Despite the rise of Wireless and the Internet, low-speed dial-up embedded modems still find wide application in many sectors. These include Vending/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 is a duplex split-band modem using frequency shift keying (FSK), 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.
V.22 is a duplex split-band modem using phase shift keying (PSK), 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.
V.22bis improves upon V.22 by using quadrature amplitude modulation (QAM), 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 (of combinations of phase and amplitude). V.22bis was adopted in 1984.
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), is another key innovation that provides forward error correction (FEC) with coding gain, enabling a more effective use of the channel's capacity. V.32 was adopted in 1984.
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.
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.
V.90 improves upon V.34 by using pulse-code modulation (PCM) for the downstream link, achieving speeds of up to 56,000 bps when connected to a digital modem, sending G.711 signals with a symbol rate of 8000 baud. V.90 was adopted in 1998.
V.92 improves upon V.90 by adding 'Quick Connect', 'Modem on Hold', 'V.PCM upstream' and 'V.44 compression' features. V.92 was adopted in 1999.