RETURN Protocol Information

 

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:

  • CNGThis signal may be sent from the Originating FAX machine after dialing is complete. The CNG signal consists of the transmission of 1100 Hz for 1/2 second, followed by a 3 second silent (2100 Hz OFF) period.

    The CNG signal can alert the answering user (misdialed number, manual FAX operation) of connection to a FAX machine.

  • CEDThis signal may be sent by the Terminating FAX machine anywhere between 1.8 to 2.5 seconds AFTER answering the call. The CED signal consists of a 2100 Hz tone that is from 2.6 to 4 seconds in duration. The CED tone is useful for disabling any echo cancellers on the line.

    The CED also incorporates a “silent” interval following the 2100 Hz tone. This interval lasts from 55 to 95 (75 +/- 20) mS. Following this interval, the Terminating FAX machine will initiate the Pre-Message Identification procedures by transmitting a 300 BPS “Line Turn-around” preamble.

Originating FAX Operating Modes

  • When a FAX machine is operating in an AUTOMATIC fashion, the following steps occur:
    1. Go Off-Hook (2-Wire Loop Closure)
    2. Detect Dial Tone ** See Note Below **
    3. Dial Number
    4. Transmit CNG
    5. Wait For 300 BPS Preamble/ID signal from Terminating unit*.
    6. Enter Pre-Message State
         * NOTE:  The timeout values and response actions for not 
              detecting Dial Tone are not defined; and as such, 
              can vary between FAX machines. The timeout value 
              from dialing to receiving a valid response (the 300 
              BPS Preamble/ID) is defined to be 35 +/- 5 Seconds 
              (30 to 40 seconds.
  • When operated in a MANUAL fashion, the following steps occur:
    1. Operator Goes Off-Hook (2-Wire Loop Closure)
    2. Operator Detects Dial Tone
    3. Operator Dials Number
    4. Operator Waits For Ringing Signal
    5. If the Terminating location is operating in a MANUAL fashion, a verbal exchange between Operators occurs. If the Operator detects the CED signal from a Terminating FAX machine operating in an AUTOMATIC fashion.
    6. The Originating FAX machine is switched into the line by the Operator.
    7. Enter Pre-Message State

Terminating FAX Operating Modes

  • When a Terminating FAX is operating in an AUTOMATIC fashion, the following steps occur:
    1. Ringing (Ring Voltage) is detected.
    2. If the Terminating FAX machine is READY-TO-OPERATE, the call is ANSWERED (Off-Hook); else, the call is NOT-ANSWERED.
    3. An optional recorded announcement may be transmitted by the Terminating FAX machine.
    4. Transmit CED signal.
    5. Enter Pre-Message State.
  • When a Terminating FAX is operating in a MANUAL fashion, the following steps occur:
    1. Ringing (Audible Ring) is detected by the Operator.
    2. The call is ANSWERED.
    3. If the Originating FAX is operating in a MANUAL fashion, a verbal exchange occurs. If the Originating FAX machine is operating in an AUTOMATIC fashion, the CNG tone is detected by the Operator.
    4. The Terminating FAX machine is switched into the line by the Operator.
    5. Optionally, the Terminating FAX machine may generate the CED signal.
    6. Enter Pre-Message State.

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

  • Originating FAX machine will output CNG signal (1100 Hz pulse every three seconds ONLY when operating in AUTOMATIC modes.
  • Terminating FAX is required to output CED (2100 Hz) signal if operating in an AUTOMATIC mode; else, CED generation is optional.
  • When Originating a call from a MANUAL FAX machine, it is important for the Operator to put the FAX machine on-line immediately after detecting the CED from an AUTOMATIC Terminating FAX machine.

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).