authored by Chuck Bolvin, VP of Technology

802.3af is a standard that possibly not everyone has heard of but I’ll bet that anyone that has installed an enterprise class wireless LAN recently has used it without realizing it. 

802.3af is the IEEE standard for Power over Ethernet. This is the ability to provide operating power for devices via the existing CAT-5 ethernet wiring. 

Prior to 802.3af there was no standard way of powering devices such as wireless access points and IP cameras without having to run AC power to each device. Running AC power to each device can typically cost more than the devices and CAT-5 cabling itself. It also required the use of licensed electricians in most cases thus adding to the overall cost of the network. 

Standards bodies such as the IEEE and ANSI did not recognize placing power on the unused pairs of twisted pair cabling. Some manufactures got around this by calling for a second pair of low voltage wire to be run to the device. A power supply located near the ethernet switch was used to send power to the devices. One example was the old Telxon Remote Power System (TRPS).  

Eventually a new standard was created by the IEEE that would address the issues. This new standard, 802.3af was ratified in July of 2003. 802.3af specified the mechanism whereby a device could be powered by either the unused pairs of a CAT-5 line or by impressing power on the used pairs. 

By using these mechanisms, manufactures of end devices such as IP Cameras and wireless access points could integrate the required circuitry in the devices themselves. These devices would be called Powered Devices (PD). Manufactures of infrastructure equipment such as ethernet switches could also integrate the power supplies into their gear. These would be called Power Sourcing Equipment (PSE).  

In its simplest form, there are two kinds of PSE. End-Span and Mid-Span. Mid-Span was the first to hit the market. Mid-Span devices are typically called Injectors. They’re used when ethernet switches aren’t 802.3af compliant. Injectors sit in between the switch and the CAT-5 to the device.  A CAT-5 jumper cable is run from the switch to the Injector. A CAT-5 is then run to the device. If the device isn’t 802.3af capable, PowerDsine has a line of splitters that can be used to breakout the power and ethernet to the device. The leading manufacturer of 802.3af Mid-Span injectors and one of the main contributors to the standard is PowerDsine.  

The other method is called End-Span. These devices are typically ethernet switches with 802.3af technology integrated in so no Mid-Span injector is needed. These kinds of switches are manufactured primarily by enterprise class vendors such as Cisco, Allied Telesyn and others. Allied Telesyn specializes in infrastructure products for the small to mid-size market and has several ethernet switches in various price ranges that are 802.3af compliant. For new installations where no infrastructure exists, Allied Telesyn is an excellent choice. 

In order for the 802.3af standard to work, it must be able to detect when devices attached aren’t capable of accepting power on the CAT-5. So any device that implements the standard must detect this and only apply power when specific conditions are met. This prevents damage to the end devices.  

There are also two ways of placing power on a CAT-5 cable. Mid-Span products use the unused pairs whereas End-Span devices can use either the unused pairs, or the used pairs. One thing to be careful of: Not all end devices implemented the complete standard early on. That means you may run into certain combinations of gear that may not work. For example, an ethernet switch that only implemented used pair power and an end device that only accepted unused pair power. 

On note of caution: There are many manufacturers that have implemented their own PoE. These manufactures are not 802.3af compliant and are not compatible. 

Fortunately WAV carries both PowerDsine for Mid-Span products and Allied Telesyn for End-Span products so we have the knowledge to help design and implement a complete 802.3af compliant system.