> 2.3 | Software Requirements

> In TTY mode data is transmitted using two tones to represent the asynchronous serial data. Characters are preceded by a start bit and followed by a stop bit.
> The TTY/CTM functionality requires audio mode 5 or 6 with all audio parameters set to their factory default. Regardless to which audio interface the CTM device is connected to, you can use the AT^SNFS command to set your GSM engine to audio mode 5 or 6, for details see [2]. You may need to switch to factory default settings which can be done by using AT^SNFD. For further information refer to [2]. Please note that the factory audio mode settings are sometimes replaced by customer specific audio mode parameter sets. You should ensure that there is at least one audio mode available that corresponds to factory mode 5 or 6, i.e., a mode where any additional audio signal processing is switched off.
> In case you wish more detailed information the following standards pertaining to TTY/CTM equipment might be useful

> • | 3GPP TS 26.226 (ETSI TS 126 226)

> • | 3GPP TS 26.231 (ETSI TS 126 231)

> and can be retrieved from http://www.3gpp.org/specs/specs.htm
> Connecting cellular devices
> To connect your GSM engine to another cellular device both, the GSM engine and the mobile device need to connect to a CTM modem. The CTM modem allows a reliable transmission of text characters via the speech channel of cellular phone systems. You can call the AT^SNFTTY command to indicate that the audio path is in TTY/CTM mode (see [2]). Figure 2 shows the structure of the cellular phone connection.
> Figure 2: Cellular phone connection
> Connections between GSM network and a public switched telephone network (PSTN)
> To establish a TTY connection between a GSM subscriber and a PSTN subscriber the GSM network must support the TTY/CTM functionality. The CTM modem (see Figure 3) transforms the text characters into audio signals that can be transmitted via the GSM network. To receive the signals at the PSTN text telephone side these signals must be regenerated into TTY signals. To do so, enter the AT^SNFTTY command which indicates to the GSM network by setting the CTM flag that the transformation of CTM signals into TTY signals is necessary, see [2] for more detailed information.
> Figure 3: PSTN connection