In addition to adhering to your local standards, all cables must conform to:

The American National Standard ATM Physical Layer Medium Dependent (PMD) standard,
ISO.166-1990,
American National Standards Institute.

The ATM cable you connect to the ATM Module must conform to the Multi-Mode Fiber (MMF-PMD) standard defined by ANSI x.3-166-1992.

3Com supports 62.5/125mm Multi-Mode Fiber (MMF-PMD). The maximum inter-station distance (including device-to-network connectors) should not exceed 2 kilometers (1.25 miles).

Table C-2 shows the cable specifications for standard multi-mode cabling.:

Table C-2 Standard Multi-mode Cable Specifications

Specification	Description
Core	62.5µm diameter
Cladding	125µm diameter nominal 122µm minimum 28µm maximum
Numerical aperture	0.275
Maximum attenuation	11dB (including 1dB allowed for reflection and dispersion penalties)
Modal bandwidth	500 MHz.km
Maximum distance between nodes	2km (1.25 miles)
Output power (from transceiver)	-20dB minimum -14dB maximum
Receiver power	-31dB minimum sensitivity -14dB maximum sensitivity

Maximum attenuation includes attenuation and the loss induced by other components such as connectors, splices, and the mating of unlike fiber types. Although some 2km (1.25 miles) cables have a total attenuation of less than 11dB, the 2km (1.25 miles) inter-station distance must be maintained to comply with modal bandwidth requirements.

The maximum attenuation value in this table is based on a cable diameter of 62.5, 80 or 100mm.

Table C-3 shows the alternative Multi-Mode fiber types that can be used.

Table C-3 Alternative Multi-mode Cable Specifications

Core (mm)	Cladding (mm)	Numerical Aperture
50	125	0.20
50	125	0.22
85	125	0.28
100	140	0.29

If you are using fiber with a diameter of 50mm and have 3Com equipment at both ends of the link, substitute 8.0dB for the maximum attenuation. If 3Com equipment is only at one end, substitute 6.0dB for the maximum attenuation value.

If you are using equipment at the end of the link from a vendor other than 3Com, you must perform a separate loss budget analysis. Contact the vendor(s) for values to use in your analysis.

Does the Cable Provide Sufficient Bandwidth?

Bandwidth for multi-mode fiber is referred to as modal bandwidth because it varies with 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 1km (0.625 miles) distance. Your cable must have a Modal bandwidth of 500MHz, which allows the cable to support end-to-end bandwidth of 250MHz at the maximum 2km (1.25 miles) distance.

To check that the bandwidth of your fiber is within an acceptable range:

1 .   Divide the amount of bandwidth available according to the fiber specification by the total length of fiber (km).

2 .   If the result is lower than 250MHz, the link may be prone to bit errors. You should shorten the length of the fiber or use a different fiber until the result of the calculation reaches 250MHz.

Example:

A cable with modal bandwidth of 500 MHz.km will have 250 MHz of bandwidth at 2km (1.25 miles).

The same cable would have a 500MHz of bandwidth at 1km (0.625 miles).

A fiber cable with a bandwidth specification of
200 MHz.km would have only 100MHz of bandwidth at 2km (1.25 miles), which would not support ATM. In this case, another type of fiber which has a bandwidth of 155MHz would be required.