Many network attached devices, such as IP cameras, VoIP phones, WiFi radios and IP door readers use power over Ethernet (PoE).  The network cable transfers both data and power to a device.  The data uses one set of wires, while the power uses another set of wires in the network cable.

The device that inserts power onto the Ethernet cable is called an “injector” or “mid-span”.  Power is injected onto the Ethernet cable using a separate power source (injector) or using a network switch that includes built in PoE.  Many devices, that use PoE, conform to the IEEE standard.  But be careful, there are some devices that don’t conform to this standard.  It’s important to know exactly what voltage and current the device needs before using PoE.   Here is a review of the technology.

Standards Used for Power Over Ethernet 
IEEE 802.3 is the standard that defines the physical layer for Ethernet cabling.  It includes the type of wire, signal levels, and the bandwidth.  In 2003 the standard was updated to include PoE.  IEEE 802.3af defined the voltage and power available power for devices on the network.  This specification provided PoE that provided up to 15.4 watts. The specification was updated again in 2009.  IEEE 802.3at provided higher powered (25.5 watts) which is referred to as PoE+.

PoE injectors that conform to IEEE standard provide between 44 and 57 volts.  The effective distance of cable is the same as the standard Ethernet distance of 100 m (328 ft.).   There are two modes, A and B, for powering the devices.  The modes define the wires pairs and polarity of the voltage used for power.

PoE Injector Technology: 

In all cases, power is sent over the extra network wires.  The wires are quite narrow (24-gauge) so cable resistance reduces the power available at the device, and that depends on the distance from the source. The standard defines the nominal PoE voltage at 48 volts.  The voltage level at the injector is designed to be about 57 volts, but is then reduced by the resistance in the wire to about 44 volts by the time it gets to a distant device.  There are various injectors available that can provide more power to the device.  This means they have a higher current capability and thus provide more power at 48 volts.

There are some passive PoE sources and there are “active” or “smart” type sources.  The passive PoE sources are less expensive, while the “active” type provides some device protection that can prevent damage caused by short circuits, as well as providing over current and over-voltage protection.

Power sources that conform to the IEE standard include network switches individual or multi-port PoE injectors (or mid-spans).

Power Source Using a Network Switch that includes PoE
There are a number of network switches that include PoE source on some or all their ports.  Even though many switches will say they provide power, they can be quite different in capability.  Most of these switches will provide 15.5 or 25.2 watts (PoE+) on the PoE port.  Some switches include power on some of their ports, and not others.  You have to be careful which port you use for the PoE device.

Some switches have PoE on all the ports, but have a power budget that limits the total power available to the connected devices.  For example, if an 8-port switch has a power budget of 32 watts, it can actually support only 2 devices that use 15.5 watts each.

Power sources using a Mid-span or Power injector

Power injectors are available as single device or multi-port units.  Multi-port injectors are available from a number of manufacturers and support standard PoE or PoE+ power to multiple devices. They are available with 4-port, 8-port, 16-port and even 24-port configurations.  Since these devices also pass the data on the network, make sure you select an injector that meets your network speed.  They are available in 100 megabit or Gigabit bandwidth.

Single device injectors are available with much higher power levels.  These very high power units can provide up to 100 watts, using 4 wires in the network cable.  This higher power level allows you to power some of the PTZ type cameras that require much higher power levels.   Always check that the PoE source is compatible with the camera.

By: Bob Mesnik