Cabletron Systems 100BASE-FX, Cabling Manual

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Hybrid Installations 7-5
Fast Ethernet Network Requirements
Delay
As fiber optic cabling is often used to make connections between Fast Ethernet
repeaters or hubs, the 100BASE-FX specification allows a multimode fiber optic
link to be configured such that the total propagation delay for the link is less than
or equal to 2.56
µ
s one-way. Keep in mind, however, that propagation delay must
be calculated for the entire network. If there are more stations than the one
connected by your fiber optic link, you must also calculate the propagation delay
for the longest of those station links.
If the total one-way propagation delay of any signal path exceeds 2.56
µ
s, the Fast
Ethernet network is out of specifications, and error conditions may result. To
eliminate propagation delay problems, incorporate some form of segmentation,
such as bridging or routing, into the network to separate the problem signal paths
from one another.
Length
The 100BASE-FX specification limits a multimode fiber optic cable segment to
412 m or less. Assuming that a fiber optic cable meets all other limitations for
100BASE-FX usage, it is possible to extend a multimode fiber optic link to an
estimated maximum of 2 km. At a length of more than 2 km, the propagation
delay introduced by the multimode fiber optic cable segment may exceed the
2.56
µ
s limit of the Fast Ethernet specification and cause excessive OOW errors.
Cabletron Systems does not recommend the installation or use of any multimode
fiber optic cable segment that exceeds 100BASE-FX limitations of 412 m.
Hybrid Installations
In Fast Ethernet networks, the combining of fiber optic and unshielded twisted
pair media in a single, repeated network requires calculating a network radius.
This is because the delay requirements for a Fast Ethernet network are so
demanding that a mixed-media network must take the differences between the
standard media into account.
The network radius is the calculation of the longest path in the Fast Ethernet
repeater domain (from one station to a Fast Ethernet repeater and out to another
station). Figure 7-1 shows an example of a mixed media Fast Ethernet repeater
domain.