Network administrators who manage optical links have the added challenge of dealing with signal attenuation—the rate at which light dissipates over a network.
Attenuation is caused by a number of factors and can affect both network performance and the ability to analyze the network.
Excessive signal attenuation can cause link failure. Understanding signal levels, selecting the right split ratio on TAPs, and carefully managing the location of repeaters can prevent problems. This section defines attenuation, explains how it is affected by fiber and other optical elements on a network, and how it can be efficiently managed.
Attenuation is the reduction of signal strength during transmission caused by the absorption of light from the materials through which it travels. Greater signal loss equals higher attenuation. A signal can lose intensity or experience increased attenuation with each surface or medium it traverses. Many factors contribute to the attenuation rate of signals including devices such as TAPs and transmission through optical cables.
Optical signal strength is measured in decibels (dB) and is based on a logarithmic scale. If a signal attenuates too much, the destination device cannot identify it or the signal may not even reach the destination. This is why some optical links depend on repeaters, which amplify the signal.
Attenuation and TAPs
As with all devices inserted into an optical link, one side effect of TAP usage is signal attenuation.
TAPs are used to provide access to the data streams passing through a high-speed, full-duplex network link. TAPs deliver a complete copy of data to a monitoring device for accurate analysis. An Optical TAP optically splits the light power of the full-duplex signal into two copies. One part of the split signal is sent to the other device on the network, while the other is simultaneously passed to the analysis or monitoring appliance.
A TAP attenuates the signal for two reasons:
A portion of the signal strength is “siphoned off” and sent to the analyzer. How much of the signal strength is redirected for analysis depends on the split ratio of the TAP.
The connections and internal TAP cables and connectors absorb and refract a small portion of the signal.
An Optical TAP contributes to signal attenuation, but typically not enough to make a significant difference on the network.
An optical split ratio must be designated on each TAP. In most cases, a 50/50 split ratio is ideal, providing sufficient light to both the network and monitoring device. However, there may be special cases that require an alternative ratio in order to meet signal power needs. For example, if a TAP is cabled close to the analyzer NIC and the link under test requires a long cable run, you may want to provide more light power back to the network than to the monitoring device. If you do choose a ratio other than 50/50, keep in mind that the signal has to be strong enough for it to be interpreted at the analyzer. More details about this are covered in Determining the best split ratio for you.
Managing attenuation
Managing signal attenuation is critical for running a network at optimal performance.
If signal attenuation is too high, destination devices may not be able to establish a link or receive network traffic. Repeaters can help, but they can be costly and inconvenient to implement. In general, unless a signal must travel a long distance or is compromised by patch panels, there should not be a problem using the 50/50 split ratio. The most efficient and cost-conscious way to manage attenuation is to measure signal levels throughout the network and place repeaters only when and where they are needed.
To determine if a light signal is at an acceptable level at any point on a network, it is helpful to use an optical power meter. Optical power meters measure signal power at a port, helping you determine whether a device is receiving a strong enough signal and thereby identifying if repeaters need to be placed. The meters are typically inexpensive and are offered from a number of vendors.
Attenuation and optical cables
Optical cables contribute to signal attenuation. As light travels through an optical cable, some of its energy gets dispersed and absorbed by the cable. The attenuation rate varies depending on the cable type used.
Depending on your transmission technology, you may be required to use a specific cable type. Examples include single-mode (for LX or LR) and multimode (for SX or SR). Multimode cable has a larger core diameter than single-mode cable, resulting in greater light dispersion. Unless the cable run is extremely long, the signal attenuation for both cable types is a minor contributor to the power loss budget. However, multimode cable does cause higher signal attenuation than single-mode cable. Check with the cable manufacturer to determine specific attenuation rates.