802.3af is the original Power over Ethernet standard, published in 2003, and IEEE lists it as a superseded standard. Its text was folded into the base Ethernet standard, where the same thing is called Type 1 and lives in Clause 33.
That is why every switch datasheet still says 802.3af while no current document by that name exists. What Type 1 means in practice: a port that sources up to 15.4 W over two of the four pairs, a device that can count on 13 W after the cable takes its share, and classes 0 through 3 to negotiate which.
The figure people quote for the device end is usually 12.95 W, which is from the 2003 document rather than the standard today.
- 802.3af-2003 is listed by IEEE as a superseded standard
- The same specification is Type 1, in Clause 33 of IEEE 802.3
- A Type 1 port sources 15.4 W; the device is assured 13 W
- Two pairs only. Four pair power arrives with 802.3bt
- Type 1 and Type 2 interoperate, the lower one setting the limit
On this page
What it isWhat 802.3af actually is now
The document exists, it is dated 17 June 2003, and its title is a mouthful: Data Terminal Equipment (DTE) Power Via Media Dependent Interface (MDI). IEEE's own page for it carries one line of status, and that line reads Superseded Standard.
Amendments to IEEE 802.3 are not permanent documents. They are drafted, approved, and then rolled into the next revision of the base standard, at which point the amendment stops being the place to look. 802.3af went in, 802.3at followed it, and the text a vendor is actually building to today sits in Clause 33 of the current revision of 802.3.
Clause 33 does not use the word 802.3af. It uses Type 1.
| The name on the datasheet | The name in the standard | Where it is written |
|---|---|---|
| 802.3af | Type 1 | Clause 33 |
| 802.3at, or PoE+ | Type 2 | Clause 33 |
| 802.3bt, or 4PPoE | Type 3 | Clause 145 |
| 802.3bt, high power | Type 4 | Clause 145 |
This is not pedantry about names. It decides where the answer to a question lives, and it explains why searching the standard for "802.3af" and finding nothing does not mean you looked in the wrong standard.
PSE and PDWhat 802.3af powers, and where the power comes from
Power over Ethernet sends DC power and data down the same Ethernet cable, so a device on the network needs no power outlet near it. The standard names the two ends. The power sourcing equipment, or PSE, supplies the power. The powered device, or PD, receives it.
The PSE is one of two things. An endspan is a PoE switch, with the power supply built into the switch ports. A midspan is a PoE injector, a small box between an ordinary switch and the PD that adds power to the cable.
Both follow the same detection and classification rules, so a PD cannot tell them apart.
The PDs are low power network devices. VoIP phones were the first large use of 802.3af PoE, and it still suits them, along with fixed IP cameras, single radio wireless access points, badge readers and small sensors.
Anything that needs higher power, such as a pan and tilt camera with a heater or a multi radio access point, needs 802.3at or 802.3bt.
The voltage is nominally 48 V DC. A Type 1 PSE supplies between 44 and 57 V at up to 350 mA, and a PD has to work on anything from 37 to 57 V, because the cable drops part of it.
The 15.4 watts at the PSE is simply 44 V multiplied by 350 mA.
The practical benefits are the ones that made the PoE standards spread. One cable per device, no electrician for a ceiling mounted access point or a camera on an outside wall, and central battery backup: put the PoE switch on a UPS and the phones and cameras stay up through a power cut.
The two numbersThe two numbers, and why they are different
Two power figures describe the same PoE port, and confusing them is the most common mistake in a PoE power budget. Both are in watts, and they are not interchangeable.
15.4 W is what the port sources. It is the maximum power a Type 1 PSE puts out at its own connector.
13 W is what the powered device can count on. Copper has resistance, the cable can be 100 m long, and the difference is lost as heat on the way. The standard assigns that figure so a PD designer knows what is safe to draw at the far end of the worst permitted cable.
The Ethernet Alliance sets out the per class figures for what a powered device is assigned:
| Class | Power assigned to the device |
|---|---|
| 1 | 3.84 W |
| 2 | 6.49 W |
| 3 | 13.00 W |
| 4 | 25.50 W, which is Type 2 |
Class 0 is not a fifth level. It is an alias for Class 3, used by a device that does not classify itself, and it means the port has to assume the full Type 1 draw.
Where 12.95 W comes from. Almost every page on this subject gives 12.95 W as the device figure, and that number is genuine: it is what the 2003 document specified, for devices up to 12.95 W at 350 mA and 48 V DC.
The current standard states 13.00 W for Class 3. Both describe the same port, one of them is from a superseded document, and a specification written today should cite the current one.
Before the powerWhat happens before any power arrives
A PSE does not put voltage on an Ethernet cable and hope. It measures first, and the numbers involved explain why plugging a laptop into a PoE port does nothing at all.
Detection. The port measures the effective resistance of whatever is on the other end, using at least two voltage and current points at least 1 V apart, taken between 2.8 V and 10 V. An effective resistance of 25 kilohms signifies a device that can accept power.
A valid signature means a resistive slope between 23.7 and 26.3 kilohms with a voltage offset no greater than 1.9 V.
The safety limits during that measurement. The port may not produce more than 30 V and may not source more than 5 mA while it looks. That is a probe, not power, and it is why an ordinary network card sees nothing.
The rule that follows from it. The Ethernet Alliance states it flatly: under no condition can a port power a pair set unless that pair set has a valid detection signature. No signature, no voltage, whatever is plugged in.
Classification. Once the PD is confirmed, the PSE asks how much power it needs, and the answer is the PoE class. That is what lets a PoE switch budget for 3.84 W rather than reserving 13 W for a device that will never draw it.
The detection method has not changed since Type 1, which is the reason a device built in 2004 negotiates correctly with a switch built this year.
Why two pairsTwo pairs, and what that costs
A Type 1 PSE delivers power over two of the four pairs in the Ethernet cable. Not four.
That single sentence is the reason the PoE ceiling sat where it did for fifteen years, because current through copper is what heats the cable, and the grade of that cable decides how well it sheds the heat.
Both wiring options exist and a device has to accept either.
| Option | Pairs carrying power |
|---|---|
| Alternative A | The pairs also carrying data on a 10 or 100 Mbit link |
| Alternative B | The two pairs unused by a 10 or 100 Mbit link |
Four pair power is what 802.3bt added, in a separate clause, and it is where the 60 W and 90 W PoE ports for higher power devices come from. The Ethernet Alliance is explicit that anything from assigned Class 5 upward requires all four pairs.
What to checkWhat to check before buying a PoE switch
Read the per port figure and the total power budget as two separate numbers. A 24 port PoE switch advertising 802.3af on every port and a 190 W budget cannot run 24 Type 1 devices at full draw. The budget is the constraint that bites, and it is the number left out of the headline.
Match the PoE type to the powered device, not to the marketing word. An access point asking for 802.3at will not run properly on a Type 1 port. It may boot, disable a radio, and look like a coverage problem for a week.
Treat passive PoE as a different technology. Passive injectors put voltage on the pairs with no detection and no classification. They work, they are cheap, and plugging one into a device that was not expecting it is the one way to actually damage hardware.
Everything in this article describes the negotiated standard, which does not power a port until it has confirmed there is something on the other end that wants power.
Check the cable, not just the switch. The 13 W figure assumes a cable within specification for 100 m. A run built from patch cable offcuts and a cheap coupler is where the missing watts go.
Ask what happens when the PoE budget runs out. Some switches deny power to the next device, some drop the lowest priority port. Both are defensible and only one of them is a surprise at three in the morning.
PitfallsWhere people go wrong
Looking for the 802.3af document. It is superseded. The text is Clause 33 of the current IEEE 802.3, under the name Type 1.
Quoting 15.4 W as what a device can use. That is the port figure. The device figure is 13 W, and the gap is cable loss.
Assuming 802.3bt means 60 W. Type 3 has a two pair variant limited to Class 4. The four pair capability is what buys the higher classes.
Treating Class 0 as the lowest class. It is an alias for Class 3, so an unclassified device is budgeted at the full Type 1 draw rather than the smallest.
Mixing passive PoE injectors into a standards based deployment. No detection, no classification, and the one path to damaged equipment in an otherwise safe technology.
Sizing a PoE switch on ports rather than watts. The per port maximum and the total power budget are different numbers, and the second one is what runs out.
ComparisonThe four PoE types, side by side
| Criterion | Type 1 | Type 2 | Type 3 | Type 4 |
|---|---|---|---|---|
| Amendment | 802.3af | 802.3at | 802.3bt | 802.3bt |
| Sold as | PoE | PoE+ | PoE++ or 4PPoE | PoE++ |
| Clause | 33 | 33 | 145 | 145 |
| Port sources | 15.4 W | 30 W | 60 W | 90 W |
| Device is assured | 13 W | 25.5 W | 51 W | 71.3 W |
| Pairs | Two | Two | Two or four | Four, always |
| Highest class | 3 | 4 | 6 | 8 |
The row that decides a purchase is the last one but one. A Type 3 port can be two pair or four pair capable, and the two pair version tops out at Class 4, which is Type 2 power in a Type 3 wrapper. A port advertised as 802.3bt is not automatically a 60 W port.
Interoperability between the PoE standards is the good news and it goes in every direction. A Type 2 port powers a Type 1 device. A Type 1 port powers a Type 2 device, which recognizes what it is connected to and limits itself to 13 W rather than failing.
Nothing is damaged by the mismatch; the device either runs in a reduced mode or does not come up. Type 2 also added a second way to negotiate power, in software over LLDP, beside the hardware classification all PoE devices support. 802.3af devices use the hardware method only.
FAQFrequently asked questions
What is 802.3af?
The first Power over Ethernet standard, published in 2003 as an amendment to IEEE 802.3 under the title DTE Power Via Media Dependent Interface. IEEE now lists it as a superseded standard, because its text was folded into the base standard, where the same specification is called Type 1 and sits in Clause 33.
How much power does 802.3af provide?
The port sources up to 15.4 W. The device at the other end is assured 13 W, with the difference lost in the cable. The older figure of 12.95 W comes from the 2003 document and describes the same thing.
What is the difference between PoE and PoE+?
PoE is the marketing name for Type 1, from 802.3af, and PoE+ is Type 2, from 802.3at. A Type 2 port sources 30 W instead of 15.4 W and assures the device 25.5 W instead of 13 W. Both use two pairs and both live in Clause 33, so the difference is power rather than mechanism.
Is 802.3af still valid?
The specification is, the document is not. A device built to Type 1 works exactly as it always did, and interoperates with every later type. What is superseded is the standalone 2003 amendment, so a purchase specification written today should point at IEEE 802.3 Clause 33 and Type 1.
Will an 802.3at device work on an 802.3af port?
It will connect, and it will be limited. A Type 2 device recognizes that it is being powered by a Type 1 port and restricts itself to 13 W. Whether that is enough depends on the device, and an access point with two radios is the usual case where it is not.
What are the PoE classes?
A negotiation of how much power the device actually needs, so the port can budget for it rather than reserving the maximum. Class 1 is assigned 3.84 W, Class 2 is 6.49 W, Class 3 is 13.00 W, and Class 4 is 25.50 W and belongs to Type 2. Classes 5 through 8 arrived with 802.3bt.
What is Class 0?
An alias for Class 3, used by a device that does not present a classification signature. The port has to assume the full Type 1 draw, which is why an unclassified device consumes the largest share of a switch's power budget.
Which pairs carry the power?
Two of the four, and both options are permitted. Alternative A uses the same pairs that carry data on a 10 or 100 Mbit link; Alternative B uses the two pairs those links leave idle. A compliant device accepts either, which is why the choice is invisible in practice.
What is 802.3bt?
The amendment that added four pair power, defining Type 3 and Type 4 in Clause 145. Type 3 assures a device 51 W, Type 4 assures 71.3 W, and anything from assigned Class 5 upward requires all four pairs to be used.
What is UPOE?
A vendor name for four pair power delivery that predates 802.3bt. It solved the same problem by the same means, powering over all four pairs, and the standards based answer to it is Type 3 or Type 4.
Can PoE damage a device that does not support it?
Not the standardized kind. A compliant port performs detection before it applies power and never energizes a port that has not answered. Passive injectors are the exception: they apply voltage regardless, and they are where damaged hardware comes from.
How do I work out how many devices a switch will power?
Divide the switch's total power budget by the assigned class power of the devices, not by the per port maximum. Twelve Class 3 devices need 156 W of budget before any headroom, which is more than several 24 port switches advertise in total.
How do the PoE standards compare on wattage?
The three PoE standards differ mainly in PoE wattage. 802.3af, also called Type 1 PoE, supplies 15.4 W at the switch port and guarantees 12.95 W at the device. 802.3at, Type 2, supplies 30 W. 802.3bt adds Type 3 at 60 W and Type 4 at up to 90 W.
Keep readingRelated concepts
Read next · Cabling and connectivity What UTP Cable Is, and Why the Twist Does the Shielding The cable the power travels down, and why its resistance is the reason the two numbers differ. Open this next9 min- Cabling and connectivity · 10 min Cat6 vs Cat6a The cable grades, and the copper cross section that decides how much heat a powered run makes.
- Switching · 10 min Top of Rack Switching, and the Number Nobody Puts on the Datasheet Where the powered ports usually live, and the decision that puts them there.
- Cabling · 13 min What Is a Patch Panel, and Why It Does Nothing At All Where a powered run is terminated, and why the panel itself is indifferent to it.