Interface Classes

As with data modems, fax modems must link up with your computer and its software. Unlike data modems, which were blessed with a standard since early on (the Hayes command set), fax modems lacked a single standard. In recent years, however, the Electronics Industry Association and the Telecommunications Industry Association have created a standard that is essentially an extension to the Hayes AT command set. The standard embraces two classes for support of Group 3 fax communications: Class 1 and Class 2.

Class 1 is the earlier standard. Under the Class 1 standard, most of the processing of fax documents is performed by computer software. The resulting fax data is sent to the modem for direct transmission. It includes requirements for autodialing; a GSTN interface; V-series signal conversion; HDLC data framing, transparency, and error detection; control commands and responses; and data commands and reception.

Class 2 shifts the work of preparing the fax document for transmission to the fax modem itself. The modem hardware handles the data compression and error control for the transmission. The Class 2 standard also incorporates additional flow-control and station-identification features, including T.30 protocol implementation, session status reporting, phase C data transfer, padding for minimum scan line time, quality check on received data, and packet protocol for the DTE/DCE interface.

These classes hint at the most significant difference between computer-based fax systems, which is software. Fax modem hardware determines the connections that can be made, but the software determines the ultimate capabilities of the system. A fax modem that adheres to various standards (classes as well as protocols) will open for you the widest selection of software and the widest range of features.