PoE, Power over Ethernet, is to supply power to connected devices through a network cable. Network cable, also called twisted pair cable. Twisted pair, as the name implies, is two wires twisted together. The general network cable is composed of 4 pairs of twisted pairs, that is, 8 wires. The four pairs of 8-core network cables are arranged according to certain rules, and there are two specifications of T568A and T568B. The specific definition requirements are as follows:
From this, we can see that in a 100M transmission network, in fact, only 2 pairs and 4 wires are needed. The remaining 4 wires are free. For gigabit networks, all 4 pairs of 8 wires are used. In addition, the T568B standard is used more, and the T568A standard is mostly used for direct connection with similar equipment, such as a connection between two computers. In addition, with the development of data exchange technology, the network ports of various devices can generally adapt to different A/B standards.
As mentioned above, in the 100M network, there are two pairs of four network cables that are idle. Can these four cables be used to transmit power and supply power to the equipment? The answer is yes. The only problem is that the receiving end equipment is different, and the required voltage is definitely different. If the supplied voltage does not meet the requirements of the receiving end equipment, such as the voltage is too high, it will definitely be easy to damage the equipment. Therefore, a unified standard is needed.
This standard is the IEEE 802.3 standard for POE power supply developed by the Institute of Electrical and Electronics Engineers (IEEE). The existing IEEE 802.3 standards on PEO power supply are summarized as follows:
|Standard||Nickname||Maximum power||Power supply line sequence|
|802.3af||PoE||15.4W||12+/36- or 45+/78-|
|802.3at||PoE+||30W||12+/36- or 45+/78-|
|802.3bt||4PPoE/PoE++/Ultra PoE/UPOE||60W||12+/36- or 45+/78- or 1245+/3678-|
From the list, we can see that there are 3 power supply line sequences, we named them A, B, 4 pairs of three modes. A mode, when using 1236 line to transmit data at the same time transmit power. B mode, use 1236 line to transmit data, use 4578 line to transmit power.
The POE power supply device (such as a POE switch) decides to use the A or B power supply line sequence mode. At the same time, the POE receiving equipment must support both A/B power supply line sequence.
4-pair line mode, that is, the power supply line sequence used by the 802.3bt standard. All 4 pairs of wires need to be used to transmit power.
The so-called non-standard PoE is simply a network cable power supply method that does not meet the 802.3af/at standard. The main point is to use the idle 4578 line pair of the 100M network cable to directly transmit power, such as 5V, 9V, 12V or 24V power directly from one end of the network cable to the other end to the power receiving device. So it is obvious that if the rated voltage required by the powered device is 5V, but the transmitted voltage is 12V, it is likely that the device will be damaged due to excessive voltage. At the same time, because it is directly powered, it is not suitable for too high voltage transmission, which also leads to a short transmission distance that can be supported (low voltage and severe voltage drop). Therefore, when using non-standard POE equipment, you must pay attention that the voltage of the POE power supply equipment and the power receiving equipment are the same, and the transmission distance cannot be too far.
Application in security
In the high-definition network video surveillance project, the limitation of the transmission distance of the POE network camera is mainly due to the loss of the network cable and the greater power consumption after the infrared light of the camera is turned on at night.
Theoretically, the maximum allowable impedance of a normal 100-meter network cable for category 3 network cable is 20Ω, and that for category 5 is 12.5Ω. Under the 802.3af standard 15.4 watts and 350mA power supply, the voltage drop is 7V, the network cable loss is 2.45 watts, and the maximum available power of the receiving terminal is 15.4-2.45 = 12.95 watts. Generally, the power consumption of a network camera during the day is about 3-5 watts, and at night when the infrared light is fully turned on, it is about 8-10W. So there is no problem with the transmission distance of the POE network camera up to 100 meters. However, if the quality of the network cable is not good, or the use of non-copper wire will increase the line loss, severely reduce the POE transmission distance and even the stability of the camera’s power supply (the performance will cause the camera to frequently drop the line at night).
If we use 802.3at standard 30 watts, 600mA power supply, 5 types of network cable transmission, under calculation, whether it can transmit 250 meters and can provide stable power supply for the camera.
The maximum resistance of the 250-meter standard category 5 network cable is 31.25Ω, the transmission voltage drop is 18.75V, the network cable loss is 11.25W, and the receiving terminal has 30-11.25=18.75 watts.
The power supply is enough, but we cannot ignore the transmission rate of the more critical ports. The transmission distance of the switch is limited to 100 meters. It is very important that the transmission bandwidth of 100M can be guaranteed within 100 meters. If it exceeds 100 meters, the network transmission bandwidth will drop sharply and packet loss will be serious. However, if the port speed is reduced to 10M, it is enough to ensure a transmission distance of 250 meters. For a single network camera, a bandwidth of 10M is sufficient.
The above is just a simple calculation and explanation to explain why there are many POE switches on the market that claim to support the transmission of ordinary POE cameras for 250 meters. From the perspective of power supply and support for the transmission rate of a single camera, this product is theoretically feasible. It should be noted that, in practice, general network cameras only support the 802.3af standard, and the heat generated by the line during the transmission process will cause a large loss of power consumption. But even so, through optimization, appropriately increase the voltage of the power supply terminal (POE switch), reduce the transmission current (reduce the voltage drop of the power receiving terminal, reduce the heat generation during the line transmission, and reduce the power loss), and reduce the port rate to 10M, it is feasible to transmit an 802.3af standard POE network camera stably for 250 meters.
PoE products in security
- PoE network camera
- PoE switch
- PoE module
- PoE splitter