In Fast Ethernet cabling, 100Base-TX uses ____ cable and has a maximum segment…

Concept — how an Ethernet physical layer is named, and what fixes its reach.

Every IEEE 802.3 physical-layer variant carries a three-part name: the signalling rate in Mbit/s, the signalling method (Base means baseband — the cable carries one unmodulated Ethernet signal), and a suffix that encodes the transmission medium. Inside that suffix the letter T stands for twisted-pair copper and the letter F stands for optical fibre. The maximum segment length is not fixed by the signalling rate; it is fixed by the physical medium together with the cabling grade and duplex mode that the variant specifies, because copper is limited by attenuation and crosstalk across balanced pairs while fibre is limited by optical loss and dispersion. Read the medium letter first: it sets the order of magnitude, and the variant's cabling and duplex settle the exact figure.

Application — decoding 100Base-TX piece by piece.

1. 100 — the signalling rate: 100 Mbit/s, which is why the family is called Fast Ethernet.

2. Base — baseband signalling: the cable carries a single Ethernet signal with no carrier modulation.

3. T — the medium letter: twisted-pair copper. An F in this position would instead mean optical fibre.

4. X — the 4B/5B block-coding scheme inherited from FDDI; the TX form runs on two pairs of Category 5 or better UTP/STP.

5. Distance — because the medium is balanced copper, IEEE 802.3 Clause 25 fixes the maximum link segment at 100 metres, which TIA/EIA-568 structured cabling budgets as roughly 90 m of solid horizontal cable plus roughly 10 m of stranded patch cord at the two ends.

Cross-check — the Fast Ethernet family side by side.

The table shows exactly what the concept predicts: every copper (T) variant stops at 100 metres however many pairs it uses, and only the fibre (F) variant reaches into hundreds or thousands of metres — with its exact figure settled by the duplex mode.

Common trap. 200 metres is a genuine Fast Ethernet number, but it is the diameter of a repeated copper collision domain — two 100-metre segments either side of a Class II repeater — not the length of a single cable segment. 2000 metres is genuine too, but it belongs to full-duplex 100Base-FX over multimode fibre, a name carrying an F rather than a T.

Putting it together: 100Base-TX is Fast Ethernet over twisted-pair copper, with a maximum segment length of 100 metres.