# Arbitration inter-frame spacing

AIFS is a time interval between frames being transmitted under the IEEE 802.11e EDCA MAC. It depends on the Access Category 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 Category. Slot time ST (also denoted by ${\displaystyle \sigma }$) is dependent on the physical layer. Short Interframe Space (SIFS) is the time between a DATA and ACK frame.