Power over Ehternet (PoE) - Powering UP VOIP phones

What it is ?     

      Power over Ethernet or POE, is a system capable of transmitting electrical power + data over a single ethernet cable. In this sense it is similar to a USB cable which also does the same thing by supplying both a data as well as power connection to low power devices like keyboards/mice etc. However, the maximum length of USB cables is about 5 meters and they provide 2.5 W of power. POE, on the other hand allows longer cable lengths and more power(IEEE 802.3at-2009 POE standard, provides up to 25.5 W of power). POE operation can be obtained with a Category-3 cable for less power, however the IEEE standard requires CAT-5 cables or higher for high power levels.

      POE is nowadays widely used to power VOIP Phones, IP Cameras and Wireless access points.

Usage with VOIP Phones - Avaya 4621SW/9641G

 

       With VOIP phones like Avaya 4621SW or 9641G, PSE (Power Sourcing Equipment) capable switches are required for the phones to pull power from. Without a POE capable switch the phone will not be able to boot up. Such a switch is called an endspan. However such switches may not be readily available. In that case an intermediary device is required between a non POE capable switch and a POE powered device(PD). Such a device is called a midspan. An example of a midspan device is a POE injector.

A POE injector typically takes two inputs :

1. Power (48V DC generally)
2. Ethernet Cable (CAT-5 generally, from a switch)

      It is able to provide a single output in the form of a CAT-3/4/5 cable that is capable of transmitting both power and data. Such a POE injector can be easily used to power up the Avaya phones listed above. The POE injector I used, was capable of supplying 15.4 W power over a distance of 100 meters over a CAT-5 cable and came at a cheap cost of about $20.