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ATM Cabling
115
Because the resulting value, 7.85, does not exceed the maximum
attenuation value, no adjustments are needed in the types of fibers joined
or how they are connected. The link meets all of the specifications of the
MMF-PMD.
Verifying Modal
Bandwidth
The bandwidth of an optical fiber is the lowest frequency where the
magnitude of the baseband frequency response has decreased by 3 dB
compared to the magnitude at zero frequency. Bandwidth for multi-mode
fiber is referred to as modal bandwidth because it varies based on the
modal field (or core diameter) of the fiber. Modal bandwidth is specified
in units of MHz
•
km, which indicates the amount of bandwidth
supported by the fiber for a one km distance.
The modal bandwidth specified in Table 25 is 500 MHz
•
km, which
allows the cable plant to support end-to-end bandwidth of 250 MHz at
the maximum 2 km distance. As a check, use the following formula to
verify that the bandwidth of the fiber is within an acceptable range:
n MHz
•
km / xkm = y MHz
In this formula,
n
is the amount of bandwidth available according to the
fiber specification. Divide this number by the total length
x
of the fiber in
kilometers. The result is the modal bandwidth
y
, measured in MHz.
If the result is lower than 250 MHz, the link may increase bit errors. To
reduce the likelihood of bit errors, shorten the length of the fiber or use
different fiber until the result of the calculation reaches 250 MHz.
Example.
Cable with a modal bandwidth of 500
MHz
•
km will have 250
MHz of bandwidth at 2 km:
(500 MHz
•
km) / 2 km = 250 MHz
The same cable would have 500 MHz of bandwidth at 1 km. A fiber cable
with a bandwidth specification of 200 MHz
•
km would have only 100
MHz of bandwidth at 2 km, which would not support ATM. In this case,
another type of fiber would be required.
Single Mode Fiber
Standards
The SMF-PMD standard defines the requirements for an ATM cable plant
to support a connection distance of up to 14.4 kilometers (8.6 miles) of
single mode fiber. The cable plant includes all fiber optic components