Planning a VoIP phone rollout: bandwidth, VLANs and PoE budgets

A phone rollout looks simple on paper: plug in the phones and start calling. In practice, three things decide whether calls sound clear on day one or whether you spend the week chasing complaints.

1. Bandwidth per call

Each call uses bandwidth in both directions, and the packet headers add a lot on top of the voice itself. A G.711 call at 20 ms uses about 87 kbps each way on Ethernet; G.729 uses about 31 kbps. Use the VoIP bandwidth calculator with your codec and the number of calls you expect at the busiest hour, then add headroom for growth.

2. A separate voice VLAN

Put phones in their own VLAN. It keeps voice traffic apart from busy PCs, makes quality of service (QoS) easy to apply, and lets you write tighter security rules. Most switches can put a phone and the PC plugged into it on different VLANs through one port. The Cisco switch commands page shows the switchport voice vlan setting.

3. A PoE budget that adds up

Desk phones usually draw 4 to 7 W, but video phones and conference units can need PoE+. The catch is the switch's total PoE budget: a 48-port switch may not power 48 devices at full load. The PoE power budget calculator shows the power each device needs at the port (including cable loss) and how much of the switch budget you will use.

Before go-live

• Test a few phones on the longest cable runs first.
• Make a call while copying a large file over the same link and listen for choppy audio.
• Document which switch port and VLAN each phone uses.

Voice is unforgiving: a little packet loss that nobody notices on a web page sounds terrible on a call. Plan for the busiest hour, not the average one.