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4th Nov, 2003
ELEC 315 Midterm Exam
Fall 2003
(1) Three departments computers are linked by Ethernet switches. Assume in the beginning all LAN switches switching tables are empty. (a) When computer A sends a packet to Computer C, how may copies will be sent by switch 2? (b) A while later, computer B sends a packets to computer C, how many copies will be generated by switch 1? (c) Still later computer C sends a packet to A, how many copies will be generated in switch 3? (Remember each LAN switch will use learning to reduce the amount of traffic generated in it). Explain your answers in detail.
(2) An Ethernet uses the CSMA/CD access protocol, and has the following parameters:
a. Cable: 1km
b. Signal speed: m/sec
c. Packet length: 500bits
d. Capacity: 3Mbps
(a) Use the simplified equation to compute the efficiency of the network. (b) If we increase the capacity to 10Mbs, What packet length should we use to keep the efficiency unchanged?
(3) Three packets A, B, C collide on the Ethernet and retransmissions will be generated based on the binary exponential back-off algorithm. For A and B, this is the first collision, for C the second collision. (a) What is the probability that the next transmission is still a collision? (b) What is the probability that the next transmission is from C? (this includes the cases where C transmits with another packet).
(4) A data link layer protocol is based on a window scheme. The sending window size is 4 and the receiver window is 4 as well. After initialization, the sending window is (0,1,2,3) and the receiving window is the same. (a) Assume the transmitter sends out 2 packets and receives acknowledgements for both of them. What is the current sending and receiving window after this event? (b) Consider a different scenario. Assume the transmitter sends out 2 packets (packet 0 and 1), and packet 0 is lost, but packet 1 has been received by the receiver. How many acknowledgements will be sent back by the receiver? (Assume accumulative ACK is used). Explain.
(5) The IP lookup table of a router is shown below.
a. 245.24.255.33/24 port 1
b. 245.24.248.67/23 port 2
c. 245.24.255.210/22 port 3
d. 245.24.128.128/23 port 4
(a) What is the lookup result for the IP address 245.24.251.34, and (b) 245.24.254.77
(6) A satellite channel is used to link two ground stations. It has the following parameters:
a. Signal delay: 270 ms (beam up and down)
b. Satellite link capacity: 2Mbps
c. Data packet length: 600 bits
d. Ack packet length: 200 bits
e. Packet processing time: 0.1 ms
(a) Suppose stop and wait is used. What is the efficiency of the channel? (b) Lets switch to go-back-n protocol. Assume we use 4 bits for the packet ID field. What is the maximum efficiency achieved for the satellite link?