Wi-Fi WMM: How Priorities Work

Wi-Fi Multimedia (WMM) lets voice and video go ahead of downloads and backups. Follow a packet from an app to one of four queues, then watch the queues race for the air: see why a voice call stays smooth while someone downloads, and what happens when WMM is off.

Four queues, four head starts

Every Wi-Fi device, access points included, keeps four transmit queues called access categories: Voice, Video, Best effort and Background. They all compete for the same air with the usual listen-before-talk rules, but with different settings: higher-priority queues wait a shorter fixed time (AIFS) after the channel goes quiet, pick their random backoff from a smaller range, and may keep the channel for several frames in a row (a TXOP). Priority isn't guaranteed; it's a much better chance of going first.

Which queue a packet uses depends on the priority marking (DSCP) the app or network put in its IP header.

1 · From app to queue

Pick what the traffic is. The DSCP value in the packet's IP header is translated to an 802.11 User Priority, which picks the queue.

Traffic

Traffic class (RFC 8325)	DSCP	User Priority	Access category

2 · The race for the air

Phones on voice calls (Voice), laptops in video calls, the first one also browsing the web (Video + Best effort), PCs downloading (Best effort) and a NAS backing up (Background), all on one 5 GHz channel. Each row is one queue. The traffic is random: voice packets every 20 ms or so, video frames in bursts of packets, and the web, downloads and backup switching on and off. New random run draws new traffic; switching WMM replays the same traffic, so you compare like with like.

WMM

WMM gives higher priorities

a shorter wait (AIFS) a smaller backoff range (CW) TXOP bursts

Voice calls Video calls Other PCs downloading

Speed

Window

Move along the timeline 0.00 ms

AIFS: fixed wait after the channel goes quiet Random backoff, counting down (paler bar) Solid: transmitting ACK / Block Ack Collision ◆ internal collision: two queues in one device tied, the lower priority backs off ▼ … waited: a voice packet arrives, and how long it waited to go on the air

3 · WMM on vs off

The first 2 seconds of , the random traffic in the timeline above, played with WMM on and off. Press New random run to see how much the results vary. Access delay is how long a frame waits, from reaching the front of its queue until it starts transmitting. Voice needs this well under 20–30 ms; a few milliseconds is ideal.

Backoff picks

Every time a queue gets a frame ready, it picks a random number of empty slots (9 µs each) to count down, from 0 up to its contention window (CW). Darker = picked more often. The outline is the starting range; picks beyond it come after collisions, when the range doubles.

4 · The WMM parameters

The EDCA parameters most APs advertise on 5 GHz: the Wi-Fi Alliance WMM defaults (slot 9 µs, SIFS 16 µs). AIFS = SIFS + AIFSN × slot. The AP advertises the values clients must use in its beacons; the AP itself uses slightly shorter waits.

Access category	User Priorities	AIFSN (client / AP)	AIFS (client)	Contention window	TXOP limit	Typical traffic
Voice (AC_VO)	6, 7	2 / 1	34 µs	0–3, up to 0–7	1.5 ms	Voice calls, some network control
Video (AC_VI)	4, 5	2 / 1	34 µs	0–7, up to 0–15	3 ms	Video calls, streaming marked by the network
Best effort (AC_BE)	0, 3	3 / 3	43 µs	0–15, up to 0–1023	one exchange	Everything unmarked: web, most apps, most streaming
Background (AC_BK)	1, 2	7 / 7	79 µs	0–15, up to 0–1023	one exchange	Backups, updates, bulk transfers marked as low priority

AIFS: the fixed wait after the channel goes quiet. Voice and video start counting down their backoff 1 slot before best effort and 5 slots before background.
Contention window: the range the random backoff is drawn from. It doubles after each collision, up to the maximum, and resets after a success. Small windows mean voice usually picks a short backoff.
TXOP limit: once a voice or video queue wins, it may send several queued frames back to back (SIFS apart) for up to this long. Best effort and background get one frame exchange per win; they rely on aggregation instead.
WMM and the 802.11 standard: the values above are the Wi-Fi Alliance WMM defaults that APs advertise in their beacons. 802.11-2024 itself (Table 9-194) lists different defaults for OFDM PHYs: TXOP limits of 2.080 ms for voice, 4.096 ms for video and 2.528 ms for best effort and background, with the same AIFSN and contention windows. A client uses those only if its AP advertises no EDCA parameters; in practice APs advertise the WMM values.

Notes and simplifications

Marking: WMM only helps traffic that is marked. Many apps send everything unmarked (Best effort); calling apps such as Teams, Zoom and FaceTime usually mark their voice and video. Networks can trust, re-mark or strip DSCP at the switch, AP or controller, so check the policy end to end. Older devices map DSCP to User Priority by its top three bits (DSCP ÷ 8), which puts EF 46 in Video; RFC 8325 fixes this by mapping EF to UP 6 (Voice).
Internal collisions: when two queues in the same device finish their backoff in the same slot, the higher priority transmits and the other behaves as if it had collided (doubles its contention window). Queues in different devices really collide.
Also part of WMM: admission control (the AP can require a device to ask before using Voice or Video, and refuse if the channel is full), and WMM power save (U-APSD), which lets phones sleep between voice packets. WMM is required for Wi-Fi 4 (802.11n) and later data rates, so it's always on in modern networks; "WMM off" here is for comparison.
This model: every device hears every other. Traffic is random but repeatable per run: each voice call sends 60–100 µs frames every 20 ms (±1 ms); each video call 2–8 packets of 300–550 µs every 33 ms, with an occasional 12–19-packet key frame; web browsing, downloads and the backup come in on/off spells (on average 40 ms on / 250 ms off, 150 / 150 ms and 400 / 300 ms) of 200 µs–1.5 ms aggregates. Each queue holds up to 40 packets. ACKs are 44 µs. Only uplink from the devices is shown, and rate changes are left out. "WMM off" gives every queue the Best-effort parameters; unticking one of WMM's differences gives every queue the Best-effort value for just that parameter.