Power over Ethernet permits network installers and administrators to control the distribution of power over a network in a manner that is reliable, safe, efficient, robust and is as simple as possible to install.
The concept of Power over Ethernet was standarised in 2003 and has found widespread application in network technologies such as VoIP, wireless LAN’s, IP-Surveillance and access control.
IEEE 802.3af-2003
The IEEE 802.3af standard was approved on June 12, 2003, by the Institute of Electrical and Electronic Engineers (IEEE)
The IEEE 802.3af standard defines the specifications to deliver power over standard Ethernet cables, and in particular the following issues:
Legacy Installation - Power over Ethernet ensures safe delivery of power to existing legacy devices as well as power-enabled terminals.
Preservation of Cabling Infrastructure - PoE avoids altering existing wiring and does not damage cabling infrastructure already in place.
Data Integrity - Power delivery over Ethernet cables does not cause data degradation or loss of data integrity.
The standard provides up to 15.4 watts of DC power (Minimum 44VDC and 350 mA) to each device. Only 12.95 watts is assured to be available at the powered device as some power is dissipated in the cable.
Concepts and Terminology
Power over Ethernet is defined across a single network link. The three basic components of a PoE connection are:
The equipment that delivers power to the cable (often referred to as a PSE, which stands for power sourcing equipment)
The device which receives its power from the cable (also known as a powered device, or PD)
The cable itself
Typical PD's include IP cameras and wireless access points (AP's).The PSE would normally be a PoE enabled network switch or a mid-span power injector, patched in to add PoE capability to a non-PoE network switch channel or similar.
A PoE network switch can deliver power to a PD, such as an IP camera, over its data cable.
A mid-span PSE can be used to add PoE capability to standard network switch channels
The terms PSE, PD and mid-span are defined in the current IEEE specification for PoE, 802.3af, which is part of the 802.3 set of standards that define Ethernet1. Nearly all devices described as incorporating PoE comply with this standard (although the manufacturers' literature should always be consulted).
In order that 802.3af PoE can coexist with existing Ethernet technology, PDs must display a signature to advertise that they can accept PoE, and optionally a power class, to indicate how much power they require. The PSE detects these before it connects power to the cable, in order to protect incompatible devices. Signature detection and power classification will be described in more detail in later sections.
PoE is also designed to operate over standard network cable: Cat 3, Cat 5, Cat 5e or Cat 6 (often collectively referred to as Cat 5), using conventional RJ452 connectors.
