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Facsimile Call Processes

CCITT (ITU-T) Recommendation T.30 describes how FAX calls are established. This document presents a top-level overview of these processes.

Types of Messages

During the call setup/establishment process, there are two tonal signals that are used:

Originating FAX Operating Modes

Terminating FAX Operating Modes

Pre-Message State Overview

The Pre-Message state is entered by the Terminating FAX machine by transmitting a "Line Turn-Around" preamble, followed by ID information.

The preamble consists of a series of flag characters lasting from 850 mS to 1.15 S (1 S +/- 15%). Further detail concerning the preamble and the Pre-Message State is not discussed herein.

Key FAX Call Establishment Points

The Terminating FAX machine will begin transmittal of the Line Turn-Around Preamble and ID information immediately (75 mS +/- 20 mS) after ceasing the 2100 Hz tone. The Terminating FAX machine will wait 3 seconds (+/- 15%) for a response from the Originating unit. Upon timeout, the process is repeated three times. At the end of the third retry, the call can be either disconnected, or the ID process restarted until the 30 - 40 second timeout window is reached.

When the Terminating FAX machine is operating in a MANUAL mode, the ID information will be transmitted every 4.5 seconds, versus 3 seconds. This facilitates operation where a MANUAL Originator calls an AUTOMATIC Terminator and wishes to RECEIVE (not Transmit).

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