Power Over Ethernet

In my opinion, this is one of the most overlooked conveniences in SOHO networking. Providing power-over-Ethernet (POE hereafter) is tremendously useful. It lets me keep my Polycom and Aastra phones powered by the same UPS as the rest of network closet. If ever I need to replace my ATAs, I will definitely seek new units that are POE capable.

I especially feel that POE is useful for Wi-Fi access points. It lets you position the AP in a location that is selected for ideal wireless propagation (even outside in a weatherproof housing), without concern for providing an AC outlet. It also makes it easy to provide physical security for your WLAN from the wiring closet. That is, when I'm out of office for a week, the AP is powered down by simply unplugging the cat 5 jumper running to the AP.

A small Linksys switch capable of POE

POE can be provided by careful selection of your network switch. Some low cost 8 and 16 port switches provide POE on a limited number of ports, ideal for SOHO use.

It's also possible to add POE via "mid-span insertion". This involves placing a small power insertion device on the network line, between the switch and the device to be powered. This is how I started using POE, as my Aastra 480i phones came with POE insertion devices. I was so happy with them that I purchased a couple more for my Polycom phones.

Mid-span POE insertion devices come in single and multi-port models. The single port models look like "line bump" power supplies but with two RJ-45 jacks. Multi-port POE insertors look a lot like network switches.

POE capable switches are definitely more expensive than non-POE switches. If you shop wisely, you may find a POE capable switch that meets your needs, while superficially more expensive, is actually cheaper than a non-POE switch and a mid-span insertion device.

If you only need a few POE ports, then using mid-span insertion is typically less expensive.

In examining POE insertors or POE capable switches, it's worth noting how much current each port can provide. The Linksys 8 port switch pictured above, for instance, provides 15 watts per port when 4 ports are powered or only 7 watts per port when all 8 ports are powered.

You need to be aware of the power requirements of each of your upstream devices and be certain that the POE power source can handle all of them. Phones are not generally a large power draw. Wi-Fi access points and security cameras draw a little more.

The standard for POE is referred to as 802.3af and specifies not only the wiring standard, but also a protocol for POE power sources to detect if the upstream device is also POE capable.

Prior to this standard becoming widespread several manufacturers made equipment based upon their own standards. This is especially true for older Polycom and Cisco IP phones. These may require special network adapter cables to be powered by standard POE power sources.

Alternatively, some larger midspan POE inserters (ex Belden Power Sense) can switch between standard and device specific POE on a per-port basis. That can be very handy if you need to power a variety of devices.

While 100% VOIP, we are still able to keep our phones, our entire network for that matter, running when the power fails. The combination of a decent UPS and POE makes this possible.

Perhaps one day I'll pull the plug on the UPS and see how long everything runs. Its never been needed for more than 10-15 minutes at a time.

It's a truly amazing and wonderful thing to be sitting at my desk when the power goes out suddenly. Then, in the silence created by the total lack of PC noise, I find myself basking in the faint glow of the backlight from an Aastra 480i. The silence is shattered by the ringing of my phone. It's my wife calling from the house telling me that the power is out.

It's even more amazing when the entire network stays up throughout a power outage and I'm able to easily transition to working on a laptop complete with internet access over Wi-Fi.

This article originally appeared on Clearing My Head.
© 2008 Michael Graves

Tags: Asterisk, m0n0wall, PoE, VoIP,

Related Articles: