802.11 wireless communication is half duplex based on CSMA-CA (Carrier Sense Multiple Access – Collision Avoidance) technique. Since it is not possible to detect collision over the air, this technique will try to prevent collisions. All the 802.11 stations like Access Points, wireless laptops, mobile phones, wireless printers, scanners etc. implement CSMA-CA for the medium access for wireless transmission. Each wireless station using CSMA-CA include two methods to access the medium and to avoid collision – Physical carrier sensing and Virtual Carrier sensing.
The former is implemented by sensing the medium – air to detect RF energy above a specific threshold using CCA (clear channel assessment). If a wireless station detects RF energy above threshold, it will continue to wait and not access the medium; assuming that another wireless station is transmitting in the air currently.
Virtual carrier sensing is another technique used by the wireless station that depends on the NAV (Network Allocation Vector) timer for medium access. If NAV timer =0, then medium is free for access. However, if this timer is not 0, then medium is busy and the station cannot contend for medium. NAV timer depends on the Duration Field in a wireless frame that is being transmitted in air. This field indicates the duration of time that is required by the frame to complete the transmission. NAV of a wireless station is set according to the Duration field. This method helps in avoiding hidden node problems. [refer to another post: “RTS-CTS and CTS-to-Self” in understanding hidden node problem].
Below are the steps involved in the CSMA-CA technique.
- Physical and Virtual carrier sensing techniques are performed simultaneously. If any of these processes indicate a busy medium, then these methods are repeated until it indicates that the medium for wireless transmission is free and available.
- After which, there is a delay period of DIFS (Distributed Interframe Spacing) timer based on the modulation technique used.
- Once DIFS timer expires, dot11 station intending to transmit in the medium should start the random backoff algorithm using contention window where a random integer is selected and multiplied with the slot time. Slot time again depends on the modulation technique used.
- The station can successfully transmit in the wireless medium only if physical carrier sensing and virtual carrier sensing indicates free medium and the random counter is 0.
- If the random counter is not 0, then it is decremented and both the carrier sensing are checked again. If at any point, one of the carrier sensing results in busy medium, the entire process starts over from Step 1 till the random backoff counter value where it has left off ; instead of calculating the random backoff timer again.
The below flow chart should give a better understanding on the method used.
References:
- CWAP Official Study guide
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