Arbitration inter-frame spacing

Arbitration Inter Frame Spacing (AIFS), in wireless LAN communications, is a method of prioritizing one Access Class (AC) over the other, such as giving voice or video priority over email. AIFS functions by shortening or expanding the period a wireless node has to wait before it is allowed to transmit its next frame. A shorter period means a message has a higher probability of being transmitted with low latency, which is particularly important for delay-critical data such as voice or streaming video.

AIFS is a time interval between frames being transmitted under the IEEE 802.11e EDCA MAC. It depends on the Access Class and generally depends on the AIFSN, or AIFS-number. AIFS is defined by the formula AIFSN[AC]*ST + SIFS, where the AIFSN depends on the Access Class. Slot time ST (also denoted by $\sigma$) is dependent on the physical layer. Short Interframe Space (SIFS) is the time between a DATA and ACK frame.