What are Class 1, Class 2, Group 3 and Group 4?

01 January 1998

Class 1 and Class 2 are standards that define commands used by PC software to control a modem.

• Class 1 is a low level interface - the PC has full control of the modem.
• Class 1 allows Unimessage to implement all of the latest ITU standards.
• Class 2 is a higher level interface - the PC must go through complex modem firmware.
• Class 2 therefore has more room for bugs in the modem firmware.
• Class 2 has been through many incompatible revisions.
• Class 2 cannot be used with all of the latest ITU standards.

Group 3 and Group 4 are ITU fax standards.

• ITU fax standards enable all fax devices to communicate with one another.
• Group 3 and Group 4 define the commands and data that go over the telephone lines.
• Group 3 is designed to work over normal (PSTN) voice grade telephone lines.
• Group 4 is designed to operate on high speed ISDN lines.

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