V.32 handshake & modulation
Handshake timing diagram
(adapted from 'ITU-T Recommendation V.32', 1993)
used in 2-wire duplex modems
ANS: Answer tone, 2100 Hz
AA/CC: 1800 Hz, 0° or 180° phase respectively
AC/CA: 600+3000 Hz, 0° or 180° phase respectively
S/S': signal states ABABAB... or CDCDCD... respectively
TRN: Training sequence consisting of scrambled marks sent at 4800 bps.
R1/R2/R3: Repeated 16-bit rate sequences, scrambled and differentially encoded
E: A single 16-bit rate sequence
B1: Binary ones scrambled and encoded for subsequent transmission of data
(Note) The inclusion of a special echo canceller training sequence at this point is optional
Analysis of a recorded call
1. Dial tone (from PSTN)
2. DTMF dialling (from calling modem)
3. Ringback tone (from PSTN)
4. Answer tone, with phase reversals (2100 Hz, from answering modem)
5. AA/CC signals (1800 Hz with phase reversal, from calling modem)
6. AC/CA/AC signals (600+3000 Hz with phase reversal, from answering modem)
7. Training sequence from answering modem, half-duplex mode
8. S/S' synchronisation sequence
9. Training sequence from calling modem,
10. S/S' synchronisation sequence, full-duplex mode
11. Second training sequence from answering modem
12. User data communication phase
Note: The annotations shown here are but an over-simplification of the intricacies of the V.32 handshake.
For more details, consult the relevant ITU-T documentation.
Annotated spectral plot of recorded audio (Frequency/Hz against Time/s)
Adopted in 1984, V.32 improves upon split-band modems, with a key innovation being echo cancellation.
Echo cancellation provides channel separation 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.