Design Reference

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,
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-dependant, 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 max 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. Due to 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 available budget and the sum of losses and dispersion, the margin can be
more or less than required, which determines whether additional consideration is needed. If
Dispersion considerations for long reach
Network Design Reference for Avaya VSP 4000 February 2014 27