WLAN networks employ CSMA/CA mechanism to gain access of the medium to transmit data packets.
CSMA stands for Carrier Sense Multiple Access. All WLAN devices sense if the medium is idle. only if the medium is sensed to be idle, the 802.11 Station device can transmit the frame.
The CA in CSMA/CA stands for Collision avoidance. A WLAN device cannot transmit and receive simultaneously. If the medium is sensed to be busy, a WLAN device will back-off for a random amount of time. An exponential back-off algorithm has been developed that determines the random time.
In addition to the Back-off time, all 802.11 stations also employ another virtual back-off mechanism that is termed as Network Allocation Vector (NAV). NAV is a time duration. This duration is received by a 802.11 station in the last packet that was sensed by the 802.11 station on air.
Only if the NAV duration and the back-off timer count down to Zero and the medium is sensed to be idle can the 802.11 transmit a packet on the air. The above concept will be further discussed in Distributed Co-ordination Function section and Enhanced Distributed Co-ordinate Access (EDCA).
Why CSMA/CD is not used in WLAN
- The reason CSMA/CD cannot be used by WLAN stations is because WLAN stations do not have separate TX and RX pairs. The WLAN stations are always in listen mode and whenever a packet needs to be transmitted, the WLAN Radio switches to Transmit mode, transmits the packet and then reverts back to listen mode. It is not possible by a WLAN station to detect a packet transmission by another WLAN station at the same time that it is transmitting
- Also, It is not possible by WLAN stations to detect transmission by hidden nodes