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Fast EtherHub-12se User’s Guide
Configuring the Network 3-3
Precise Guidelines for Configuring Multiple Hubs
A detailed formula for calculating maximum cable length was provided in the
section on
3UHFLVH&DEOH/HQJWK%XGJHWLQJ in Chapter 2. This formula stated that the
maximum length of cabling that can be run from end-node to end-node is:
367m
≥ S(repeater equivalent length)
+ S(length of category 5 twisted-pair cable)
+ (0.9 * S(length of fiber optic cable
1
)) 1 - if fiber is used
For the daisy chain of three hubs shown in Figure Chapter 3:-4,
communications must
be supported for node pairs AB and AC. In this case, first determine the
limitations on cable length found on the longest path (i.e., the path with the
longest delay time).
Remember that this Class II device has an inherent repeater delay of 90 bit time
or 80 meters of twisted-pair cable. Therefore, if node A and B are linked to
repeaters 1 and 3 respectively (each using 50 meters of twisted-pair cable), then
the maximum length from node A to B is 367m - (3 x 80m) = 127m and the
total inter-hub cabling will be limited to 27 meters (i.e., 127m - 2x50m = 27m).
Therefore, if you use 15 meters of twisted-pair cable to connect hubs 1 and 2,
then 12 meters of cable can be used to connect hubs 2 and 3.
Moreover, if node A and C are linked to adjacent hubs 1 and 2 in this system,
then the maximum length of fiber optic cable that can be used to connect node C
to hub 2 is 367m - 2*80m - (50m + 15m) = 0.9 * Length
FX
, or 157 meters.
However, because this value exceeds the maximum permitted length for fiber
optic cable, it must be reduced to 112 meters.