Fast EtherHub-12se User’s Guide
Installing the System 2-3
Distance Limit for Fast Ethernet Cabling
There are two common ways to calculate maximum cable length. The first
way, which is refered to as the “Simplified Cable Length Constraints” method
assumes that the maximum permissible cable length is used to connect various
end-node devices (such as workstations or servers) to the hub. A more careful
method of calculating maximum cable length, referred to as the “Precise Length
Budgeting” method, requires you to measure the exact cable length for each
pair of nodes that need to communicate, and to provide the signal delay time for
each hub in your system. Both of these methods are described in this section.
Simplified Cable Length Constraints
The Fast EtherHub-12se 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 112 meters
for fiber optic 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:
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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.
