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Authors:
James F. Kurose, Keith W. Ross
Chapter:
The Link Layer: Links,access Networks, And Lans
Exercise:
Problems
Question:18 | ISBN:9780132856201 | Edition: 6

Question

Suppose nodes A and B are on the same 10 Mbps broadcast channel, and the propagation delay between the two nodes is 325 bit times. Suppose CSMA/CD and Ethernet packets are used for this broadcast channel. 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 bits, A begins transmitting a frame. In the worst case, A transmits a minimum- ized 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?

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Answer

Given data:

The speed of broadcast channel=10mbps.

The propagation delay between node A and node B=325times.

Yes, node A finish transmitting before it detects that node B has transmitted.

Suppose that at time t=0, node A transmits the frame and at time t=576, node A finish transmitting the frame.

  • In the worst case, the node B begins transmission at time just before the first bit of node A arrives at node B which means before the time of propagation delay. So, node B starts its transmission at time.
  • The propagation delay between node A and node B is 325 times. So, the first bit of node B arrives at node A at time =324+325 = 649.
  • Hence, Node A finishes it transmission before detecting that transmission by the node B is started.

Thus, Node A incorrectly believes that the frame is transmitted successfully without a collision.

 

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