Eli5: How does Ethernet over Power works?

Question

And while we’re at it, how does Power over Ethernet (PoE) works?

Thanks

In: Engineering

Answer 1

Power over Ethernet literally just puts 48V down the data cables if there is a given resistance between 2 cables in an Ethernet cable.

To request power, a device just needs to put that exact resistance on the right cables, and the PoE switch/injector will detect that and supply 48V down the cables. (And it does this by “biasing” the cables by 48V – they still have the same data going down them as normal but the switch/injector just starts the electrical signal at 48V and the PoE device knows to take off 48V before it starts processing the data in those signals).

Local Ethernet over powerline works in a similar, but much more complicated way. The usual 110/220v power that runs around your house works on a regular cycle of 50 or 60Hz… rising and falling regularly.

However all devices connected to that power circuit are basically just wired to the same set of cables running around the house. So if a Ethernet-over-main device modifies that regular power signal ever so slightly, the signal it does that with is sent everywhere on the same cable. It basically overlays the Ethernet signal (with a lot of processing and encryption and other changes) onto the mains cable that powers it. Another device on the other side of the house can then read those signals off the incoming power and send their own to form a network.

This has problems – first it’s shared so the more devices you have, the slower things get. Also it interferes with the electrical signal quite a bit and people like ham radio operators have complained about the signals it produces by accident coming out of people’s house wiring. Also it only works on the same phase – in large sites, each building will have a different electrical phase, so they are basically “disconnected”. I once had to explain to a bunch of teachers that thought they knew better about how to wire a large school site and insisted we use these devices. With power phases differing between buildings, there is basically no connection between devices like this, so they don’t work, or only work around one single building (and often quite poorly compared to a cheap cable).

Some places have this same system but where the signal goes back over the local electrical distribution network outside the home – for example some types of smart meter are able to communicate back along the electrical transmission network to the local power provider. This is far more complicated again and not widely deployed.

But pretty much they all rely on overlaying a small signal over a large amount of power (DC in the case of Poe, AC in the case of Ethernet-over-powerline) so that you can later split them back into signal and power separately.