Power Sourcing Equipment

Power Sourcing Equipment

Power over Ethernet or PoE describes any of several standardized or ad-hoc systems which pass electrical power along with data on Ethernet cabling. This allows a single cable to provide both data connection and electrical power to devices such as network hubs or closed-circuit TV cameras. Unlike standards such as Universal Serial Bus which also power devices over the data cables, PoE allows long cable lengths. Power may be carried on the same conductors as the data, or it may be carried on spare pairs of the cable.

There are several common techniques for transmitting power over Ethernet cabling; two of them have been adopted and standardized by the IEEE 802.3 committee.

Power may be transmitted on the unused (spare) conductors of a cable, since only two of the four pairs are needed for the commonly used 10Mbit/s–100Mbit/s physical layers. In the IEEE standards, this is referred to as Alternative B.

Power may be transmitted on the data conductors by applying a common-mode voltage to each pair. Because Ethernet uses differential signalling, this does not interfere with data transmission; and the common mode voltage (which may be, e.g., positive for the TX pair and negative for the RX pair) is easily extracted using the center tap of standard Ethernet magnetics. This is similar to the phantom power technique commonly used for powering audio microphones. In the IEEE standards, this is referred to as Alternative A.

In addition to standardizing existing practice for spare-pair and common-mode data pair power transmission, the IEEE PoE standards provide for signalling between the power source equipment (PSE) and powered device (PD). This signaling allows the presence of a conformant device to be detected by the power source, and allows the device and source to negotiate the amount of power required or available. Up to 25 watts is available for a device, depending on the version of the standard in use.