V.22 handshake & modulation

Handshake timing diagram

tl-v22-hs
(adapted from 'K-Series MODEM Design Manual', by Silicon Systems, 1992)

Analysis of a recorded call

  1. Dial tone (from PSTN)
  2. DTMF dialling (from calling modem)
  3. Calling tone (from calling modem)
  4. Ringback tone (from PSTN)
  5. Answer tone, with phase reversals (2100 Hz, from answering modem)
  6. Guard tone (1800 Hz, from calling modem)
  7. Unscrambled 'high-band' marks
  8. Originate 'low-band' signal (Fc=1200 Hz, from calling modem)
  9. Answer 'high-band' signal (Fc=2400 Hz, from answering modem)
Note: The use of Calling tone and Guard tone is country-specific.

tl-v22-call
Annotated spectral plot of recorded audio (Frequency/Hz against Time/s)
tl-qpsk

Modulation

Adopted in 1980, V.22 is a duplex split-band modem using phase shift keying (PSK) , supporting a maximum speed of 1200 bps. The transition from FSK to PSK makes better use of the available channel capacity, however, it also necessitates the use of a scrambler and more complex modulation and demodulation circuitry. Each channel uses a symbol rate of 600 baud, with each symbol having four states (0°/90°/180°/270°).

ITU-T V.22 
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Related product: 3am LineScope dial-up analyzer