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csma+wireless-sols

# csma+wireless-sols - CS 118 Spring 2008 Problems Problem 1...

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CS 118 Spring 2008 : Problems Problem 1 Problem 1 Suppose nodes A and B are on the same 10Mbps Ethernet bus, and the propagation delay between the two nodes is 255 bit times. Suppose node A begins transmitting a frame and, before it finishes, node B begins transmitting a frame. Can A finish transmitting before it detects that B has transmitted? Why or why not? If the answer is yes, then A incorrectly believes that its frame was successfully transmitted without a collision. Hint: Suppose at time t = 0 bit time, A begins transmitting a frame. In the worst case, A transmits a minimum-sized frame of 512+64 bit times. So A would finish transmitting the frame at t = 512 + 64 bit times. Thus, the answer is no, if B’s signal reaches A before bit time t = 512 + 64 bits. In the worst case, when does B’s signal reach A? At t = 0 A transmits. At t = 576 , A would finish transmitting. In the worst case, B begins transmitting at time t = 224 . At time t = 224 + 225 = 449 B ’s first bit arrives at A. Because 449 < 576, A aborts before completing the transmission of the packet, as it is supposed to do. Thus A cannot finish transmitting before it detects that B transmitted. This implies that if A does not detect the presence of a host, then no other host begins transmitting while A is transmitting.

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csma+wireless-sols - CS 118 Spring 2008 Problems Problem 1...

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