V.22 bis handshake & modulation
used in 2-wire duplex modems

  • Handshake timing diagram
  • (adapted from 'K-Series MODEM Design Manual', by Silicon Systems, 1992)

  • Analysis of a recorded call
  • 1. PABX dial tone (from PSTN, 350+425Hz)
  • 2. Single DTMF '0' digit (from calling modem)
  • 3. Dial tone (from PSTN)
  • 4. DTMF dialling (from calling modem)
  • 5. Answer tone (2100 Hz, from answering modem)
  • 6. Unscrambled 'high-band' marks
  • 7. S1 signal (unscrambled double dibit pattern, from calling modem)
  • 8. S1 signal reply (from answering modem)
  • 9. Originate 'low-band' signal (Fc=1200 Hz, from calling modem)
  • 10. Answer 'high-band' signal (Fc=2400 Hz, from answering modem)
  • Note: The use of Calling tone and Guard tone is country-specific.

    Annotated spectral plot of recorded audio (Frequency/Hz against Time/s)


  • Modulation
  • V.22 bis - Adopted in 1984, V.22 bis improves upon V.22 by using quadrature amplitude modulation (QAM), supporting a maximum speed of 2400 bps. The transition from PSK to QAM necessitates the use of adaptive equalization and more complex signal processing. Each channel uses a symbol rate of 600 baud, with each symbol having 16 states (of combinations of phase and amplitude).