I’ve been researching Power over Ethernet (PoE) lately, as I have two PoE switches at home. It wasn’t a requirement I had in mind when I bought them, but I’m happy I did now because it allows me to play around with it. I also strongly recommend to look into PoE for you lab if you’re going for Voice. I have some IP Phones I use as end devices in labs: they can be pinged, have a build-in webserver on port 80, are cheaper than a seperate computer and consume less power, especially on PoE.

To the point: it is remarkably hard to find simple information about the mechanics of PoE without having to go through many (sometimes contradicting) resources. There are three methods of PoE I’m going to discuss: Cisco’s Inline Power (ILP), the 802.3af standard and the 802.3at standard. Besides that, there are many vendor-specific implementations.

ILP was, like most vendor-specific PoE solutions, developed before the standards. It uses a 340kHz tone of AC current which it continuously sends through one of the data pairs. A Cisco IP Phone will loop back the tone (only that tone, using a filter, no actual data frames) to the PoE switch, which will detect it and grant power to the line. Once the IP Phone becomes active, in low-power mode (6.3W), it will send a CDP message stating the correct power requirements. The switch will adjust it’s power to that, so the IP Phone can work properly. Documentation is vague about the maximum supported output, but it seems to be 15.4W, like 802.3af.

802.3af detects a powered device by sending a low DC current through the transmit and receive pairs. If a resistance of 25,000 Ohm is measured, caused by a PoE supported device attached to the port, power will be granted. After the device powers up, it will be classified into a category. The device will create a load, the switch will recognise the load and determine the class. If this is not done, the default class 0 is used. Classes are:

0 – 15.4W (default, the maximum supported)
1 – 4.0W
2 – 7.0W
3 – 15.4W
4 – More, 802.3at

There are two power modes: mode A and mode B. In mode A, pairs 1,2 and 3,6 are used for power. These are also the data wires in 10Mbit and 100Mbit connections, a ‘phantom technique’ is used to put both streams on the wires. In mode B, pairs 4,5 and 7,8 are used, which are spare pairs in 10/100Mbit networks. In gigabit networks all pairs are used for communication. Note that a device requiring 10W for example, must use class 0 or 3, so the 5.4W that is allocated too much to that device can’t be used for another device on the same switch. Cisco’s ILP does not have this problem.

And last: 802.3at, commonly named PoE+. It supports up to 25,5W officially, but since both modes (Mode A: pins 1,2 and 3,6; mode B: pins 4,5 and 7,8) can be used at the same time, devices exist that can draw 51W of power using this standard.

So those are the standards, but what devices can provide that power, and use that power? Providing is done by either an end-span device (a switch or switch chassis blade) or a mid-span device (a power injector). Record holder for the most output seems to be the Mega PoE by Phihong for the moment, at 95W per port. No idea whether this has any real use, and whether the cables will heat significantly during usage.

As far as devices that can use PoE goes, while it originally started with IP Phones, many other devices support it now too, e.g.:

  • Wireless Access Points, for places where no power cables are, like ceilings.
  • IP Cameras, for the same reason as above.
  • Switches, like the WS-C2960PD-8TT-L.
  • Complete thin clients and computers, like the ones by SkinnyBytes, supporting 802.3at.
  • Print servers
  • Electric guitars (yes, I’ve found multiple articles, just not in English but my native language).

For an even more in-dept read, this was some of my best source material. The only thing I’m unable to find out so far is which modes are used by the switches used in my lab. The IP Phones power up completely without CDP, indicating that 802.3af is present and working, but if I turn on CDP, which method is used? Still 802.3af, or Cisco’s ILP? There are no commands to toggle these modes, just a general ‘power inline auto’ and ‘power inline never’. If anybody knows, please let me know (in the comments).

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