You’d think that now that the Power over Ethernet (PoE) standard is ratified, life would be simple. But when it comes to requesting power from the local LAN switch, it seems that some end stations are more equal than others.

The PoE standard specifies how to deliver DC power over normal Cat 5 Ethernet cable, alongside the data signal (see Power over Ethernet for Office WLANs). But different devices require different amounts of power. If you know the power needed for the kit you’re connecting, you can work out how much power (and how big a PSU) you really need, and maybe save yourself some money on AC to cool down all that unnecessary power.

Power Classifications
Part of the 802.3af specification—but it is optional—defines several power classes into which end stations, be they wireless access point (AP), IP telephony handsets or CCTV cameras, will fall, based on the actual amount of power they require. These are detailed below:

  • Class 0 (default): Power reserved for device at switch port is 15.4W. Actual requirement may be much less
  • Class 1: Power reserved for device at switch is 4W (will deliver between 0.44W & 3.84W to device)
  • Class 2: Power reserved for device at switch is 7W (will deliver between 3.84W & 6.49W to device)
  • Class 3: Power reserved for device at switch is 15.4W (will deliver between 6.49W & 12.95W to device)
  • Class 4: Reserved for future use

The switch classifies the end device by injecting a low voltage signal towards the device. The device puts a load on the wire, and the switch uses the load current returned to determine into which class it should put that device, and hence how much power should be sent out of that port. If either the switch or the end device doesn’t support this process, then the default class will be used, and 15.4W delivered.

Now before we go any further, we need to explain where the 15.4W number comes from, since we’ve said in the past that up to around 13W is delivered to the device.

This is to do with the power efficiency of the switch that provides the power, and also the loss over the Ethernet cable. Working backwards, the spec calls for a maximum of 12.95W at the device. With a 100m cable run, this means the switch port needs to deliver up to 15.4W. Because of losses over the line cards and switch backplane, the switch PSU may actually need to provide for up to about 17W per port to be able to guarantee full power to every port. That’s a difference of 4W per port between what the end device gets and the raw power that has to be provided, so it’s important to take that into account when figuring out what power supplies you need.

This is where the advantage of using equipment that does support power classification can make a difference. Some wireless access points, for instance only use 6W. This means they fall into class 2, and the switch need only provide 7W. Which, in theory, if the switch was populated with APs, means you’d only need half the power you might expect.

Vendor options
Of course, the whole point of a standard (as far as the manufacturers are concerned) is to provide something for them to add their own proprietary improvements to, to make you go with their products for the ‘added-value’. And 802.3af is no different.

So you’ll be offered a couple of extra features that add flexibility to your network — but you’ll maybe have to stick with one vendor to get the benefits.

One option that some manufacturers provide over and above the standard is the ability to prioritise the ports on your switch, so that if you run out of power (by adding a new line card you hadn’t counted on in your power calculations, or if one PSU fails), you can define which powered ports will be kept going in preference over others. One system is the PowerDsine 6012 reviewed here.

Another possibility is the option to be even more granular than the classifications allow. Cisco Catalyst switches, for instance use the Cisco Discovery Protocol (CDP) to find out exactly how much power is needed. A Cisco IP phone, for example, that requires 9.3W would fall into class 3. Connected to a non-Cisco switch, it would use up the full class 3 power draw; connected to a Cisco switch, it will negotiate down to just what it actually requires.

If you’re looking for Power over Ethernet switches and end devices, it’s worth taking a look into how they handle power management and control. Buying a bigger PSU than you need to can have knock-on effects in terms of the UPS you need, and the AC you need to install—you may be able to do with less.