CS 635 Lecture 4, Part 2

Exponential Backoff

After the MAC has completed transmitting the jam sequence, it must wait, or "Back Off", before attempting to transmit again. The amount of time it must wait is determine by one of two Backoff algorithms: the Standard Backoff algorithm (ISO/IEC 4.2.3.2.5) or the Modified Backoff algorithm.

Standard Backoff

The Standard Backoff algorithm, also called the "Truncated Binary exponential Backoff", is described by the equation:

0 <= r <= 2**k

where r ( a random integer) is the number of slot times tha MAC must wait (1 slot time = 512 bit times), and k ios the smaller of n or 10, where n is the number of re-transmission attempts

Modified Backoff

The Modified Backoff is described by the equation

0<=r<=2**k
where r (a random integer) is the number of slot times the MAC must wait, and k is 3 for n< 3 and k is the smaller of n or 10 for n>=3, where n is the number of re-transmission attempts.
The modified backoff reduces the possibility of multiple collisions on the first three retries. The disadvantage is that it extends the maximum time needed to gain access to the network on the first three tries.