List of ITU-T V-series recommendations
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 the Latin words for "twice" and "thrice").
Source: Wikipedia — List of ITU-T V-series recommendations (CC BY-SA 4.0)