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Power over Ethernet - PoE, IEEE802.3af standard

Zbynek Soban, 20. November 2003 - 0:00

Power-over-Ethernet (PoE) is an emerging new technology that enables delivery of data and power to internet appliances over existing CAT-5 Ethernet cables. PoE is rapidly gaining in popularity with manufacturers and end users of devices such as VoIP phones, wireless access points, security cameras and point-of-sale terminals. PoE is governed by the standards defined in the recently ratified IEEE802.3af standard.

802.3af

802.3af, also known as Power over Ethernet, defines a way to build Ethernet power-sourcing equipment and powered terminals. The specification involves delivering 48 volts of AC power over unshielded twisted-pair wiring. It works with existing cable plant, including Category 3, 5, 5e or 6; horizontal and patch cables; patch-panels; outlets; and connecting hardware, without requiring modification. Compatibility with IEEE Standard 802.3, it is compatible with 10BASE-T and 100BASE-TX UTP, and do no harm to 1000BASE-T, with no changes to the existing MAC.

Power delivery

The current delivered to each node is limited to 350 milliamps. The total amount of continuous power that can be delivered to each node, taking into account some power loss over the cable run, is 12.95 watts. IP phones and wireless LAN access points typically consume 3.5 to 10 watts.

Power is carried on two wire pairs, to comply with safety standards and existing cable limitations. 802.3af power sourcing equipment contains a detection mechanism to prevent sending power to noncompliant devices. Only terminals that present an authenticated Power over Ethernet signature will receive power, preventing damage to other equipment.

Applications

  • IP Telephony
  • Web Cameras
  • Wireless Access Points
  • Industrial Automation
  • Home Automation
  • Security Access Control and Monitoring Systems
  • Point of Sale Terminals
  • Lighting Control
  • Gaming and Entertainment Equipment
  • Building Management

Power source equipment: end-span and mid-span

End-span refers to an Ethernet switch with embedded Power over Ethernet technology. These new switches deliver data and power over the same wiring pairs - transmission pairs 1/2 and 3/6.

Mid-span devices resemble patch panels and typically have between six and 24 channels. They are placed between legacy switches and the powered devices. Each of the mid-span ports has an RJ-45 data input and data/power RJ-45 output connector. Mid-span devices tap the unused wire pairs 4/5 and 7/8 to carry power, while data runs on the other wire pairs.

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