Wi-Fi QoS for VoIP: DSCP & WMM Settings

Great microphones and codecs won’t save a call if your network treats real-time packets like background syncs. On Wi-Fi 6/6E, quality of service (QoS) is the difference between crisp audio and “can you hear me now?”. This guide explains how DSCP and WMM map voice traffic across wired and wireless hops—and the practical tweaks that actually move the needle.

The building blocks (in plain English)

• DSCP (Differentiated Services Code Point) marks IP packets so the network can prioritise them. It’s the foundation of DiffServ. See the standard in RFC 2474 and the recommended classes in RFC 4594.
• WMM (Wi-Fi Multimedia) is the 802.11e QoS model that buckets frames into four access categories (Voice, Video, Best Effort, Background). Overview: Wi-Fi Alliance: WMM.
• Wi-Fi 6/6E improves scheduling and airtime fairness (OFDMA, TWT), which helps latency-sensitive traffic when configured correctly: Wi-Fi CERTIFIED 6.

DSCP → WMM: getting the mapping right

Most enterprise APs map DSCP 46 (EF) to WMM Voice and DSCP 34 (AF41) to WMM Video by default. Verify and, if necessary, enforce these on the controller:

• Signalling (SIP/WebRTC control): AF31/CS3 → WMM Video or Best Effort (keeps call setup responsive without jumping the voice queue).
• Media (RTP/SRTP): EF (46) → WMM Voice (strictest contention parameters).
• Background sync and updates: CS1/Best Effort → WMM Background.

If you run multiple SSIDs, keep the voice SSID minimal and enable 802.11k/v/r only if roaming glitches are resolved in your handset estate.

Practical access-point settings that matter

• Enable WMM (and don’t disable it to “fix” throughput tests).
• Airtime fairness / OFDMA: leave on, but monitor; mis-sized RU allocation can starve legacy devices.
• Minimum data rate policy: raise the basic rates to evict very low-rate clients from hogging airtime.
• Band steering: prefer 5 GHz/6 GHz for voice; 2.4 GHz is noisy and often congested.
• Client load balancing: keep it gentle; aggressive steering during calls can spike jitter.

Edge and WAN considerations

QoS is only as strong as the narrowest link:

• Mark at the source. Set DSCP in your client or SBC, then trust and preserve markings on switches, controllers and firewalls.
• Shape, don’t drop. Use fair queueing with active queue management; FQ-CoDel tames bufferbloat better than FIFO/RED.
• Split tunnels wisely. Local-breakout voice avoids tromboning to a distant DC.
• Test with loss/jitter. Synthetic RTP probes reveal if the WAN policy actually honours EF.

Application-level tips (WebRTC and SIP)

• Opus first, 20 ms frames, FEC on where bandwidth allows; reserve PCMU/PCMA only for legacy interop.
• Prefer ICE + TURN for hostile networks; keep TURN quotas sane to avoid runaway cost.
• If your estate mixes softphones and browsers, align DSCP policies across both so media and control stay consistent. A helpful deployment baseline (even if your app isn’t Teams) is Microsoft’s QoS guidance—its DSCP values are widely interoperable.

Troubleshooting quick wins

• “Rings but no audio/video” on Wi-Fi: confirm WMM is enabled and DSCP isn’t being cleared at the AP or firewall.
• Choppy audio at peak times: check airtime utilisation, then validate EF frames actually map to WMM Voice.
• Roaming drops: ensure 802.11r FT matches handset capabilities; disable for problematic models.
• Great Wi-Fi, poor off-net calls: the bottleneck is the WAN—apply FQ-CoDel and honour EF upstream.

Where a browser SIP client fits

If you’re rolling out a browser-based SIP/WebRTC client, QoS still matters—the browser will mark traffic but your network must carry that intent end-to-end. Platforms like Siperb benefit immediately when DSCP and WMM are configured correctly, especially on high-density Wi-Fi.

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