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This article mainly introduces how the POE system is composed, has a certain reference value, interested friends can refer to, I hope you can learn a lot after reading this article, the following let the editor take you to understand it.

Brief introduction of POE system composition

A complete PoE system includes two parts: power supply end equipment (PSE, Power Sourcing Equipment) and power receiving end equipment (PD, Power Device). PSE devices, such as POE switches, are devices that power Ethernet client devices and are also managers of the entire PoE Power over Ethernet process, while PD devices are PSE payloads that receive power, that is, client devices of PoE systems, such as IP phones, network security cameras, wireless AP and many other Ethernet devices such as PDA or mobile phone chargers. Based on the IEEE 802.3af standard, the two establish information about the connection of the receiving end device PD, device type, power consumption level and so on, and use this as the power supply to PD through Ethernet according to PSE.

There are four pairs of twisted pair in the standard five types of network cables, and IEEE80 2.3af allows two wire sequence power supply methods:

1) one is to transmit current on the pairs of 4, 5, 7 and 8 lines, and stipulate that 4 and 5 are positive electrodes and 7 and 8 are negative electrodes.

2) the other kind of power supply is the transmission power supply on the 1, 2, 3, 6 lines, the polarity is arbitrary, 1, 2 is positive, 3, 6 is negative or 1, 2 is negative, 3, 6 is positive, one of the polarity of power supply.

The working process of IEEE802.3af:

In June 2003, IEEE approved 802. 3af standard, which clearly defines the items of power detection and control in remote systems, and specifies the way in which routers, switches and hubs supply power to IP phones, security systems and wireless LAN access points through Ethernet cables.

1. Detection: at the beginning, before PSE supplies power to the powered equipment, it outputs a low voltage to check whether the powered equipment (PD) conforms to the IEEE802.3af standard. If it meets the standard, it is generally in the powered equipment to select a resistance of 24.9K to confirm that it meets the IEEE802.3af power supply standard.

2, grading: when the PSE detects a resistance value that meets the requirements, it will further increase the output voltage to grade the powered equipment and evaluate the power loss required by the PD equipment. If the powered device does not respond to the hierarchical confirmation current at this time, PSE defaults the powered device to level 0, providing it with an output power of 15.4W.

3. Power supply: during a start-up period of configurable time (generally less than 15 μ s), PSE devices begin to supply power to PD devices from low voltage until 48V DC power is provided. After confirming the classification, PSE will output 48V direct current to the powered equipment, and confirm that the powered equipment does not exceed the power requirement of 15.4W. When the powered equipment is overloaded or short-circuited, PSE stops supplying power for it and enters the testing phase again.

4. Power outage: if the PD device is disconnected from the network, the PSE will quickly (usually within the 300~400ms) stop supplying power to the PD device and repeat the detection process to check whether the cable terminal is connected to the PD device.

Main parameters of IEEE802.3af power supply system:

The DC voltage is between 44V and 57V and the typical value is 48V.

The typical working current is 10~350mA and the typical output power is 15.4W.

The overload detection current is 350~500mA.

Under no-load condition, the maximum required current is 5mA.

Provide electric power requests of four Class levels from 3.84W to 12.95W for PD equipment.

Grading parameters of IEEE802.3af:

The maximum operating power required by the Class 0 device is 0mm 12.95W

The maximum operating power required for Class 1 equipment is 0mm 3.84W

The operating power required by the Class 2 device is between 3.85W~6.49W

The power range of the Class 3 device is between 6.5 and 12.95W.

Introduction of IEEE 802.3at Standard

Due to the IEEE 802.3af specification, the power consumption of PoE on powered equipment (PD) is limited to 12.95W, which is sufficient to meet the demand for traditional network powered devices. However, with the emergence of IP telephones and high-power applications such as webcams, dual-band access, video telephones and PTZ video surveillance systems, 13W power supply is obviously unable to meet the demand, which limits the application range of Ethernet cable power supply. In order to overcome the power budget limitations of PoE and promote it to new applications, IEEE has set up a new task force to explore ways to improve the power limits of the international power standard. In order to evaluate the possibility of implementing IEEE802.3at technically and economically, the new standard is called IEEE802.3at, which defines devices with power requirements higher than 12.95W as Class 4, which can be extended to 25W or higher. The new standard was released in early 2009.

Compared with 802.3af, IEEE 802.3at can output more than 2 times of electricity, and the output power of each port can be more than 30W. As far as the standard is concerned, the two have different definitions of power and classification.

In the IEEE802.3at regulation, the powered equipment PD can be up to 29.95W PSE will provide it with more than 30W DC power supply. PD responds with Class4-graded current, telling PSE whether it can provide it with the higher power specified by 802.3at.

Main parameters of IEEE802.3at power supply system:

The DC voltage is between 50 and 57V, and the typical value is 50V.

The typical working current is 10~600mA and the typical output power is 30W.

The powered device PD supports Class4 rating.

Examples are as follows:

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