In computer networks, Signal-to-Noise Ratio (SNR) Margin and Line Attenuation are parameters applied to evaluate the quality of data transmission. Briefly, the SNR margin quantifies the signal quality relative to the noise, while the line attenuation measures the loss of signal strength throughout the transmission.
In this article, we’ll explore these concepts and their implications and review methods for optimizing transmission quality in copper and fiber optic cables.
2. SNR Margin
The SNR margin is a measure of signal quality that checks the level of noise in the environment. In short, it’s the difference between the signal’s power and the noise’s power. In general, computer networks measure this in decibels (dB).
A higher SNR margin indicates better signal quality, which generally results in more reliable transmission and fewer errors. To clarify, a high SNR means that the signal in the cable is easy to detect. On the other hand, a low SNR indicates that noise corrupts the signal, making it difficult to interpret.
The formula for measuring SNR is: . With that, the signal power () and noise power () should be measured in the same location or similar areas within the same system.
Additionally, below are listed the recommended SNR values for LAN connections:
|10dB – 20dB
|20dB – 29dB
Note that various factors can cause noise. In the context of copper wires, noise is typically generated by electromagnetic interference, resulting in undesired signals that affect the transmission quality. For instance, equipment contributing to generating noise and signal interference includes transformers, wireless devices, and faulty connectors.
However, optical fibers are less susceptible to electromagnetic interference than copper cables. Nevertheless, situations such as random fluctuations of electrons in the fiber conductors or variations in the phase of the different modes that make up the optical signal are some of the noises that interfere with the optical fiber signal.
As there’s no cable type that eliminates noise interference, one can use additional technologies to enhance the SNR value. Hence, these technologies include increasing signal strength through the use of repeaters or amplifiers. Also, the reduction of noise levels through the use of signal isolators, among others, can be effective.
3. Line Attenuation
Line attenuation refers to the reduction in signal strength along a transmission line, be it a copper cable or optical fiber. Thus, this measurement is often expressed in decibels per unit of distance (e.g., dB per kilometer).
As a signal travels through a transmission link, it naturally loses power. That is, the signal weakens as the transmission line extends or as the signal frequency increases. Therefore, attenuation affects any type of signal, whether digital or analog.
For attenuation measurements, it’s necessary to have an instrument capable of signal power measurement. So, the equipment must first be calibrated with a reference source to capture the input power. For instance, this reference source can be a copper cable with an electrical signal or an optical cord with a light source. Next, during the test, a signal is sent from one end (initial) of the cable and received by the meter at the other end.
The formula for determining line attenuation is: . Where is the initial power of the signal and is the power of the signal received after traveling through the cable.
Thus, the result is the line attenuation expressed in decibels. In this case, the lower the value, the more efficient the transmission. Conversely, high values indicate a significant loss of signal power.
The following table shows the attenuation values generally recommended for network connections:
|20dB – 30dB
|30dB – 40dB
|40dB – 50db
Although attenuation is natural, several situations can interfere with the signal. Thus, copper cables made from low-quality materials can show greater attenuation due to imperfections. In optical fibers, excessive bending can increase attenuation due to modal dispersion. Also, in both cases, faulty connectors or poorly made splices directly affect the signal power.
As with SNR margin, the right choice of components, implementation following installation recommendations, and preventive maintenance all help minimize attenuation.
In this article, we studied Signal-to-Noise Ratio (SNR) Margin and Line Attenuation. We learned that both concepts are related to the quality and efficiency of data transmission over physical media, such as copper cables or optical fibers.
In short, the SNR margin measures the overall quality of the signal about the noise. At the same time, line attenuation indicates how much the signal weakens as it travels along a transmission line.