Installation manual

Fast EtherHub-8se 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 “Precise Cable Length Budgeting” in Chapter 2. This formula stated
that the maximum length of cabling that can be run from end-node to end-node
is:
367m (1204 feet)
≥ S(repeater equivalent length)
+ S(length of category 5 twisted-pair cable)
For the daisy chain of three hubs shown in Figure 3.3, 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 (262 feet) 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
(416 feet) and the total inter-hub cabling will be limited to 27 meters (i.e.,
127m - (2 x 50m) = 27m, or 88 feet). Therefore, if you use 20 meters (65 feet)
of twisted-pair cable to connect hubs 1 and 2, then 7 meters (22 feet) 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 twisted-pair cable that can be used to connect node
C to hub 2 is 367m - (2 x 80m) - (50m + 20m) = 137 meters (449 feet).
However, because this value exceeds the maximum permitted length for any
twisted-pair connection, it must be reduced to 100 meters (328 feet).