Voicepath GSM modem a single wave file.

Question

When a GSM modem is receiving voice(i.e., multiple wave files continuously) in the voice path, how will the modem able to identify the completion of a single wave file? Is there any software that could perform this?

Answer 1

The modem doesn't actually send wave files (in the sense of .wav files), because these require a header that describes how long the file is. Instead, the modem keeps sending raw data until either the call ends or the computer tells it to stop. The modem signals the end of the data with a DLE byte followed by a ! - see the wikipedia article about voice modem commands for details of this (and for how a DLE byte is sent in the datastream).

As for converting the data to .wav or another usable format: many audio conversion programs can do this; soxis one such command-line program.

Answer 2

A GSM mobile station may support different bearers. Depending on device capability this includes

• voice
• circuit switched data
• packet switched data

A mobile phone typically supports voice, CS data and PS data. A GSM modem may support PS data only. Each of these bearers are implemented differently in order to utilize the radio channel effectively.

You can transport a WAV file via an packet switched bearer, however then it will be transported using IP technology. This needs an application on both endpoints handling the transport and the presentation to the user, e.g. playout.

A voice bearer supplies a continuous audio stream, starting with connecting the call and ending with terminating the call. GSM does not support discrete portions of audio on the voice bearer. Playing a WAV file on the voice bearer is a non-standard functionality for virtually all commercial devices. You may need to find test devices supporting this.

If you use the voice bearer to play out the WAV file like a normal telephone call and want to detect completion of a playout you may detect a pattern in the audio (requires an algorithm to calculate similarity since your WAV file will be encoded several times and may be mixed with noise) or use some sort of out-of-band signalling, some phones support dual mode, i.e. running voice bearer and PS bearer in parallel. Both not trivial to do.