How many meters can a network cable transmit at most? Several common classifications of network cables

Maximum distance of data transmission via network cable

Those who are familiar with the Internet know that twisted pair cables have an "insurmountable" transmission distance of "100 meters". Whether it is a Category 3 twisted pair cable with a transmission rate of 10M, a Category 5 twisted pair cable with a transmission rate of 100M, or even a Category 6 twisted pair cable with a transmission rate of 1000M, the maximum effective transmission distance is 100 meters. The integrated wiring specification also clearly requires that the horizontal wiring cannot exceed 90 meters and the total link length cannot exceed 100 meters. In other words, 100 meters is a limit for wired Ethernet, and this limit is the link length from the network card to the hub device.

How is the 100m maximum distance obtained?

What causes the 100-meter transmission distance limit of twisted pair cables? This requires a deep dive into the underlying physical principles of twisted pair cables. Network transmission is actually the transmission of network signals on twisted pair cables. As an electronic signal, when it is transmitted in twisted pair cables, it is inevitably affected by resistance and capacitance, which leads to attenuation and distortion of network signals. When the attenuation or distortion of the signal reaches a certain level, it will affect the effective and stable transmission of the signal. Therefore, twisted pair cables have a transmission distance limit, so how is the 100-meter upper limit calculated?

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Category 5 UTP and Category 5e UTP are mainly used for computer networks. According to the 100Base-TX specification of Fast Ethernet, the communication rate is 100 mbps. The time it takes for 100 mbps Ethernet to transmit 1 bit of data (also known as "bit time") can be calculated as follows: 1 bit time = 1/100 mbps = 10ns

Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection), which is a carrier sense multiple access technology with collision detection (Carrier Sense Multiple Access/Collision Detection) to share communication channels. When switching is introduced, this technology is still indispensable. There are devices connected to both ends of a link. These two devices may send data at the same time, causing a conflict. At this time, the collision domain is 2. The conflict will cause the loss of data packets. To avoid packet loss caused by conflicts, Ethernet uses collision detection and back-off retransmission technology. In order to back off and retransmit, it must be ensured that one end can detect the conflict before transmitting a data packet. The minimum frame length of Ethernet is 64 bytes, that is, 512 bits. Calculated at a rate of 100 mbps, it takes 512 bits*10ns=5120ns to transmit 512 bits.

When data information is transmitted in the network, it will be delayed when passing through different components. The delay of Category 5 UTP is 5.56ns/m. When designing Ethernet, a relay rule must be followed. This rule 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 must not exceed 512 bits, which is 5120ns for a transmission rate of 100mbps. In the ring, network elements include cables, relay units, MAUs, and DTEs. Add up their delays and multiply by 2 to get the ring delay. At the same time, the ring collision diameter can also be calculated. According to this theory, the longest distance that a signal can be transmitted before a minimum frame is sent can be calculated. This is why the link span is limited to 100 meters.

When the length exceeds 100 meters, the collision cannot be detected in time. The information packet damaged by the collision is transmitted and received by the receiver. This information packet cannot pass the verification and is forced to be discarded. At this time, the fallback retransmission mechanism is not activated, so the packet will be lost. When the transmission rate is lower than 100 mbps, the 100-meter limit can be appropriately relaxed in actual applications. It must be stated that although this is actually effective, it does not meet the standard. It must be explained during the certification test, otherwise it may cause some problems, such as product warranty.

Optimal cable distance during actual construction

From the above, we can see why the maximum length of the network cable is stipulated to be no more than 100 meters when using PoE power supply. However, in actual construction, in order to ensure the quality of the project, it is generally 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 PoE power supply transmission distance is not determined by the PoE technology, but by the type and quality of the network cable.

Although in actual construction, high-quality network cables 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 gradually appear over time, which will cause subsequent maintenance problems. The simplest case is, for example, the bandwidth upgrade, which makes the equipment that can work normally at a distance of more than 100 meters no longer work properly after the network speed is greatly increased.

The impact of cable type and quality on transmission distance

Category 5 cable (Cat5) is the most common standard network cable on the market, but the quality of products produced by different manufacturers varies greatly. Especially in the price-oriented environment in China, many manufacturers replace copper wire with copper-clad iron or copper-clad steel in order to reduce costs, resulting in a decrease in the transmission distance of the network cable and even network instability and packet loss. Equipment manufacturers are often blamed, which is really unfair.

Therefore, if you want to make the best use of PoE, you must use good quality network cables. Don't lose sight of the big picture and affect the overall quality of the project.

• Category 5e (Cat5e): Compared with Category 5 twisted pair, Category 5e twisted pair has less attenuation and crosstalk, can provide a more solid network foundation, meet most application requirements (especially support Gigabit Ethernet 1000Base-T wiring), bring convenience to network installation and testing, and become a better solution in current network applications. The transmission characteristics of Category 5e are the same as those of ordinary Category 5, but the Category 5e wiring standard stipulates that all 4 pairs of Category 5e cables can achieve full-duplex communication.
• Category 6 (Cat6): The transmission frequency of this type of cable is 1MHz to 250MHz. The Category 6 wiring system should have a large margin in the PS-ACR at 200MHz. It provides twice the bandwidth of Category 5e. The transmission performance of Category 6 wiring is much higher than the Category 5e standard and is most suitable for applications with a transmission rate higher than 1Gbps.

An important difference between Category 6 and Category 5e is that the performance in crosstalk and return loss is improved. For the new generation of full-duplex high-speed network applications, excellent return loss performance is extremely important. The Category 6 standard cancels the basic link model, and the wiring standard adopts a star topology structure. The required wiring distance is: the maximum link length cannot exceed 90 meters, and the channel length cannot exceed 100 meters.

There is no strict difference in transmission distance between Category 6 and Category 5e, that is, the maximum transmission distance of a single segment is 100 meters. Of course, the transmission distance of Category 6 can be appropriately increased. The so-called 100 meters means that if it exceeds the distance, it will not meet the relevant technical indicators such as 1000M bandwidth, resulting in speed reduction and other problems.