A combination fax board and data modem available as an external unit that plugs into the serial port of the computer or as an expansion card for internal installation. It includes a switch that routes the call to the fax or data modem, and incoming faxes are printed on the computer's printer. Most modern modems are fax/modems. A fax/modem requires software that generates the fax transmission from typed-in text, a disk file or from a screen image. Fax/modems often transmit a sharper image than a fax machine, which obtains its source material by scanning the page. Group 3 Service Classes Group 3 fax/modems provide different levels of processing based upon their service class. Class 1 devices perform basic handshaking and data conversion and are the most flexible because much of the work is done by the computer's CPU. Class 2 devices establish and end the call and perform error checking. There are a variety of de facto Class 2 implementations and one Class 2.0 standard. See fax and Internet faxing.