Fast EtherHub-12se User’s Guide
Precise Cable Length Budgeting
If you plan on installing a network for a small office, it may not be convenient to place hubs within 5 meters of each other, and then run up to 100 meters of cabling for each end user. This kind of installation can be very messy and wastes a lot of cabling. Depending on how your users are spread out, it may be more convenient to run a longer length of inter-hub cabling (say 50 meters or more), and then run shorter lengths of cable from each hub to the end users. Depending on the inherent delay in the hub and the maximum cable lengths required, it may even be possible to cascade up to three Fast Ethernet hubs.
However, when setting up these kinds of distributed cascades, you have to be very careful in calculating the maximum length for each node pair. All end- node devices (e.g., workstations or servers) must be within 100 meters of the connected hub when using twisted-pair cabling, or within 112 meters when using fiber optic cabling. However, when linking one or more Fast Ethernet repeaters together (i.e., in the same collision domain), the overall length of cable used between any two nodes should not exceed the limits outlined in the IEEE 802.3u standard. In precise terms, the maximum length of cabling that can be run from end-node to end-node is
367m ³ S(repeater equivalent length)
+ S(length of category 5 twisted-pair cable)
+ (0.9 * S(length of fiber optic cable1)) 1 - if fiber is used
This repeater has an inherent delay of 90 bit time or 80 meters of twisted-pair cable. Taking a daisy chain of two hubs connected with twisted-pair cabling for an example, the maximum length from node to node is 367m - (2 x 80m) = 207m. Therefore, if both node A and B are linked to separate repeaters in a two hub system, each using 100 meters of cable to connect to their respective hub, then the inter-hub cabling will be limited to 5 meters (i.e., 207m - 2x100m = 7m, which is then reduced to 5 meters according to a common wiring rule).
