Juniper EX4100, Hardware Manual, Page: 115 / 283

occurs in both multimode and single-mode transmissions. An efficient optical data link must transmit
enough light to overcome attenuation.
Dispersion is the spreading of the signal over time. The following two types of dispersion can affect
signal transmission through an optical data link:
• Chromatic dispersion, which is the spreading of the signal over time caused by the different speeds
of light rays
• Modal dispersion, which is the spreading of the signal over time caused by the different propagation
modes in the fiber
For multimode transmission, modal dispersion usually limits the maximum bit rate and link length.
Chromatic dispersion or attenuation is not a factor.
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 (
P
B
) for the link:
1. Determine values for the link's minimum transmitter power ( P
T
) and minimum receiver sensitivity
(
P
R
). In the following example, we measure both (
P
T
) and (
P
R
) in decibels relative to one milliwatt
(dBm).
P
T
= – 15 dBm
P
R
= – 28 dBm
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