CS 218 - Sample Midterm. Answers due Feb 28 1. (10) How does slow frequency hopping improve the performance of a GSM system? 2. (20) Differentiated QoS multicast A source transmits a file to a multicast group using a shared tree approach (eg, sparse mode PIM). Suppose that 10 of the nodes can receive the file @ 10MBPS, and the other 10 @ 1MBPS. You want to minimize average time to complete the file transfer (averaged over all destinations), subject to the obvious trunk capacity constraints. (a) Define the optimal transmission policy (b) How can the source get the information necessary to implement the optimal policy? Assume OSPF (ie, link state) routing. (c) Can you generalize the optimal policy to arbitray number of nodes with arbitrary receive rate values? 3.(20)(a) Explain how deflection routing prevents switch buffer overflow if switches have same number of inputs as outputs, no matter how high the offered load is. (b) Next, explain how Myrinet avoids buffer overflow at heavy load. (c) How do Myrinets and deflection routing nets (eg, Manhattan network) avoid deadlocks? 4.(20) Why is deflection routing more appropriate than wormhole routing in a multihop optical network with "all-optical" switching (no electronics)? 5. (20) Why is it more difficult to implement "fair queueing" in a broadcast channel (such as a wireless channel) than in a wired channel? 6. (10) Mention pro's and con's of on-demand routing versus periodic table update routing in a wireless networks