V.22bis handshake & modulation

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)


Adopted in 1984, V.22bis 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).

ITU-T V.22bis

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Related product: 3am LineScope dial-up analyzer