Arbitration inter-frame spacing

Arbitration Inter Frame Spacing (AIFS) is a time interval between frames being transmitted under the IEEE 802.11e MAC. It depends on the Access Class (AC) and generally depends on the AIFSN, or AIFS-number.

AIFS is defined as: AIFSN[AC]*ST+SIFS, where the AIFSN depends on the Access Class. Slot time ST (also denoted with $\sigma$ is physical layer-dependent. SIFS is the Short Inter Frame Spacing, the time between a DATA and ACK frame.