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transmission. Passive media components such as cables, cable splices, and connectors cause
attenuation. Although attenuation is significantly lower for optical fiber than for other media, it still
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 caused by the different speeds of light rays.
• Modal dispersion, which is 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 attentuation does not usually limit the maximum bit rate or link length.) 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 the 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.
Calculate the Fiber-Optic Cable Power Budget for an MX10004 Router
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 the parts of a configured system don't operate at the
worst-case levels. We cannot use the phrase "all parts . . . don't operate." You can write "none of the
parts operate," however.
To calculate the worst-case estimate for the fiber-optic cable power budget (P
1. Determine values for the link's minimum transmitter power (P
(P
). For example, in the following example, we measure (P
R
P
= –15 dBm
T
P
= –28 dBm
R
B
) for the link:
) and minimum receiver sensitivity
T
) and (P
) in decibels per milliwatt (dBm):
T
R
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