Terminology » Networking Basics » Power over Ethernet (PoE) Explained

Power over Ethernet (PoE) Explained

As the name suggests, PoE, also referred to as Power over Ethernet, is a device that provides direct current electrical power over a twisted pair Ethernet cable. This makes it possible to carry both Ethernet data and power over the same port. The standard for Power over Ethernet was first introduced in the year 2003; later, 802.3af was rectified. This is also referred to as clause 33 in the 802.3 standard.

A PoE device as per the 802.3af standard delivers 15.4 Watts of DC power. Power over Ethernet works with 10baseT, 100baseT, and 1000baseT. Another version of the standard was developed in 2009 and is referred to as the 802.3at supplement, which provides power up to 34.2 Watts.

Standard versions such as Universal PoE provide 60 Watts and are developed by Cisco. Power over HDBASE-T delivers 100 Watts over cat-5e and cat-6 cable over all four pairs. The Power over Ethernet standard defines two device types: power sourcing equipment (PSE) and powered device (PD).

PSE is the one that delivers power over a twisted pair Ethernet cable. Examples of PSE are Ethernet switch ports, power injectors, and adapters. PD is usually powered by a PSE device. Examples of PD are wireless APs (Access Points), VOIP-compliant phones, IP-based video cameras, and so on. PD is also referred to as DTE (Data Terminal Equipment) as per the standard.

PoE

Fig.1 depicts a typical power over Ethernet device which takes AC power and Data as input and provides data and DC power on the same single port.

Fig.2 describes the application of a Power over Ethernet adapter used in a typical WiMAX system. The fig.2 describes the use of PoE adapter used in wimax system designed for high speed internet connectivity. Data-IN port is connected with the ethernet port of the laptop to access/browse the internet.

Power over Ethernet

The second input port is connected with a 3-pin AC port. The output port will have both the data as well as DC power on the same port which is interfaced with the SU-ORF in this application example. SU-ORF is the subscriber terminal typically installed in the user premises. The fig.2 is the lab set up. As shown further SU-ORF is interfaced with Base Station(AN-100U) from redline communication. BS is interfaced with LAN to provide backbone internet connectivity.

PoE Ethernet Switch

Another example application is as shown in the fig.3 wherein a PoE Ethernet switch takes AC power and delivers multiple POE ports. These multiple ports are interfaced with a VOIP phone, an IP-enabled video camera, and a wireless access point. All these devices get power as well as data from the switch.

The Power over Ethernet standard specifies two types of PoE systems: Type 1 and Type 2. The table mentions different parameters for both of these types. Type 1 PoE device is meant for low-power systems while Type 2 is designed for higher-power applications.

Specifications	PoE 802.3af Type1	PoE 802.3at Type2
Power at the PD	12.95W	25.5W
Maximum power using PSE	15.4W	34.2W
Voltage range(at PSE)	44 to 57	50 to 57
Voltage range(at PD)	37 to 57	42.5 to 57
Maximum Current	350 mA	600 mA
Maximum cable loop resistance	20 Ohm with cat.3/Cat.5 or better	12.5 Ohm with cat.5 or better