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How does a PoE switch provide PoE power

How does a PoE switch provide PoE power? PoE power supply principle overview

The principle of PoE power supply is actually very simple. The following takes a PoE switch as an example to explain in detail the working principle of a PoE switch, the PoE power supply method and its transmission distance.

How PoE Switches Work

After connecting the power receiving device to the PoE switch, the PoE switch will work as follows:

 overview

Step 1: Detect the powered device (PD). The main purpose is to detect whether the connected device is a real powered device (PD) (in fact, it is to detect the powered device that can support the power over Ethernet standard). The PoE switch will output a small voltage at the port to detect the power receiving end device, which is the so-called voltage pulse detection. If the effective resistance of the specified value is detected, the device connected to the port is the real power receiving end device. It should be noted that the PoE switch is a standard PoE switch, and the non-standard PoE switch of the single-chip solution will not perform this detection without a control chip.

Step 2: Classification of Powered Devices (PD). When a Powered Device (PD) is detected, the PoE switch classifies it, classifies it, and evaluates the power consumption required by the PD.

grade

PSE output power (W)

PD input power (W)

0

15.4

0.44–12.94

1

4

0.44–3.84

2

7

3.84–6.49

3

15.4

6.49–12.95

4

30

12.95–25.50

5

45

40 (4-pair)

6

60

51 (4-pair)

8

99

71.3 (4-pair)

7

75

62 (4-pair)

Step 3: Start the power supply. After the level is confirmed, the PoE switch will supply power to the receiving end device from a low voltage until a 48V DC power is provided within less than 15μs configuration time.

Step 4: Power on normally. It mainly provides stable and reliable 48V DC power for the receiving end equipment to meet the power consumption of the receiving end equipment.

Step 5: Disconnect the power supply. When the power receiving device is disconnected, the power consumption is overloaded, the short circuit occurs, and the total power consumption exceeds the power budget of the PoE switch, the PoE switch will stop supplying power to the power receiving device within 300-400ms, and will restart the power supply. test. It can effectively protect the power receiving device and the PoE switch to prevent damage to the device.

PoE power supply mode

It can be seen from the above that the PoE power supply is realized through the network cable, and the network cable is composed of four pairs of twisted pairs (8 core wires). Therefore, the eight core wires in the network cable are the PoE switches that provide data and The medium of power transmission. At present, the PoE switch will provide the receiving end device with compatible DC power through three PoE power supply modes: Mode A (End-Span), Mode B (Mid-Span) and 4-pair.

PoE power supply distance

Because the transmission of power and network signals on the network cable is easily affected by resistance and capacitance, resulting in signal attenuation or unstable power supply, the transmission distance of the network cable is limited, and the maximum transmission distance can only reach 100 meters. The PoE power supply is realized through the network cable, so its transmission distance is affected by the network cable, and the maximum transmission distance is 100 meters. However, if a PoE extender is used, the PoE power supply range can be extended to a maximum of 1219 meters.

How to troubleshoot PoE power failure?

When the PoE power supply fails, you can troubleshoot from the following four aspects.

Check whether the power receiving device supports PoE power supply. Because not all network devices can support PoE power supply technology, it is also necessary to check whether the device supports PoE power supply technology before connecting the device to a PoE switch. Although PoE will detect when it is working, it can only detect and supply power to the receiving end device that supports the PoE power supply technology. If the PoE switch does not supply power, it may be because the receiving end device cannot support the PoE power supply technology.

Check whether the power of the power receiving device exceeds the maximum power of the switch port. For example, a PoE switch that only supports the IEEE 802.3af standard (the maximum power of each port on the switch is 15.4W) is connected to a power receiving device with a power of 16W or more. At this time, the power receiving end The device may be damaged due to power failure or unstable power, resulting in PoE power failure.

Check whether the total power of all connected powered devices exceeds the power budget of the switch. When the total power of the connected devices exceeds the switch power budget, the PoE power supply fails. For example, a 24-port PoE switch with a power budget of 370W, if the switch complies with the IEEE 802.3af standard, it can connect 24 power receiving devices that follow the same standard (because the power of this type of device is 15.4W, connecting 24 The total power of the device reaches 369.6W, which will not exceed the power budget of the switch); if the switch complies with the IEEE802.3at standard, only 12 power receiving devices that follow the same standard can be connected (because the power of this type of device is 30W, if the switch is connected 24 would exceed the switch’s power budget, so only a maximum of 12 can be connected).

Check whether the power supply mode of the power supply equipment (PSE) is compatible with that of the power receiving equipment (PD). For example, a PoE switch uses mode A for power supply, but the connected power receiving device can only receive power transmission in mode B, so it will not be able to supply power.

Summarize

PoE power supply technology has become an important part of digital transformation. Understanding the principle of PoE power supply will help you protect PoE switches and power receiving devices. At the same time, understanding PoE switch connection problems and solutions can effectively avoid deploying PoE networks. waste unnecessary time and cost.


Post time: Nov-09-2022