ECE/CSC 570 Section 001. Final test. December 12, 2007

Transcription

1 ECE/CSC 570 Section 00 Final test December 2, 2007 Questions 0 each carry 3 marks. Answer only by placing a check mark to indicate whether the statement is true of false in the appropriate box, and leaving the other box blank. Qustions 5 carry 4 marks each. Answer by circling a, b etc. (for each question, circle all the correct answers). Questions 6 8 carry 20 marks each. Answer each in the space provided, supplying adequate intermediate steps, calculation or reasoning you have to use. It should be possible to answer the questions in the space provided. Use the back of the paper or attach extra sheets only if you absolutely have to. You may consult any books, notes, or other printed or handwritten material that you bring to the test, but no other person. You must not use a cellphone, and if you are using a computer, calculator, PDA etc., you must demonstrate to the proctor s satisfaction that you are not using it for any communication. The only conditions under which you are allowed to access a computer network are to consult the lecture slides on the class website, or the electronic version of the textbook on the D. H. Hill library website. This is a three hour test. Maximum possible score is 00. First Name: Last Name: Student ID: (9 digit) Pledge of Honor: I pledge on my honor that I have answered the questions in this test purely on my own individual ability, and have neither given help to nor received help from another person. Signature:

2 . An IP forwarding layer could be stacked over an ATM forwarding layer. TRUE FALSE 2. For long-haul TCP connections, the transmission delay is likely to be negligible to the end-to-end propagation delay. 3. The FIB of a router contains the route /2. This is a valid route. 4. An iterative server is best suited for implementation as multiple concurrent processes. 5. Of any two Error Control Coding schemes, the one which employs a larger length of the error code will always be the stronger code. 6. In transferring a very large file over a packet-switched network, the size of the packet payload has no effect on the overall length of the time required for the transfer. 7. In a Markov Chain representing the slotted ALOHA medium, where the state is the number of backlogged stations, a transition from a state i to a state i+j may or may not be possible (non-zero probability), depending on i and j. 8. In a GBN system, simply providing the receiver with a buffer for out of order packets (without changing the sender s protocol) would not improve efficiency at all. 9. A MAN router serves outgoing traffic from ten 00 Mbps LANs which always sends traffic at maximum possible rate. The TDM ratio of LAN frames into MAN frames must be exactly 0: 0. At any time, if the TCP congestion window cwnd is larger than the receiver s window size, the cwnd overrides the receiver s window size.

3 . In a slotted ALOHA system with a very large number of users, it is observed that collisions occur in about 7% of the slots. The proportion of idle slots during the same time is approximately a. 28% b. 8% c. 8% d. Vanishingly small 2. The following four routes, all with the same next hop router, exist in an FIB: / / / /2 a. They can be aggregated into a single route b. They can be reduced to two routes by aggregation c. They can be reduced to three routes by aggregation d. They cannot be aggregated at all 3. In a shift register designed to produce a CRC code, the first few steps of operation produced the register states shown in the following table: Message Bit Fed R6 R5 R4 R3 R2 R R a. The generator polynomial is x 6 + x 2 + b. The generator polynomial is x 7 + x 4 + x 2 + c. The generator polynomial is x 7 + x 4 + d. The CRC, when finally produced, will be 8 bits long e. It is impossible to determine without knowing the length of the dataword 4. Dijkstra s algorithm is being used to compute the shortest paths from Node to every other node in a 0-node network. At the end of one iteration, the distances estimates are as shown below; the shaded distances have been made permanent. The last action was to make the distance to node 5 permanent. The outgoing links from Node 5 are: (i) To Node 6, cost 3, (ii) To Node 2, cost 4, (iii) To Node 9, cost 7. To To 2 To 3 To 4 To 5 To 6 To 7 To 8 To 9 To a. The distance estimate to Node 6 will be updated immediately next b. The distance estimate to Node 9 will be updated immediately next c. The next node to be made permanent will be 3 d. It is impossible to determine these with the amount of information given e. The situation shown above is impossible in correct operation of the algorithm 5. On a communication channel, the signal to noise power ratio is known, so is the width of the band available for communication and the number of discrete voltage symbols used by the endpoint communication equipments. In an effort to estimate an upper bound on available bitrate, the predictions from both Shannon s and Nyquist s theorems are calculated, and they return different estimates. a. The bitrate given by Shannon s theorem must be lower than Nyquist s b. Either may be the larger estimate c. This is impossible the two results must agree d. If the number of discrete symbols is known, Shannon s may be ignored

4 6. Consider a GBN ARQ system, with constant size frames. Each frame requires a time τ to transmit. The propagation delay is ρ in any one direction, ρ > τ. The GBN system uses an enhancement which uses two timeout, T fast and T > T fast, measured from the end of packet transmission. Both T fast and T are greater than 2τ + ρ. All times are given in units of seconds. The first time a new packet is transmitted, the timeout T fast is used to determine when the packet should be retransmitted. However, starting with this retransmission, the normal timeout T is used, no matter how many other retransmissions are required. Assume that ACKs require zero time to transmit, and are never lost. Each packet transmission succeeds with a probability of p. The sending DLC always has more frames to send. (a) What is the expected time required to transmit a given frame successfully, counting the time from the beginning of the first transmission attempt of that frame, and till the end of the last (successful) transmission of that frame? (b) What is the average throughput of the system, in frames per second?

5 7. In the network shown below, the routes from Node 4 to all other nodes are shown in bold lines each link of the network is bidirectional. Assume propagation delays along each link is the same. Assume that simultaneous reception cannot occur the receiving process at any node will arbitrarily sequentialize multiple packets that arrive at that node at nearly the same time (a) Show how a broadcast initiated by Node 4 proceeds by filling in the following table. (add as many rows as are required). Round Transmissions 4 3, 4 5 (b) How many extra transmissions does this method make over the minimum number which would be taken by an optimal system, such as a spanning tree broadcast?

6 8. Consider the following network topology. Each undirected arc in the graph below actually represents a pair of directed links, one in each direction along the arc, each with the cost shown. Traffic between the following node pairs must be routed (other node pairs do not have traffic demand between them): A F A D C D C F C G B D B E B G B F C 3 3 D F A B 3 E (a) Route all the traffic by shortest paths. Under this routing, identify the link(s) over which the maximum number of traffic demands are routed, and state what this number is. Similarly identify the link(s) over which the minimum traffic flows, and this minimum. (Shortest path determination can be done by inspection, or using some approach such as Dijkstra s algorithm; calculations leading to shortest path determination need not be shown, but show the shortest paths.) Remember A-B and B-A are two distinct links, etc. G (b) Determine an alternate routing for some or all of the traffic demands such that the maximum number of demands traversing any given link is lowered. Describe your methodology. State, if possible, some logical argument that leads you to think that this number cannot be lowered further.