Power over Ethernet (PoE) sends power and data on the same network cable. For IP phones it means no power adapter at the desk, fewer cables, and phones that keep working during a power cut as long as the network switch is on a UPS. This guide explains how PoE works for IP phones and how to plan switches and backup power.
How PoE works
A PoE switch (or an injector) puts a small DC voltage on the Ethernet cable. The phone detects it and powers itself from the cable. The switch first checks that a PoE device is connected, so plugging a laptop into a PoE port is safe.
| Standard | Common name | Power at the switch port |
|---|---|---|
| IEEE 802.3af | PoE | Up to 15.4 W |
| IEEE 802.3at | PoE+ | Up to 30 W |
| IEEE 802.3bt | PoE++ | Up to 60 W or 90 W |
Most desk IP phones need standard 802.3af PoE. Phones with large colour screens, expansion modules or video may need PoE+. The BitVoice B301 PoE variant uses IEEE 802.3af.
Plan the switch power budget
Every PoE switch has a total power budget, which is often less than the number of ports times the per-port maximum. To size it:
- Find the power each phone draws on its datasheet. Entry-level phones usually draw a few watts, well below the 15.4 W limit.
- Multiply by the number of phones on the switch, and add any other PoE devices: wireless access points and cameras use much more.
- Add headroom of 20 to 30 percent for growth and start-up peaks.
- Compare with the switch’s PoE budget, not just its port count.
Keep phones up during power cuts
This is where PoE helps most in India. With PoE, the phones are powered from the server room, so one UPS covers them all.
- Put the PoE switches, the IP PBX, the router and any VoIP gateways on the same UPS.
- Size the UPS for the full PoE load plus the switches, and for the backup time you need.
- Without PoE, each desk phone needs its own adapter and its own backup, which is rarely done.
Cabling tips
- Use Cat5e or Cat6 cable; PoE works up to the normal 100 m Ethernet limit.
- Avoid cheap passive injectors; use a proper 802.3af or 802.3at switch or injector.
- Use the phone’s second Ethernet port to connect the PC, so one cable serves both. The phone passes data through; the PC is not powered over it.
Without a PoE switch
If your switches do not support PoE, you can:
- Use the phone’s power adapter at each desk, or
- Add a PoE injector between the switch and the phone, or
- Replace switches in stages, starting with reception and critical desks.
Common mistakes
- Checking port count but not the switch’s PoE budget.
- Putting the PoE switch on a UPS but not the PBX or the internet router.
- Buying PoE+ for every phone when standard PoE is enough.
Planning a rollout of IP phones? BitVoice supplies the B301 IP phone, the Voxdial IP PBX and the network planning. Talk to us.
FAQs
What is PoE in an IP phone?
PoE (Power over Ethernet) lets an IP phone take its power from the network cable, so no separate power adapter is needed at the desk.
Which PoE standard do IP phones use?
Most desk IP phones use IEEE 802.3af (standard PoE, up to 15.4 W per port). Phones with large screens or video may need 802.3at (PoE+).
Will PoE IP phones work during a power cut?
Yes, if the PoE switch, IP PBX and router are on a UPS. The phones take power from the switch, so one UPS covers all of them.
Can I use a PoE phone without a PoE switch?
Yes. Use the phone’s power adapter or a PoE injector between the switch and the phone.
