8.1 Overview

26.2263GPPCellular text telephone modemGeneral descriptionRelease 17TS

The Cellular Text Telephone Modem allows a reliable transmission of text characters via the speech channel of cellular phone systems. The structures of the CTM transmitter and receiver are depicted in Figure 2. The specification of the Cellular Text Telephone Modem holds for implementations on the mobile side as well as for the network side.

Figure 2 – Overview of the CTM Transmitter and Receiver

The CTM transmitter expects text input with the character encoding according to the international character set ISO10646-1. It performs character encoding, FEC error protection, insertion of synchronization information, interleaving and modulation. Additionally, the CTM signal is periodically suspended and the output is muted in order to avoid a condition where any voice activity detectors inside the cellular phone system might classify the modem signal as non‑speech. In case there is nothing to transmit (i.e. no text input present), the CTM modulator generates a zero‑valued output signal and the switch S1 is set to the ‘closed’ position so that a speech or audio signal can bypass the CTM transmitter.

At the receiving end, the CTM signal is detected and decoded by the corresponding functions of the CTM receiver. The decoded characters are available at the CTM receiver’s "text" output, again in ISO10646-1 encoding. If the CTM demodulator does not detect a CTM signal, the speech or audio signal coming from the speech decoder is forwarded via the switch S2 to the "speech" output of the CTM receiver in order to support alternating between text and voice..