V.32 handshake & modulation
used in 2-wire duplex modems

  • Handshake timing diagram
    • (adapted from 'ITU-T Recommendation V.32', 1993)
  • Phase annotations
  • 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)

  • Modulation
  • 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.