The University of Tennessee Department of Electrical and Computer Engineering
ECE 553: Computer Networks Spring 2007
Assignment Set #2
Problem 1 (20)
A 1 Mbyte file is to be transmitted over a 1 Mbps communication line that has a bit error rate (BER) of p=10-6. Answer the following questions: (a) What is the probability that the entire file is transmitted without errors? Note that for n large and p very small, (1 p)n enp . (b) Next assume that the file is partitioned into N equal-sized blocks that are transmitted separately. What is the probability that all the blocks arrive without error? Is dividing the file into blocks useful? Problem 2 (10)
Why are three ACKs required for the fast retransmit process in TCP?
Problem 3 (20)
Consider the Go-Back-N protocol with a sender window size of 3 and a sequence number range of 1024. Suppose that at time t, the next-in-order packet that the receiver is expecting has a sequence number of k. Assume that the medium does not reorder messages. Answer the following questions:
(a) What are the possible sets of sequence numbers inside the sender’s window at time t? Justify your answer. (b) What are all possible values of the ACK field in all possible messages propagating back to the sender at time t. Explain your answer. Problem 4 (20)
Answer true or false to the following questions and briefly justify your answer: (a) With the SR protocol, it is possible for the sender to receive an ACK for a packet that falls outside of its current window. (b) With GBN, it is possible for the sender to receive an ACK for a packet that falls outside of its current window.
1 (c) The alternating-bit protocol is the same as the SR protocol with a sender and receiver window of size 1. (d) The alternating-bit protocol is the same as the Go-Back-N protocol with a sender and receiver window size of 1.
Note: The alternating-bit protocol refers to the protocol described in lecture 9 (for channels with error and loss).
Problem 5 (10)
Suppose that frames are 1000 bits long and that ACK messages are 100 bits long. Calculate the efficiency of Stop-and-Wait ARQ in a system that transmits at R = 1 Mbps and with RTT = 10 ms for channels with a bit error rate (BER) of 10-6. Assume bit errors are independent. Note: Please refer to the supplementary handout on Performance of ARQ protocols.
Problem 6 (20) (a) Consider the case of RTT = 1 second and O = 100 kbytes. Prepare a chart that compares the minimum latency to the latency with slow start for R = 28 kbps, 100 kbps, 1 Mbps and 10 Mbps. (b) Suppose TCP increased its congestion window by two rather than by one for each received ACK during slow start. Thus, the first window consists of one segment, the second of three segments, the third of nine segments, and so on. Using the technique shown in class, express K in terms of O and S.
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