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Attenuation
Chapter 3 Optical TAPs 32
rev. 1
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. As with all devices inserted into an optical link,
one side effect of TAP usage is signal attenuation.
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.
Determining the best split ratio for you
Fiber optic data travels on light power. A fiber optic TAP makes a
copy of the data for the analyzer by splitting the light power.
To ensure that all of the devices receive enough light power to
establish and maintain a connection, you must understand where light
can be “lost” as it travels between the network devices connected to
the TAP and from the TAP to the analyzer.
After the send strength and receive sensitivities of the ports and cable
distances are known, a “power loss budget” can be calculated. The
power loss budget can be helpful in determining if there is enough
signal strength left at the analyzer receive port for a desired split ratio.
The primary factors that need to be collected to determine loss budget
are the: