Calculate The Fiber-Optic Cable Power Budget For Ex Series Devices - Juniper EX4300 Series Hardware Manual

For single-mode transmission, modal dispersion is not a 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, you can consider its effect 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 power loss.

Calculate 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. This planning helps you 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. You use a worst-case analysis even though not all the parts of an
actual system operate at the worst-case levels.
To calculate the worst-case estimate for a fiber-optic cable power budget (
1. Determine values for the link's minimum transmitter power (
P
( ). In the following example, we measure both (
R
(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
) and (
T
P P
) by subtracting ( ) from (
B R
P
) for the link:
B
P
) and minimum receiver sensitivity
T
P
) in decibels relative to one milliwatt
R
P
):
T
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