C-15
BASIC FDDI NETWORKS
In some multimode applications, existing 50/125
µ
m or 100/140
µ
m
fiber can be used over longer distances. However, the cable must
conform to the FDDI standard for bandwidth and attenuation to
remain compliant with the FDDI standard.
Attenuation
The maximum attenuation (attenuation budget) between any two
active connections to the ring, as defined by the FDDI standard, is 11
dB. The budget includes the attenuation of the cabling, splices,
connections, and optical bypass switches. 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 optical
bypass switches are installed, 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, the maximum number of bypass switches is two.
Bandwidth
The minimum modal bandwidth of fiber optic cable used in an FDDI
network is 500 Mhz at 1300 nm.
Number of Stations
The maximum number of devices in a single FDDI ring is 500. This
limit is determined by the propagation delay from 1000 physical
connections. With the exception of optical bypass switches, all FDDI
devices are counted as two connections against the 1000 physical
connection budget. It is easy to see how connections are counted when
only dual attached stations are used (1000 divided by 2 connections
for each DAS = 500 nodes), but to understand how connections are
counted for other device types, refer to Figure C-11. A DAC without
attached devices is counted as two connections (main ring
connections), the same as a DAS. As each SAS or SAC is attached to
the DAC, two connections must be counted 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 are not counted until a device is attached.