Arbitration inter-frame spacing (AIFS), is an optional technique used to prevent collisions in IEEE 802.11e based WLAN standard (Wi-Fi), in the medium access control (MAC) layer. It determines the time interval that a station should wait before it sends its request frame, by prioritizing the stations based upon the Access Category, i.e. the type of data to be transmitted.
In a wireless communication, it is very difficult to resolve collisions. So, collision avoidance schemes are used when multiple stations want to access the channels. Inter-frame spacing are inserted before transmissions for avoiding collisions. Unlike other inter-frame spaces, AISF assigns priorities to frames depending upon their Access Category (AC). For example, video streams may be given priority over emails.
In this scheme, the waiting period (equal to the AISF) of a wireless station, is shortened or expanded before the station can transmit its frame. Higher priority stations are assigned shorter AISF. This implies that a higher priority station has to wait for a shorter time interval before it can transmit its frame. This is particularly important in delay − critical transmissions like video streaming or voice streaming.
Arbitration inter-frame spacing is given by the following formula −
AISF = (AISFNAC ST) + SIFS
AISFN is AISF-number which is dependent upon Access Category (AC)
ST is the slot time as laid down by the physical layer
SIFS is Short Inter-Frame Space. SIFS is the time interval required by a wireless device in between receiving a frame and responding to the frame.