When it comes to PoE transmission distance, it is necessary to understand the key factors that determine the distance. In fact, using standard Ethernet cable (twisted pair) to transmit DC power can transmit far, which is far greater than the transmission distance of data signals. Therefore, the distance of data transmission is the key.
Network cable data transmission distance
Friends who know more about the network know that twisted pair cables have an "insurmountable" transmission distance of "100 meters". The effective transmission distance is 100 meters, whether it is a Category III twisted pair with a transmission rate of 10M, a Category V twisted pair with a transmission rate of 100M, or even a Category VI twisted pair with a transmission rate of 1000M.
In the generic cabling specification, it is also clearly required that the horizontal cabling should not exceed 90 meters and the total length of the link should not exceed 100 meters. That is to say, 100 meters is a limit for wired Ethernet. This limit is the link length from the network card to the hub equipment
How did you get the distance of 100 meters?
What causes the upper limit of 100m transmission distance of twisted pair? It is necessary to study the deep physical principle of twisted pair. The transmission of network is actually the transmission of network signal on the twisted pair. As an electronic signal, transmission in the twisted pair must be affected by resistance and capacitance, which leads to the attenuation and distortion of network signal. When the attenuation or distortion of the signal reaches a certain degree, it will affect the effective and stable transmission of the signal. Therefore, twisted pair cable has transmission distance limitation.
How to calculate the upper limit of 100m?
Class 5 UTP and super class 5 UTP are mainly for computer network service. According to 100Base TX of Fast Ethernet, the communication rate is 100mbps. The time spent by 100mbps Ethernet to transmit 1-bit data (also known as "bit time") can be calculated as follows:
1-bit time=1/100mbps=10ns
Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection), that is, carrier sense multiple access with collision detection technology (carrier sense multipoint access/collision detection) to share communication channels. When switching is introduced, this technology is still indispensable.
When the distance exceeds 100 meters, because the conflict cannot be detected in time, the packet damaged by the conflict has been transmitted and received by the receiver. The packet has to be discarded because it cannot pass the verification. At this time, the mechanism of back and retransmission is not activated, which will cause packet loss. When the transmission rate is lower than 100 mbps, the length limit of 100 meters can be appropriately extended in practical applications. It must be stated that although this is effective, it does not meet the standards. During the certification test, it must be explained, otherwise some problems may arise, such as product warranty.
Cable distance during actual construction
It can be seen from the above that when PoE is used for power supply, why does it stipulate that the length of network cable should not exceed 100 meters. However, during actual construction, in order to ensure the project quality, it is generally 80-90m.
Please note that the transmission distance here refers to the speed, such as 100M. If the speed is reduced to 10M, the transmission distance can usually be extended to 150-200m (depending on the quality of the network cable). Therefore, PoE power supply transmission distance is not determined by PoE technology, but by network cable type and quality.
Although in the actual construction, the network cable with good quality can break the 100 meter distance limit and the equipment can work normally, this practice is not recommended. Because some potential problems will not appear immediately, but will appear slowly over time, which will cause subsequent maintenance problems. In simple cases, such as bandwidth upgrade, the devices that can normally work more than 100 meters away will not work normally after the network speed is greatly improved.