What Is PoE?

What PoE is

PoE stands for Power over Ethernet. A regular network cable contains eight thin wires, four pairs. The same wires that carry data can also carry a low-voltage electric current — enough to power a phone, a camera or a Wi-Fi access point.

Without PoE, every phone on a desk needs two connections: a network cable to the wall and a power adapter to the outlet. With PoE — a single cable. It sounds like a small detail, but in an office with thirty workstations that means thirty fewer adapters, thirty free outlets, and much tidier desks.

The method became an official standard in the early 2000s, and since then almost every desktop IP phone, security camera and professional Wi-Fi access point supports it.

How it works

The device that supplies the power is called a PSE (source) in technical language, and it is usually a PoE switch located in the network cabinet. The device that receives it is called a PD (powered device) — the phone.

1. You connect the phone with a network cable.
2. The switch sends a very low voltage and checks: is there a PoE-capable device on the other end? If you connect a regular computer, the switch detects that there is no such device and sends no power — so it is safe.
3. The phone reports which "class" of power it belongs to — how many watts it needs.
4. The switch turns on the power, and the phone lights up. All of this takes seconds.

Power and data travel over the same cable without interfering with each other. No special cable is needed — a standard CAT5e or CAT6 network cable works, up to the same 100 meters as any network cable.

Standards and power levels

Some of the power is lost in the cable, so the device receives slightly less than the switch sends. For a desk phone this doesn't matter: most actually use only a few watts.

Beware of passive PoE: some cheap equipment pushes constant power through the cable with no detection and no standard. Connecting it to a device that doesn't expect it can cause damage. In office equipment, stick to standard PoE.

The switch's power budget

This is the detail worth knowing before you buy. Every PoE switch has a total power budget — how many watts it can give to all ports together. A 24-port switch can't necessarily give 30 W to each of them at the same time.

A sample calculation: 20 phones at about 5 W = 100 W. Two Wi-Fi access points at about 12 W = 24 W. Four cameras at about 6 W = 24 W. In total about 150 W. A switch with a 120 W budget won't be enough, and the last two devices to connect simply won't power on, or will power on and off intermittently.

The rule: add up the consumption of all the devices, add a reserve, and choose a switch whose budget is higher than the sum. Phones with a large touchscreen or with extra key modules consume more, so it is important to know in advance which model sits at each workstation.

PoE during a power outage

This may be the most important advantage for IP phones. When all the phones get their power from the switch, it is enough to connect the switch, together with the router, to a single UPS in the cabinet — and all the phones in the office keep working during a power outage, as long as the battery lasts and the internet is up.

Without PoE, every phone depends on its own outlet. In a power outage, even if the router is backed up, the phones themselves shut off, and the office is left without a phone.

Note that the UPS must be suited to the load: a switch powering twenty phones consumes more than a router alone, so the backup time depends on the battery size and the number of devices. It is worth checking in advance how long it actually lasts.

And if the switch doesn't support PoE

There are two solutions. The first: a PoE injector — a small box that connects to an outlet and to the network cable, and adds power to the cable of one device. Suitable for a single workstation or one Wi-Fi access point. The second: a power adapter for each phone, which comes with most models or is sold separately.

But when you set up an office with more than a few phones, one PoE switch in the cabinet is simpler, tidier and cheaper over time than a pile of adapters and injectors.

Common mistakes

• Not calculating the power budget — and then discovering that half the phones power on and half don't.
• A PoE switch without a UPS — you miss the big advantage: phones that work during a power outage.
• Cheap cables — a cable with wires that are too thin or not solid copper heats up and loses power over long runs.
• Connecting both an adapter and PoE — usually harmless, but unnecessary and confusing on the day of a fault.
• Confusing "PoE ports" with "all ports" — on some switches only some of the ports deliver power. Check the specifications.

A detailed example: 20 phones and four Wi-Fi access points

Take a real estate office with 20 phones and four Wi-Fi access points, and do the calculation the way it is done before ordering. The detail that matters is not "how many phones" but which phones:

Now three additions that people forget. Loss in the cable: on a 70-meter run a few more percent are lost. Allocation by class: many switches reserve for each port the maximum of the class the device declared, not the actual consumption, so a Wi-Fi access point that requests PoE+ "takes" up to 30 W of the budget even when it consumes 13. Reserve: more phones next year, and a camera at the entrance.

The conclusion: a 24-port switch with a 190 W budget looks sufficient by the first sum, but in practice will be at or over the limit. A switch with a 370 W budget leaves room. And if the four Wi-Fi access points require PoE+, check that the switch gives PoE+ on all ports, not just the first four.

PoE classes: what the phone "asks" the switch for

In the first seconds after connection, the phone and the switch hold a short negotiation, and the phone reports which class it belongs to. The class determines how much power the switch reserves for the port:

An old or cheap device that doesn't declare a class is treated as class 0, and the switch reserves the full standard PoE maximum for it. That is why on some switches the budget "runs out" long before real consumption gets close to it: twenty ports that each reserve 15 watts add up to 300 watts on paper, even when the phones together draw 90.

Smart and managed switches can switch to allocation by actual consumption, and many also support a protocol in which the phone reports exactly how much power it needs (LLDP-MED). When the budget is tight, this is a setting worth checking in the spec sheet before you replace a switch.

Another spec-sheet detail: port priority. On many switches you can mark ports as "critical," so that when the budget runs out the switch shuts off the less important ports first (for example, the warehouse camera) and not the reception phone. On a simple switch the decision is made by port number, so the important phones are connected to the lower-numbered ports.

When something doesn't work: a phone that won't power on

The most useful diagnostic tool is also the simplest: swap locations. A phone that works on port 3 and not on port 20 means the problem is in the port or the line, not in the phone. A phone that doesn't work anywhere is a phone that needs to be replaced.

Tips from the field

• Remote restart. On a smart or managed switch you can turn a single port off and on from the browser. A phone that gets stuck gets an "unplug from the wall" without anyone crawling under the desk.
• PoE off on computer ports. It's not required, but it's tidy: mark the ports meant for computers and printers as non-PoE, so the budget isn't reserved for them by mistake.
• Measure after installation. Smart switches show how many watts each port actually draws. One look at the table after a week of work tells you whether the plan was right.
• Heat. A switch that powers 150 watts of phones also gives off heat. Leave 1U empty above and below it in a crowded cabinet.
• An injector is temporary, not permanent. A PoE injector is good for a workstation added in a hurry. If it stays more than a few months, it's worth planning a real PoE port.