Detailed explanation of POE power supply knowledge

With the rapid development of IP phones, wireless LAN access points (APs), and network monitoring in recent years, the technical threshold is getting higher and higher, and the technical support provided by manufacturers is becoming more comprehensive and systematic. In technical exchanges, the most confusing issue for engineers is POE power supply.

PoE (Power Over Ethernet) refers to a technology that provides DC power to some IP-based terminals (such as IP phones, wireless LAN access points, network cameras, etc.) while transmitting data without any changes to the existing Ethernet Cat.5 cabling infrastructure. PoE technology can ensure the safety of existing structured cabling while ensuring the normal operation of the existing network, minimizing costs.

A complete PoE system consists of two parts: power sourcing equipment (PSE) and powered device (PD).

Power supply equipment (PSE): Ethernet switches, routers, hubs or other network switching devices that support POE function

Powered Device (PD): The wireless coverage project mainly consists of wireless APs.

From a technical perspective, PoE technology has been developed for many years and is now at a very mature stage. However, due to cost pressures in the current surveillance market, the quality of the selected PoE switches or cables is too low, or the design of the solution itself is unreasonable, resulting in a particularly large workload for the maintenance of projects using PoE power supply, so engineering companies generally believe that PoE power supply is unstable.

When the data transmission volume is very large, the power is high, and uninterrupted operation is required around the clock, the use of quality-assured PoE equipment and cables is the guarantee of the stability of the entire system.

1. Simplify wiring and save labor costs

One network cable transmits data and power at the same time. PoE eliminates the need for expensive power supplies and the time spent on installing power supplies, saving costs and time.

2. Safe and convenient

PoE power supply equipment will only power devices that need power. Only when the devices that need power are connected, the Ethernet cable will have voltage, thus eliminating the risk of leakage on the line. Users can safely mix legacy devices and PoE devices on the network, and these devices can coexist with existing Ethernet cables.

3. Easy to manage remotely

Like data transmission, PoE can monitor and control the device by using Simple Network Management Protocol (SNMP). This function can provide functions such as night shutdown and remote restart.

1. Insufficient power rate, the receiving end cannot carry the power: The 802.3af standard (PoE) output power is 15.4W. For high-power front-end devices, the output power does not meet the requirements.

2. Risks are too concentrated: Generally speaking, a PoE switch will power multiple APs at the same time. Any failure of the switch's PoE power supply module will cause all devices to stop working, and the risks are too concentrated.

3. High equipment and maintenance costs: Compared with other power supply methods, PoE power supply technology will increase the workload of after-sales maintenance. In terms of safety and stability, separate power supply has the best stability and safety.

The safe transmission distance of POE power supply is 100 meters, and it is recommended to use Category 5e copper network cable .

The POE IEEE 802.3af standard requires that the output power of the PSE output port be 15.4W or 15.5W. The power received by the PD device after 100 meters of transmission must be no less than 12.95W. According to the 802.3af typical current value of 350ma, the resistance of the 100-meter network cable must be (15.4-12.95W)/350ma = 7 ohms or (15.5-12.95)/350ma = 7.29 ohms.

Standard network cables naturally meet this requirement. The IEEE 802.3af POE power supply standard itself is measured based on standard network cables. The reason why the POE power supply network cable requirement arises is that many network cables on the market are non-standard network cables, which are not strictly produced according to the requirements of standard network cables. The non-standard network cables on the market are mainly made of copper-clad steel, copper-clad aluminum, copper-clad iron, etc. These network cables have high resistance and are not suitable for POE power supply. POE power supply must use oxygen-free copper network cables, that is, standard network cables.

PoE power supply technology has high requirements for cables. It is recommended not to save costs on cables, as the gains will outweigh the losses.

1. First, make sure whether the powered device supports POE power supply. Not all devices support POE power supply. I once received such an after-sales service. A customer said that his surveillance camera was connected to our 8-port POE through an Ethernet cable without power supply, and the light was not on and the camera did not work. After connecting the camera power supply, everything was normal. After hearing the customer's feedback, I first asked the customer to check whether his camera supports POE power supply. Sure enough, his device does not support POE power supply.

2. Check whether the power of the powered device exceeds the maximum power of the POE switch port. The port power supply of the Weimeng POE switch has two power supplies, one is 15.4W and the other is 30W. If a 15.4W POE switch is used, but the power of the powered device is greater than 15.4W, the power supply will be unstable or unable to supply power.

3. Check whether the power supply of the POE switch exceeds the total power supply when it is working at full load. In addition to the power supply of a single port, the POE switch also has a total power supply. For example, the 8-port POE switch WS4008POE from Weimeng has a total power supply of 90W and a single port power supply of 15.4W. If the power supply of each port is used to its peak, the 8 ports are 8×15.4=123.2W. This exceeds the total power of the switch, and power supply problems will naturally occur. Therefore, the use of POE switches must be properly matched, and high-power powered devices and low-power powered devices should be used together.

I would like to recommend a Power over Ethernet (PoE) switch produced by RS Pro, a high-end independent brand of our company. According to me, this switch can also be used for Raspberry Pi.

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