V.32/V.32bis handshake & modulation

Handshake timing diagram

(adapted from 'ITU-T Recommendation V.32', 1993)

Handshake 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. 1. Dial tone (from PSTN)
  2. 2. DTMF dialling (from calling modem)
  3. 3. Ringback tone (from PSTN)
  4. 4. Answer tone, with phase reversals (2100 Hz, from answering modem)
  5. 5. AA/CC signals (1800 Hz with phase reversal, from calling modem)
  6. 6. AC/CA/AC signals (600+3000 Hz with phase reversal, from answering modem)
  7. 7. Training sequence from answering modem, half-duplex mode
  8. 8. S/S' synchronisation sequence
  9. 9. Training sequence from calling modem
  10. 10. S/S' synchronisation sequence, full-duplex mode
  11. 11. Second training sequence from answering modem
  12. 12. User data communication phase
Note: The annotations shown here oversimplify 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 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

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