The cellular text telephone modem - the solution for supporting text telephone functionality in GSM networks

Text telephone devices are text-based terminals that allow the users to communicate by text via fixed-line telephone networks. Since cellular phone systems are sometime subject to severe radio channel impairments and the modem signals of these text telephones are therefore not always transmitted reliably, the Federal Communications Commission (FCC) has required a solution to guarantee a reliable transmission of text telephone data for emergency calls via cellular phone systems. For the North American PCS-1900 cellular phone systems recently the Standards Committee T1 has standardized a solution for this requirement, which is based on a new modem protocol, the Cellular Text Telephone Modem (CTM), whose signals can be reliably transmitted via the speech channel of cellular phone systems. After a short introduction into text telephony, this contribution provides a description of this solution for PCS-1900 systems using CTM signals. The solution is indeed independent of the cellular system and works on de-facto all speech channels.