Bell modem standards.
In the 1960s and 1970s, Bell Labs created carrier standards for use with Bell equipment and lines to accommodate customers'  networks.

Bell 103—Asynchronous data transmission, full-duplex operation over 2-wire dialup or leased lines; 300-bps data rate. Ideal for the “low-demand” user who exchanges files infrequently with another PC user or an on-line bulletin board. Comparable to ITU V.21.

Bell 201B, Bell 201C—
201B: Synchronous data transmission, full-duplex operation over 4-wire leased lines and half-duplex operation over 2-wire leased lines; 2400-bps data rate.
201C: Synchronous data transmission, half-duplex operation over 2-wire dialup lines; 2400-bps data rate. Both are comparable to ITU V.26. Bell 201B compatible modems are the first of the fast modems. Many businesses still use them in typical terminal-to-host, multidrop applications. Bell 201C modems are for use with dialup lines and are basically a complement to the 201B. If you need to transmit synchronous data (for example, IBM 3780/2780 applications) at 2400 bps, but you don't need the on-demand performance and cost of a leased line, then consider buying a Bell 201C.

Bell 208A, Bell 208B—
208A: Synchronous data transmission, full-duplex operation over 4-wire leased lines, half-duplex operation over 2-wire leased lines; 4800-bps data rate.
208B: Same as 208A, but over 2-wire dialup lines; comparable to ITU V.27. The first standards to enable higher-speed data transmission (4800 bps) over leased lines for multipoint networks. For example, these standards enable users to connect IBM mainframes at a central office to terminals in branch offices with fewer modems and more efficient in-house cable runs.

Bell 212A—Synchronous/asynchronous data transmission, full-duplex operation over 2-wire leased or dialup lines; 1200-bps data rate. This versatile standard provides for efficient full-duplex operation over 2-wire dialup lines. This is a big advantage for small businesses that need to avoid the expense of leased lines. If you add a leased line, however, you can still use Bell 212A. Comparable to ITU V.22.

The International Telegraphic Union (ITU) is an international association that creates worldwide standards. These standards are prefaced by a letter “V”.

V.13—Provides for simulated half-duplex (switched-carrier) control. V.32- and V.33-compliant modems that support V.13 can be used in sync IBM® RJE environments, so these networks can also take advantage of V.32/V.33 technology.

V.22, V.22 bis—Synchronous/asynchronous data transmission, full-duplex operation over 2-wire leased or dialup lines; 1200-bps data rate (V.22 bis, 2400 and 1200 bps). Small businesses can beat the expense of leased lines by using V.22 modems with dialup lines. If they add a 2-wire leased line later, they can still use this modem. V.22 accommodates the equipment found in today's typical “hybrid” network: synchronous mainframes and terminals and asynchronous PCs. V.22 bis doubles data throughput (to 2400 bps) for quick service of large file transfers.

V.25, V.25 bis—Provides for automatic calling and answering circuitry for use on dialup lines. V.25 defines a dialup parallel interface; V.25 bis defines a dialup serial interface. These standards enable any computer, sync or async, to perform autodialing functions with a V.25-compliant modem.

V.32—Synchronous/asynchronous data transmission, full-duplex operation over 2-wire dialup or 2-wire leased lines; 9600-bps data rate (fallback to 4800 bps). The V.32 recommendation is the first universal standard for 9600-bps modems on dialup or leased phone lines. V.32-compliant modems are the industry standard for high-speed networks. Trellis-encoding modulation enables high data speeds and reduces errors. Data can be sent over standard dialup lines, which are a lot less expensive to use than leased lines. And V.32-compliant modems will work anywhere in the world. Any business, large or small, that handles large quantities of data transfers or huge data files will benefit from the fast, accurate data transfers of V.32 technology.

V.32 bis—Synchronous/asynchronous data transmission, full-duplex operation over 2-wire dialup or leased lines; 14.4-, 12-kbps, and 9600-, 7200-, 4800-bps data rates. It offers two advantages over V.32. A V.32 bis modem transmits data up to 14.4 kbps. And V.32 bis redefines modem-connection negotiations (called training and retraining). Training is the procedure two modems use to make a connection; they “discuss” and “agree upon” a data rate. Retraining is a subsequent negotiation after data exchange is under way—both modems “agree” to reduce transmission to a slower speed to overcome ambient line noise. Retraining is triggered by the line noise. After modems retrain, data transfer resumes. V.32 bis also provides a procedure called “fastrain,” which enables the modem to fall either backward to a slower speed or forward to a faster speed. When two V.32 bis modems do a fastrain, they stop, determine that they can run faster, and then switch speed in a few milliseconds. One advantage of V.32 bis over V.33 (see below): With V.33 you can send sync data at 14.4 kbps over a 4-wire leased line, but with V.32 bis, you can do the same over a 2-wire dialup line—much less expensive to use than a leased line.

V.33—Synchronous data transmission, full-duplex operation over 4-wire leased lines, 14.4- or 12-kbps data rate. A V.33-compliant modem uses the same signal-modulation techniques that are used by V.32 modems, but restricts operation to 4-wire leased lines. If your network requires sync data transmission up to 14.4 or 12 kbps (as do many IBM and all supercomputer environments), choose a V.33 modem.

V.34—Synchronous/asynchronous data transmission, full-duplex operation over 2-wire dialup, and 2- or 4-wire leased line; up to 28.8-kbps data rate with automatic fallback to compatible lower modems such as V.32 bis, V.32, V.22 bis, and V.22. V.34 supports speeds from 1200 bps to 28.8 kbps.

V.34+—Synchronous or asynchronous data transmission, full-duplex over 2-wire dialup and 2- or 4-wire leased lines; up to 33.6-kbps data rate with automatic fallback to compatible lower modems such as V.34, V.32, V.22 bis and V.22. V.34+ supports speeds from 1200 bps to 33.6 kbps.