Why can't a network cable transmit beyond one hundred meters?

According to the International cabling guideline standards for structured cabling, network cables are limited to 100 meters. But why should network cables not exceed 100 meters?

1. First, it's important to understand what this 100-meter limit refers to.

The 100 meters for network cable laying refers to a permanent cabling link of no more than 90 meters from the equipment room patch panel to the faceplate, plus two patch cords at each end. The combined length of these two patch cables and the buried permanent cabling should not exceed 100 meters.

2. So, why can't network cables be laid beyond 100 meters?

Everyone knows that signals are transmitted in binary form, represented by "0" and "1". Generally, a voltage of 515V is taken to represent "1", and -5-15V to represent "0". Voltages between -3~3V are considered meaningless signals, or white noise, by the receiving logic circuit and are discarded. This is where the packet loss rate often mentioned in the industry comes from. Therefore, the error tolerance voltage for signal transmission within the network cable is U=2V, meaning the signal can tolerate a voltage attenuation of 2V as it travels through the cable.

We know that the characteristic impedance of network cables is |Z|=100Ω; using Ohm's Law, we can calculate I=0.02A.

Ohm's Law

Here, taking standard Cat5e cables as an example, their circular cross-sectional area is:

; we can calculate the cross-sectional area as: S=0.19625mm2.

Using the formula for resistance, R=ρL/S, and knowing the resistivity of oxygen-free copper, which is 0.0175Ωmm²/m, we can calculate that L is approximately 112 meters. Additionally, a 12-meter cable would lose about 0.02V. Therefore, a 100-meter cable would lose 1.98V.

Thus, it can be calculated that to ensure stable signal transmission, the maximum length of the cable can reach 112 meters. Although the error tolerance for voltage is 2V, when the signal attenuates to 3V, it will be considered lost. Adding the intermediate connectors, such as module IDC pins and RJ45 connectors, which are not made of oxygen-free copper, will result in even greater voltage loss.

Moreover, during transmission, information can also be subject to interference, which might change the direction of the voltage and cause errors.

Some suggest that increasing the output power could increase the error voltage tolerance. However, this requires more from the equipment, increasing costs and proving counterproductive.

3. In summary, to ensure stable signal transmission and reduce packet loss rates, the laying distance of network cables should be kept within 100 meters.