Data transmission over voice dedicated channels using digital modulations

With the actual expansion in communication systems, data channels are taking more importance, especially in mobile data networks. But in some cases the transmission over data channels is not possible or not convenient, which makes the data transmission over voice channel an interesting alternative. Firstly, the operators of mobile phone networks (GSM, GPRS) tend to limit the access to the Internet in order to prevent Voice over IP (VoIP) for economical reasons. Secondly, the cross- networks bridges used in VoIP (VoIP-ISDN, VoIP-GSM, VoIP-PSTN) give an opportunity to transmit data over this voice channel without any additional data bridges. Thirdly, for security matters, transiting data dissimulated in a voice-like signal is more discrete. Lastly, some communication networks such as THURAYA are dedicated to voice communications.