PoE switches
A PoE switch that will actually power what you plug into it
Every PoE switch publishes two power figures, and buyers read the wrong one. The per-port wattage says what a single port may offer; the total budget says how many ports may offer it at the same time — and it is the budget that decides whether the last access point comes up.
By Jean-Benoit Buisson
Read the Type, not the trade name
The specification is a Type number, published by the IEEE, and it comes with a pair of wattages.
802.3af, Type 1: up to 15.4 W at the switch port, 12.95 W guaranteed at the device, delivered over two of the four pairs.
802.3at, Type 2: up to 30 W at the port, 25.5 W at the device, with an additional classification step through which the source and the device confirm each other. The higher allowance is granted by that exchange rather than assumed, which is why two devices that both say “PoE+” can behave differently on a switch that supports only one of the two methods.
802.3bt, Types 3 and 4: 60 W and 90 W respectively at the port, on all four conductors, each with its own lower guaranteed figure at the device.
Names printed on boxes and injectors — the various “plus” and “ultra” words — are marketing rather than specification. The Type number and the published wattages are what two data sheets can be compared on.
The budget is the number that decides your installation
A switch with eight PoE ports rated at 30 W each does not necessarily supply 240 W. Its total budget is a separate figure, often considerably lower, and it is the ceiling for everything drawing at once.
So the calculation is: add the requirement of every powered device you intend to attach, taken from each device’s own data sheet, and compare the sum with the budget. Then leave headroom, because devices draw their maximum at inconvenient moments — an access point with several clients, a device starting up — and a budget spent exactly is a budget that fails intermittently, which is the worst way for it to fail.
Where a switch mixes PoE and non-PoE ports, check which ports are which. It is a common and sensible design, and it is also a common reason for a device not powering up.
Match the switch to the devices, in that order
Work from the far end back. List what has to be powered — access points on ceilings, a phone, a small appliance that takes power over its network cable — and read the Type each one requires from its own documentation.
A device specified for Type 2 will not settle on a source that offers only Type 1; it will either fail to start or restart repeatedly, which looks like a fault in the device. Buying the switch first and discovering this afterwards is the ordinary sequence, and it is worth reversing.
Then check the run. The two published wattages differ because the cable between them takes a share, and a long run to a loft is where that share is largest.
Port speed and PoE are independent decisions
A switch can deliver a great deal of power over gigabit ports, or very little over multi-gigabit ones. Read the port table for speed and the power table for wattage, and do not let one imply the other.
For access points, the uplink speed of the switch port is the ceiling for everything passing through that unit. It is generally a gigabit and generally sufficient; the point is to know it rather than to discover it.
Noise, heat and where a PoE switch can live
Delivering power makes heat, so PoE switches run warmer than their plain equivalents and reach active cooling at lower port counts. A fan-cooled unit in a hallway cupboard is fine; the same unit on a shelf in a study is not.
Read the maker’s published clearance and ventilation requirements before choosing the position, and leave a rack unit of space above the switch if it is going into a cabinet.
Injectors, and when they are the better answer
For one or two powered devices, a single-port injector supplied with the unit does the job for a fraction of the price of a PoE switch, and it can sit next to an existing switch quite happily.
The switch becomes the better answer at three or four devices, when the injectors and their mains sockets have become a clutter, and when having one place to read the total budget starts to matter. Anything involving a socket added to feed an injector in a loft or a cupboard is governed by the manufacturer’s instructions and the wiring rules where you live.
