CIS 427: Computer Networks and Distributed Processing Read all questions carefully and show all your work toward the answer. Make sure your answers are clear and easy to follow. There are eight questions. Q1) Why might a wireless mesh topology be superior to a base station topology for communications in a natural disaster? (5 points) Q2) Suppose the following sequence of bits arrives over a link: 011010111110101001111111011001111110 Show the resulting frame after any stuffed bits have been removed. (5 points) Consider this is the end of a sequence sent from sender to receiver, no more bits to be sent. Indicate any errors that might have been introduced into the frame. (5 points) Page 17 Q3) Suppose we want to transmit the message 11100011 and protect it from errors using the CRC polynomial x³ + 1. (a) Use polynomial long division to determine the message that should be transmitted. (5 points) (b) Suppose the leftmost bit of the message is inverted due to noise on the transmission link. What is the result of the receiver's CRC calculation? How does the receiver know that an error has occurred? (10 points) Page 27 Q4) Show the 4B/5B encoding and the resulting NRZI signal for the following bit sequence: 1110 0101 0000 0011. (5x2 points) Q5) Assuming a framing protocol that uses bit stuffing, show the bit sequence transmitted over the link when the frame contains the following bit sequence: 110101111101011111101011111110. Mark the stuffed bits. (5 points) Page 37 Q6) Consider an ARQ algorithm running over a 40-km point-to-point fiber link. (a) Compute the propagation delay for this link, assuming that the speed of light is 2×108 m/s in the fiber. (5 points) (b) Suggest a suitable timeout value for the ARQ algorithm to use. (5 points) (c) Why might it still be possible for the ARQ algorithm to time out and retransmit a frame, given this timeout value? (5 points) Page 417 Q7) Draw a timeline diagram for the sliding window algorithm with SWS = RWS = 3 frames, for the following two situations. Use a timeout interval of about 2×RTT. (a) Frame 4 is lost. (10 points) (b) Frames 4 to 6 are lost. (10 points) Page 57