Most complaints about IP phones, such as choppy voice, robotic audio or calls that drop when someone starts a large download, are not phone problems. They are network problems. VoIP QoS (quality of service) and a voice VLAN are the two settings that keep calls clear when voice and data share the same switches and internet link. This guide explains what each one does, how to set them up for an office, how much bandwidth each call needs and what numbers to aim for.
Why voice needs special treatment
Data traffic such as email or a file download does not mind small delays. If a packet arrives late, it is simply waited for. Voice is different. An IP phone sends a small packet every 20 milliseconds, and each one must arrive on time and in order. If packets are delayed, arrive unevenly or are lost, the listener hears gaps, echoes or a robotic voice.
On a busy network, a large backup or a video call can fill a switch port or the internet link for a few seconds. Without QoS, voice packets wait in the same queue as everything else. With QoS, they jump to the front.
Targets for good call quality
| Measure | Aim for | What happens beyond it |
|---|---|---|
| One-way delay (latency) | Under 150 ms | Callers talk over each other |
| Jitter (variation in delay) | Under 30 ms | Choppy or robotic audio |
| Packet loss | Under 1% | Words and syllables go missing |
Inside a well-built office network, all three should be close to zero. Problems usually appear on the internet link, on Wi-Fi or on an overloaded switch. For home and remote users, see VoIP call quality for remote agents.
Step 1: Put phones on a voice VLAN
A VLAN splits one physical network into separate logical networks. A voice VLAN keeps IP phones apart from computers, even when both plug into the same switch, or when the computer plugs into the PC port on the back of the phone.
Benefits:
- Priority is easier to apply, because everything on the voice VLAN is voice.
- Broadcast traffic from computers does not reach the phones.
- Security improves, because a laptop cannot easily reach the phone system’s management ports.
- Addressing stays tidy, with phones in their own IP range and DHCP scope.
How phones learn the voice VLAN
You can set the VLAN ID on each phone by hand, but on more than a few phones it is better to let the network tell them. The common ways are:
- LLDP-MED, where the switch announces the voice VLAN and the phone joins it automatically.
- A DHCP option, where the data network’s DHCP server tells the phone which VLAN to use, and the phone then asks again on that VLAN.
- Provisioning, where the phone system pushes the VLAN setting in the phone’s configuration file.
Check which methods your phone model and switch support before you start. With Voxdial IP PBX, BitVoice IP phones are auto-provisioned, which keeps settings consistent across every desk. See IP phone auto provisioning.
Step 2: Mark voice packets with DSCP
DSCP is a small field in every IP packet that says how important it is. Phones and phone systems mark their packets, and switches and routers use the marking to decide what to send first.
| Traffic | Common DSCP value | Meaning |
|---|---|---|
| Voice media (RTP) | 46 (EF) | Expedited forwarding: always first |
| Call signalling (SIP) | 24 (CS3) or 26 (AF31) | High, but below voice |
| Everything else | 0 (best effort) | Normal data |
Then configure your switches and router to trust these markings on voice VLAN ports and give EF traffic a priority queue. Marking alone does nothing if the network ignores it.
On the internet link, priority only works on the side you control, which is your outbound traffic. Your operator decides what happens to inbound traffic. That is why a dedicated link or a properly sized connection for SIP trunks matters more than any router setting.
Step 3: Size the bandwidth
Each call uses bandwidth in both directions at the same time. The codec sets the voice payload, and packet headers add a fixed amount on top. At the standard 20 ms packet size:
| Codec | Voice payload | On the IP network | On an Ethernet LAN |
|---|---|---|---|
| G.711 | 64 kbps | about 80 kbps | about 87 kbps |
| G.722 (HD voice) | 64 kbps | about 80 kbps | about 87 kbps |
| G.729 | 8 kbps | about 24 kbps | about 31 kbps |
So 20 calls at the same time on G.722 need roughly 1.7 Mbps in each direction on the LAN. Add headroom of at least 25 to 30 percent and never plan voice to fill the internet link. For codec choice, read VoIP codecs: G.711, G.729, G.722 and Opus.
Inside the office, a gigabit switch has more than enough bandwidth for voice. The internet link and site-to-site links between branches are where sizing matters.
Step 4: Use PoE switches and a UPS
IP phones can take power from the network cable using PoE, so you do not need a power adapter at every desk. Choose switches with enough PoE budget for all phones, and put them on a UPS together with the IP PBX. Then phones keep working through short power cuts. Read PoE for IP phones for budget planning.
Common mistakes
- Creating a voice VLAN but not trusting DSCP on the uplink ports, so priority is lost at the first switch.
- Running phones over Wi-Fi in areas with weak coverage. Use wired phones for desks.
- Using cheap unmanaged switches that cannot handle VLANs or QoS.
- Forgetting the PC port. If a computer plugs into the phone, its traffic must stay on the data VLAN.
- Turning on SIP ALG on the router. It often rewrites SIP messages badly and causes one-way audio or dropped registrations. Turn it off unless your operator asks for it.
Want your network checked before an IP phone rollout? Talk to BitVoice.
FAQs
What is QoS in VoIP?
QoS, or quality of service, is a set of network settings that gives voice packets priority over other traffic, so calls stay clear even when the network is busy.
What DSCP value should I use for voice?
Mark voice media as DSCP 46 (EF) and call signalling as DSCP 24 (CS3) or 26 (AF31). Then make sure switches and routers trust and act on those markings.
Do I need a separate VLAN for IP phones?
It is strongly recommended. A voice VLAN makes priority easier to apply, keeps computer broadcasts away from phones and improves security.
How much bandwidth does a VoIP call need?
At 20 ms packets, a G.711 or G.722 call needs about 87 kbps each way on an Ethernet LAN, and a G.729 call about 31 kbps. Multiply by your busiest-hour calls and add headroom.
