What is the Standard implementation of PoE injector ?

Standard implementation:

Power over Ethernet based on standards is implemented according to the specifications outlined in IEEE 802.3af-2003, which was later included as clause 33 in IEEE 802.3-2005, or the updated version from 2009 known as IEEE 802.3at. These guidelines mandate the use of at least category 5 cable for delivering high power levels, although category 3 cable can be utilized if lower power requirements are needed.

Power is delivered in the form of a common-mode signal through multiple differential pairs of wires in Ethernet cables. This power is provided by a power supply in a PoE-enabled networking device, like an Ethernet switch, or can be added to a cable using a midspan power supply. The midspan power supply is an extra PoE power source that can be used with a non-PoE switch.

Phantom power is used in Ethernet cables to enable powered pairs to carry data. This allows for compatibility with various Ethernet standards, including 10BASE-T, 100BASE-TX, 1000BASE-T, 2.5GBASE-T, 5GBASE-T, and 10GBASE-T. The technique works by utilizing differential data transmission over each pair with transformer coupling, allowing for the DC supply and load connections to be made to the transformer center-taps at each end. The polarity of the DC supply can be inverted using crossover cables, with the powered device able to operate with either spare pairs 4 5 + and 7 8 – or data pairs 1 2 + and 3 6 -. Polarity standards are defined for spare pairs, but can be ambiguous for data pairs, often requiring the use of a diode bridge.

Comparison of PoE parameters

Property	802.3af (802.3at Type 1), PoE	802.3at Type 2, PoE+	802.3bt Type 3, 4PPoE or PoE++	802.3bt Type 4, 4PPoE or PoE++
Power available at PD	12.95 W	25.50 W	51 W	71.3 W
Maximum power delivered by PSE	15.40 W	30.0 W	60 W	90 W
Voltage range (at PSE)	44.0–57.0 V	50.0–57.0 V	50.0–57.0 V	52.0–57.0 V
Voltage range (at PD)	37.0–57.0 V	42.5–57.0 V	42.5–57.0 V	41.1–57.0 V
Maximum current	350 mA	600 mA	600 mA per pair	960 mA per pair
Maximum cable resistance per pairset	20 Ω	12.5 Ω	12.5 Ω	12.5 Ω
Power management	Three power class levels (1–3) negotiated by signature	Four power class levels (1–4) negotiated by signature or 0.1 W steps negotiated by LLDP	6Six power class levels (1–6) negotiated by signature or 0.1 W steps negotiated by LLDP	Eight power class levels (1–8) negotiated by signature or 0.1 W steps negotiated by LLDP
Derating of maximum cable ambient operating temperature	None	5 °C (9 °F) with one mode (two pairs) active	5 °C (9 °F) with one mode (two pairs) active	10 °C (20 °F) with temperature planning required
Supported cabling	Category 3 and Category 5	Category 5	Category 5	Category 5
Supported modes	Mode A (endspan), Mode B (midspan)	Mode A, Mode B	Mode A, Mode B, 4-pair Mode	4-pair Mode Mandatory

Powering devices

There are three available power modes for PoE: Mode A, Mode B, and 4-pair. In Mode A, power is delivered on the data pairs of 100BASE-TX or 10BASE-T. In Mode B, power is delivered on the spare pairs. The 4-pair mode delivers power on all four pairs. PoE can also be used on Ethernet standards such as 1000BASE-T, 2.5GBASE-T, 5GBASE-T, and 10GBASE-T, where the phantom technique is used to deliver power as there are no spare pairs.

Mode A has two alternative configurations (MDI and MDI-X), utilizing the same pairs but with differing polarities. In this mode, pins 1 and 2 (pair #2 in T568B wiring) create one side of the 48 V DC, while pins 3 and 6 (pair #3 in T568B) create the other side. These same pairs are utilized for data transmission in 10BASE-T and 100BASE-TX, enabling the transmission of both power and data over only two pairs in these networks. The flexible polarity of Mode A allows for Power over Ethernet (PoE) to adapt to crossover cables, patch cables, and Auto MDI-X.

In mode B,it requires the use of a 4-pair cable, with pins 4-5 forming one side of the DC supply and pins 7-8 providing the return. These pairs are considered the “spare” pairs in both T568A and T568B and are used for 10BASE-T and 100BASE-TX connections.

The decision of whether to use power mode A or B is made by the PSE, not the PD. The standard prohibits PDs that only support mode A or mode B. The PSE has the capability to support mode A, mode B, or both. PDs signal their compliance with the standard by including a 25 kΩ resistor between the powered pairs. If the resistance detected by the PSE is too high, too low, or indicates a short circuit, no power will be supplied. This protects devices that do not have PoE support. Additionally, a power class feature allows the PD to communicate its power requirements by adjusting the sense resistance at higher voltages.

To retain power, the PD must use a minimum of 5-10 mA continuously for at least 60 ms. If the PD fails to meet this requirement for more than 400 ms, the PSE will deem the device disconnected and will remove power for safety reasons.

There are two types of PSEs:endspans and midspans. Endspans, also known as PoE switches, are Ethernet switches that incorporate power over Ethernet transmission technology. Midspans, on the other hand, are power injectors that sit between a regular Ethernet switch and the powered device, providing power without affecting data transmission. Endspans are typically used in new installations or when replacing switches for various reasons, such as upgrading from 10/100 Mbit/s to 1 Gbit/s, as they offer the convenience of adding PoE capability. Midspans, on the other hand, are utilized when there is no need or desire to replace and reconfigure the existing Ethernet switch, and only PoE functionality needs to be added to the network.

Steps to power up a PoE link

Step	Action	Volts specified (V) 802.3af	Volts specified (V) 802.3at
Detection	PSE detects if the PD has the correct signature resistance of 19–26.5 kΩ	2.7–10.1	2.7–10.1
Classification	PSE detects resistor indicating power range	14.5–20.5	14.5–20.5
Mark 1	Signals PSE is 802.3at capable. PD presents a 0.25–4 mA load.	—	7–10
Class 2	PSE outputs classification voltage again to indicate 802.3at capability	—	14.5–20.5
Mark 2	Signals PSE is 802.3at capable. PD presents a 0.25–4 mA load.	—	7–10
Startup	Startup voltage	> 42	> 42
Normal operation	Supply power to device	37–57	42.5–57

Type 2 devices are another term for IEEE 802.3at capable devices. An 802.3at Power Sourcing Equipment (PSE) can also utilize LLDP communication for indicating its 802.3at capability.

Available power levels

Class	Usage	Classification current (mA)	Power range at PD (W)	Max power from PSE (W)	Class description
0	Default	0–5	0.44–12.94	15.4	Classification unimplemented
1	Optional	8–13	0.44–3.84	4.00	Very Low power
2	Optional	16–21	3.84–6.49	7.00	Low power
3	Optional	25–31	6.49–12.95	15.4	Mid power
4	Valid for Type 2 (802.3at) devices, not allowed for 802.3af devices	35–45	12.95–25.50	30	High power
5	Valid for Type 3 (802.3bt) devices	36–44 & 1–4	40 (4-pair)	45
6	Valid for Type 3 (802.3bt) devices	36-44 & 9–12	51 (4-pair)	60
7	Valid for Type 4 (802.3bt) devices	36–44 & 17–20	62 (4-pair)	75
8	Valid for Type 4 (802.3bt) devices	36–44 & 26–30	71.3 (4-pair)	90

Only IEEE 802.3at (Type 2) devices are permitted to use Class 4, and they must have valid Class 2 and Mark 2 currents for powering up. If an 802.3af device claims to have a Class 4 current, it will be deemed non-compliant and treated as a Class 0 device instead.

The setup phases are as follows: