Ethernet - Computer Busses - page 431

• 100BASE-FX (fiber-optic cable) - which is the physical layer standard for 100 Mbps
over fiber-optic cables.

Fast Ethernet, or 100BASE-T, is simply 10BASE-T running at 10 times the bit rate. It is a
natural progression from standard Ethernet and thus allows existing Ethernet networks to be
easily upgraded. Unfortunately, as with standard Ethernet, nodes contend for the network,
reducing the network efficiency when there are high traffic rates. Also, as it uses collision
detect, the maximum segment length is limited by the amount of time for the farthest nodes
on a network to properly detect collisions. On a Fast Ethernet network with twisted-pair cop-
per cables this distance is 100 m, and for a fibre-optic link, it is 400 m. Table 26. 2 outlines
the main network parameters for Fast Ethernet.

Table 26.2 Fast Ethernet network parameters

100BASE-TX 100VG-AnyLAN

Standard IEEE 802.3u IEEE 802.12

Bit rate 100 Mbps 100 Mbps

Actual throughput up to 50 Mbps up to 96 Mbps

Maximum distance
(hub to node)

100 m (twisted-pair, CAT-5)
400 m (fiber)

100 m (twisted-pair, CAT-3)
200 m (twisted-pair, CAT-5)
2 km (fibre)

Scaleability none up to 400 Mbps

Advantages Easy migration from

10BASE-T

greater throughput, greater distance

As 100BASE-TX standards are compatible with 10BASE-TX networks then the network al-
lows both 10 Mbps and 100 Mbps bit rates on the line. This makes upgrading simple, as the
only additions to the network are dual-speed interface adapters. Nodes with the 100 Mbps
capabilities can communicate at 100 Mbps, but they can also communicate with slower
nodes, at 10 Mbps.

The basic rules of a 100BASE-TX network are:

• The network topology is a star network and there must be no loops.
• Cat-5 cable is used.
• Up to two hubs can be cascaded in a network.
• Each hub is the equivalent of 5 m in latency.
• Segment length is limited to 100 m.
• Network diameter must not exceed 205 m.

26.9.1 100BASE-T4
100BASE-T4 allows the use of standard Cat-3 cables. These contain eight wires made up of
four twisted-pairs. 100BASE-4T uses all of the pairs to transmit at 100 Mbps. This differs
from 10BASE-T in that 10BASE-T uses only two pairs, one to transmit and one to receive.