Design Reference

Optical power considerations
When you connect the device to collocated equipment, ensure that enough optical attenuation exists
to avoid overloading the receivers of each device. You must consider the minimum attenuation
requirement based on the specifications of third-party equipment. For more information about
minimum insertion losses for Avaya optical products, see Installing Transceivers and Optical
components on the Avaya Virtual Services Platform 4000 Series, NN46251-301.
Dispersion considerations for long reach
Precise engineering of transmission links is difficult; specifications and performance are often
unknown, undocumented, or impractical to measure before equipment installation. Moreover, the
skills required to perform rigorous link budget analysis are extensive. Fortunately, a simple,
straightforward approach can assure robust link performance for most optical fiber systems in which
you use Avaya switches and routers.
This method uses an optical power budget, the difference between transmitter power and receiver
sensitivity, to determine whether the installed link can operate with low bit error ratio for extended
periods. The power budget must accommodate the sum of link loss (that is, attenuation), dispersion,
and system margin, described in the following paragraphs.
Link losses are the sum of cabled fiber loss, splices, and connectors, often with an allocation for
additional connectors. Cabled fiber loss is wavelength and installation dependent, and is typically in
the range of 0.20 to 0.5 dB/km. See the cable plant owner or operator for specifications of the cable
you use, particularly if the available system margin is unsatisfactory. Engineered links require
precise knowledge of the cable plant.
For long, high bit rate systems, pulse distortion, caused by the transmitter laser spectrum interaction
with fiber chromatic dispersion, reduces receiver sensitivity. Transceivers for long reach single mode
fiber systems have an associated maximum dispersion power penalty (DPP
max
) specification, which
applies to G.652 (dispersion unshifted) single mode fiber and the rated transceiver reach. The actual
power penalty that you must use is
DPP
budget
= [link length(km) / transceiver maximum reach (km)] * DPP
max
For example, if an 80 km transceiver is specified as having DPP < 3 dB, and if the actual link length
will be 40 km, DPP
budget
is one-half the maximum, or 1.5 dB.
Link operating margins are sometimes allocated for impairments such as aging, thermal, or other
environmental effects. Because of the potentially large number of factors that can degrade
performance, you can usually rely on statistics to represent these factors as a single margin value,
in dB, to cover all effects. Margin is life and design dependent, but is typically 3.5 to 4.5 dB,
minimum. Whether you require additional margin depends on the details, such as whether actual or
specified transmitter power and receiver sensitivity are used. Avaya specifications represent worst-
case values.
The sum of margin, dispersion power penalty, and passive cable plant losses must be less than the
available power budget. Alternatively, if you calculate available power margin as the difference
between the available budget and the sum of losses and dispersion, the margin can be more or less
Dispersion considerations for long reach
January 2015 Network Design Reference for Avaya VSP 4000 Series 29
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