Modem Standards

Guidelines for communication.

ITU modem standards.
The International Telegraphic Union (ITU) is an international association that establishes worldwide communication standards. Its standards are prefaced by the letter -V- and include the following:

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.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.

Today, a vast installed base of Bell Standard customers relies heavily on these modems. Black Box offers a line of Bell Standard modems. Bell Standards include the following:

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 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.

V.42, V.42 bis-These -V Series- protocols are internationally recognized standards for error control and data compression. V.42 is the recommendation of the ITU for error control; it contains two algorithms (LAPM, or Link Access Protocol, and MNP 1-4). When two V.42-compliant modems establish a connection, they use LAPM to control
data errors and retransmit -bad- data blocks. If one modem supports V.42 and the other supports only MNP, then the two negotiate to use MNP protocol. In both cases, the error-control process is automatic and requires no special user actions or software programs. V.42 bis corresponds roughly to MNP Level 5. The difference is the amount of data compressed: V.42 can usually generate a 4:1 ratio of data compression, depending on the type of file transmitted.