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ITU-T V.34 defines the modulation methods and operating sequences for a modem used on General Switched Telephone Networks (GSTNs) and on point-to-point 2-wire leased telephone-type circuits. Both full duplex and half-duplex modes are supported. Data rates in the range of 2400 bps to 33600 bps, in incremental steps of 2400 bps are supported. Symbol rates of 2400, 2743, 2800, 3000, 3200, and 3429 symbols per second are supported. Using QAM with synchronous line transmission, selectable mandatory rates are 2400, 3000, and 3200 symbols/s with optional rates of 2743, 2800 and 3429 symbols/s. Asymmetric symbol and data rates are also supported. Negotiation and training sequences at startup establish the data rate. Control channel rates are 1200 and 2400 bps, with an optional auxiliary channel with a synchronous data signalling rate of 200 bit/s.
Trellis coding for all data signaling rates and shell mapping permit satisfactory performance even over noisy channels. Adaptive techniques enable the modem to achieve close to the maximum data signalling rate a channel can support on each connection. VOCAL's V.34 facsimile modem software uses state-of-the-art line probing analysis to automatically determine maximum channel capacity. It also supports MSE fallback/fallforward threshold control to adjust the tradeoff between higher data channel rates and symbol error rates. Channel separation is accomplished using echo cancellation techniques.
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.