Power over Ethernet (PoE)

With the introduction of new Ethernet-enabled devices expanding geometrically, the need to power these devices from standard AC power outlets has become a limiting factor. IP telephones, wireless access points, IP cameras and device servers are examples of devices limited by the need to have an AC power outlet nearby to plug in a DC power adapter. At best, power supply installation and wiring adds labor and results in the mess of extra wiring; worst case, the lack of nearby AC power means devices cannot be installed where they are needed.

In response to this need, IEEE developed IEEE802.3af to standardize a system of supplying low voltage power to networked devices via the communications line. Released in mid-2003, the standard was entitled: Data Terminal Equipment (DTE) Power via Media Dependant Interface (MDI). It is more commonly referred to as Power over Ethernet (POE). PoE supplies power to network devices over the same standard Category 5e (CAT-5) cabling that carries the data. Devices can be installed wherever structured Ethernet wiring is located, without the need for AC power outlets nearby.

The benefits of PoE include increased mobility for end devices, added safety (no AC power involved), simplicity of installation, reliability, security and cost savings. These advantages have led to the development of a variety of new PoE-enabled products (including B&B Electronics’ PES1A and PESV1A RS-232 to Ethernet Converters).
How PoE Works

In the PoE scheme, two different types of devices are involved: Power Sourcing Equipment (PSE) and Powered Devices (PD).

PDs are PoE-enabled network end devices equipped to accept low voltage power transmitted over structured Ethernet CAT-5 cabling.

PSEs provide the DC power to PDs. PSEs source up to 12 watts at 48 volts to each PD. A PSE may be an endspan device or a midspan device. An endspan device typically is a network switch enabled to provide PoE power on each port. A midspan device is connected in-line to each end device and adds power to the line.

Power is carried over the cabling using two techniques: Alternative A and Alternative B

Alternative A implements a simplex, or ‘phantom feeding’ method for delivering power to the end device. Power is carried on the same conductors as data. CAT-5 cabling for standard 10BaseT and 100Base-TX Ethernet uses two data/signal pairs connected to pins 1 and 2 and pins 3 and 6 on RJ-45 connectors. Power sourcing equipment superimposes power onto these pins via the center tap of internal signal coupling transformers. In the powered device (PD) the power is derived from these lines using the reverse technique.