In some projects, PoE switches are needed to power wireless AP, network phone, network camera and other devices. Although this practice is very simple, but the real involves the choice and use, I am afraid that it is not so simple, otherwise there would not be so many so-called "unstable", "unnecessary" and other views.
1. PoE transmission distance
The power supply distance using PoE is the same as the signal transmission distance of the network cable, which is 100 meters. Generally, PoE uses 4/5 and 7/8 pairs of cables in the network cable for power supply, so when using PoE power supply, try to choose a good quality 8-core copper wire (standard network cable).
2, power supply, voltage
PoE power supply has two standards, one is IEEE802.3af, the other is IEEE802.3at.
IEEE802.3af: power supply voltage 44~57V, power supply current 10~350mA, maximum power supply power 15.4W, maximum electric power 12.95W.
IEEE802.3at: power supply voltage 50~57V, power supply current 10~600mA, maximum power supply power 30W, maximum electric power 25.5W.
Why is the voltage 40V, 50V so large, will not burn the equipment? Of course not, the cameras that support PoE power supply have PoE modules, which can convert 40V and 50V voltage into the voltage required by the equipment.
What is the power supplied and the power received? The power supply is the output voltage from the switch or the video recorder port, and the electric power is the voltage received by the camera, which is calculated after the attenuation of 100 meters CAT-3 cable line.
The electric power 25.5W is too big, will it burn the camera? No, the PoE module will "AC" during power supply to determine the power supply, not necessarily according to the maximum power output.
3. Power supply process
Detection: Before power supply, the power supplier outputs a small voltage to detect whether the device supports PoE. If it does not support PoE, no power is supplied. If yes, go to the next step.
Classification: After confirming that the receiving party supports PoE, the power supply party will conduct further testing to determine the power required by the receiving party and classify the receiving party.
Start power supply and continue to provide stable power supply.
4. Answer PoE questions
Can af and at be universal? Not universal, but at is backward compatible with af. That is to say, if the switch supports the at standard and can provide a maximum of 30W power supply, it can fully power the 7W camera that supports the af standard. However, if the camera supports at, 20W of power is required, and the switch that only supports af cannot supply power to this camera normally.
Can PoE switches be switched at two levels? PoE is only to solve the power supply problem, will not affect the signal, if you want to use a multi-layer switch, only need to let the camera closest to the switch to power the camera is good, the switch behind the switch does not need to have any contact with PoE.
Buy PoE switch must know three points:
1. Don't try to be cheap
China is a large market with a variety of products, and PoE switches are no exception. There are countless brands of PoE switches in the market, both large and small, and the price and quality vary greatly. Some people who have first contact with PoE power supply think that as long as it is a PoE switch, it does not matter if it is good or bad, so in order to save costs, they often choose the lowest price compared with more than N.
Not that cheap is bad, cheap also have good goods, but after all, less. Generally speaking, the higher the price of a thing, the hardware, parts, accessories, chips and so on used are high-quality, guaranteed, very cheap things, there are many hidden dangers and shortcomings. For PoE switches, the price is too low may have many problems in later use, such as unstable power supply, no power supply, after-sales can not be carried out.
2, try to stay away from non-standard
PoE switches are mostly standard, but there are also a small number of non-standard. The reason why the non-standard still exists is because the cost is low, but because some places can use non-standard. However, the non-standard PoE switch has security risks, such as burning the network device connected to it. When using a non-standard PoE switch, the power supply voltage and the power device voltage must be consistent. Standard PoE switch is relatively safe, can automatically connect with the connected network equipment, only support PoE power supply network equipment PoE switch will start to power supply, to ensure the safety of the power equipment.
3, choose the right model
There are many PoE switch models, the number of ports is 4, 8, 16, 24, the port transmission rate is 100 and gigabit, the power supply power is 60W, 150W, 250W, 400W, what port number and port transmission rate need to be selected correctly. The port transmission rate of PoE switches is about 40 to 60%. If a network theoretically uses 100 gigabit PoE switches, select Gigabit switches. In addition, for surveillance cameras such as infrared lights, we must consider the power of infrared lights.
The use of PoE switches must know two points:
1, high-quality network cable The quality of the network cable is critical to the success of PoE power supply. It is generally recommended to use Category 5 network cables, super Category 5 network cables and category 6 network cables. The internal material of the network cable is also very important, do not use what aluminum wire, copper-covered aluminum wire, copper-covered iron wire, iron core wire, etc., the use of pure copper wire is ideal. In a word: go to regular shopping malls, supermarkets to buy regular network cables.
2, the theoretical distance of PoE power supply within 80 meters is 100 meters, but in view of the quality of the market Internet line and the impact of the environment, the actual power supply distance is maintained within 80 meters as far as possible.