edited by
7,480 views
14 votes
14 votes
Suppose two nodes, A and B, are attached to opposite ends of a 900 m cable, and that they each have one frame of 1000 bits (including all headers and preambles) to send to each other. Both nodes attempt to transmit at time t=0. Suppose there are four repeaters between A and B, each inserting a 20 bit delay. Assume the transmission rate is 10 Mbps, and CSMA/CD with backoff intervals of multiples of 512 bits is used. After the first collision, A draws K=0 and B draws K=1 in the exponential backoff protocol. Ignore the jam signal.

Ques-1. What is the one-way propagation delay (including repeater delays) between A and B in seconds. Assume that the signal propagation speed is 2 * 108m/sec.

Ques-2. At what time (in seconds) is A's packet completely delivered at B.

Ques-3. Now suppose that only A has a packet to send and that the repeaters are replaced with bridges. Suppose that each bridge has a 20 bit processing delay in addition to a store-and-forward delay. At what time in seconds is A's packet delivered at B?

My work:- propagation tim = 4.5 *10-6 sec

Transmission time by A = 100  *10-6 sec.

For 2nd ques.I am not getting how to take collision and back off delay...

then how to solve for bridges.
edited by

1 Answer

Best answer
13 votes
13 votes

A]900/(2 * 10^8)m/s + (4*20)/10000000) = 12.5 micro seconds

B]frame transmission time is (1000bits/10*106bps)=100 μsec
At time t = 0 , both A and B transmit.
At time t = 12.5μ sec , A detects a collision.
At time t = 25μ sec last bit of B 's aborted transmission arrives at A .
At time t = 37.5μ sec first bit of A 's retransmission arrives at B .
At time t= 37.5 μsec + 100 μsec =137.5 μsec. A 's packet is completely delivered at B .

C]Each Bridge introduces additional 1000-bit store-and-forward delay and 20-bit processing delay. Total delay introduced is 4080-bit time or 408 µs. Transmission delay is 1000-bit time or 100 µs. Propagation delay is 4.5 µs. A’s packet reaches B at time 408 + 100 + 4.5 = 512.5µs 

selected by

Related questions

0 votes
0 votes
0 answers
4
srestha asked Dec 9, 2017
299 views
What is contention window for CSMA/CD? What is it's difference with persistence method?