What it is
Power over Ethernet is a way to send data and direct current power over one standard Ethernet cable. The device at the far end gets its network link and its electricity from the same port. That means one cable to a wireless access point, a camera, a desk phone, a badge reader or a sensor. No local outlet and no power brick at the device.
Two roles matter. The power sourcing equipment, or PSE, is the switch or injector that supplies power. The powered device, or PD, is the camera or phone that draws it. A PoE switch is the usual office setup. A midspan injector adds power to a link when the existing switch has no PoE. Both work, but a switch is easier to monitor because power and data share one port.
The main standards are IEEE 802.3af, 802.3at and 802.3bt. 802.3af, often called PoE, supplies up to 15.4 watts at the source and about 12.95 watts at the device. 802.3at, or PoE+, raises that to 30 watts at the source and 25.5 watts at the device. 802.3bt adds Type 3 at 60 watts and Type 4 at 90 watts at the source, with less usable power at the device after cable loss. The gap between source and device watts is real: cable resistance turns some power into heat.
How it works
When you plug in a compliant PD, the PSE applies a low voltage and checks for a signature. If it sees the right signature, it negotiates a power class. Only then does it raise the voltage to the active range, typically 44 to 57 volts DC. The device starts, and the switch records the class and the draw. A non PoE device sees no full power, so a laptop or plain switch does not get fried by a PoE port.
Power travels on the same copper pairs as data. 802.3af and 802.3at use two pairs, either the data pairs or the spare pairs. 802.3bt uses all four pairs to reach higher wattage. That is why cable matters: all four pairs must be present and terminated. A cable with only two working pairs may pass data but fail to deliver bt power.
The channel limit is 100 meters from switch to device, including patch cords. Longer runs drop voltage and can cause reboots or failed startup. Thin cable makes it worse. So does bundling many powered cables in a hot ceiling. Managed switches add per port power limits, on and off control, and real time wattage. Some use LLDP or a vendor protocol to negotiate power with the device. Check the total power budget, not just the port count. A 24 port switch with a 370 watt budget cannot run 24 ports at 30 watts each.
Why it matters for a business
PoE moves the power source to the wiring closet. Each access point or camera can sit on a battery backed switch, so it stays up during a short outage. You also get remote power cycling. If a camera locks up, you can bounce the port from the switch instead of sending someone to unplug it.
It simplifies moves. A phone or badge reader needs one cable, not a cable and a nearby outlet. In older buildings with few outlets, that keeps the install clean. It also keeps power adapters out of ceilings, where they are hard to service.
The trade-off is that the switch becomes a shared resource. A bad power budget or a failed switch takes down every device on it. Plan spare ports, spare budget and a UPS for the closet. A cheap unmanaged PoE switch may power devices fine, but it gives you little visibility when a camera reboots at night.
Common mistakes
The first mistake is buying by port count. A 48 port PoE switch sounds generous until you add up the wattage. Check the total power budget and the per port limit. Leave headroom for startup draw and for devices you will add later.
The second is mixing standards without checking. An 802.3af camera on a bt port is fine. A bt pan tilt zoom camera with a heater on an af port will not start, or it will reboot when the heater kicks in. Read the datasheet for maximum draw, not average draw.
Cheap cable is a common trap. Copper clad aluminum has higher resistance than solid copper. It may pass data on a short run and fail on a long PoE run. Use solid copper Cat5e or Cat6 and keep the run under 100 meters. Label every port and record the device, the class and the wattage.
How to get started
Start with a list. For each device, write down the model, the PoE standard it needs, the maximum watts, and whether it has a heater or motor. Add the cable length. Group devices by closet, then count ports and watts. That gives you a real switch specification instead of a guess.
Choose a switch with a power budget above your current total, plus room for growth. Pick managed if you want per port control and monitoring. Size the UPS for the switch and the devices together. Label both ends of every cable and keep a simple record.
Roll out one closet at a time. Test devices under load and watch the switch power graph for a week. If a device reboots at night, check the cable, the port limit and the startup draw before you replace hardware. If your device inventory is unclear, one option among others is Cloudverg's Hardware and endpoint review at $4,500/month. It covers an inventory of endpoints, servers, network gear and licenses, a standard device specification per role, and a three year refresh plan. It runs as a fixed scope over two weeks and starts within 10 business days.
Frequently asked questions
Does PoE work on any Ethernet cable?
It needs solid copper cable with all four pairs for the higher bt types. Cat5e or Cat6 is the usual choice. Keep the run under 100 meters and avoid copper clad aluminum. A short flat patch cord may work on the bench and fail in a ceiling.
Can PoE damage a device?
Standard PoE checks for a signature before it sends full power, so a compliant non PoE device should not be damaged. The risk comes from passive injectors that send power all the time, or from a nonstandard pinout. Use compliant switches and injectors, and label ports you have set to forced power.
How many PoE devices can one switch power?
It depends on the switch power budget, not the number of ports. Add the maximum watts for each device and compare that total to the budget. Leave room for startup draw and growth. A port count alone tells you nothing about available power.