Arbitration inter-frame spacing

Short description: Medium access control prioritization scheme

Arbitration inter-frame spacing (AIFS),[1]: 1886 in wireless LAN communications, is a method of prioritizing one access category (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 AIFS 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. AIFS is defined byAIFS=AIFSN[AC]⋅ST+SIFS,where AIFSN (the so-called AIFS-number, a 4-bit integer with a minimum value of 2) depends on the AC.[1]: 2000 [2] Slot time ST (also denoted by σ) is dependent on the physical layer. Short Interframe Space (SIFS) is the time between a DATA and ACK frame.

AIFSN[AC] will be set by the AP in the EDCA Parameter set in beacon and probe response frames. If it is not set then the STA has to use the following default values:[1]: 1068–1069

Default values for AIFSN  STA Type  | OCB Activated[lower-alpha 1] | AC_BK[lower-alpha 2] | AC_BE[lower-alpha 3] | AC_VI[lower-alpha 4] | AC_VO[lower-alpha 5]

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\-  | True  | 9  | 6  | 4  | 2
Non-sensor STA  | False  | 7  | 3  | 2  | 2
Sensor STA  | \-  | 7  | 2  | 5  | 4

1. ↑ Controlled by dot11OCBActivated. OCB stands for "outside the context of a BSS", and BSS stands for "basic service set".
2. ↑ Background.
3. ↑ Best Effort.
4. ↑ Video.
5. ↑ Voice.

The IEEE 802.11e EDCA MAC has been adopted as part of the IEEE 802.11p standard for Wireless Access in Vehicular Environments (WAVE).

## References

1. ↑ 1.0 1.1 1.2 "IEEE Standard for Information Technology—Telecommunications and Information Exchange between Systems Local and Metropolitan Area Networks—Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications". IEEE Std 802.11-2024 (Revision of IEEE Std 802.11-2020) (IEEE). 28 April 2025. doi:10.1109/IEEESTD.2025.10979691. ISBN 979-8-8557-1835-5.
2. ↑ Mangold, S.; Sunghyun Choi; Hiertz, G.R.; Klein, O.; Walke, B. (2003-12-31). "Analysis of IEEE 802.11e for QoS support in wireless LANs" (in en). IEEE Wireless Communications 10 (6): 40–50. doi:10.1109/MWC.2003.1265851. ISSN 1536-1284. http://ieeexplore.ieee.org/document/1265851/.

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