The original coaxial Ethernet used half-duplex transmission so only one device could transmit at a time. However, in 1997, Ethernet was expanded to include a full duplex capability that allowed more than one PC on a network to transmit at the same time. Switches increasingly replaced hubs. These switches had the capability of full duplex and rapid handling of Ethernet frames.

100BASE-TX uses 4B/5B encoding, which is then scrambled and converted to multi-level transmit-3 levels or MLT-3. In the example, the highlighted window shows four waveform examples. The top waveform has no transition in the center of the timing window. No transition indicates that a binary 0 is present. The second waveform shows a transition in the center of the timing window. A binary 1 is represented by a transition. The third waveform shows an alternating binary sequence. The absence of binary transition indicates a binary 0, and the presence of a transition indicates a binary 1. Rising or falling edges indicate 1s. Very steep signal changes indicate 1s. Any noticeable horizontal line in the signal indicates a 0.

Figure shows the pinout for a 100BASE-TX connection. Notice that the two separate transmit-receive paths exist. This is identical to the 10BASE-T configuration.

100BASE-TX carries 100 Mbps of traffic in half-duplex mode. In full-duplex mode, 100BASE-TX can exchange 200 Mbps of traffic. The concept of full duplex will become increasingly important as Ethernet speeds increase.

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