MHz km / xkm = y MHz, MHz km / 2 km = 250 MHz

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 The bandwidth of an optical fiber is the lowest frequency where the Bandwidth 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 The SMF-PMD standard defines the requirements for an ATM cable plant Standards 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

7000 specifications

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