Recommendation G.957 (06/99)
13
performance must be employed. Alternatively, the number of connectors must be reduced. It also
may be necessary to limit the number of connectors or to use connectors
having improved reflectance
performance in order to avoid unacceptable impairments due to multiple reflections. Such effects
may be particularly significant in STM-16 and STM-4 long-haul systems.
In Tables 2 to 4, the value –27 dB maximum discrete reflectance between points S and R is intended
to minimize the effects of multiple reflections (e.g. interferometric noise). In Tables 3 and 4, the
value
−
27 dB for maximum receiver reflectance will ensure acceptable penalties due to multiple
reflections for all likely system configurations involving multiple connectors, etc. Systems
employing fewer or higher performance connectors produce fewer multiple reflections and
consequently are able to tolerate receivers exhibiting higher reflectance. As an extreme example, if
only two
connectors exist in the system, a 14 dB receiver return loss is acceptable.
For systems in which reflection effects are not considered to limit system performance, no values are
specified for the associated reflection parameters and this is indicated in Tables 2 to 4
by the entry
NA (not applicable). However, when using this Recommendation for a particular application, it
should be noted that if upgradability to other applications having more stringent requirements is
contemplated, then these more stringent requirements should be used.
The possible need to develop a specification for transmitter signal-to-noise
ratio under conditions of
worst-case optical return loss for the applications in Tables 2 to 4 is for further study.
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