V.32/V.32bis handshake & modulation
Handshake timing diagram
(adapted from 'ITU-T Recommendation V.32', 1993)
Handshake phase annotations
Analysis of a recorded call
Annotated spectral plot of recorded audio (Frequency/Hz against Time/s)
Adopted in 1984, V.32 improves upon split-band modems, with one key innovation being echo cancellation. Echo cancellation provides channel separation over a telephone line without splitting the available bandwidth. While still using QAM, V.32 supports a maximum speed of 9600 bps. Each channel uses a carrier of 1800 Hz, and a symbol rate of 2400 baud, with each symbol having 16 states, or optionally 32 states with Trellis coding. QAM with Trellis coding, also known as trellis-coded modulation (TCM), is another key innovation that provides forward error correction (FEC) with coding gain, enabling a more effective use of the channel's capacity.
V.32bis improves upon V.32 following further developments in signal processing electronics that make it possible to implement more complex modulation schemes. Using TCM, V.32bis supports a maximum speed of 14,400 bps. Each channel uses a carrier of 1800 Hz, and a symbol rate of 2400 baud, with each symbol having 128 states. V.32bis was adopted in 1991.ITU-T V.32 ITU-T V.32bis