Network cable data transmission maximum distance


Friends who know more about the network know that twisted pair has an "insurmountable" transmission distance of "100 meters". Whether it is Category 3 twisted pair with 10M transmission rate, Category 5 twisted pair with 100M transmission rate, or even Category 6 twisted pair with 1000M transmission rate, the longest effective transmission distance is 100 meters.


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


How did you get the maximum distance of 100 meters?

What caused the upper limit of the 100-meter transmission distance of the twisted pair? This requires a deep dive into the deep physical principles of the twisted pair. The transmission of the network is actually the transmission of the network signal on the twisted pair line. As an electronic signal, when it is transmitted in the twisted pair line, it must be affected by resistance and capacitance, which leads to the attenuation and distortion of the network signal. When the attenuation or distortion of the signal reaches a certain level, the effective and stable transmission of the signal will be affected.

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Therefore, the twisted pair has a transmission distance limit, so how to calculate the upper limit of 100 meters?

Category 5 UTP and Category 5 Super UTP are mainly used for computer networks. According to the 100Base-TX regulations of Fast Ethernet, the communication rate is 100mbps, and the time it takes for 100mbps Ethernet to transmit 1 bit of data (also known as "bit time"). ) can be calculated as follows:


1 bit time = 1/100mbps = 10ns

When data information is transmitted in the network, there will be delays when passing through different components. The delay of the five types of UTP is 5.56ns/m. When designing Ethernet, it is required to comply with a relay rule, which is also called the golden rule or the 5-4-3-2-1 rule. This rule applies not only to 10mbps Ethernet, but also to Fast Ethernet. This rule requires that the ring collision delay should not exceed 512 bits, which is 5120ns for a transmission rate of 100mbps.


In the loop, the network elements include cables, relay units, MAU and DTE, etc., add up their delays and multiply by 2 to obtain the loop delay, and also calculate the loop conflict diameter. According to this theory, it is possible to calculate the maximum distance that the signal can travel before the transmission of a minimum frame is completed. This is why the link span is limited to 100 meters.


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


Maximum cable distance during actual construction

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


Please 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 transmission distance of PoE power supply is not determined by PoE technology, but by the type and quality of the network cable.


Although in actual construction, a network cable with better quality can break through the 100-meter distance limit and the equipment can work normally, but this approach is not recommended. Because some potential problems do not appear immediately, but gradually appear over time, which can cause subsequent maintenance problems. The simplest case is the upgrade of bandwidth, so that the equipment that can work normally at a distance of more than 100 meters will not work normally after the network speed is greatly improved.

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