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Many friends asked Security Jun, why the network cable only transmits 100 meters, today we will come to understand this aspect.
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
Data information is transmitted over the network and is delayed when passing through different components. The delay for Category 5 UTP is 5.56 ns/m. When designing Ethernet, it is required to follow a relay rule, also known as 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 loop collision delay should not exceed 512 bits, which is 5120 ns for a transmission rate of 100 mbps. In the loop, the network components have cables, relay units, MAUs, and DTEs. Adding their delays and multiplying by 2, the loop delay is obtained, and the loop collision diameter can also be calculated. According to this theory, the farthest distance that a signal can transmit before a minimum frame is transmitted can be calculated. 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 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 is not recommended. 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.
Impact of cable category and quality on transmission distance
Category 5 (Cat 5): It is the most common standard network cable on the market, but the quality of different manufacturers is very different. Especially in the domestic price-oriented environment, many manufacturers will reduce copper in order to reduce costs. The line is replaced by copper-clad iron and copper-clad steel, which causes the transmission distance of the network cable to drop, and even the phenomenon of network instability and packet loss, and the equipment manufacturers often take the black pot, which is really embarrassing. Therefore, if you want to make PoE play a good effect, you must use a good quality cable, not because of small loss, affecting the overall quality of the project.
Super Category 5 (Cat 5e): Compared to Category 5 twisted pair, Super Category 5 twisted pair has less attenuation and crosstalk, providing a more solid network foundation for most applications (especially supporting Gigabit) Ethernet 1000Base-T cabling) brings convenience to network installation and testing, and has become a better solution for current network applications. The transmission characteristics of the Super Category 5 line are the same as those of the ordinary Category 5 line, but the Category 5 cabling standard stipulates that all four pairs of lines of the Category 5 cable can achieve full duplex communication.
Category 6 (Cat 6): This type of cable has a transmission frequency of 1MHz to 250MHz. The Category 6 cabling system should have a large margin at 200MHz. The integrated attenuation crosstalk ratio (PS-ACR) should have a large margin. It provides 2 times the super five. The bandwidth of the class. The transmission performance of Category 6 cabling is much higher than the Category 5 standard and is best suited for applications with transmission rates above 1Gbps.
An important difference between Category 6 and Category 5 is that it improves performance in terms of crosstalk and return loss. For a new generation of full-duplex high-speed network applications, excellent return loss performance is extremely important. The basic link model is eliminated in the six standards. The cabling standard adopts a star topology. The required cabling distance is: the length of the link must not exceed 90 meters, and the channel length cannot exceed 100 meters.
There is no strict difference in the transmission distance between the Category 6 and Super Category 5 lines, that is, the maximum transmission distance of a single segment is 100 meters. Of course, the six types of lines can be appropriately increased in the transmission distance. The so-called 100 meters refers to the technical indicators that can not meet the 1000M bandwidth, and the speed is reduced.