What is PoE Alternative B?
Alternative B is the second of the two power-feeding methods defined in IEEE 802.3af (PoE) and 802.3at (PoE+). The power sourcing equipment (PSE) puts DC power on the pairs at pins 4-5 and 7-8. On 10BASE-T and 100BASE-TX links those two pairs carry no data, so they're called the spare pairs: one pair carries the positive side of the supply and the other carries the return.
It's also called Mode B or PoE Mode B. "Alternative" is the PSE-side term and "Mode" is the powered-device-side term for the same pinout, and most people use them interchangeably.
Spare pairs vs. gigabit
The "spare pairs" label only holds for 10/100 Ethernet. Gigabit Ethernet (1000BASE-T) and faster use all four pairs for data, so on those links Alternative B power shares pairs that are also carrying data, just as Alternative A does. The power is applied as a common-mode voltage, so it doesn't interfere with the data either way.
| Alternative A (Mode A) | Alternative B (Mode B) | |
|---|---|---|
| Power pins | 1-2 and 3-6 | 4-5 and 7-8 |
| 10/100 link | Power shares the data pairs | Power uses the spare pairs |
| Gigabit link | Power shares data pairs | Power shares data pairs |
| Two-pair cabling (pins 1, 2, 3, 6 only) | Works | No power |
How PoE equipment handles it
- A PSE uses one alternative per port. Many midspan injectors use Alternative B, while many PoE switches use Alternative A.
- A compliant powered device (PD), such as a VoIP phone, access point or IP camera, accepts power on either pair set, so it works with either method automatically.
- IEEE 802.3bt (PoE++) can power all four pairs at once, using the Alternative A and B pairs together to deliver more wattage.
Troubleshooting notes
- Alternative B needs all four pairs. A cable with only pins 1, 2, 3 and 6 terminated can pass 10/100 data but won't deliver Alternative B power, so the PD stays dark.
- Old cable-sharing tricks that ran a second 10/100 connection or a phone line over the spare pairs of one cable can't coexist with Alternative B power.
- Check the injector or switch status lights and the port's PoE status, then run a cable tester to confirm all eight conductors are connected.
For the CompTIA A+ exam
Core 1 (220-1201) objective 2.5 covers PoE injectors, PoE switches and the PoE standards. Alternative B isn't named in the objectives, but remember the pinout split: A uses pins 1-2 and 3-6, B uses 4-5 and 7-8, and a standards-based powered device accepts either.