It doesn't "stand for" anything. When we were developing the 100 Mb/s Ethernet standard, a proposal was initially presented for an encoding scheme that supported both twisted pair and fiber. To facilitate discussion, it was called "100BASE-X", where the "X" was a placeholder for whatever medium would ultimately be used. The symbolism stuck and we kept the "X" designation to indicate the use of 4B/5B block encoding;
-TX meant that coding on twisted pair, and -FX meant that encoding used on fiber.
-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX
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