Group 4 fax is the facsimile standard for ISDN. The current standard for analog phone lines is Group 3, which uses a V.29 modem communicating at a maximum rate of 9,600 bps over conventional analog lines. Group 4 will use a B-channel running at 64 kbps and will not have the additonal overhead of converting the digitally scanned image to analog in the modems at both ends of a connection.
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