APPENDIX A: BASIC FDDI NETWORKS
The attenuation of the typical multimode fiber optic cable used in FDDI networks is 2.5 dB/km or 5 dB for the 2 km maximum node separation. When installing optical bypass switches, each switch could add 2.5 dB to the attenuation. With an 11 dB budget to work with, and 5 dB expended on the cable, you can install a maximum of two bypass switches.
A.7.4 Bandwidth
The minimum modal bandwidth of fiber optic cable used in an FDDI network is 500 Mhz at 1300 nm.
A.7.5 Number of Stations
The number of devices in a single FDDI ring cannot exceed 500. The amount of propagation delay generated by 1000 physical connections determines this limit.
With the exception of optical bypass switches, all FDDI devices count as two connections against the 1000 physical connection budget. You can easily see how to calculate this number when only dual attached stations reside on the network (1000 divided by 2 connections for each DAS = 500 nodes). However, to understand how to count connections for other device types, refer to Figure
A DAC without attached devices counts as two connections (main ring connections), the same as a DAS. As you attach each additional SAS or SAC to the DAC, you must count two connections against the budget, one for the concentrator port and one for the attached device.
This same logic applies to counting connections for a SAC. The multiple ports of the concentrator do not count until you attach a device.
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