Juniper SRX 5600 Hardware Manual page 224

SRX 5600 Services Gateway Hardware Guide
Attenuation and Dispersion in Fiber-Optic Cable
Correct functioning of an optical data link depends on modulated light reaching the
receiver with enough power to be demodulated correctly. Attenuation is the reduction
in power of the light signal as it is transmitted. Attenuation is caused by passive
media components, such as cables, cable splices, and connectors. While attenuation
is significantly lower for optical fiber than for other media, it still occurs in both
multimode and single-mode transmission. An efficient optical data link must have
enough light available to overcome attenuation.
Dispersion is the spreading of the signal in time. The following two types of dispersion
can affect an optical data link:
For multimode transmission, modal dispersion, rather than chromatic dispersion or
attenuation, usually limits the maximum bit rate and link length. For single-mode
transmission, modal dispersion is not a factor. However, at higher bit rates and over
longer distances, chromatic dispersion rather than modal dispersion limits maximum
link length.
An efficient optical data link must have enough light to exceed the minimum power
that the receiver requires to operate within its specifications. In addition, the total
dispersion must be less than the limits specified for the type of link in Telcordia
Technologies document GR-253-CORE (Section 4.3) and International
Telecommunications Union (ITU) document G.957.
When chromatic dispersion is at the maximum allowed, its effect can be considered
as a power penalty in the power budget. The optical power budget must allow for
the sum of component attenuation, power penalties (including those from dispersion),
and a safety margin for unexpected losses. For more information about power budget,
see "Calculating Power Budget for Fiber-Optic Cable" on page 198.
Calculating Power Budget for Fiber-Optic Cable
To ensure that fiber-optic connections have sufficient power for correct operation,
you need to calculate the link's power budget, which is the maximum amount of
power it can transmit. When you calculate the power budget, you use a worst-case
analysis to provide a margin of error, even though all the parts of an actual system
do not operate at the worst-case levels. To calculate the worst-case estimate of power
budget (
sensitivity (
P
The following hypothetical power budget equation uses values measured in decibels
(dB) and decibels referred to one milliwatt (dBm):
198
Network Cable Specifications and Guidelines
Chromatic dispersion—The spreading of the signal in time resulting from the
different speeds of light rays.
Modal dispersion—The spreading of the signal in time resulting from the different
propagation modes in the fiber.
), you assume minimum transmitter power (
P
B
):
P
R
= P P
B T R
) and minimum receiver
P
T