User Manual

Appendix
BreezeNET PRO.11 Series 8-27 User’s Guide
8.5.3.6 Exponential Backoff Algorithm
Backoff is a well known method used to resolve contention between
different stations wanting to access the medium. The method requires each
station to choose a Random Number (n) between 0 and a given number, and
wait for this number of Slots before accessing the medium, always checking
if a different station has accessed the medium before.
The Slot Time is defined in such a way that a station will always be capable
of determining if another station has accessed the medium at the beginning
of the previous slot. This reduces collision probability by half.
Exponential Backoff means that each time the station chooses a slot and
happens to collide, it will increase the maximum number for the random
selection exponentially.
The 802.11 standard defines an Exponential Backoff Algorithm, that must
be executed in the following cases:
• When the station senses the medium before the first transmission of a
packet, and the medium is busy
• After each retransmission, and
• After a successful transmission
The only case when this mechanism is not used is when the station decides
to transmit a new packet and the medium has been free for more than DIFS.
The following figure shows a schematic of the access mechanism:
Figure 9.17: Access Mechanism