Question

1. [5 marks] Why must the backoff algorithm used in a CSMA/CD network be exponential backoff? (Your explanation must not exceed 250 words)
2. [5 marks] After four collisions, what is the probability that the random value K=5 will be chosen by a node? How long does a node wait before attempting to transmit the frame when K=5, assuming that the network data rate is 100 Mbps?

Answer 1

Why must the backoff algorithm used in a CSMA/CD network be exponential backoff?

ANS- The backoff algo used in CSMA/CD network must be exponential backoff due to the following reasons-

The station continues transmission of the current frame for a specified time along with a jam signal, to ensure that all the other stations detect collision.

The station increments the retransmission counter, c, that denote the number of collisions.

The station selects a random number of slot times in the range 0 and 2^c – 1. For example, after the first collision (i.e. c = 1), the station will wait for either 0 or 1 slot times. After the second collision (i.e. c = 2), the station will wait anything between 0 to 3 slot times.

If the station selects a number ? in the range 0 and 2c – 1, then

Back_off_time = k × Time slot,

And the end of the backoff time, the station attempts retransmission by continuing with the CSMA/CD algorithm.

If the maximum number of retransmission attempts is reached, then the station aborts transmission.

After four collisions, what is the probability that the random value K=5 will be chosen by a node? How long does a node wait before attempting to transmit the frame when K=5, assuming that the network data rate is 100 Mbps?

ANS- K = 5 selected from {0, 1, 2, 3, 4,5, ... 16} since 2^4 = 16

o Probability of any specific value being drawn = 1/16 = 0.0625

o for 100Mbps Ethernet, each bit takes 1µs to transmit. then K * 512-bit-times is (5)*(512)*(1µs)

##That is all about your answer..........please upvote my answer..........please............