100BASETX

100BASE-TX, commonly referred to as 100BASE-TX Ethernet, is an IEEE 802.3u standard for local area networks. It provides a nominal data rate of 100 megabits per second over copper twisted-pair cabling and is part of the 100BASE family, which also includes 100BASE-FX fiber variants and 100BASE-T4 for older copper configurations. 100BASE-TX is the most widely deployed variant of the 100BASE standard.

Physical layer and cabling: 100BASE-TX uses two pairs of Category 5 (or better) unshielded twisted pair cabling, with an RJ-45 connector. Data is transmitted using 4B/5B encoding followed by MLT-3 line encoding, at a signaling rate of 125 megabaud, yielding 100 Mbps of payload. The standard specifies a maximum segment length of 100 meters for copper.

Topology and operation: The physical topology is a star, typically realized with hubs or switches. In older networks operating in half-duplex mode through hubs, CSMA/CD is used to manage access to the shared medium. Modern installations use switches in full-duplex mode, which eliminates collisions and requires auto-negotiation to determine speed and duplex settings.

Relationship to other standards and evolution: 100BASE-TX is part of the 100BASE family within IEEE 802.3 and is followed in later years by higher-speed standards such as 1000BASE-T. While largely supplanted for new designs by Gigabit Ethernet, 100BASE-TX remains in use in some legacy or cost-sensitive networks.