Dumb power-over-ethernet question

[Boydie]

Do PoE devices need to be connected directly to the injector via a single cable?
Or is it acceptable to do injector to RJ45 wall plate, with receiving device connected to a wall plate in a different room?
If so, what’s the best cable to use to connect the two wall plates - Cat 5/5e/6? Limited research suggests Cat6 has lower power dissipation due to thicker conductors, but may get data crosstalk for the same reason.

I suspect it’ll be fine but all the examples online show direct connection between injector and device (and the two I use myself have this arrangement), so I though I’d better check.

[jp6741]

You can definitely use PoE via sockets, for example this would be how you connect wall sockets to a PoE rack switch in a business.

All you have to watch for is plugging non-PoE, or the wrong PoE, devices into a “hot” socket and blowing it up.

As for cable, Cat 6 should be fine, just watch for cheap sources or CCA markings.

[Mince]

Yes — this is how it's normally done: you put the power supplying device (i.e. the PoE switch) in the closet and use the structured cabling in the building to feed the data and power into the offices where you connect a camera or wireless AP. The fixed part of the cabling is usually no more than about 80-90m with the assumption that the drops at either end are only 2-3m.

The cabling is typically Cat6A these days (at least in our environment, which replaced Cat5E — we never really used Cat6). 6A is a bit annoying in cabinets as the cables are fairly thick and the bend radius is not small. Luckily, around that time, someone (at least in our environment) worked out it was better to interleave the switches and patch panels so the cables can usually just jump the 10cm or so between them and don't need to chase up and down the vertical rails — the only problem is where some switches supply higher power or >1Gbit/s speeds and we have the ports needing those scattered around.

Plugging non PoE devices into PoE switches is not particularly problematic as there is a protocol for signalling that the power is to be supplied: the cable doesn't just have 48V at 60W chugging down it from the get-go — the device has to indicate what it wants at the electrical level, to get it turned on, then it can use LLDP-MED to signal more exact requirements, after it's booted. Even more complicated is the protocol for switching the power off again!

One of the confusing things is heat dissipation as the power is supplied in the cabinet and some heat is generated by the switches but some is also lost in the cables and then at the device (e.g. wireless AP), which tends to confuse people designing power and cooling requirements.