V.34 is an ITU-T recommendation for modems intended for use on connections on generalized switched telephone networks (GSTN) and point-to-point, two-wire leased telephone type circuits. V.34 permits a bidirectional transfer of up to 28.8 Kbps with additional defined data transfer rates of 24 Kbps and 19 Kbps respectively.

V.34 improves the handshake and connection process through a line-probing feature. It allows a V.34 device to choose operating parameters for a given connection. Following this stage, complex signals are transmitted that allow distant receivers to analyze connection characteristics before the data transmission stage. All connected devices make use of this line analysis to choose key operating parameters. A line-probing operation is performed on all new connections and is sometimes performed at selected times during connection as part of the retaining process. This allows devices to adapt to a broad range of distortions from one call to another and hold different line conditions over long time periods.

The main characteristics of the V.34 standard are:

• Duplex and half-duplex modes of operation on GSTN and point-to-point, two-wire leased circuits
• Auto moding to V-series modems supported by V.32bis auto mode procedure and Group 3 (digital) fax machines
• Quadrature amplitude modulation for every channel with synchronous line transmissions at selectable symbol rates
• Trellis coding for data signaling rates
• Channel separation through echo cancellation techniques
• Exchange of rate sequences during the startup to establishing the data-signaling rate
• Synchronous primary channel data signaling rates