The series of ITU-T Recommendations specifying protocols relating to data communication over the over the public switched telephone network (PSTN). V-series specifications largely relate to modems, data interfaces on modems (e.g.,V.35, discontinued as a modem recommendation but still alive as a connector), and modem communications protocols, over dial-up and leased lines in both full duplex (FDX) and half-duplex (HDX). See Table V-3 for selected V-series Recommendations. For a full listing of ITU-T Recommendations, see the contact information in Appendix A.

Table V-3: Selected ITU-T V-Series Recommendations

Recommendation Number	Description
V.17	14.4 kbps two-wire modems for facsimile transmission over dial-up PSTN circuits
V.21	300 bps FDX modems for use over dial-up PSTN circuits
V.22	1200 bps FDX modems for use over dial-up PSTN circuits and point-to-point two-wire leased lines
V.22bis	2400 bps FDX modems using the frequency division multiplexing (FDM) technique for use over dial-up PSTN circuits and point-to-point two-wire leased lines
V.23	600/1200-baud synchronous and asynchronous HDX modems for use over dialup PSTN circuits and point-to-point two-wire leased lines
V.24	Definitions for interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE)
V.26	2400 bps FDX modems for use on four-wire leased lines
V.26bis	1200/2400 bps FDX modems for use over dial-up PSTN circuits
V.26ter	2400 bps FDX modems with echo cancellation for use over dial-up PSTN circuits and point-to-point two-wire leased lines
V.27	4800 bps FDX modems with manual equalization for use on leased lines
V.27bis	2400/4800 bps FDX modems with automatic equalization for use on leased lines
V.27ter	2400/4800 bps FDX modems for use for use over dial-up PSTN circuits
V.28	Electrical characteristics for unbalanced double-current interchange circuits
V.29	9600 bps HDX and FDX modems for use on point-to-point four-wire leased lines
V.32	9600 bps FDX modems with echo cancellation for use over dial-up PSTN circuits and point-to-point two-wire leased lines
V.32bis	4800/7200/9600/12,000/14,4000 bps FDX modems with echo cancellation for use over dial-up PSTN circuits and point-to-point two-wire leased lines
V.33	12,000/14,400 bps synchronous FDX modems for use on point-to-point four-wire leased lines
V.34	28.8 kbps modems for use over dial-up PSTN circuits and point-to-point twowire leased lines
V.34bis	33.6 kbps modems for use over dial-up PSTN circuits and point-to-point twowire leased lines
V.35	All that remains of this modem specification is a clunky connector for serial data.
V.41	Code-independent error-control system
V.42	Error-correcting procedures for DCE using asynchronous-to-synchronous conversion
V.42bis	Data compression procedures for DCE using error correction procedures
V.43	Data flow control
V.44	Data compression procedures
V.54	Loop test devices for modems
V.56	Comparative tests of modems for use over two-wire voice grade PSTN connections
V.56bis	Network transmission model for evaluating modem performance over two-wire voice grade PSTN connections
V.56ter	Test procedure for evaluation of two-wire 4 kHz voiceband FDX modems
V.61	4800 bps simultaneous voice plus data modems, with optional automatic switching to 14.4 kbps data-only signaling rates of up to 14 400 bit/s for use over dialup PSTN circuits and point-to-point two-wire leased lines
V.90	56 kbps downstream and 33.6 kbps upstream digital and analog modem pairs for use over dial-up PSTN circuits
V.91	64 kbps digital modems for use over four-wire circuit-switched connections and point-to-point leased four-wire digital circuits
V.92	Enhancements to Recommendation V.90
V.110	Support by an ISDN of DTE with V-Series type interfaces
V.120	Support by an ISDN of DTE with V-Series type interfaces with provision for statistical multiplexing
V.130	ISDN terminal adaptor framework
V.140	Procedures for establishing communication between two multiprotocol audiovisual terminals using digital channels at a multiple of 64 kbps or 56 kbps
V.150.0	Modem-over-IP networks: Foundation
V.150.1	Modem-over-IP networks: Procedures for the end-to-end connection of V-series DCE
V.151	Procedures for end-to-end connection of analog PSTN text telephones over an IP network utilizing text relay
V.152	Procedures for supporting voice-band data over IP networks
V.300	A 128 (144) kbps DCE for use on digital point-to-point leased circuits