What is 100Base-T4?

100BASE-T4 is an early implementation of Fast Ethernet over twisted pair cables, carrying data traffic at 100 Mbps (Mega bits per second) in local area networks (LAN). It was launched as the IEEE 802.3u standard in 1995. Here, 100 represents the maximum throughput (100 Mbps), BASE denotes the use of baseband transmission, and T4 denotes the use of four twisted pair cables in Fast Ethernet.

Key Properties

• Cable requirement − Uses four pairs of unshielded twisted pair (UTP) Category 3 cables, also known as voice grade cables.

• Wire configuration − Two pairs are bidirectional and two pairs are unidirectional. The unidirectional pairs are dedicated for transmit and receive operations. Three pairs can transmit data simultaneously in each direction.

• Data transmission rate − Each twisted pair transmits at a maximum of 25 Mbaud. With three pairs working together, the system achieves 75 Mbaud total capacity.

• Encoding scheme − Utilizes 8B/6T (eight binary/six ternary) encoding, which converts 8-bit binary data into 6-symbol ternary codes for transmission.

How 100BASE-T4 Works

100BASE-T4 achieves 100 Mbps throughput by using three of the four available wire pairs simultaneously. The 8B/6T encoding scheme converts each 8-bit data byte into a 6-symbol ternary code, where each symbol can have one of three voltage levels (+1, 0, -1). This ternary signaling allows more efficient use of the available bandwidth on Category 3 cables.

The system splits the 100 Mbps data stream across three 25 Mbaud channels, with each pair carrying approximately 33.3 Mbps of actual data after accounting for the encoding overhead.

Advantages and Disadvantages

Conclusion

100BASE-T4 was an important early Fast Ethernet standard that enabled 100 Mbps networking over existing Category 3 cabling. While it has been largely superseded by 100BASE-TX, it played a crucial role in the transition from 10 Mbps to 100 Mbps Ethernet networks.