WMM access category (AC)
One of four transmit queues in Wi-Fi Multimedia (voice, video, best effort, background), each with its own channel-contention settings that make winning the channel more or less likely.
The network analogy
A weighted queue, not a reserved lane: the priority queue gets served more often, but nothing sets aside bandwidth for it.
Why it matters for wireless
Marking control traffic for a higher category only shifts its odds against other traffic on the same channel, so the worst-case delay still has to be proven against the application's timeout.
WMM sorts every outgoing frame into one of four queues, called access categories: voice (AC_VO), video (AC_VI), best effort (AC_BE) and background (AC_BK). The WMM specification defines each category as a label for a set of contention parameters: the wait before counting down (AIFSN), the contention window range (CWmin and CWmax) and a limit on how long a winner may hold the channel (TXOP limit). Each category gets its own values, which shifts the odds at each contention round. It does not reserve airtime. A voice-category frame still queues behind whatever wins the channel first. The argument is made in full in Prioritization is not reservation.
The category a frame lands in comes from its IP marking. RFC 83251 recommends a DSCP to 802.11 user priority (UP) mapping, and the user priority selects the category. A few rows show the pattern:
| DSCP | User priority | Access category |
|---|---|---|
| EF | 6 | AC_VO |
| AF41, AF42, AF43 | 4 | AC_VI |
| DF (default) | 0 | AC_BE |
| CS1 | 1 | AC_BK |
RFC 8325 also notes that many implementations simply copy the top three bits of the DSCP into the user priority. That older default does not always land a class where the RFC recommends, so check what the AP and the client actually do. Unmarked traffic falls into best effort, which is the case on the missing QoS marking page. For how categories share a channel with everything else, see airtime is finite; the camera and sensor network page shows the effect of heavy video-category traffic beside control traffic.
The marking is a request. The proof is a measured worst case against the timeout.
About this page
Built from 2 sources: 2 standards bodies and labs. Researched and drafted with AI assistance, then reviewed and approved by Ben Rutter on . How pages are made
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Cite this page
Plain
Ben Rutter. "WMM access category (AC)." OT Wireless, published October 6, 2026. https://otwireless.com/glossary/wmm-access-category/
APA 7
Rutter, B. (2026, October 6). WMM access category (AC). OT Wireless. https://otwireless.com/glossary/wmm-access-category/
BibTeX
@misc{rutter2026wmmaccesscategory,
author = {Rutter, Ben},
title = {{WMM access category (AC)}},
year = {2026},
howpublished = {\url{https://otwireless.com/glossary/wmm-access-category/}},
organization = {OT Wireless},
}