Telecoms glossary • P
PoE (Power over Ethernet)
One cable to the desk instead of two. The specification to check is not the per-port rating but the switch’s total power budget.
What is PoE (Power over Ethernet)?in VoIP and business telephony
1. Power over Ethernet delivers electrical power and network data down the same Ethernet cable, so an IP desk phone needs one cable and no plug point. The switch supplies the power, the device negotiates how much it needs, and the whole arrangement is defined by the IEEE 802.3af, 802.3at and 802.3bt standards.
2. For voice it is mostly an installation convenience, with one operational benefit that matters more: a single UPS on the switch keeps every phone in the building powered, instead of a plug-pack and a battery at every desk.
The standards, and which one a phone needs
Three IEEE standards matter, and they stack: a device built for the oldest works on a switch built for the newest. The figures differ at the switch and at the device because some power is lost in the cable, which is why every standard quotes both.
| Standard | At the switch | At the device | Pairs | Typical use |
|---|---|---|---|---|
| 802.3af (2003) | 15.4 W | 12.95 W | Two | IP phones, basic access points, most small cameras. |
| 802.3at (2009) | 30 W | about 25 W | Two | Higher-power access points, pan-tilt-zoom cameras. |
| 802.3bt Type 3 (2018) | 60 W | at least 51 W | Four | Wireless access points at the top of the range, displays. |
| 802.3bt Type 4 (2018) | 90 W | at least 71 W | Four | Laptops and lighting. Nothing to do with telephony. |
Desk phones sit comfortably at the bottom. A Yealink T31G is 802.3af class 2; a T46U is 802.3af class 3 and draws between 1.6 W and 3.8 W over PoE. Anyone quoting you 802.3at switching on the grounds that the phones need it is selling capacity for something other than phones.
Power budget is the number that catches people
Per-port ratings are advertised. Total budgets are in the specification sheet, and they are what actually limits a switch. TP-Link’s TL-SF1008LP is a fair example: eight ports, PoE on the first four, 802.3af support for up to 15.4 W on each of those ports, and a total budget of 41 W for the switch.
Four ports at 15.4 W is 61.6 W, which the switch cannot supply. Four desk phones at 3.8 W each is about 15 W, which it can supply several times over. The port rating is a ceiling per port; the budget is the pool they all draw from.
That distinction only becomes a problem when the switch is shared. A cabinet running twelve phones, three access points and a couple of cameras is a different sum from one running twelve phones. Add up the worst-case draw of everything you intend to plug in, compare it to the budget rather than the port rating, and leave headroom for the next thing someone adds.
What it changes in a South African office
- One UPS instead of twenty
- Put the PoE switch on a UPS and every phone it powers rides through a power interruption. This is the single most useful thing PoE does for a voice site, and it is why the cabinet is where continuity planning starts. It only works if the router and the fibre terminating equipment are on the same UPS, because phones with power and no internet are just phones with lights on.
- Fewer plug points
- A twenty-desk office needs twenty fewer sockets and twenty fewer plug-packs. Less clutter, and one fewer thing for a cleaner to unplug.
- Phones go where the network goes
- Reception desks, conference rooms, a warehouse wall. Cat5e or better, and the usual 100 metre Ethernet limit still applies, power or no power.
- Injectors for the odd device
- A single-port injector adds PoE to one connection without replacing a switch. Sensible for one or two devices, false economy beyond that, and it reintroduces the plug point you were trying to avoid.
One caveat worth carrying into a quote: expansion modules on a receptionist’s phone often need their own adapter, so that desk needs a socket regardless.
Sizing a switch honestly
Count every device that will draw power from the switch, use each one’s worst-case figure rather than its idle figure, and compare the total against the switch’s power budget rather than its per-port rating. Then check that the phones you are buying ship with a power supply, because many business SKUs do not.
If continuity is the point of the exercise, size the UPS for the switch, the router and the fibre terminating equipment together, and test it by pulling the mains rather than by reading the runtime on the box. Ten minutes of testing is worth more than any specification sheet.
About this entry
Definitions are written for South African business buyers and checked against primary sources – provider documentation, standards bodies and ICASA – not vendor marketing. Reviews are independent and sponsors are always disclosed. Read our editorial policy and scoring methodology.
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