PoE is one of those topics everyone assumes is solved. You plug the access point in, it powers up, the survey looks fine, and the project moves on. Then someone measures two spatial streams on a 4x4 access point, or notices that the 6 GHz radio never came up, and the investigation ends at the switch.
I get this question a lot in the field: which PoE standard do I actually need for these access points? Let us go through the standards, the names the industry uses for them, the classes underneath, and what all of it changes for a Wi-Fi deployment.
I always say: a PoE problem is hard to troubleshoot but very easy to prevent.
The Standards, and the Names We Give Them
There are three IEEE amendments that matter, and each one carries a marketing name that is used far more often than the standard number. The confusion in the field almost always comes from mixing the two vocabularies.
| Standard | Name | Ratified | Power at Source | Power at Device | Pairs Used |
|---|---|---|---|---|---|
| 802.3af | PoE, Type 1 | 2003 | 15.4 W | 12.95 W | 2 |
| 802.3at | PoE+, Type 2 | 2009 | 30 W | 25.5 W | 2 |
| 802.3bt Type 3 | PoE++ | 2018 | 60 W | 51 W | 4 |
| 802.3bt Type 4 | PoE++ (high power) | 2018 | 90 W | 71.3 W | 4 |
The gap between the two power numbers in each line is not a rounding error. It is the power dissipated in up to 100 m of copper. The figure on the access point datasheet is what the powered device draws, and the figure you have to budget on the switch is the higher one.
The Classes, and What Actually Gets Negotiated
Underneath the standards sit the classes. The class is how the powered device tells the switch how much power it needs, so that the switch does not reserve 51 W for a device that will never draw more than 12 W.
| Class | Standard Range | Power at Device |
|---|---|---|
| 0 – 3 | 802.3af | up to 12.95 W (Class 0 is the default) |
| 4 | 802.3at | 25.5 W |
| 5 – 6 | 802.3bt Type 3 | 40 W / 51 W |
| 7 – 8 | 802.3bt Type 4 | 62 W / 71.3 W |
The negotiation happens in two stages, and this is the part worth knowing.
- First, detection and classification happen at the physical layer: the switch looks for the 25 kΩ signature resistance, then reads the class through current signatures on the pairs.
- Second, LLDP takes over at the data link layer and lets the access point request the exact power it wants, adjust it as features come up, and confirm what the switch has allocated.
What This Changes for Your Access Points
Modern tri-radio access points do not fail when they are underpowered. They degrade, and the vendors document exactly how. Take the Juniper Mist AP47 as a concrete example, since Juniper documents exactly what happens at each power level:
| PoE Level | Wi-Fi Radios | Power Required |
|---|---|---|
| 802.3af | Not supported - the AP will not power on | — |
| 802.3at | 2x2:2 on all three Wi-Fi radios, or 4x4:4 with only two of the three enabled | Reduced functionality |
| 802.3bt | Full 4x4:4 on all three Wi-Fi radios | ~29 W |
The scanning radio, along with BLE, GPS, and UWB, stays active regardless of power source.
Read that table again from the point of view of a design. On 802.3at, that access point drops to 2x2 across the board or gives up a whole radio, and nothing about the client count or coverage map warns you. The capacity model you built is impacted.
This is also why the power decision belongs in the design phase and not in the installation phase. You can plan your PoE budget based on the AP models and AP quantity you have in your design to make sure all APs have all the necessary power to operate as expected.
How I Spec PoE for a Wi-Fi Project
- Read the power table on the datasheet before you pick the switch. Every serious vendor publishes a table of what the access point does at af, at, and bt. That table, for the exact model being deployed, is the requirement. Not a generic wattage.
- Budget the switch, not the port. A switch advertising 802.3bt on every port rarely has the total budget to deliver it on every port at once. Add up the worst case draw of the access points on that switch, and check what happens when a redundant power supply is lost.
- Enable LLDP on the access layer. It costs nothing and it is the difference between the access point asking for what it needs and accepting what it was handed.
- Cable for the power, not only for the data. Four pair delivery at Type 3 and Type 4 puts real heat into a bundle. Cat6A is the sensible default for new installations, and the 100 m limit is a cabling limit that PoE does not change.
- Verify that the mode is the one you specified. Check the power mode the access point reports and the power the switch says it allocated, on a sample of ports, before the site is handed over. This takes minutes and it catches the ports that quietly fell back. You can automate this with APIs now.
- Keep injectors as the exception. They are the right answer for one access point in a remote location, and the wrong answer for a floor.
One caveat worth stating plainly. Not every deployment needs 802.3bt. A 2x2 access point in a small office runs perfectly on PoE+, and specifying Type 4 everywhere is money spent on capability nobody will use. The objective is not the largest power budget available. It is knowing, before the switch is ordered, which mode your access points are going to run in.
Resources
These are the references worth keeping on this topic:
- What is Power over Ethernet. A thorough overview of the standards, the classes, the difference between power sourcing equipment and powered devices, and the endspan and midspan distinction. Start here if the vocabulary itself is what is unclear.
- Understand Aironet and Catalyst AP Power Requirements Reference. Cisco documents model by model what happens at each power level, including the fact that its Wi-Fi 7 access points require standards based 802.3bt rather than proprietary UPOE.
- IEEE 802.3bt-2018. The amendment itself, which specifies four pair delivery and the Type 3 and Type 4 classes. It is available at no cost through the IEEE GET program if you want the authoritative wording rather than a summary.
- Ethernet Alliance PoE Certification. The certification program built specifically because the term PoE is not trademarked and proprietary implementations do not always interoperate. The certified product registry is useful when you are specifying gear from more than one vendor.