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What is Power over Ethernet (PoE)?


Traditional standards of telephone power
When using a residential telephone line, a phone still functions in the event of a power outage. The reason is that the phone company sends low voltage power over the phone line. This power is separate from the electric company’s network, so that even if the power goes out, the phone will still function.

Business telephone systems power analog and digital telephone extensions in the same way. Manufacturers of these systems provide standards for sending low voltage power through digital and analog extension ports. This allows the extensions to have features such as backlit displays, general functioning, and ringing. In some cases, consoles with large displays or telephones with adjuncts like speakerphones also require a separate power supply. However, the majority of extensions use direct line power.

IP phones and the history of PoE
IP telephones are connected to your data network and receive power differently. Because Ethernet wiring predates IP telephony, cabling standards never included requirements for directly powering devices like IP phones. The first IP telephones that were released came with external power supplies. Because businesses typically deploy more phones than data equipment, provisions for power outlets at each workstation were required. This proved to be more difficult than traditional digital and analog phone installations which simply involved plugging the phone into the jack.

In order to eliminate local power supply requirements for IP phones, Cisco introduced a proprietary standard in March 2000 (Cisco Inline Power) to send power over Ethernet wiring. This innovation received recognition industry-wide and began a movement to standardize power over Ethernet (PoE) for multi-vendor interoperability.

The Institute of Electrical and Electronics Engineers (IEEE), one of the world’s leading technical professional societies, ratified the 802.3af Power over Ethernet standard in June of 2003. It describes a system to pass electrical power safely, along with data, on Ethernet cabling.  The 802.3af PoE standard allows

IP Phone External Power Supplies
The Avaya 1151D1 and Cisco Power Cube are commonly used to power IP phones where PoE is not available.

manufacturers of network hardware and endpoint devices to produce switches, IP telephones, wireless access points, and IP cameras with standardized PoE support. Consequently, proprietary forms of power over Ethernet are now obsolete.

Today, most IP phones support both 802.3af PoE and external power supplies. If a company has yet to deploy switches supporting PoE, then they can still use power supplies for their IP phones.