Signal loss is an issue in both copper and fiber cabling infrastructures. This is represented by the percentage of signal lost at the destination based on the initial transmission sent. In other words, the signal received is not as strong as the signal sent.
Signal loss is also called attenuation and it increases as the signal travels through the media, which can include:
- Copper (electrical signals)
- Fiber (light beams)
- Wireless (radio waves)
The amount of signal loss depends on a number of different criteria, including:
- Distance
- End connectors
- Cable quality
- Patches between the endpoints
- Electrical interference (for copper cables)
- Radio interference (for wireless traffic)
| Note: Signal strength is measured in units called Bells (B) that are divided into decibels (dB), which are one-tenth of a Bell. The B is capitalized because the unit is named after Alexander Graham Bell, the inventor of the first practical telephone.
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Signal modification can be expressed in two ways:
- Signal gain: the signal received is higher than the signal transmitted
- Signal loss: the signal received is lower than the signal transmitted
Signal loss also involves using a logarithmic scale. This logarithmic behavior means that the signal degradation does not follow a linear scale based on the dB number; instead, it increases much faster. To exemplify this, let’s use the following examples: If you have a difference of 3dB lost between the original signal and the signal received, this means that the signal received is half that of the transmitted signal. If the difference is 10dB, this means that the signal has been degraded 20 times. At 20dB there is a 100 times difference in signal loss and at 30dB there is a 1000 times difference in signal loss.
In the case of optical fibers, when installing them over long distances, you should analyze the MMF or SMF cable specifications to find the rate of signal loss. For example, the signal loss specified by the cable manufacturer can be 3dB for each km. On top of that, you should add the signal loss on the patch panels, also specified by the manufacturer, which is usually below 1dB (e.g., 0.3dB). Each time you go through a patch panel you lose signal strength, so a fiber cable that travels across a 2 km distance and through two patch panels has an end-to-end signal loss of 6.6dB based on the figures mentioned earlier (2x3dB per km and 2×0.3dB per patch panel).
Depending on the initial signal strength and the receiving end requirements (e.g., applications, services, etc.), careful analysis should be done in order to calculate the exact signal loss over long distances. Based on the level of signal loss, a connection can have a number of symptoms:
- Lack of connectivity (as a result of severe signal loss)
- Intermittent connectivity (enough signal strength to bring up the link but not enough to maintain it)
- Slow performance
- Interface errors
- Data corruption
Dedicated link quality testing equipment can help troubleshoot signal loss issues and can provide useful information on the health of the physical media (i.e., copper, fiber, or wireless).