ZyXEL Communications COMET 336 User Manual page 59

Data/Fax Discrimination
If the DTE wishes to allow for a data or fax call, the +FCLASS and +FAA or +FAE
commands can be configured for adaptive answer between data and Class 1 or Class 2
fax.
Voice/Fax Discrimination
This is the most important discrimination capability needed from the user's standpoint.
The modem must be configured for Voice/Audio (#CLS=8), causing the modem to enter
Online Voice Command Mode immediately upon going off-hook. In Voice/Audio Mode,
the DTE automatically receives indications of DTMF tones and Calling Tones. The
DTE can now switch to Voice Transmit Mode in order to play a greeting message,
perhaps one which instructs the caller how to enter specific DTMF sequences to switch
modes. The DTE can then react to the response, or the lack thereof, to such a message.
The modem supports switching to Class 1 or Class 2 answer mode by virtue of the
#CLS=1 or #CLS=2 command, and if such a switch is made and fails, the modem
reports the failure but does not hang up, allowing the DTE further experimentation time.
If the user wishes to switch to Class 1 or Class 2, but also wants the DTE to indeed
hang up the line if the fax fails, the +FCLASS command should be used instead of the
#CLS command. The only difference between these commands is that issuing +FCLASS
cancels the modem's memory of voice, whereas #CLS causes the modem to remain off-
hook, even if a fax or data handshake fails, until it receives an H command.
Voice/Data/Fax Discrimination
The DTE can try data modem operation after an answer by changing the #CLS setting to
0. A data handshake attempt can be added based upon DTMF responses or lack thereof.
7.2.3 Voice/Audio Data Transfer
A significant area of concern when handling the transfer of voice/audio data is the data
transfer rate on the modem/DTE interface. Data transfer rates can be expressed as the
number of interrupts which must be serviced per time period to keep up. This is a
function of the sampling rate and compression method (if any) used by the modem, and
the DTE interface speed required to handle the data flow on the telephone line side.
Tables 7-1a and 7-1b demonstrate the relationship between the sampling rate, interrupt
rate, and DTE speed necessary for the modem to support various compression ratios.
The built-in 16C550A UART-compatible interface in the MCU hardware is required to
support Voice/Audio Mode operation in the Microsoft Windows environment.
Table 7-1. DTE Speeds a. 7.2 kHz Sample Rate
Modem Samples per 8-bit Byte
Required Asynchronous DTE Speed (8N1 Format)
Implied DTE Speed Supported
1 (8-bits)
52 Voice Operation