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ITU-T V.29 defines the modulation methods and operating sequences for a 9600 bps modem used primarily for use on point-to-point 4-wire leased telephony circuits. Data transmission is at 9.6 kbps, with fallback rates of 7.2 kbps and 4.8 kbps, with symbol rate of 2400 symbols per second. V.29 may operate in either full duplex or half-duplex mode.
All of the facsimile modulations and data streams can be controlled with VOCAL's T.30 library or with VOCAL's facsimile service Class 1 Command Set(EIA-578 and T.31). The T.30 library could in turn be controlled by VOCAL's facsimile service Class 2 Command Sets T.32 (Fax Class 2.1), EIA-592 (Fax Class 2.0), or industry standard (Fax Class 2). With Fax Class 1, most of the protocol is executed in software, with the advantage that any changes don't require hardware modifications. This is in contrast to Fax Class 2, where image processing is handled by the modem.
The line interface may be an analog front end (codec and DAA) or a digital interface such as T1/E1, Switched 56, and ISDN. The upper end of this software offers a direct binary and HDLC interface.