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").

General standards

Applies to V.1–V.9

Interfaces and voiceband modems

Applies to V.10–V.34

Ad hoc standards

In order to gain first-mover advantage, many modem companies introduced models based on upcoming V-series standards before they reached final ratification. In other cases, companies introduced non-standard systems but gave them ITU-like names.

Wideband modems

Applies to V.35–V.39

Error control and data compression

Applies to V.40–V.49

Simultaneous transmission of data and other signals

Applies to V.60–V.99

Interworking with other networks

Applies to V.100–V.199

Interface layer specifications for data communication

Applies to V.200–V.249

Control procedures

Applies to V.250–V.299

Digital circuit modems

Applies to V.300–V.399