Modems for Desktop PC

Microcom Systems was a major modem vendor during the 1980s, although they were never as popular as the "big three", Hayes, U.S. Robotics (USR) and Telebit. more...

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Nevertheless they hold an important place in modem history due to their introduction of the MNP error-correction and compression protocols, which were widely used under license by most modem manufacturers in the 1990s. Compaq purchased the company in 1997.

In the mid-1980s several companies introduced new modems with various "high-speed" features in order to differentiate themselves from the growing legion of Hayes 1200 bit/s clones that were flooding into the market. Developing such a protocol was not all that easy, and generally required a fairly powerful and expensive microcontroller to handle the modulation. For companies with limited resources, entering this market was difficult.

Microcom took another approach, addressing the feature gap not through higher speeds, but through additional capabilities. They developed a series of protocols that implemented simple packet-based file transfer protocols suitable for implementation on very simple microcontrollers. The differences were primarily in how difficult the protocol was to implement, with MNP 1 being extremely simple allowing it to be placed on existing modems with no changes, while MNP 4 offered much better throughput at the cost of increased memory needs, which modems typically had little of (40 bytes was common).

Microcom introduced their own modems starting with the AX/1200 and AX/2400 modems, which featured MNP 4 error correction in an otherwise standard 1200 bit/s Bell 212/v.22 or 2400 bit/s v.22bis modem. When a Microcom modem was used by both ends of a connection, the connection was entirely error-free.

In order to control these new features, Microcom introduced a series of new command switches prefixed with the backslash, \, while retaining the extended commands used in the Hayes Smartmodem 2400, prefixed by the ampersand, & for things like carrier detection and speed selection. As other companies increasingly used the MNP protocols, many chose to keep the original commands specified by Microcom, notably AT&T Paragon's chipsets which were fairly popular in the early 1990s. Hayes instead chose to introduced their own set with additional &-prefixed commands, USR an incompatible set of &-prefixed commands, and Telebit added to their already bewildering array of setup registers. It would be many years before the complete dominance of the Rockwell chipsets would re-standardize the market on the Hayes-based commands.

Microcom continued developing the MNP standards, and later introduced the MNP 5 standard, which compressed the data in the modem before sending it, thereby actually increasing the data rate while still being error-free. MNP 5 was introduced on the AX/1200c and AX/2400c, the "c" for "compression". MNP 1 through 5 were later handed to the ISO for standardization, and became widely available.

Read more at Wikipedia.org