Video surveillance system POE switch power supply problem

The problem of PoE transmission distance is to first figure out the key factors determining the maximum distance. In fact, the transmission of DC power with a standard Ethernet cable (twisted pair) can be transmitted very far, which is much longer than the transmission distance of the data signal. Therefore, the maximum distance of data transmission is the key.

Network cable data transmission maximum distance

Friends who know more about the network know that the twisted pair has an "insurmountable" "100 meters" transmission distance. Whether it is a three-type twisted pair with a transmission rate of 10M, or a Category 5 twisted pair with a transmission rate of 100M, or even a Category 6 twisted pair with a transmission rate of 1000M, the farthest effective transmission distance is 100 meters.

In the integrated wiring specification, it is also explicitly required that the horizontal wiring cannot exceed 90 meters, and the total length of the link cannot exceed 100 meters. That is to say, 100 meters is a limit for wired Ethernet, which is the link length from the network card to the hub device.

How did the maximum distance of 100 meters come from?

What caused the upper limit of the 100-meter transmission distance of the twisted pair? This is to take a deeper look at the deep physics of the twisted pair. The transmission of the network is actually the transmission of the network signal on the twisted pair. As an electronic signal, when transmitted in the twisted pair, it is inevitably affected by the resistance and capacitance, which leads to the attenuation and distortion of the network signal. Attenuation or distortion of the signal to a certain extent will affect the effective and stable transmission of the signal. Therefore, the twisted pair has a transmission distance limit.

So how do you calculate the upper limit of 100 meters?

Category 5 UTP and Super Category 5 UTP are mainly for computer network services. According to Fast Ethernet 100Base-TX, the communication rate is 100mbps, and the time taken for 100mbps Ethernet to transmit 1 bit of data (aka "bit time" ) can be calculated as follows:

1 bit time = 1/100mbps = 10ns

Ethernet is a carrier-assisted multi-access technology (Carrier Sense Multipoint Access/Collision Detection) with CSMA/CD (Carrier Sense Multiple Access with Collision Detection), which is shared by the carrier. Do not open this technology.

When it exceeds 100 meters, because the conflict cannot be detected in time, the packet destroyed by the conflict is transmitted and received by the receiver. The packet is forced to be discarded because it cannot pass the verification. At this time, the mechanism for resending the retransmission is not It is activated, which will result in the loss of the package. When the transmission rate is lower than 100 mbps, the limit length of 100 meters can be appropriately relaxed in practical applications. It must be stated that this is actually effective, but it does not meet the standards. In the certification test, it must be explained, otherwise there will be some problems, such as product warranty.

Maximum cable distance during actual construction

It can be seen from the above that when using PoE power supply, why should the maximum length of the network cable not exceed 100 meters? However, in actual construction, in order to ensure the quality of the project, it is generally 80-90 meters.

Note that the transmission distance here refers to the maximum rate, such as 100M. If the rate is reduced to 10M, the transmission distance can usually be extended to 150-200 meters (depending on the quality of the network cable). Therefore, the PoE power transmission distance is not determined by PoE technology, but by the cable type and quality.

Although the actual quality of the network cable can break through the limit of 100 meters in the actual construction, the equipment can work normally, but this practice is not worth recommending. Because some potential problems are not immediately presented, but appear slowly over time, which can cause subsequent maintenance problems. The simplest case, such as the upgrade of the bandwidth, makes the equipment that can work normally at a distance of more than 100 meters will not work properly after the network speed is greatly improved.