100BASET4

100BASE-T4 is a Fast Ethernet physical-layer standard defined by IEEE 802.3u in 1995. It provides 100 Mbit/s Ethernet over copper cabling by using four twisted-pair conductors. Unlike the more common 100BASE-TX, which relies on two pairs, 100BASE-T4 was designed to operate on Category 3 or better copper and could leverage existing, older cabling installations.

The technology uses four pairs to carry signal traffic, enabling 100 Mbit/s without requiring the newer cabling typically associated with Fast Ethernet. The standard specifies a star topology with hubs or switches and a maximum segment length of up to about 100 meters per Ethernet segment, in line with other copper-based Ethernet standards. Devices designed for 100BASE-T4 require compatible PHY hardware, and interoperability with 100BASE-TX equipment is limited without appropriate bridging or conversion.

In practice, 100BASE-T4 offered a way to upgrade to 100 Mbit/s on networks that had predominantly Category 3 cabling, avoiding immediate rewiring. However, the approach was more complex and often more costly than upgrading to 100BASE-TX, which used two pairs and became the dominant Fast Ethernet solution. As cabling standards improved and 100BASE-TX equipment became cheaper and more widespread, 100BASE-T4 saw declining adoption. Today it is considered largely obsolete, with most modern networks relying on 100BASE-TX or faster technologies such as 1000BASE-T or fiber variants.

Overall, 100BASE-T4 represents an early attempt to deliver 100 Mbps over legacy copper cabling, offering backward compatibility advantages in some environments but eventually being superseded by simpler, more flexible Fast Ethernet implementations.