Power over Ethernet

802.3af is the original PoE standard, 48VDC at up to 350mA - not to exceed 15W - originally designed to power IP telephones.
802.3at (also known as PoE+) provides up to 25.5 W of power, and prohibits Inline Power, enabling 1000 Base-T support.

Power is carried over the (Category 3, 5, 5e and 6) cabling by one of the following techniques:

• Mode A (Endspan) delivers Phantom Power on the data pairs: 2 and 3. Data switches are Endspan.

  Mode A has two configurations: MDI (normal), and MDIX - as MDI, but with reversed polarity.
  - MDIX accommodates crossover cables, patch cables, and Auto-Crossover interfaces.

• Mode B (Midspan) delivers Inline Power on the unused pairs: 1 and 4, and therefore requires 4-pair cable.

  Midspan power supplies (external PoE injectors) provide inline power.

• 1000 Base-T has no unused pairs, so power is delivered using Phantom Power in both Mode A and B.

  Midspan power supplies can't be used with Gigabit Ethernet because all four pairs are required for data!

The power source (not the device) decides whether Mode A or B is used. Compliant devices must support both Modes.
Avaya IP Phones support 10/100 Base-T: Mode A (2 pairs) with a PoE data switch, and/or Mode B (4 pairs) with a midspan power injector.

Note: T568A and T568B are the two wiring standards for 4-pair data cable. The only difference is the position of the orange and green wire pairs. T568B is preferred, and should be used wherever possible. A 'crossover' cable has 568A at one end, and 568B at the other.

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