Fast EtherHub-12s User’s Guide
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 ≥ S(repeater equivalent length) + S(length of CAT-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 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.
Then, if node A and C are linked to adjacent repeaters 1 and 2 in this system, the maximum length of twisted-pair cable that can be used to connect node C to hub 2 is 367m - 2*80m - 50m - 15m = 142 meters. However, the maximum length for any twisted-pair connection cannot exceed 100 meters.
Configuring the Network | 3-3 |
