Interframe spaces play an important role in 802.11 medium contention. They are used as additional medium contention protocols in addition to CCA and NAV. An interframe space, or IFS, is a quiet period access points and stations must wait before transmitting a frame.
There are shorter interframe spaces used for higher priority data transmissions. Think about QoS in that regard.
There are four rules that define which IFS will be used before frame transmission:

* If arbitration is complete, then reduced IFS (RIFS) or short IFS (SIFS) will be used.
* If arbitration has not been determined, then arbitration IFS (AIFS) or DCF IFS (DIFS) will be used. DIFS is used for 802.11 WLANs that don’t support 802.11e QoS.
* If AP or station received a corrupted frame, then extended IFS (EIFS) will be used.
* PCF IFS (PIFS) is part of PCF and not used in the real world.




Links and Resources Mentioned

* CTS 041: 802.11 Medium Contention
* 802.11 Arbitration White Paper from CWNP
* CWAP – 802.11 Medium Contention from mrn-cciew