Accton Technology FAST ETHERHUB-12S manual Precise Cable Length Budgeting

Fast EtherHub-12s User’s Guide

Simplified Cable Length Constraints

The Fast EtherHub-12s is a Class II repeater. IEEE recommends that Class II hubs be limited to a cascade of two hubs. The simplest rule is that workstation connections be limited to 100 meters for twisted-pair cabling, and that interhub cabling be limited to 5 meters. If you are stacking hubs in a wiring closet, this is good advise. When a hub is located in central wiring closet, connections to end users tend to be very long. Using these assumptions, then maximum cable lengths depend on the device type you want to attach to the hub, as shown in the following table:

cable type

twisted-pair

workstation

100 meters

another hub

5 meters

switch

100 meters

When connecting to another hub, traffic is confined to a single collision domain. However, when connecting to a collapsed backbone (e.g., a switch or router), the collision domain is broken up, enabling the use of longer cable lengths as illustrated in the preceeding table.

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

2-4 Installing the System