Power over Ethernet (PoE) delivers electrical power and network data over a single Ethernet cable, so an IP camera needs only one run back to the switch instead of a separate power supply at each location. This is the quiet enabler of modern surveillance installs: it removes an electrician from most of the job and lets cameras go where there is no convenient mains socket.
The IEEE standards set the power budget. 802.3af (PoE) supplies roughly 15.4 W at the switch port (about 12.95 W at the device); 802.3at (PoE+) about 30 W, enough for PTZ cameras and small heaters; and 802.3bt (PoE++) up to roughly 90 W for heated PTZ domes and other hungry devices. A standard run is limited to about 100 m, the normal Ethernet distance.
Two practical pitfalls. First, the switch has a total power budget across all ports — populate every port with a PTZ camera and you can exceed it, so the budget must be planned, not assumed. Second, distance and cable quality matter: thin or long runs drop voltage, and a camera that boots fine but resets under load (IR illuminators at night, heater on a cold morning) is often a power-budget or cable problem, not a camera fault.
