And while we’re at it, how does Power over Ethernet (PoE) works?
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In: Engineering
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basically, Ethernet signals are just pluses of electrical energy down a wire. EOP and POE pretty much just increase the “carrier” signal for the messages up to a useful amount of power. instead of watching for a shift between 0.1 and 0.2 volts, you are looking for a shift between 120.1 and 120.2 volts instead.
its a bit more complex than that, but thats the ELI5 version
Let’s start with PoE. If you have a switch that can provide power, when it connects to something that needs power, it has a low voltage bit of power already being sent down the line (think, like an old phone or a doorbell). This is enough to power the device up enough to make a handshake that it needs power. This causes the switch to send power down the line and actually fully power on the device.
Ethernet over Power, also commonly called powerline, basically uses the power lines in a building to transmit network traffic. Depending on the wiring and what else the lines are being used for, it can work decently, or have all sorts of issues. Basically, in two different plugs, you plug in a device that translates from a regular ethernet connection to what it can transmit over the powerlines, and then the other end does the reverse.
The great advantage is that you don’t need to provide separate power lines to the device. In buildings, a big chunk of the cost goes to the installation guys. Sometimes more than the electronic devices. With PoE you save these costs.
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.
For PoE:
The 8 wires in an ethernet cable operate in differential pairs. This means that when you subtract the voltage of one wire from the other, you get the value of the pair. Normally the goal is 1-1=0 or 1-0=1, however, there’s nothing that prevents you from using larger numbers. If you take one of the pairs and increase both wires by 48, you get 48-48=0 and 49-48=1. This means the data can still transfer. You can then use one of the pairs as your 0V (it doesn’t actually have to be 0V because everything’s relative) and another pair as your 48V. The other side can then use those two pairs as their power source while data can still be sent over those pairs. Nowadays there’s active PoE where the devices negotiate the power, but the concept is still the same.
You have 3 people, 1 person doesn’t make any noise and just listens. One of the other two people is making a constant hum noise, the other is talking normally. As the listener it’s pretty easy to ignore the person making the hum noise and just listen to the person talking, right? PoE basically works this way, but the person making the hum noise is DC power and the person talking is the data. DC power is a constant voltage and easy to separate from the data (which is not constant voltage). There are some variations of PoE where the power and data are on separate wires, but for most modern PoE over gigabit ethernet they share the same wires.
Ethernet over Power is a big more complicated but works similarly. The AC power in your house is of a constant frequency so you can put some digital data (at a different frequency) on the same wires then separate them with a filter at the other end.
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