Power over Ethernet – PoE

What is Power over Ethernet – PoE? What’s the difference between 802.3af and 802.3at (also known as PoE+)? All of the essential information about PoE modes, pinouts and voltages in a single article.

• 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.

RJ45 Pin #	T568A		T568B		10/100 Base-T			1000 Base-T
	Pair	Colour	Pair	Colour	A-MDI	A-MDIX	Mode B	Mode A	Mode B
1	3		2		Rx+	Rx+	Rx+	TxRx A+	TxRx A+
2					Rx-	Rx-	Rx-	TxRx A-	TxRx A-
3	2		3		Tx+	Tx+	Tx+	TxRx B+	TxRx B+
4	1		1		unused	unused	PWR	TxRx C+	TxRx C+
5								TxRx C-	TxRx C-
6	2		3		Tx-	Tx-	Tx-	TxRx B-	TxRx B-
7	4		4		unused	unused	PWR	TxRx D+	TxRx D+
8								TxRx D-	TxRx D-

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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The content in this article was mirrored from Mike’s PBX Cookbook for preservation and it is licensed under CC BY-NC 4.0.