the network (i.e., a 5 x 5 matrix), for each iteration of the algorithm.Your answer should include 3 such matrices, including the one for the original state (i.e.,nodes only know of themselves and their 1-hop neighbours).(d) At the network layer, the Internet is a connectionless best-effort packet network. On theother hand, the traditional telephone network provides a reliable circuit switched service.Discuss what are the advantages and disadvantages of these two approaches to networkdesign. [6](e) You have written a Basic server program in the first assessed exercise for this course, usingthe Python programming language and the Berkeley Sockets API. List the basic stepsrequired to set up a server socket, receive a connection and serve it to completion. You canprovide either code or a description of the function of the respective code. [6](f) Explain What important problem is solved by public-key cryptography that is not solved bysymmetric cryptography. Briefly explain why and how public-key cryptography solves thisproblem. [4]1 CONTINUED OVERLEAF2. Operating Systems(a) Consider a cache with 3 slots and the following stream of requests:C, D, A, B, D, A, C, B, C, D, AGive the contents of the cache after each request and indicate cache misses, when the cacheis using (i) the LRU algorithm (5 marks) and (ii) the LFU algorithm (5 marks). Show yourwork (i.e., recentness order for LRU, frequency of access for LFU). [10](b) Consider the following set of processes:Process ID Arrival Time Burst Time PriorityShow the Scheduling order and execution times of individual process, and compute theturnaround time of each process and the average waiting time over all processes, for each ofthe following scheduling algorithms: FCFS, SJF, non-preemptive priority (a smaller prioritynumber implies a higher priority), and pre-emptive RR (quantum = 3). [16]2 END OF QUESTION PAPER如有需要，请加QQ：99515681 或邮箱：99515681@qq.com

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