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Review of Networking
2-4 Fast Ethernet
This signal path, two end stations and the repeaters between them, is called the
network radius. Unlike standard Ethernet networks, Fast Ethernet networks have
a maximum network radius that may restrict the lengths of station cabling to less
than the maximum allowable distances for single links. Typically, network radius
calculations are only important when mixing 100BASE-TX and 100BASE-FX
networks. The maximum network radius limits are provided later in this section.
As the imposition of a maximum network radius on mixed 100BASE-TX and
100BASE-FX networks severely limits the design options of Fast Ethernet
networks, Fast Ethernet devices may incorporate buffered uplinks. A buffered
uplink is a Fast Ethernet port on a repeater which allows the repeater to ignore the
collision domain of the uplink. This allows the buffered uplink to be a
maximum-length segment even in mixed media environments.
Fast Ethernet LANs must meet the following media and network restrictions in
order to adhere to IEEE standards:
• Cabling Quality: All 100BASE-TX links require UTP cabling meeting or
exceeding the Telecommunications Industry Association (TIA) Category 5
specification. The link must be compliant from end to end, including all
connectors and patch panels.
• Link Length: No single link in the Fast Ethernet network may exceed the
limitations given below, including jumper cables and patch cables:
- 100 m for 100BASE-TX networks
- 400 m for 100BASE-FX networks
• Network Radius: Network radius is the distance traveled from the station with
the longest media link to the Fast Ethernet repeater and out to the station with
the second-longest media link. In order to meet IEEE standards, Fast Ethernet
networks constructed with Class I repeaters must not exceed the following
maximum network radii:
- 200 m for homogenous 100BASE-TX networks
- 260 m for mixed 100BASE-TX and 100BASE-FX networks
- 272 m for homogenous 100BASE-FX networks
NOTE
A buffered uplink is considered a bridged or switched
connection only for purposes of determining cable length.
NOTE
These media lengths are fixed values. Deviation from these
maximums will lead to poor network performance.