Internet Router Hardware Guide

M160 Internet Router Hardware Guide
Dispersion is the spreading of the signal in time. The following two types
of dispersion can affect an optical data link:
Chromatic dispersion—The spreading of the signal in time resulting from
the different speeds of light rays.
Modal d ispersion—The spreading of the signal in time resulting from the
different propagation modes in the fiber.
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. Howev er,
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 r eceiver r equires 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 t he 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 72.
Calculating Power B u dget for Fiber-Optic Cable
To ensure that fiber-optic connections hav e 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 (
P
B
), you assume minimum transmitter
power (
P
T
) and minimum receiver sensitivity (P
R
):
P
B
=P
T
–P
R
The f ollowing hypothetical power budget equation uses values measured in
decibels (dB) and decibels referred to one milliwatt (dBm):
P
B
=P
T
–P
R
P
B
=–15dBm–(–28dBm)
P
B
=13dB
72 Networ k Cable Specifications and Guidelines