Dial-up modem technology primer: V.21 to V.92
ITU-T V.xx modulations used in 2-wire duplex data modems

tl-modem

Despite the rise of Wireless and the Internet, low-speed dial-up embedded modems still find wide application in many sectors. These include Vending and Gaming machines, Set-top-box back-channel, Direct broadcast satellite, Utility meters, Alarm systems and Point-of-sale terminals. They are also used for remote diagnostics, telemetry, monitoring and control.

V.21

V.21 is a duplex split-band modem using frequency shift keying (FSK)icon-link, supporting a maximum speed of 300 bps. It is designed to be simple enough to permit an implementation using passive (LC) filters. V.21 was adopted in 1964.

ITU-T V.21icon-link

V.22

V.22 is a duplex split-band modem using phase shift keying (PSK)icon-link, supporting a maximum speed of 1200 bps. The transition from FSK to PSK makes better use of the available channel capacity, however, it also necessitates the use of a scrambler and more complex modulation and demodulation circuitry. Each channel uses a symbol rate of 600 baud, with each symbol having four states (0°/90°/180°/270°). V.22 was adopted in 1980. More...

ITU-T V.22icon-link

V.22bis

V.22bis improves upon V.22 by using quadrature amplitude modulation (QAM)icon-link, supporting a maximum speed of 2400 bps. The transition from PSK to QAM necessitates the use of adaptive equalization and more complex signal processing. Each channel uses a symbol rate of 600 baud, with each symbol having 16 states. V.22bis was adopted in 1984. More...

ITU-T V.22bisicon-link

V.32

V.32 improves upon split-band modems, with one key innovation being echo cancellation. Echo cancellation provides channel separation over a telephone line without splitting the available bandwidth. While still using QAM, V.32 supports a maximum speed of 9600 bps. Each channel uses a carrier of 1800 Hz, and a symbol rate of 2400 baud, with each symbol having 16 states, or optionally 32 states with Trellis coding. QAM with Trellis coding, also known as trellis-coded modulation (TCM)icon-link, is another key innovation that provides forward error correction (FEC)icon-link with coding gain, enabling a more effective use of the channel's capacity. V.32 was adopted in 1984. More...

ITU-T V.32icon-link

V.32bis

V.32bis improves upon V.32 following further developments in signal processing electronics that make it possible to implement more complex modulation schemes. Using TCM, V.32bis supports a maximum speed of 14,400 bps. Each channel uses a carrier of 1800 Hz, and a symbol rate of 2400 baud, with each symbol having 128 states. V.32bis was adopted in 1991. More...

ITU-T V.32bisicon-link

V.34

V.34 improves upon V.32bis in a number of ways: in adaptivity, constellation shaping and precoding. Adaptivity is achieved by using a range of carrier frequencies and symbol rates, including some beyond those in V.32bis. Shaping is a technique used to minimize the average signal power for a given constellation, and a technique known as shell mapping is used, yielding a 0.8dB shaping gain. Precoding is equalization applied at the transmitter based on information from the receiver, ensuring better use of the full bandwidth. V.34 uses a larger, 1664-point super-constellation, with all symbols being a subset of this super-constellation. For example, at 28,800 bps, Fc=1800 Hz, Rs=3000 baud, Symbols=1408; at 33,600 bps, Fc=1959 Hz, Rs=3429 baud, Symbols=1664. V.34 was adopted in 1994. More...

ITU-T V.34icon-link

V.90

V.90 improves upon V.34 by using pulse-code modulation (PCM)icon-link for the downstream link. It achieves speeds of up to 56,000 bps when connected to a digital modem, by sending G.711 symbols at the system sampling rate of 8000 Hz. V.90 was adopted in 1998.

ITU-T V.90icon-link

V.92

V.92 marginally improves upon V.90 by adding "Quick Connect", "Modem on Hold", "V.PCM upstream" and "V.44 compression" features. V.92 was adopted in 2000.

ITU-T V.92icon-link