Notes:
From Ethernet - The Definitive Guide
All 100Base standards use 4B/5B encoding, NRZI (non return to zero, ones inverted) on fiber. NRZI minimizes the number of transitions to transmit a transmit its data and is self-clocking. The additional symbols available in the 4B/5B are used to carry additional control signals link "IDLE", "JAM", etc. 4B/5B was originally part of the ANSI X3T9.5 FDDI standard.
2Km is standard for multimode fiber, but considerably longer distances are possible using single mode fiber (distances over 40Km routinely acheived).
100BaseF send data continuously even if no data is being transmitted, in the form of IDLE signals (part of the control signals available because of 4B/5B encoding). This is used to continuously monitor the link. If no signal comes from the other end (at least an IDLE should always come through), a Far End Fault (FEF) is reported by the hardware.