Interframe Space Types - IFS

Six of the most common

From shortest to longest:

• RIFS (Reduced Interframe space): Highest priority - Introduced with 802.11n to provide a shorter interframe space during a 802.11n transmission burst. RIFS is not used with 802.11ac.
• SIFS (Short Interframe space): Second highest priority after 802.11n was introduced. - SIFS are used when STAs have seized the medium and need to keep it for the duration of the frame exchange sequence. Using By using the smallest space between transmissions within the frame exchange sequence prevents other STAs from attempting to use the medium, since they are required to wait for the medium to be idle for a longer space of time. Thus giving priority to the completion of the frame exchange sequence in progress.
• PIFS (Priority Interframe Space): Middle priority - Used with the PCF mode of operation and is largely unused today, with the one exception; the channel switch announcement (action frame). Since a channel switch announcement will typically follow a DFS eveent, it is important for the AP to be able to communicate with its BSS, so that it can switch channels. For that reason a channel switch frame take priority over other data frames.
• DIFS (DCF Interframe Space): Lowest priority - Used for first attempt at data and management frame transmission in a DCF network.
• AIFS (Arbitration Interframe Space): Used by QoS stations (802.11e) - Used the same as DIFS are used, but only by Q0S stations under EDCA contention mechanisms.
• EIFS (Extended interframe space): Used after the receipt of corrupt frames - When a STA loses it’s timing, duration and length of the current transmission, due to corrupt packets it must wait the for an EIFS to regain it’s timing, unless a properly received frame resets the NAV before the duration of the EIFS expires.