Cellular Text Telephone Modem
CTM ( Cellular Text Telephone Modem ) is a method standardized by the 3GPP for the reliable and changing transmission of text and voice over the normal voice channel in cellular cellular networks or in the PSTN .
target
Deaf, hard-of-hearing and speech-impaired people use a special writing telephone (TDD - Telecommunications device for the deaf) to transmit text over the voice channel of the traditional landline network . However, modern digital and cellular telephone systems do not offer satisfactory error rates for the transmission of text, since the coding and compression methods are optimized for voice signals and distort the modem signal too much. CTM should therefore offer a solution for all text telephony systems worldwide by improving the modulation techniques including FEC, interleaving and synchronization. The following requirements must be met by the new transmission method:
- Text telephony is a recognized method of making an emergency call. This functionality must be supported.
- CTM must be able to work with current and future GSM codecs.
- CTM must be applicable to existing and future voice channels in GSM and UMTS.
- The effects on cellular networks should be minimal, the voice transmission must not be delayed and the text transmission a maximum of 800 ms.
- Roaming between different operator networks should be possible without text loss.
- A transmission rate of 10 characters / s should be achieved with a net bit rate of 80 bit / s.
construction
The CTM adapter can be integrated directly into the mobile phone or the TDD of the mobile subscriber; an external adapter can also be used. In the case of a landline subscriber, the signal adjustment for CTM must, however, be processed beforehand within the telephone network. The CTM adapter contains a CTM transmitter and a CTM receiver. Incoming text-telephone characters are recognized by the text-telephone detector and converted into a robust signal by the CTM transmitter, which can then be transmitted over the voice channel. Voice signals are passed through from the transmitter without modification by closing switches S1 and S3, which are marked in Figure 1. In the same way, a valid CTM signal is transformed back in the CTM receiver and audio signals are only passed through.
In the CTM transmitter, the incoming text is transformed by 5 different components. The text coming from the text telephone detector is first converted into UTF-8 characters and then transmitted serially for subsequent error protection. An r = 1/4 convolutional encoder is used in the FEC component, i.e. 1 net bit is mapped to 4 gross bits. Then synchronization information is inserted and the generation of a burst is initiated. Since voice activity detectors are used in cellular telephone systems, which could classify a continuous data stream as “no voice”, an 80 ms long mute interval must be periodically inserted at this point in the CTM transmitter. The data is then better protected against the high error rates of the radio channel by interleaving and scrambling and sent to the CTM modulator. There the signal is finally frequency-modulated, whereby the predefined frequencies 400 Hz / 600 Hz / 800 Hz / 1000 Hz and 0 Hz for "mute" are taken by assigning 2 bits each. Control characters can be inserted at any point within the character stream, where <ENQUIRY> is used to determine CTM support at the communication partner or for resynchronization if the connection is interrupted. Another control character <IDLE> is used as a filler if the current burst is to be continued, but the user is not currently entering any characters.