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