3Com DUA3700-0BAA04 manual n MHz km / xkm = y MHz, Table E-5 Link Loss Attenuation Calculation

ATM Cabling E-7

The following calculation would arrive at the link loss attenuation value for this linked fiber (Table E-5):

Table E-5Link Loss Attenuation Calculation

62.5µ m	+ 50µ m	+ splice	+ ST con-	+ insertion	= Total link
cable loss	cable loss	loss	nector loss	loss*	attenuation
1 km(1.75	+1 km(3	+0.3 dB	+0.6 dB	+2.2 dB	=7.85 dB
dB/km)	dB/km)

* for mating unlike fiber types

Because the resulting value, 7.85, does not exceed the maximum attenuation value listed in Table E-1, 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 E-1 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: