Fast Ethernet: Physical Layers and Repeaters
100BASE-TX Physical Layer
This physical layer defines the specifications for 100BASE-T Ethernet over two pairs of Category 5 UTP (unshielded twisted pair) wire, or two pairs of Type I STP (shielded twisted-pair) wire. With one pair for transmit and the other for receive, the wiring scheme is identical to that used for 10BASE-T Ethernet.
100BASE-T4 Physical Layer
This physical layer defines the specification for 100BASE-T Ethernet over four pairs of either Category 3, 4, or 5 UTP wire. With this signaling method, three wire pairs are used for transmit and receive, while the fourth pair listens for collisions.
100BASE-FX Physical Layer
This physical layer defines the specification for 100BASE-T Ethernet over two strands of 62.5/125 micron fiber cable. One strand is used for transmit, while the other is used for receive.
All 10BASE-T repeaters are considered to be functionally identical. Fast Ethernet repeaters, however, are divided into two distinct types: Class I and Class II.
A Class I repeater transmit or repeats the incoming line signals on one port to the other ports by first translating them to digital signals and then retranslating them to line signals. The translations are necessary when connecting different physical media (media conforming to more than one physical layer specification) to the same segment.
A Class II repeater immediately transmits or repeats the incoming line signals on one port to the other ports; it does not perform any translations. This repeater type connects identical media to the same segment (i.e. TX to TX). At most, two Class II repeaters can exist within a single network segment. The cable used to cascade the two devices is called an unpopulated segment, or IRL (Inter-Repeater Link).
Both Class I and Class II repeaters have multiple shared ports; ports that are on the same network segment. Since they share a fixed bandwidth, nodes attached to these ports must operate at the same speed, either 10 Mbps or 100 Mbps.
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