VoIP QoS settings explained

VoIP QoS settings explained in simple terms

A router or switch sends packets out in the order they arrive. Without QoS there is one queue, so a voice packet can wait behind a large file upload. A few tens of milliseconds of waiting is nothing for a download but very noticeable on a call.

Microsoft describes QoS as creating carpool lanes in your network. It works in three stages:

1. Classify: identify which packets are voice, using the marking the phone or app sets, the port range it uses, or the voice VLAN it sits on
2. Mark: label each packet with a DSCP (Differentiated Services Code Point) value so every device on the path knows what it is. Voice media is marked EF, value 46
3. Queue: give marked voice a priority queue on each switch and router, and shape traffic leaving the site so the router, not the provider, decides what waits

For QoS to work, the same settings have to apply end to end: phones and computers, switches, Wi-Fi and the router to the internet. Any part of the path that ignores the markings can undo the rest. Microsoft also notes that a small business with few users might not need QoS, but should consider it if it sees network delays. The wider checklist is in VoIP requirements.

DSCP markings by platform

The voice audio itself is marked EF (46) on all the platforms below. Signalling, video and screen sharing get their own values, and each vendor documents them:

Two practical points follow. In Teams, the "Insert Quality of Service (QoS) markers for real-time media traffic" setting has to be turned on in the Teams admin centre, and Microsoft notes that Mac, iOS, Android and Teams Phone devices use fixed values (EF for audio). Windows clients are marked by policy. Microsoft's best practice is to combine client marking with port-based rules on routers, which catch devices that cannot mark their own traffic.

For 3CX, Forever UC and SIP trunks, the phones and the platform set their own markings; we can confirm them for your platform and devices during set-up. More on each platform: Teams VoIP, Webex VoIP and 3CX VoIP.

Voice VLANs and LLDP-MED

A voice VLAN puts phones on their own virtual network. It keeps broadcast traffic from PCs away from phones, makes QoS rules simpler (everything on the voice VLAN is voice) and lets you apply tighter security to phones.

Phones can learn which VLAN to join without anyone touching them:

• LLDP-MED: the switch advertises the voice VLAN and QoS policy to the phone when it connects, and the phone can report its power needs for PoE
• DHCP options: the phone gets the voice VLAN from the data network's DHCP server, then restarts on the voice VLAN

With a PC plugged into the phone's pass-through port, the phone tags its own traffic onto the voice VLAN and passes the PC's traffic through untagged. One cable serves both, which is why PoE and gigabit pass-through are worth having on VoIP desk phones.

Trust boundaries: which markings to believe

Any device can set a DSCP value, so the network has to decide where to trust it. That point is the trust boundary.

• Trust the phone: on voice VLAN ports, trust the markings IP phones set, since they mark audio correctly
• Re-mark or police the rest: on ordinary PC ports, either clear markings or trust only the platform's known port ranges, so a misconfigured application cannot claim the priority queue
• Mark at the edge where clients cannot: port-based rules on the router or firewall mark traffic from devices that do not mark their own, as Microsoft recommends for Teams
• Keep the priority queue small: size it for your peak voice calls rather than a large share of the line, so voice cannot starve everything else

A good QoS design is short to write down. Most of the effort is in applying it consistently on every switch, access point and router.

Wi-Fi: WMM and planning for voice

Wi-Fi has its own QoS, called WMM (Wi-Fi Multimedia), which maps DSCP markings to access categories so voice is sent ahead of background traffic over the air. Microsoft's guidance for Teams calls over Wi-Fi is to:

• Turn on QoS or WMM so media traffic is prioritised
• Prefer the 5 GHz band, which suits real-time media better but needs more access points for coverage
• Use band steering on dual-band networks to move capable devices to 5 GHz
• Keep neighbouring access points on non-overlapping channels so they do not compete

Coverage matters more than speed for voice. Plan access points around where people walk and talk, not only where they sit. Cloud-managed access points make it easier to keep the same settings across every site; see cloud managed Wi-Fi on our sister site.

WAN shaping and a simple VoIP QoS calculator

Most congestion happens where the fast office network meets the slower internet connection. If the router sends faster than the line can carry, packets queue inside the provider's equipment, where your markings may count for nothing. Shaping outbound traffic a little below the line's real upload speed keeps the queue on your router, where the priority queue works.

A simple VoIP QoS calculator for the priority queue:

1. Count your peak simultaneous calls on the site
2. Multiply by the codec's bandwidth per call: Cisco puts G.711 and G.722 at about 87.2 kbps and G.729 at about 31.2 kbps, including Ethernet overhead at 20 ms packets
3. Add a margin for signalling and growth
4. Check the result against the shaped upload speed, leaving plenty for everything else

For example, 10 G.711 calls need roughly 872 kbps for the priority queue in each direction. If that is a large share of your upload, the answer is more bandwidth rather than more QoS; see VoIP requirements.

What QoS cannot fix

QoS shares out the bandwidth you have. It does not create more, and it only works on links you control.

• The public internet generally ignores your DSCP markings, so QoS protects calls inside your network and at your router, not across the internet
• Home workers on their own broadband are outside your QoS policy; Microsoft notes that QoS works as expected only when implemented on all links between callers
• VPNs add overhead and delay; Microsoft recommends a split-tunnel VPN so Teams traffic bypasses it
• A line that is simply too small at peak times needs more bandwidth, a second connection or a leased line

To check QoS is doing its job, Microsoft suggests capturing traffic at the network edge, for example with port mirroring, and confirming the DSCP values have not been stripped. If calls still suffer, our VoIP call quality guide walks through the tests and fixes.

How we help

Forever Systems can design and apply VoIP QoS settings as part of a phone system project, from voice VLANs and PoE on the switches to markings, Wi-Fi settings and shaping on the router. If you use Cisco Meraki, the same policy can be set across every site from one dashboard: see cloud managed switches on our sister site.

Contact us with your platform, switches and connection, and we can set out the QoS settings your network needs.