Asynchronous or synchronous modems? —————————————————- Formerly, data communication was done by sending job commands to a mainframe computer, and having the result returned in one batch. The modems were called synchronous. Such modems (and computers) are still in use in some large corporations. Most of today's online services are based on an interactive dialog between the user and the remote computer. The user enters a command, for example a letter or a number in a menu, and the result is returned almost immediately. The modems used for such work are called asynchronous (See "Explanation of some words and terms" in appendix 4). Unless you know that you must have a synchronous modem, buy an asynchronous one.
Choice of speed ———————- Speed is measured in many ways. One method is to use baud. Another is to use characters per second (cps) or bits per second (bps). Bps is a measure of how many data bits that can be transferred over a data channel in one second. (Each byte is split up into bits before transfer during serial communication.) The relationship between baud and bits per second is complex, and often misused. Bits per second is unambiguous. In this book, we will use it as bps. We can estimate the number of characters per second by dividing the number of bps by ten. For example. 1200 bps is roughly 120 cps. In 1987, 300, 1200 and 2400 bps asynchronous modems were the standard in many countries. Around 1990, the growth in 9600 bps modems and modem with faster speeds gained momentum. Modem user manuals often give transfer speed by referring to some international classification codes. Here are some CCITT codes with explanation:
V.21 0-300 bps Still used by a small group. Cannot
full duplex communicate with the American Bell
103 standard.
V.22 1200 bps Partly compatible with the American
full duplex Bell 212a standard. Sometimes it
works, sometimes it fails.
V.22bis 2400 bps Used all over the world. Very
full duplex common.
V.23 600 & 1200 Rare protocol. Used mainly in Europe. bps w/75 Half duplex. bps return ch.
V.26ter 2400 bps Used mainly in France
full duplex
V.27ter 2400/4800 bps Used in Group III fax
half duplex
V.29 4800, 7200 and Used in gr. III fax and in some (Ame- 9600 bps rican) modems. Do not buy V.29 if you half duplex want a 9600 bps modem.
V.32 4800/9600 bps Current standard for 9600 bps modems
full duplex