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A simple network consist of two nodes X and Z which are 800 m apart. Each node has a single frame of F = 1500 bits (including all header and preambles) to send to each other. Both nodes attempt to transmit at ‘t = 0’. In case that a node executes the exponential Back-off algorithm. In the algorithm, a node experiencing the nth collision in a row for given frame, randomly chooses a value of k from {1, . . . 2m} where m is min (n, 10). The node then wait 8k μs before attempting to retransmit the frame. It is also given that the propagation speed of the signal over the network is 2 × 108 m/s and a data rate of 150 Mbps. Assume that collision occur at time t = 0, and that the node X draws k = 1 and Z draws k = 2. The time taken by X’s packet (from t = 0) to be completely delivered to Z is ________ μs.

asked in Computer Networks by Active (3.1k points)
edited by | 143 views
22microsecond i m getting.?(8+10+4)?
I don't the answer but can you share your approch??
I think its all CSMA/CD but the catch is how they have used backoff algo.They are limiting the value domain(1-1024) only and moreover if a node gets the control to transfer frame it has to wait for 8k us where k is a randomly chosen number.[Rest things are similar to the basic procedure I guess.]
I think to solve this we have to see that we can send only after  after tp(to get to know about collision )+8 *1(k=1 for X) time and we can send till the collision again happens at tp+8*2(k=2 for Z)
i think it should be like,

4us(to know about collision) + 8us(wait time) + 2*4us(dummy packet to ensure that there is no collision) + 10us(transmission) + 4us(propagation) = 34us

and ignoring the fact to use dummy packet(as it is used in traditional CSMA/CD), it should be = 26us
At 0us --collision happens
At 4us--Both stations get to know about collision
At 4us --X draws k=1 and Z draws k=2
At 4us +4 us=8us Last collided bit will leave the station
At 8us+8*1us(wait)=16us X starts transmitting
But My doubt is that Z will also start transmitting at 8us+8*2(wait)=24us and which will again cause collision at t=24us So should X transmit only during 16us to 24us??

joshi please help!!
i think its 26 micro second

Tx=10 micro second; Tp=4 micro second

collision happened at t=0;

after 8k micro second  retransmission happen as k=1 by station X and k=2 by Y station

8k us=8us---------------1

so X waits again 4 u second to transmit as k=1;---------------2

then tx+tp=10 us + 4 us=14 us ------------3

add 1 +2 + 3 equation = 8us + 4us + 14 us=26 us

...if i m wrong plz correct me.
2*tp should be added once only to detect collision i think

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