Calculating The Fiber-Optic Cable Power Budget For Ex Series Devices - Juniper EX9253 Hardware Manual

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 (
1. Determine values for the link's minimum transmitter power (
P
( ). For example, here, (
R
milliwatt (dBm).
P
= – 15 dBm
T
P
= – 28 dBm
R
NOTE: See the specifications for your transmitter and receiver to find the minimum
transmitter power and minimum receiver sensitivity.
2. Calculate the power budget (
– 15 dBm – (–28 dBm) = 13 dBm
P P
) and ( ) are measured in decibels, and decibels are referred to one
T R
P P
) by subtracting (
B
P
) for the link:
B
P
) and minimum receiver sensitivity
T
P
) from ( ):
R T
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