Trellis coded modulation

In the late 1980s, modems operating over plain old telephone service (POTS) typically achieved 9.6 kbit/s by employing four bits per symbol QAM modulation at 2,400 baud (symbols/second).

[citation needed] 14 kbit/s is only 40% of the theoretical maximum bit rate predicted by Shannon's theorem for POTS lines (approximately 35 kbit/s).

[1] Ungerboeck's theories demonstrated that there was considerable untapped potential in the system, and by applying the concept to new modem standards, speed rapidly increased to 14.4, 28.8 and ultimately 33.6 kbit/s.

[clarification needed] Though hard to visualize in multiple dimensions, a simple one-dimension example illustrates the basic procedure.

A flurry of research activity ensued, and by 1984 the International Telecommunication Union had published a standard, V.32,[2] for the first trellis-modulated modem at 9.6 kilobit/s (2,400 baud and 4 bits per symbol).