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@           FAX CLASS 1 CALL SELECTION SPECIFICATION       @ 
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						U.S. Robotics, Inc. 
						8100 North McCormick Boulevard 
						Skokie, IL 60076-2999 
 
 
	INTRODUCTION 
	------------ 
	The following defines the DCE Control mechanism for 
	U. S. Robotics Data & Facsimile modems that provide  
	automatic received Call Discrimination and Selection. 
 
	This definition assumes that the DTE initiates in a 
	fax state and turns control over to a Data mode 
	program when a data call is detected by the DCE. 
 
 
	DETECTING A FAX CALL 
	-------------------- 
	The DTE software starts by placing the modem in Fax 
	mode by issuing an AT+FCLASS=1, and then turning on 
	Call Selection with AT+FAA=1. 
 
	The modem answers an incoming call using an ATS0 or an  
	ATA from the DTE, and then goes off hook and looks for  
	100ms of 1100Hz of calling tone.  This 1100Hz detector  
	runs for 3.2 seconds.  If a calling tone is detected,  
	the modem sends the "FAX" message to the DTE, and  
	then sends 2100Hz for 3 seconds, followed by V.21  
	channel 2 flags.  A CONNECT response is sent to the DTE  
	at the start of the flag data.  The DTE software then  
	negotiates in Fax mode. 
 
	If a calling tone is not detected during the 3.2 
	second period, the normal Data mode answer tones are  
	sent.  If a connection is established in Data mode, the  
	modem sends the "DATA" message to the DTE, and goes  
	into on-line command mode.  The DTE sends an  
	initialization string, followed by an ATO.  The ATO  
	command instructs the modem to go back on-line and  
	issue the CONNECT [rate] message. 
 
	When the data pump determines that a Data mode 
	connection cannot be established, the modem falls back  
	to Fax mode.  A 2100 Hz tone is sent, followed by V.21  
	channel 2 flags and a CONNECT message. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
	INVOCATION OF MODE 
	------------------ 
	NOTE:   This command is only valid while in Fax mode  
	(+FCLASS=1). 
 
	Command: +FAA=[mode] 
 
		Options:	AT+FAA  =0 Disable Call Selection 
    			        	   =1 Enable Call Selection 
	 
					   =? Returns possible +FAA values 
						 [0,1] 
 
	Command: AT+FAA? returns the current value of +FAA=  
	[0 or 1]. 
 
	NOTE:  The +FAA parameter is set to zero when the  
	AT+FCLASS=0 command is issued. 
 
 
 
	CONNECT RESPONSES 
	----------------- 
 
	Verbal response:  FAX 
	Numeric response: 64 
 
	The FAX connect response is returned after a call 
	has been answered under Call Selection (+FAA=1) 
	and detected as a fax call.  After the FAX 
	response is returned, the DCE sends a 2100 Hz fax 
	answer tone for 3 seconds. 
 
 
	Verbal response:  CONNECT 
	Numeric response: 1 
 
	The CONNECT response is returned after the DCE 
	begins transmitting HDLC flags in the V.21 HDLC  
	transmit state.  This indicates to the DTE that the DCE  
	is ready to receive HDLC T.30 frame data.  The CONNECT  
	response follows the FAX connect message after a fax 
 	call is detected.  If data is not received from the DTE  
	in response to this CONNECT message, the modem will  
	return to the on-hook state.   
 
	Any character from the DTE in response to the CONNECT 
	message will cause the DCE to remain in the off-hook 
	state.  We recommend that the S19 inactivity timer be  
	used to prevent the modem from remaining in the off- 
	hook state if the fax/data software does not properly  
	distinguish the FAX response. 
 
 
	Verbal response:  DATA 
	Numeric response: 63 
 
	The DATA connect response is returned after a call 
	has been answered under Call Selection (+FAA=1) and is  
	detected as a data call.  The DATA message is sent when  
	the DCE successfully establishes a data connection, and  
	switches to the command state.  The ATO command is then  
	used by the DTE to return the DCE to the on-line state. 
 
	Response:  CONNECT [rate] 
 
	The CONNECT [rate] response is returned after the DTE  
	issues the ATO command to return the DCE to the on-line  
	state.  This is in response to a DATA connect message  
	(see DATA response).  The [rate] value is the rate at  
	which the modem connects in answer mode. 
