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Unformatted text preview: Mid-Term Exam for Computer Networks Fall 2009 >>> SOLUTIONS <<< Welcome to the Mid-Term Exam for Computer Networks . Read each problem carefully. There are eight required problems (each worth 12 points – you get 4 points for correctly following these instructions). There is also an additional extra credit question worth 10 points. You may have with you a calculator, pencils and/or pens, erasers, blank paper, and one 8.5 x 11 inch “formula sheet”. On this formula sheet you may have anything you want (definitions, formulas, homework answers, old exam answers, etc.) as handwritten by you in pencil or ink on both sides of the sheet. Photocopies, scans, or computer generated and/or printed text are not allowed on this sheet. Note to tablet PC users – you may not print-out your handwritten text for the formula sheet. You have 75 minutes for the exam. Please use a separate sheet of paper for the answer to each question . Good luck and be sure to show your work! Problem #1 Answer the following questions regarding the basics of computer networks and the Internet. a) What are the fundamental measures of a communications system? 0.25 each and round-up (2 pts total) Throughput, delay, loss, cost, mobility, robustness, and secrecy b) What are the basics tasks of a communications system? 0.25 each and round-up (2 pts total) Message formatting, error detection and recovery, addressing, routing, flow control, system management, security, and QoS c) Consider the definitions of protocol and interface. Identify what is the same and what is different in a protocol and interface. 1 each for same and difference (3 pts total) Both are a complete set of rules for information exchange (same). A protocol is between same level layers , an interface is between adjacent layers (different). A protocol is between sites , an interface is within a single site (different). d) Name the major components that typically constitute an end node. Do the same for an intermediate node. Given an example of an end node and of an intermediate node. 0.25 each and round-up – identifying adapters is critical (2 pts total) An end node will have a bus , CPU , memory , I/O , and an adapter . An intermediate node will have a bus , CPU , memory , and more than one adapter . A PC (or iPhone or etc.) is an example of an end node. A router (or switch or access point) is an example of an intermediate node. e) Sketch the OSI model. Describe the purpose of each layer in one sentence or less. 0.5 each and round-up (3 pts total) +------------------+ | Application | 7 – Provides access to applications +------------------+ | Presentation | 6 – Provides data independence +------------------+ | Session | 5 – Manages end-to-end connections +------------------+ | Transport | 4 – Provides reliable end-to-end data transfer +------------------+ | Network | 3 – Maintains point-to-point connections +------------------+ | Data link | 2 – Provides reliable error-free point-to-point data transfer +------------------+...
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