Three standards, four numbers worth memorising
IEEE 802.3af (2003) delivers up to 15.4W at the switch port, guaranteeing 12.95W at the powered device after cable losses. IEEE 802.3at (2009, 'PoE+') doubles it: 30W at the port, 25.5W at the device. IEEE 802.3bt (2018, 'PoE++') splits into Type 3 — 60W at the port, 51W at the device — and Type 4 — 90W at the port, 71.3W at the device. Everything in PoE design is downstream of those eight numbers and one rule: the device figure is what your equipment can actually spend.
The standards also differ electrically. af and at can deliver over two pairs of the cable; bt uses all four pairs, which is why its per-port figures jump and why cable quality and bundle heating enter the conversation at Type 4. Negotiation is standardised throughout: the switch detects a valid PoE signature, classifies the device's requested class, and only then energises the port — a non-PoE device plugged into any of these ports is never powered.
Which devices belong on which standard
af territory: fixed bullet and dome cameras (typical draw 4–10W), VoIP phones, simple sensors — the original PoE world. at territory: PTZ cameras with motors, cameras with heaters and long-range IR, Wi-Fi 6 access points at full load, video phones, door controllers driving locks. bt territory: large PTZ domes with blowers, Wi-Fi 6E/7 radios, digital signage, PoE LED luminaires, and powered USB-C docks through bt splitters.
The practical trap is seasonal: a PTZ camera that draws 18W in the showroom may draw 28W on a January night with its heater and IR at full power. Devices must be budgeted at worst-case draw — which is why so many 'af-era' cameras quietly became at devices. When you browse a switch catalogue like ONV's, this maps directly: the mainstream gigabit range is af/at at 30W per port (stated verbatim in the datasheets), and the bt models exist for the devices that broke that ceiling.
Classes, LLDP and the budget arithmetic
Each standard refines how a device declares its appetite. af/at use physical-layer classification (classes 0–4); bt extends to class 8 and adds finer negotiation, including LLDP-based power adjustment where both ends support it. For the buyer, the mechanics matter less than the arithmetic they enable: sum every device's worst-case watts, multiply by 1.25, and that is the total PoE budget your switch must carry. The per-port standard only has to cover the single hungriest device on that switch.
Two switches can both advertise '30W per port' and differ threefold in total budget — a 16-port switch at 150W cannot run sixteen 30W devices. The total budget, printed in every serious datasheet, is the number that decides. Our power-budget guide works the arithmetic end-to-end with a camera estate example.
Compatibility: the good news
The standards are backward compatible by design. A bt port powers an af camera correctly; an at switch runs af devices; negotiation ensures each device receives its class and no more. Mixed estates are the norm, not an exception. The incompatibilities that do exist are outside the standards: proprietary 'passive PoE' (often 24V) injectors for legacy wireless gear, which deliver power without negotiation and must never be mixed into standards-based infrastructure. Keep passive gear segregated and labelled, and the standards-based world stays safe to mix.
Reading a switch datasheet through standards eyes
With the three standards in hand, a PoE datasheet becomes a short story you can actually audit. Look for four lines: the standard(s) named per port ('Max Power Per Port: 30W, IEEE 802.3 af/at' is the honest mainstream phrasing), the total budget as a separate figure, the budget versions if any (a 400W/600W choice on the same model tells you the manufacturer expects both fixed-camera and PTZ duty), and — on bt models — the pair usage (four-pair delivery, stated as pin assignments for both pair sets). A datasheet missing the total budget is not being modest; it is hiding the binding number.
Apply the same lens to the powered devices. A camera specified as 'PoE (802.3af/at)' with a maximum consumption figure is telling you its class ceiling and its worst-case draw in two lines; a camera specified only as 'PoE supported' is telling you nothing you can budget with. The standards exist so that strangers' equipment interoperates on arithmetic alone — insist on the arithmetic being published, on both sides of the cable.
Worked example: one floor, three standards
A hotel floor makes the standards concrete. Twenty-four fixed cameras at 8W worst case: af territory, 192W total. Four corridor access points (Wi-Fi 6, 20W at full radio load): at territory, 80W. Two PTZ domes watching the car park, heaters specified to 45W peaks: bt territory, 90W. One floor, three standards, and the switch arithmetic writes itself — the floor's access switch must offer at ports for the many, bt ports for the two, and a total budget clearing 192 + 80 + 90 = 362W times headroom, call it 450W. This is why ranges like ONV's are shaped as they are: a broad af/at mainstream (30W ports, budgets to 600W) and bt models as the specialist tier, rather than a single 'maximum everything' line.
The same example exposes the two classic errors. Error one is buying an all-bt switch 'to be safe': you have paid for 90W silicon and power supplies on twenty-four ports that will never draw nine. Error two is the mirror image — speccing the whole floor at af because the cameras are af, then discovering the dome heaters in the first cold week. The standards are a sizing language; fluency is using all three words where each belongs, and no more.
The decision, compressed
Budget every device at its worst-case draw; assign it the standard that covers that draw with margin — af to ~12W, at to ~25W, bt Type 3 to ~50W, Type 4 to ~71W at the device; sum the estate, multiply by 1.25, and let that number choose the switch's total budget while the hungriest single device chooses its port class. Keep passive, non-standard power segregated and labelled wherever legacy gear forces it into the building. Do those four things and the three standards stop being a spec-sheet fog and become what they were designed to be: a sizing language precise enough to buy a network with.
One last practical note on labelling: standards fluency decays unless the estate carries it visibly. Label each PoE segment with the standard and budget it was designed to — 'at, 250W budget, 24 cams at 180W design load' on the cabinet door — and the next engineer inherits the arithmetic instead of rediscovering it at a dead dome in February. The same discipline applies at purchase review: any device arriving on site without a standards class and a worst-case wattage on its datasheet goes back, however good the demo was. The standards give you a language; labels are how the building keeps speaking it after the installers leave.