in Hardware, Telephony

the pros and cons of Power over Ethernet

Spurred on by a recent thread on the TAUG mailing list, I’ve been thinking about the cost vs. benefits of Power over Ethernet (PoE). PoE is a way to use the extra 4 wires in 10/100/1000-baseT Ethernet wiring for powering devices. The obvious application is to power IP telephones. Cisco has been doing this for a while using a proprietary solution which is similar to the PoE standard, IEEE 802.3af, which defines a protocol for detecting whether a device (or PD) is capable of receiving power prior to applying it, so that non-PDs don’t get fried by sending 48VDC down the wire. The protocol also determines how much current the PD will draw, and classifies the PD into a device class. All of this is well-described by this Wikipedia article.

There are two main advantages to using PoE, so far as I can see:

  1. No need to get a separate power brick for each device
  2. PDs can be centrally powered from a UPS in case of mains failure

The main con is the added cost, since you now need to purchase a PoE-enabled switch (which will then draw quite a bit of power, as it needs to power the devices).

What kind of cost are we talking about here?

  1. HP Procurve 2650 48-port switch (non-powered): USD $812.99
  2. HP Procurve 2650-PWR 48-port switch (PoE): USD $3,312.99

This works out to about USD $50/port, which I still think is a bit high. But maybe once the cost drops to around USD $25/port, it will be worth it.

Originally I was going to mention that purchasing PoE power injectors is another solution. A power injector has two Ethernet ports and a power brick attached to it; one of the Ethernet ports is an input from the regular non-powered network, and the other is a PoE output port to your PD. But I did some digging and find that the cost of a power injector is greater than USD $50, so I suppose this would only be feasible for a small deployment.

Step back for a moment and you realize that the cost of implementing the logic for PoE is far, far less than USD $50/port. The manufacturers are clearly just capitalizing on the fact that this is “new” technology and making a huge profit on it. As such I predict that there will definitely be wiggle room on the price in the future which will make PoE deployments more cost-effective.