factor. However, at higher bit rates and over longer distances, chromatic dispersion limits the 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 within 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.
Calculating the Fiber-Optic Cable Power Budget for EX Series Devices
To ensure that fiber-optic connections have sufficient power for correct operation, calculate the link's
power budget when planning fiber-optic cable layout and distances to ensure that fiber-optic
connections have sufficient power for correct operation. The power budget is the maximum amount of
power the link 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 for fiber-optic cable power budget (
P
B
) for the link:
1. Determine values for the link's minimum transmitter power (
P
T
) and minimum receiver sensitivity
(
P
R
). For example, here, (
P
T
) and (
P
R
) are measured in decibels, and decibels are referred to one
milliwatt (dBm).
P
T
= – 15 dBm
P
R
= – 28 dBm
NOTE: See the specifications for your transmitter and receiver to find the minimum
transmitter power and minimum receiver sensitivity.
2. Calculate the power budget (
P
B
) by subtracting (
P
R
) from (
P
T
):
– 15 dBm – (–28 dBm) = 13 dBm
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