An example Back-off mechanism is depicted pictorially below
FIG COURTESY: IEEE Std 802.11-2012TM (section 9.3.4.3)
- The figure depicts 5 802.11 stations trying to compete for the air medium using DCF mechanism
- Station A has the transmitting right at the beginning – all the other stations back-off for the duration of transmission by Station A and an additional DIFS time frame
- Once the DIFS time frame is completed – all the different Stations start counting down their exponential Back-off window time.
- Station C counts down to zero first senses the medium to be free at this point.
- It transmits a frame. The rest of the stations sense the physical medium to be busy and stop their back off counters till the end of frame transmission by Station C and DIFS time-frame before re-starting their back-off timers
- Whichever station is able to complete the contention window back-off timer count to zero and sense the free medium wins right to transmit
- The above procedure is repeated by all stations for gaining access to the medium