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(COMP362)362-2008fall-mid-sol.pdf
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COMP362 2008 Fall Midterm Exam - Suggested Solutions
1. In indirect routing approach, the correspondent simply addresses the datagram to the mobile nodes permanent address and sends the datagram into the network, blissfully unaware of whether the mobile node is resident in its home network or is visiting a foreign network; mobility is thus completely transparent to the correspondent. In direct routing approach, a correspondent agent in the correspondents network first learns the COA of the mobile node. This can be done by having the correspondent agent query the home agent, assuming that, the mobile node has an up-to-date value for its COA registered with home agent. The correspondent agent then tunnels datagrams directly to the mobile nodes COA, in a manner analogous to the tunneling performed by the home agent.



2. The output of sender1 is (1, 1, -1, -1, -1, 1, -1, -1, -1, -1, 1, 1, 1, -1, 1, -1) and the output of sender2 is (1, 1, 1, -1, 1, 1, 1, -1, 1, 1, 1, -1, 1, 1, 1, -1). The aggregated output is (2, 2, 0, -2, 0, 2, 0, 0, 0, 0, 2, 0, 2, 0, 2, -2). Applying the decode formula
, we can obtain the output of sender1 is (-1, 1) and sender2 is (1, 1).



3. Suppose station A and C are transmitting to station B. A and C can not hear each other's transmissions. However A's and C's transmissions are indeed interfering at the destination B. We can use the Request to Send (RTS) and Clear to Send (CTS) to mitigate this problem. When a sender wants to send a DATA frame, it can first send an RTS frame to the AP, indicating the total time required to transmit the DATA frame and the acknowledgement (ACK) frame. When the AP receives the RTS frame, it responds by broadcasting a CTS frame. This CTS frame serves two purposes: It gives the sender explicit permission to send and also instructs the other stations not to send for the reserved duration.




4. (a)
(b) Yes. Its unique.
For (c) and (d), note that only the edges in the tree can be used.
(c) The cost of transmission using IP multicast is 16+5+18=39.
(d) The cost of transmission using overlay multicast is the sum of the cost of two unicast, that is (AB+BD)+(AB+BD+DE)=16*2+5*2+18=60.



5. CSMA (Carrier Sense Multiple Access) means that each station senses the channel before transmitting and refrains from the transmitting when the channel is sensed busy.


CD, collision detection, is used to improve CSMA by terminating transmission as soon as a collision is detected, and reducing the probability of a second collision.
CA, collision avoidance, is used to improve CSMA by attempting to be less "greedy" on the channel. If the channel is sensed busy before transmission, the transmission is deferred for a "random" interval.
CSMA/CA is used in 802.11. The reason is
1) Its almost impossible to listen while sending in wireless LANs, therefore collision detection is infeasible.
2) Some collisions cannot be detected due to the hidden termi