Binary Exponential BackoffBinary 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.AuthorPhani Swathi ChittaMentorProf. Saravanan Vijayakumaran

Course Name: Computer Networks Level : UG

Learning ObjectivesAfter interacting with this Learning Object, the learner will be able to:Explain how the binary exponential backoff algorithm works

Definitions of the components/Keywords:53241 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.

After a collision, time is divided into discrete slots whose length is equal to 2, where is the maximum propagation delay in the network. The reason for this choice is that 2 is the minimum amount of time a source needs to listen to the channel to always detect a collision.

The stations involved in the collision randomly pick an integer from the set {0,1}. This set is called the contention window. If the sources collide again because they picked the same integer, the contention window size is doubled and it becomes {0,1,2,3}. Now the sources involved in the second collision randomly pick an integer from the set {0,1,2,3} and wait that number of slot times before trying again. Before they try to transmit, they listen to the channel and transmit only if the channel is idle. This causes the source which picked the smallest integer in the contention window to succeed in transmitting its frame.

Definitions of the components/Keywords:53241 In general, after collisions, a random number between 0 and is chosen.

After a station detects collision, it aborts its transmission in the slot duration itself in which it started transmitting.

In Ethernet, the doubling of the contention window stops after 10 collisions and the contention window remains {0,1,...,1023}.

After 16 collisions , the process is aborted and the source stops trying.

Master Layout 153241Source S1Source S2Part 1 Collision with two sourcesPart 2 Collision with three sourcesAssuming two sources are involved in collisionTimeTimePlace a drop down box to choose number of sources 2 and 3

Step 1: 15324Source S1Source S2TimeTimeC W: {0, 1}CW: {0, 1}

Instruction for the animatorText to be displayed in the working area (DT)The figure in master layout should appear firstThe first statement in DT should appear Next the numbers 0 and 1 in braces should appear along with the second sentence in DTAssume first collision took place, The initial contention window for two sources is {0, 1}

Step 2: 15324Source S1Source S2TimeTime 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8CW :{0,1}CW: {0,1}

Instruction for the animatorText to be displayed in the working area (DT)With the first sentence in DT, the time line should be divided.Next the two sentences in DT should appearAfter 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.

Step 3: 15324Source S1Source S2TimeTime 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8CW :{0,1}CW :{0,1}

Instruction for the animatorText to be displayed in the working area (DT)The first sentence appears firstWith the second sentence in DT, the blue lines should blink and the red lines at S1 should appearThe third and fourth lines should appear and then the red lines at S2 should appear Ideal case that S1 and S2 picks different slots randomly for the transmission of the frame.Suppose S1 picks 0 and S2 picks 1, S1 starts sending the frame in slot 0 S2 should start sending the frame in slot 1 but it hears the channel is busy and waits till the channel is free.

Step 4: 15324Source S1Source S2TimeTime 0 0 1 2 3 4 5 6 7 0 0 1 2 3 4 5 6 7 CW: {0,1,2,3}CW: {0,1,2,3}Collision occurs

Instruction for the animatorText to be displayed in the working area (DT)The first sentence in DT should appear firstNow the numbers at the time line should move right and the first zero should disableThen the blue lines should blinkAfter that with the second line in DT, the numbers in the braces should appearSuppose S1 and S2 picks the same slot number 0 to transmit the frame, they collide again.After second collision, the contention window changes to {0,1,2,3}

Step 5: 15324Source S1Source S2TimeTime 0 0 0 1 2 3 4 5 6 0 0 0 1 2 3 4 5 6CW: {0,1,2,3,4,5,6,7}CW: {0,1,2,3,4,5,6,7}Collision occurs

Instruction for the animatorText to be displayed in the working area (DT)The first sentence in DT should appear firstNow the numbers at the time line should move right and the first two zeros should disableThen the blue lines should blinkAfter that with the second line in DT, the numbers in the braces should appearThen the third line should appearSuppose S1 and S2 picks the same slot number 2 to transmit the frame, they collide again.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.

Master Layout 253241Source S1Source S2Part 1 Collision with two sourcesPart 2 Collision with three sourcesAssuming three sources are involved in collisionTimeTimeSource S3Time

Step 1: 15324Source S1Source S2TimeTimeSource S3TimeCW: {0,1}CW: {0,1}CW: {0,1}

Instruction for the animatorText to be displayed in the working area (DT)The figure in master layout should appear firstThe first statement in DT should appear Next the numbers 0 and 1 in braces should appear along with the second sentence in DTAssume first collision took place, The initial contention window for three sources is {0, 1}

Step 2: 15324Source S1Source S2TimeTimeSource S3Time 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8CW: {0,1}CW: {0,1}CW: {0,1}

Instruction for the animatorText to be displayed in the working area (DT)With the first sentence in DT, the time line should be divided.Next the two sentences in DT should appearAfter 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.

Step 3: 15324Source S1Source S2TimeTimeSource S3TimeCW: {0,1,2,3}CW: {0,1,2,3} 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8CW: {0,1}

Instruction for the animatorText to be displayed in the working area (DT)The first sentence in DT should appear firstThen the blue lines should blinkAfter that with the second line in DT, the numbers in the braces should appearAfter that the third sentence should appearSuppose S1 and S2 picks the same slot number 0 to transmit the frame and S3 picks 1, then S1 and S2 collide again. After second collision, the contention window changes to {0,1,2,3}The contention window of S3 remains same.

Step 4: 15324Source S1Source S2TimeTimeSource S3TimeCW: {0,1,2,3,4,5,6,7}CW: {0,1,2,3} 0 0 1 2 3 4 5 6 7 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8CW: {0,1,2,3}

Instruction for the animatorText to be displayed in the working area (DT)The first sentence in DT should appear firstThen the blue lines should blinkAfter that with the second line in DT, the numbers in the braces should appearAfter that with the third line in DT, the numbers in the braces should appearSuppose S1 picks slot number 0 to transmit the frame after second collision and S3 picks 1 after first collision and S2 picks 3 after second collision, then S1 and S3 collide again.After third collision, the contention window of S1 changes to {0,1,2,3,4,5,6,7}After second collision, the contention window of S3 changes to {0,1,2,3}

IntroductionCredits*DefinitionsTest your understanding (questionnaire)Lets Sum up (summary)Want to know more(Further Reading)Try it yourselfInteractivity:AnalogySlide 1Slide 3Slide 17Slide 19Slide 18Electrical Engineering Place a dropdown box for each source

Place a dropdown box to select the slot number

The range of the slot number is 0 to 7

Place a dropdown box to select the contention window

The range of the contention window is {0,1}, {0,1,2,3}, {0,1,2,3,4,5,6,7}

Source S2TimeTimeSource S3Time{0, 1}{0, 1}{0, 1}Source S1Source S4Time{0, 1}

Questionnaire1. After how many collisions will the process be aborted?Answers: a) b)16 c)20 d)

2. If a 5th collision occurs, the contention window is Answers: a) 35 b) 0,1c) 0 to 35 d) 0 to 5

3. If a 12th collision occurs, the contention window isAnswers: a) 4095 b) 0 to 4095 c) 1023 d) 0 to 1023

The answers are given in red15243

Links for further readingReference websites:http://en.wikipedia.org/wiki/Exponential_backoffBooks: Computer Networks Andrew S Tanenbaum, fourth edition, Prentice HallResearch papers:

SummaryBinary 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.

After a collision, time is divided into discrete slots whose length is equal to 2, where is the time taken by the frame to reach other end.

In general, after collisions, a random number between 0 and is chosen

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