ZyXEL Elite 2864 User Guide - Page 178

of data. It is the recommended 3-bit ADPCM scheme. It will automatically recover from data errors when playing without producing a noisy voice signal. The modem also supports simultaneous DTMF (Dual Tone Multiple Frequency) tone detection, thus enabling the use of the keypad of a tone-dialing push-button telephone set to control, instruct, or answer the modem. Automatic Detection of Voice, Fax and Data Since there is no standard and there is no way to specify how a human voice should behave in telephone calling and answering, it is difficult, if not impossible, to automatically make a distinction between voice, fax, or data calls. A common method of accomplishing this detection and switch-over is the use of a human voice announcement as the first step in the answering process. The calling party is instructed to push one number in the push-button pad for fax calling and to push another number for voice calling. By detecting which DTMF tone is received, the computer software and the modem can switch to the appropriate mode accordingly. If no tone is received until timeout, the modem assumes the call to be a data call and starts modem handshaking. The shortcoming of this method is that some modems will get confused by the initial voice message announcement and will not get connected. Even if the data call is connected successfully, the longer handshake delay may not be acceptable in some applications. Some software packages use an initial silence in answering and detect if there is voice energy to decide if it is a voice call. The modem can recognize a fax or data call quicker if a calling tone is received. The voice announcement can be omitted in this case. ZyXEL will continue to work on improvements of the automatic detection of voice, fax and data. To better support the distinction between voice, data and fax calls, a new feature called EDR (Extended Distinctive Ring) has been added to the firmware of all of the modems of the 2864 series. Refer to chapter 13 and S-registers S40 and S51 regarding this feature. New capabilities or enhancements will be released in future versions of the firmware. Voice States and Operation Modes The Voice Mode DCE control interface for the 2864 series modems basically follows the TIA TR29.2 committee IS-101 Interim Standard. ZyXEL is continuously enhancing its modems' voice capability and voice feature implementation. Please refer to the manual amendment or firmware release notes for updated details. ZyXEL reserved S-register S48b5 for the IS-101 compatibility flag. When this bit is off (default), the command set is compatible with the U-1496 modem. If this bit is set, the 2864 series is compatible with the IS-101 standard. 16-2