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Black Box LH1500 Media Converters Installation and User Guide (08/05)
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“Rules-of-thumb” Collision Domain Calculations
Rules-of-thumb, while inexact, may be helpful in planning network topology.
As a rule-of-thumb, a Class II Repeater has a PDV of about 90 to 95 BTs, and twisted-
pair or fiber media has a PDV of about 1 BT per meter of length. The Black Box
LH1500 has a PDV of 80 BT. Therefore, in shared Fast Ethernet applications, the
LH1500 uses about 80 meters of equivalent cable distance to convert from TX media to
fiber FX media, i.e., it consumes almost as much of the available PDV as a Class II
repeater. Since a 512BT collision domain will almost always include at least one
repeater and two media segments, the remaining amount of Bit Times left after allowing
for a LH1500 and a length of fiber media indicates that the available fiber length will be
much less than the 412 meters that is the known maximum for fiber. Therefore, in shared
environments, Black Box LH1500 Media Converters will be of benefit when they allow