V29
The ITU-T V-Series Recommendations on Data communication over the telephone network specify the protocols that govern approved modem communication standards and interfaces.
Note: the bis and ter suffixes are ITU-T standard designators of successive iterations of a standard (bis and ter are derived from the Latin for "twice" and "thrice").
Read more about V29: General Standards, Interfaces and Voiceband Modems, Wideband Modems, Error Control and Data Compression, Simultaneous Transmission of Data and Other Signals, Interworking With Other Networks, Interface Layer Specifications For Data Communication, Control Procedures, Digital Circuit Modems, See Also