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ITU standards are those promulgated by the International Telecommunications Union, part of the United Nations. These standards are used throughout the world.

V.22 is the CCITT equivalent of the Bell 212A standard. It delivers a transfer rate of 1200 bits per second at 600 baud. It actually uses the same form of modulation as Bell 212A, but it is not compatible with the Bell standard because it uses a different protocol to set up the connection. In other words, although Bell 212A and V.22 modems speak the same language, they are unwilling to start a conversation with one another. Some modems support both standards and allow you to switch between them.

V.22bis was the first true world standard, adopted into general use in both the United States and Europe. It allows a transfer rate of 2400 bits per second at 600 baud using a technique called trellis modulation that mixes two simple kinds of modulation, quadrature and amplitude modulation. Each baud has 16 states, enough to code any pattern of four bits. Each state is distinguished both by its phase relationship to the unaltered carrier and its amplitude (or strength) in relation to the carrier. There are four distinct phases and four distinct amplitudes under V.22bis, which, when multiplied, yield the 16 available states.

V.32 is an international high speed standard that permits data transfer rates of 4800 and 9600 bits per second. At its lower speed, it uses quadrature amplitude modulation similar to Bell 212A but at the higher baud rate of 2400 baud. At 9600 bits per second, it uses trellis modulation similar to V.22bis but at 2400 baud and with a greater range of phases and amplitudes.

Note that, while most Group III fax machines and modems operate at 9600 bits per second, a fax modem with 9600 bps capability is not necessarily compatible with the V.32 standard. Don't expect a fax modem to communicate with V.32 products.

V.32bis extends the V.32 standard to 14,400 bits per second while allowing fallback to intermediary speeds of 7200 and 12,000 bits per second in addition to the 4800- and 9600-bits-per-second speeds of V.32. (Note that all these speeds are multiples of a basic 2400 baud rate.) The additional operating speeds that V.32bis has and V.32 does not are generated by using different ranges of phases and amplitudes in the modulation.

At 14,400 bits per second, there are 128 potentially different phase/amplitude states for each baud under V.32bis, enough to encode seven data bits in each baud. Other data rates (including V.32) use similar relationships for their data coding. Because there are so many phase and amplitude differences squeezed together, a small change in the characteristics of a telephone line might mimic such a change and cause transmission errors. Consequently, some way of detecting and eliminating such errors becomes increasingly important as transmission speed goes up.

V.34 is the official designation of the high speed standard once known as V.fast. Under the V.34 standard, as adopted in June 1994, modems can operate at data rates as high as 28,800 bits per second without compression over ordinary dial-up telephone lines. More recently, the standard was amended to permit operation as fast as 33,600 bps. The V.34 standard also allows lower transmission rates at 24,000 and 19,200 bits per second and includes backward compatibility with V.32 and V.32 bits.

The V.34 standard calls for modems to adapt to telephone line conditions to eke out the greatest usable amount of bandwidth. Where V.32 modems operate at a fixed bandwidth of 2400 Hz, with a perfect connection V.fast modems will be able to push their operating bandwidth to 3429 Hz. V.34 modems will use line-probing techniques to try each connection, and then apply advanced equalization to the line. To squeeze in as much signal as possible, V.34 modems use multidimensional trellis coding and signal shaping.

V.34 was immediately preceded by two non-standards, V.32 terbo and V.FC (or V.Fast Class), that were supported by various modem industry groups and chip manufacturers, but which were not formally sanctioned by the ITU. Because technology provided the power to allow higher speeds before the ITU could reach a consensus on a standard, these quasi-standards were widely available until V.34 products became available.

V.32 terbo appeared in June 1993, when AT&T Microelectronics introduced modem chips that operated at 19,200 bits per second using an extension of the V.32 modulation scheme. The "terbo" in the name is a poor play on words. "Bis" in modem standards stands for "second;" similarly "ter" means "third" (as in "tertiary"). "Terbo" means nothing but conjures up the sound of respectability ("ter" for the third iteration of V.32) and speed ("turbo" as in a high performance automobile with a turbocharger). The technology was originally designed for picture phone technologies. V.32 terbo is backwardly compatible with V.32bis standards and will connect with older modems at the highest speed at which both can operate. With compression such as MNP5 or V.42 bis, V.32 terbo modems can operate at effective data rates as high as 115,200 bits per second.

V.FC modems represent the interpretation of some modem makers and a chip maker (Rockwell International) of a preliminary version of the V.34 standard. These V.FC modems deliver true 28,800 bits per second speed using V.34 technology, but they don't use the same handshaking to set up communications as V.34. (The V.FC products pre-date the final V.34 agreements.)

V.34bis has not been officially recognized as a standard. Many of the latest modems that operate at 33.6Kbps refer to themselves with this designation in anticipation of its acceptance.

V.42 is a world wide error correction standard that is designed to help make V.32, V.32bis, and other modem communications more reliable. V.42 incorporates MNP4 as an "alternative" protocol. That is, V.42 modems can communicate with MNP4 modems but a connection between the two won't use the more sophisticated V.42 error correction protocol. At the beginning of each call, as the connection is being negotiated between modems, a V.42 modem will determine whether MNP4 or full V.42 error correction can be used by the other modem. V.42 is preferred, and MNP4 is the second choice. In other words, a V.42 will first try to set up a V.42 session; failing that, it will try MNP4: and failing that, it will set up a communications session without error correction.

V.42bis is a data compression protocol endorsed by the CCITT. Different from and incompatible with MNP5 and MNP7, V.42bis is also more efficient. On some forms of data, it can yield compression factors up to four, potentially quadrupling the speed of modem transmissions. (With PCs, the effective maximum communication rate may be slower because of limitations on serial ports, typically 38,400 bits per second.) Note that a V.42bis-only modem cannot communicate with a MNP5-only modem. Unlike MNP5, a V.42 modem never increases the transmission time of "incompressible" data. Worst case operation is the same speed as would be achieved without compression.