What is PoE Alternative A?
Alternative A is one of the two ways the IEEE 802.3af (PoE) and 802.3at (PoE+) standards let power sourcing equipment (PSE) put DC power on an Ethernet cable. With Alternative A, power rides on the same two pairs that carry 10BASE-T and 100BASE-TX data: pins 1-2 and 3-6. The PSE applies the voltage as a common-mode offset between the two pairs (often called phantom power), so the data signal on each pair isn't disturbed.
You'll also see it called Mode A or PoE Mode A. Strictly, "Alternative" describes the PSE side and "Mode" describes the powered device side, but both name the same pinout and people use them interchangeably.
Alternative A vs. Alternative B
| Alternative A (Mode A) | Alternative B (Mode B) | |
|---|---|---|
| Power pins | 1-2 and 3-6 | 4-5 and 7-8 |
| On a 10/100 link | Power shares the data pairs | Power uses the spare pairs |
| On a gigabit link | Power shares data pairs (all four pairs carry data) | Power shares data pairs (all four pairs carry data) |
| Often used by | Many PoE switches (endspan PSE) | Many midspan injectors |
How PoE equipment handles it
- The PSE, a PoE switch or a midspan injector, uses one alternative on a given port. Under 802.3af and 802.3at it doesn't power both pair sets at once.
- A standards-compliant powered device (PD), such as a VoIP phone, wireless access point or IP camera, must accept power on either pair set and in either polarity, so it works with Alternative A or B without any setting.
- Before power flows, the PSE detects a valid PD signature and classifies how much power the device needs. An ordinary laptop NIC doesn't present that signature, so a standard PSE won't power it.
- IEEE 802.3bt (PoE++) adds four-pair power: the Alternative A and B pairs carry power at the same time for higher wattage.
Troubleshooting notes
- A two-pair cable wired only on pins 1, 2, 3 and 6 can still carry Alternative A power and 10/100 data, but not Alternative B power or gigabit Ethernet.
- If a PD won't power up, check the switch's per-port PoE status and its total power budget, then test the cable for opens and shorts.
- "Passive PoE" gear skips detection and keeps voltage on the cable all the time. It can damage equipment that isn't built for it, so don't mix it with standard 802.3af/at devices.
For the CompTIA A+ exam
Core 1 (220-1201) objective 2.5 covers PoE switches, injectors and the PoE standards. The objectives don't name Alternative A or B, but knowing that PoE can use the data pairs (A) or the spare pairs (B), and that a compliant device accepts either, helps you reason through PoE questions and pinout distractors.