M160 Internet Router Hardware Guide
Dispersion is the spreadingof the signal in time. The following two types
of dispersion can affect an optical data link:
Chromatic dispersion—Thespreading of the signal in time resulting from
the different speeds of light rays.
Modal dispersion—The spreading of thesignal in time resulting from the
different propagation modes in the fiber.
For multimode transmission, modal dispersion, rath er than chromatic
dispersion or attenuation, usually limitsthe maximum bit rate and link length.
For single-mode transmission, modal dispersion is no ta factor. However,
at higher bit rates and over longerdistances, chromatic dispersion rather
than modal dispersion limits maximum link length.
An efficient optical data linkmust have enough light to exceed the minimum
power that the receiver requires to operate within its specifications. In addition,
the total dispersionmust be less than the limits specified for the typeof link in
Telcordia Technologiesdo cument 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 powerbudget. The optical power budget mustallow
for the sum of component attenuation, power penalties(including those from
dispersion), and a safety marginfor unexpected losses. For more information about
power budget, see “CalculatingPower Budget for Fiber-Optic Cable” on page 72.
Calculating Power Budget for Fiber-Optic Cable
To ensure that fiber-optic connections have sufficient power for correct operation,
you need to calculate thelink’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 operateat the worst-case levels. To calculate the
worst-case estimate of power budget (PB), you assume minimum transmitter
power (PT) and minimum receiver sensitivity (PR):
PB=P
T–P
R
The following hypothetical power budget equation uses value s measured in
decibels (dB) and decibels referredto one milliwatt (dBm):
PB=P
T–P
R
PB=–15dBm–(–28dBm)
PB=13dB
72 Network Cable Specifications and Guidelines