Wired QoS decides which packet leaves the interface first. Wireless QoS decides which station gets to transmit at all. That is a different problem, and it is why a perfectly marked voice packet can still arrive late.
WMM in one table
| Access category | Typical traffic | DSCP mapping |
|---|---|---|
| Voice (VO) | RTP | EF (46) |
| Video (VI) | Conferencing | AF41 (34) |
| Best effort (BE) | Web, file | 0 |
| Background (BK) | Backups, updates | CS1 (8) |
Each category gets different contention parameters: a shorter arbitration inter-frame space and a smaller contention window mean VO wins the medium more often. It is a statistical advantage, not a reservation.
There is no bandwidth guarantee on Wi-Fi
Even with WMM, a voice call competes for airtime with every other station. The guarantee is "you get to try first," not "you get 87 kbps." Capacity planning is the only real control.
Airtime is the budget
A 20 MHz channel at 40% utilization has 60% of its time available. A G.711 call at 87 kbps occupies roughly 0.3% of a 300 Mbps effective channel — but management overhead, retries, and beacons consume airtime too.
Practical planning numbers for one radio:
| Traffic | Recommended maximum per AP radio |
|---|---|
| Voice calls | 12–20 |
| Video conferences | 8–12 |
| Mixed voice + data | Reduce both by 30% |
Beyond that, calls still connect and quality degrades — which is worse than refusing them.
What WMM needs to work
- WMM enabled on the SSID and the AP (it is on by default on most enterprise gear).
- DSCP mapped to access category, either by the AP or by the upstream switch using CoS.
- Client support. Most phones and softphones mark correctly; some do not.
- A clean RF environment. QoS cannot fix 30% retries.
On UniFi, WMM is enabled per WLAN and DSCP-to-WMM mapping is automatic for standard markings.
Worked example: 20 calls on one AP
- One AP, 5 GHz, 40 MHz, 25 clients, 12 of them on calls.
- Channel utilization 35%, retries 8%.
- Calls are marked EF by the phones; WMM maps them to VO.
- A backup starts on the same AP and is mapped to BK.
- Voice quality stays at MOS 4.1 while the backup slows.
Remove WMM, and the same scenario produces MOS 3.2 with audible gaps. The bandwidth did not change; the contention did.
Roaming is part of voice quality
A call that roams badly is worse than one that never roams. See the roaming article for 802.11k/v/r, but the voice-specific points:
- Secondary coverage must be −72 dBm or better everywhere a call might move.
- Fast transition (802.11r) matters most for voice; test the phone fleet.
- A 300 ms roam is a syllable; a 2 s roam is a dropped call.
What breaks in production
- WMM disabled on an SSID "for compatibility," which removes voice's only advantage.
- All traffic marked BE, because the AP does not trust DSCP from the upstream.
- Too many calls per AP, producing gradual degradation rather than a clean refusal.
- A backup or update running during class, consuming airtime at BK but still consuming it.
- Poor roaming, so calls drop in stairwells and hallways.
- Multicast video on the same radio, which eats airtime at the basic rate.
Checklist
- WMM is enabled on every SSID that carries voice or video.
- DSCP mapping is verified end to end, including the AP.
- Calls per radio are capped by policy, not hope.
- Backups and updates are scheduled outside instructional hours.
- Roaming coverage and fast transition are tested with the actual handsets.
- Voice quality is measured during peak, on Wi-Fi, not only on wired.
Next
QoS closes the series. The next one goes below the packets: Physical Layer and Cabling.