PoE  refers to a technology that transmits power through network cables. It  uses existing Ethernet to simultaneously transmit data and supply power  to IP terminal devices (such as: IP phones, APs, IP cameras, etc.)  through network cables.

PoE is also known as Power over LAN or Active Ethernet, sometimes referred to as Power over Ethernet.

In  order to standardize and promote the development of PoE power supply  technology and solve the compatibility problems between power supply and  receiving equipment of different manufacturers, the IEEE Standards  Committee has issued three PoE standards: IEEE 802.3af standard, IEEE  802.3at standard, IEEE 802.3bt standard.

With  the increasing popularity of IP phones, network video surveillance and  wireless Ethernet devices in the network, the requirement to provide  power support through Ethernet itself is becoming more and more urgent.  In most cases, terminal equipment needs DC power supply, and terminal  equipment is usually installed on the ceiling or outdoors relatively  high from the ground, and it is difficult to have a suitable power  socket nearby. Even if there is a socket, it is difficult to find a  place for the AC-DC converter required by the terminal equipment  placement. In addition, in many large-scale LAN applications,  administrators need to manage multiple terminal devices at the same  time. These devices require unified power supply and unified management.  Due to the limitation of power supply location, it brings great  inconvenience to power supply management. Power over Ethernet PoE just  solves this problem.

PoE  is a wired Ethernet power supply technology. The network cable used for  data transmission also has the capability of DC power supply,  effectively solving the centralized power supply of IP phones, wireless  APs, portable device chargers, card readers, cameras, data acquisition  and other terminals. . PoE power supply has the advantages of  reliability, simple connection and unified standard:

·Reliable:  One PoE device can supply power to multiple terminal devices at the  same time, realizing centralized power supply and power backup at the  same time.

·Easy connection: Terminal equipment does not need an external power supply, only a network cable is needed.

·Standard:  It complies with international standards and uses the globally unified  RJ45 power interface, which can ensure the docking with equipment from  different manufacturers.

As shown in the figure below, the PoE power supply system includes the following two device roles:

·Power-sourcing  equipment PSE (Power-sourcing Equipment): A PoE device that supplies  power to the powered device through Ethernet, providing functions such  as detection, analysis, and intelligent power management, such as a PoE  switch.

·PD  (Powered Device): Such as wireless AP, portable device charger, credit  card machine, camera and other powered devices. PDs are classified into  standard PDs and non-standard PDs according to whether they comply with  IEEE standards.

According  to the definition of the IEEE standard, PSE devices are divided into  two types: MidSpan (PoE function module is outside the device) and  Endpoint (PoE function module is integrated into the device). The PoE  functional modules of Huawei Cloudengine S series PoE switches are all  integrated inside the device, which belongs to the Endpoint PSE device.

Endpoint  PSE devices are divided into two power supply modes: Alternative A (1/2  and 3/6 wire pairs) and Alternative B (4/5 and 7/8 wire pairs)  according to the power supply wire pairs used.

Alternative  A power supply mode supplies power through the data pair. The PSE  supplies power to the PD through the 1/2 and 3/6 wire pairs, the 1/2  connection forms the negative pole, and the 3/6 connection forms the  positive pole. 10BASE-T and 100BASE-TX interfaces use 1/2 and 3/6 wire  pairs to transmit data, and 1000BASE-T interfaces use all 4 wire pairs  to transmit data. Since DC direct current and data frequency do not  interfere with each other, current and data can be transmitted  simultaneously on the same pair of wires.

Alternative  B power mode supplies power through the idle pair. The PSE supplies  power to the PD through the 4/5 and 7/8 wire pairs, the 4/5 connection  forms the positive pole, and the 7/8 connection forms the negative pole.

The  IEEE standard does not allow the above two power supply modes to be  applied at the same time. The power supply equipment PSE can only  provide one usage, but the powered equipment PD must be able to adapt to  both situations at the same time.

After  the PSE is powered on and the PD is connected to the PSE through the  network, the PSE and the PD start power supply negotiation:

1.  Detection of PD: PSE periodically outputs a small voltage with limited  current on the port to detect the existence of PD equipment. If a  resistor with a specific resistance value is detected, it means that the  terminal of the cable is connected to a power receiving device that  supports the IEEE 802.3af standard or the IEEE 802.3at standard (a  specific resistor with a resistance value of 19kΩ～26.5kΩ, and the usual  small voltage is 2.7V～ 10.1V, the detection period is 2 seconds).

2.  Power supply capability negotiation is the process of classifying PD  equipment: PSE classifies PDs and negotiates power supply power. Power  supply capability negotiation can be achieved not only by analyzing the  resistance sent by the PSE and PD, but also by link layer discovery  protocol LLDP (Link Layer Discovery Protocol) protocol to discover and  advertise the power supply capability for negotiation.

3.  Start power supply: During the startup period (generally less than  15μs), the PSE device starts to supply power to the PD device from a low  voltage until it provides a DC voltage of 48V.

4.  Normal power supply: After the voltage reaches 48V, the PSE provides a  stable and reliable 48V DC power for the PD device, and the power  consumption of the PD device does not exceed the maximum output power of  the PSE.

5.  Power failure: During the power supply process, the PSE will  continuously monitor the PD current input. When the PD current  consumption drops below the minimum value, or the current surges, such  as unplugging the device or encountering a PD device power consumption  overload, short circuit, or exceeding the power supply of the PSE load,  etc., the PSE will disconnect the power supply and repeat the detection  process.

We  mentioned earlier that the IEEE Standards Committee has issued three  PoE standards to solve the compatibility problem between power supply  and receiving equipment of different manufacturers: IEEE 802.3af  standard, IEEE 802.3at standard, IEEE 802.3bt standard. So what is the  difference between these three standards?

In  June 2003, the IEEE 802.3 working group formulated the IEEE 802.3af  standard. As an extension of the Ethernet standard, it made detailed  regulations on the power supply, transmission and reception of network  power supply. For example: The IEEE 802.3af standard stipulates that PSE  devices need to provide up to 15.4W of DC power on each port.

Due  to some power dissipation in the cable, only 12.95W is available to the  powered device. In October 2009, the IEEE 802.3at standard was born to  meet the needs of high-power terminals. On the basis of being compatible  with the 802.3af standard, the IEEE 802.3at standard provides a maximum  power of 25.5W to meet new requirements.

In  September 2018, in order to further improve the PoE power supply and  optimize the standard, the IEEE Standards Committee released the IEEE  802.3bt standard. The IEEE 802.3bt standard further improves the power  supply capability. Type 3 can provide up to 51 W of power supply, and  Type 4 can provide up to 71.3W of power supply. Additionally, support  for 2.5GBASE-T, 5GBASE-T and 10GBASE-T is included, expanding the use of  applications such as high-performance wireless access points and  surveillance cameras.

Generally,  the power supply technology corresponding to the IEEE 802.3af standard  is called PoE power supply, the power supply technology corresponding to  the IEEE 802.3at standard is called PoE+ power supply, and the power  supply technology corresponding to the IEEE 802.3bt standard is called  PoE++ power supply, also known as 4PPoE. The corresponding parameters of  the three power supply technologies are shown in the figure below.