Jumping to the other end of the protocol stack, we visit Power-over-Ethernet, which specifies how a switch or other power-sourcing device provides DC power to remote devices, alongside the data, over the network cable.

I’ll let the wikipedia writeup explain the technical details. Essentially, it’s an electrical/wiring protocol, not a data protocol.

Devices at PSU that use PoE include Cisco phones and wireless access points, and probably others. Older Cisco phones, such as the 7910, 7940 and 7960, use Cisco’s pre-standard PoE implementation. Newer phones use the standard IEEE 802.3af implementation.

PoE is not dangerous to devices that do not need power through the Ethernet cable. Power is only sent if the correct resistance is detected, and otherwise is switched off for that port.

Desktop VoIP phones today are typically offered “power brick sold separately.” In a deployment of 15,000 IP phones, we could occupy 15,000 power outlets and waste a lot of money on power bricks, or provide power at the switch port and save wiring and money. Moreover, VoIP equipment in the telecom closets is protected by a UPS, meaning that if the phone is drawing power from the network switch (as opposed to the wall outlet) it benefits from the battery backup as well.