1. Definition and characteristics of POE switch
POE (Power over Ethernet) switch, in layman's terms, is a switch that supports power supply through network cables. It can not only realize the data transmission function of ordinary switches, but also power network terminals at the same time. Without making any changes to the existing Ethernet Cat.5 wiring infrastructure, it can provide DC power supply while transmitting data signals for some IP-based terminal devices, such as IP phones, wireless LAN access points AP, network cameras, etc.
POE switches follow international standards to ensure safety and compatibility. For example, POE switches that comply with the IEEE802.3af standard have a port output power of 15.4W, and those that comply with the IEEE802.3at standard have a port output power of 30W. For POE switches that comply with the IEEE802.3at standard, the port output power can reach 30W, and the power available to the powered device is 25.4W. In addition, POE switches have many other features. The first is flexibility. Only one cable needs to be installed and supported, which is simple and space-saving, and the device can be moved at will. Secondly, it saves costs. For devices in places where it is difficult to deploy AC power, such as video surveillance cameras, POE technology saves expensive power supplies and the time and cost of installing power supplies. In addition, POE can monitor and control devices using the Simple Network Management Protocol (SNMP) to achieve intelligent power supply. At the same time, the POE power supply device will only supply power to the devices that need power, eliminating the risk of leakage on the line and ensuring power supply safety. In addition, a single UPS can provide power for all related devices when the power is off, which has the advantage of centralized power supply. In addition, users can automatically and safely mix original devices and POE devices on the network, and the device compatibility advantage is obvious. When the remote device is connected to the network, it can be remotely controlled, reconfigured or reset, making the network equipment easy to manage.
2. Working principle of POE switch
(I) Detection stage
The POE switch first outputs a very small voltage. This process is like reaching out a hand in the dark to explore the unknown. According to the information searched, this voltage is usually very small. The purpose is to detect whether the device connected to the cable terminal is a powered device that supports the IEEE802.3af standard, which is what we often call a PD device. If the detected device does not support the standard, the POE switch will not perform subsequent power supply operations.
(II) PD-end device classification stage
When the powered device is detected, the POE switch will classify it. This classification process is like grouping different students and arranging them according to their characteristics and needs. At the same time, the POE switch will evaluate the power loss required by the PD device. For example, some devices may have low power requirements, while some devices, such as surveillance cameras with heating functions, will increase power requirements under certain circumstances. By accurately evaluating power loss, the POE switch can better provide appropriate power supply for different devices.
(III) Start power supply stage
In a configurable time, generally less than 15μs, the POE switch starts to supply power to the PD device from a low voltage. This process is like slowly turning on a faucet, and the water flow gradually increases from a trickle. Until a stable 48V DC power supply is provided, ready for the normal operation of the PD device.
(IV) Power supply stage
At this time, the POE switch provides a stable and reliable 48V DC power to the PD device. According to the data, this process must meet the power consumption of the PD device not exceeding 15.4W. Just like refueling a car, the amount of refueling needs to be controlled according to the car's tank capacity and fuel consumption to ensure that the car can run normally without problems.
(V) Power failure stage
If the PD device is disconnected from the network, the POE switch will quickly stop supplying power, generally within 300-400ms. This is like when an electrical appliance is suddenly unplugged, the power supply will be quickly cut off to ensure safety. In addition, the POE switch will repeat the detection process to detect whether the terminal of the cable is connected to the PD device, ready to provide power support for the newly connected device at any time