Johnson Controls Metasys Intelligent Fire Network Technical Manual page 24

Distance
Capabilities
24 IFC Networking—Metasys Intelligent Fire Network
After completion of installation, it is recommended to measure
attenuation of the fiber optic segment (from end to end including
cable and connectors) with appropriate fiber optic testing equipment.
4. Sum all the losses.
5. Add the attenuation factors obtained in Steps 1, 2, and 3 for a total.
This will provide an approximate attenuation total. The actual
attenuation can be measured end-to-end with standard equipment
using a wavelength of 820 nanometers.
Fiber Optic Cable and Style 4
If you have a Style 4 FN with one or more fiber optic circuit segments, and
a single fiber optic connection of a 2-way pair is lost, a major
communication loss can occur. The one remaining fiber of the pair with
the fault will continue to inject signals into one portion of the FN, causing
that part to stop operating.
If this problem occurs and the single fiber optic break cannot be
immediately repaired, break the FN into two separate segments by
manually disconnecting both fibers of the faulted segment so no signals
can pass in either direction. This only needs to be done at one end of the
cable. Disconnecting the cable will allow both remaining sections of the
FN to operate, although they will not be able to communicate with one
another.
Two terms are used when referring to distance on the FN: the distance of
the entire FN system path (one end of the network to the other) or circuit
segments (between nodes). The circuit segment distance limitations vary
according to the media type. See either the Fiber Optic Cable Circuits or
Twisted Pair Copper Wire Circuits sections for details. The total number
of MIB and repeater devices, and medium type along the circuit segments
influences the system path length.
In addition, the total system path length is related to the worst case delay
allowing for sending a signal from one part of the network to any other
part. If this delay becomes excessive, the ARCNET protocol may no
longer operate correctly, resulting in data loss.