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4 Overview
Hubs
Unlike 10BASE-T hubs which are all functionally identical, Fast Ethernet
hubs are divided into two distinct types: Class I and Class II. A Class I hub
repeats all incoming signals on one port to the other ports by first translating
them to digital signals and then retranslating them back to line signals. These
translations are necessary when connecting various network media to the
same collision domain, such as when combining two wire-pair 100BASE-TX
media with four wire-pair 100BASE-T4 media. Only one Class I hub can
exist within the same collision domain, thus this type of hub cannot be
cascaded. A Class II repeater, on the other hand, immediately repeats all
incoming line signals on one port to the other ports; no translations are
performed. This type of hub connects identical media to the same collision
domain; for example, TX to TX. At most, two Class II hubs can exist within
the same collision domain. The cable used to cascade these hubs is called an
inter-repeater link (IRL).
As mentioned earlier, stackable hubs can be used to increase the number of
available nodes in a collision domain. An entire hub stack counts as a single
repeater.
Connectivity Rules
• The maximum length of a twisted-pair segment (that is, distance
between a port in the hub to a single-address network device such as
a PC, server, or LAN switch) is 100 meters.
• The maximum diameter in a collision domain is about 205 meters
using two Class II hubs (or hub stacks) and 200 meters using one
Class I hub.
• Between any two end-stations in a collision domain, there may be up
to three segments and two Class II hubs or two segments and one
Class I hub.