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Department of Mathematics and Computer Science Examination Computer Networks (2IC15) on Thursday, August 27 th 2009, 9.00h-12.00h. First read the entire examination. There are 8 exercises in total. Grades are included between parentheses at all parts and sum up to 11 points. Good luck! 1. (1.0 pt) Bit Error Detection - Cyclic Redundancy Check and checksum are two well known methods to detect bit errors that can occur in computer networks. Compare these two methods in terms of their error detection rates and speed. Answer Checksums have a greater probability of undetected errors than do CRCs. That is, CRCs are better at detecting errors and will result in less undetected errors than checksums. CRCs can easily be computed in hardware, but not very easily in software. Checksums can be computed in software much faster than CRCs. 2. (1.0 pt) Analog vs Digital - Assume that Bob would like to send a signal to Alice over an intercontinental distance. The goal is that the received signal is as close as possible to the original transmitted signal. Which type of signal is more suitable: a digital or an analog signal? Justify your answer. HINT: Repeaters are needed in both cases. Answer In digital transmission, the noise component in the signal is removed at each repeater when the digital signal is regenerated at a higher amplitude. For analog trans- mission, the noise component is simply amplified at each repeater and is thus additive between repeaters for the length of the transmission link. 3. Router Queueing Delay - Consider the queuing delay in a router buffer with infinite size. Assume that each packet consists of L bits. Let R denote the rate at which packets are pushed out of the queue (bits/sec). (a) (0.5 pt) Suppose that the packets arrive periodically every L/R seconds. What is the average router queueing delay? Answer Zero. (b) (0.5 pt) Suppose that N packets arrive simultaneously every ( L/R ) N seconds. What is the average router queueing delay? Answer L R × ( N - 1) 2 seconds. 4. Transmission Control Protocol (TCP)- Consider the following plot of TCP window size as a function of time. Assuming TCP Reno is the protocol experiencing the behavior shown above, answer the following questions. In all cases, you should provide a short discussion justifying your answer. (a) (0.2 pt) Identify the intervals of time when TCP slow start is operating.
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