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multimode and single-mode transmission. 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, rather than chromatic dispersion or attenuation, usually
limits the maximum bit rate and 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.
Calculating Power Budget and Power Margin for Fiber-Optic Cables
Use the information in this topic and the specifications for your optical interface to calculate the power
budget and power margin for fiber-optic cables.
TIP: You can use the Hardware Compatibility Tool to find information about the pluggable
transceivers supported on your Juniper Networks device.
To calculate the power budget and power margin, perform the following tasks:
1.
Calculating Power Budget for Fiber-Optic Cable | 96
2.
Calculating Power Margin for Fiber-Optic Cable | 96
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