Binary Exponential Backoff Binary exponential backoff refers to

Binary Exponential Backoff Binary exponential backoff refers to a collision resolution mechanism used in random access MAC protocols. This algorithm is used in Ethernet (IEEE 802. 3) wired LANs. In Ethernet networks, this algorithm is commonly used to schedule retransmissions after collisions.

Exponential Backoff Algorithm § Set “slot time” equal to 2*maximum propagation delay + Jam sequence transmission time (= 51. 2 usec for Ethernet 10 -Mbps LAN) § The reason for this choice is that, it is the minimum amount of time a source needs to listen to the channel to always detect a collision. § After Kth collision, select a random number (R) between 0 and 2 k – 1 and wait for a period equal to (R*slot time) then retransmit when the medium is idle, for example: § After first collision (K=1), select a number (R) between 0 and 21 – 1 {0 , 1} and wait for a period equal to R*slot times (Wait for a period 0 usec or 1 x 51. 2 usec) then retransmit when the medium is idle § Do not increase random number range, if K=10 § Maximum interval {0 – 1023} § Give up after 16 unsuccessful attempts and report failure to higher layers

Flow Diagram: CSMA/CD (i) If a collision is detected during transmission of a packet, the node immediately ceases transmission and it transmits jamming signal for a brief duration to ensure that all stations know that collision has occurred. (ii) After transmitting the jamming signal, the node waits for a random amount of time and then transmission is resumed. The random delay ensures that the nodes, which were involved in the collision, are not likely to have a collision at the time of retransmissions.

Collision with two sources Assuming two sources are involved in collision Source S 1 Time Source S 2 Time

First collision Source S 1 Assume first collision took place C W: {0, 1} Time Source S 2 CW: {0, 1} Time CW: Contention Window

After a collision, time is divided into discrete slots. • Frame length is much larger than the slot size. • A single slot is sufficient to detect a collision Source S 1 0 1 2 3 4 5 6 CW : {0, 1} 7 8 Time Source S 2 0 CW: {0, 1} 1 2 3 4 5 6 7 Time 8

Ideal case that S 1 and S 2 picks different slots randomly for the transmission of the frame. Source S 1 0 1 2 3 4 5 6 CW : {0, 1} 7 8 Time Source S 2 0 CW : {0, 1} 1 2 3 4 5 6 7 8 Time Suppose S 1 picks 0 and S 2 picks 1, S 1 starts sending the frame in slot 0 S 2 should start sending the frame in slot 1 but it hears the channel is busy and waits till the channel is free.

Suppose S 1 and S 2 picks the same slot number 0 to transmit the frame, they collide again. Source S 1 0 0 1 2 3 4 5 6 CW: {0, 1, 2, 3} 7 Time Collision occurs Source S 2 0 CW: {0, 1, 2, 3} 0 1 2 3 4 5 6 7 Time After second collision, the contention window changes to {0, 1, 2, 3}

Suppose S 1 and S 2 picks the same slot number 2 to transmit the frame, they collide again. Source S 1 0 0 0 1 2 3 CW: {0, 1, 2, 3, 4, 5, 6, 7} 4 5 6 Time Collision occurs Source S 2 0 CW: {0, 1, 2, 3, 4, 5, 6, 7} 0 0 1 2 3 4 5 6 Time • After third collision, the contention window changes to {0, 1, 2, 3, 4, 5, 6, 7} • After 16 such attempts of transmission, the process is aborted

Exponential Backoff Algorithm • Reduces the chance of two waiting stations picking the same random waiting time • When network traffic is light, it results in minimum waiting time before transmission • As congestion increases ( traffic is high), collisions increase, stations backoff by larger amounts to reduce the probability of collision. • Exponential Back off algorithm gives last-in, first-out effect • Stations with no or few collisions will have the chance to transmit before stations that have waited longer because of their previous unsuccessful transmission attempts.