Power over Ethernet (PoE) Devices

Power over Ethernet (PoE) is a method of delivering DC voltage to an access point, wireless bridge, or wireless workgroup bridge over the Cat5 Ethernet cable for the purpose of powering the unit. PoE is used when AC power receptacles are not available where wireless LAN infrastructure devices are to be installed. The Ethernet cable is used to carry both the power and the data to the units.

Consider a warehouse where the access points need to be installed in the ceiling of the building. The labor costs that would be incurred to install electrical outlets throughout the ceiling of the building to power the access points would be considerable. Hiring an electrician to do this type of work would be very expensive and time consuming. Remember that Ethernet cables can only carry data reliably for 100 meters and, for any distance more than 100 meters, PoE is not a viable solution. The following figure illustrates how a PoE device would provide power to an access point.


Common PoE Options
PoE devices are available in several types.

• Single-port DC voltage injectors
• Multi-port DC voltage injectors
• Ethernet switches designed to inject DC voltage on each port on a given pair of pins

Single-port DC Voltage Injectors

Access points and bridges that specify mandatory use of PoE include single-port DC voltage injectors for the purpose of powering the unit. See Figure 5.17 below for an example of a single-port DC voltage injector. These single-port injectors are acceptable when used with a small number of wireless infrastructure devices, but quickly become a burden, cluttering wiring closets, when building medium or large wireless networks.


Multi-port DC Voltage Injectors

Several manufacturers offer multi-port injectors including 4, 6, or 12-port models. These models may be more economical or convenient for installations where many devices are to be powered through the Cat5 cable originating in a single wiring closet or from a single switch. Multi-port DC voltage injectors typically operate in exactly the same manner as their single-port counterparts. See Figure 5.18 for an example of a multi-port PoE injector. A multi-port DC voltage injector looks like an Ethernet switch with twice as many ports. A multi-port DC voltage injector is a pass-through device to which you connect the Ethernet switch (or hub) to the input port, and then connect the PoE client device to the output device, both via Cat5 cable. The PoE injector connects to an AC power source in the wiring closet. These multi-port injectors are appropriate for mediumsized wireless network installations where up to 50 access points are required, but in large enterprise rollouts, even the most dense multi-port DC voltage injectors combined with Ethernet hubs or switches can become cluttered when installed in a wiring closet.


Active Ethernet Switches

The next step up for large enterprise installations of access points is the implementation of active Ethernet switches. These devices incorporate DC voltage injection into the Ethernet switch itself allowing for large numbers of PoE devices without any additional hardware in the network. See Figure 5.19 for an example of an Active Ethernet switch. Wiring closets will not have any additional hardware other than the Ethernet switches that would already be there for a non-PoE network. Several manufacturers make these switches in many different configurations (number of ports). In many Active Ethernet switches, the switch can auto-sense PoE client devices on the network. If the switch does not detect a PoE device on the line, the DC voltage is switched off for that port. As you can see from the picture, an Active Ethernet switch looks no different from an ordinary Ethernet switch. The only difference is the added internal functionality of supplying DC voltage to each port.