V.22 bis handshake & modulation
used in 2-wire duplex modems |
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Handshake timing diagram
(adapted from 'K-Series MODEM Design Manual', by Silicon Systems, 1992)
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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)
Annotated spectral plot of recorded audio (Frequency/Hz against Time/s)
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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).
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