7 Text Telephony architecture based upon CTM

26.2263GPPCellular text telephone modemGeneral descriptionRelease 17TS

An overall architecture for supporting traditional text telephone communication via cellular telephone systems is depicted in figure 1. This Technical Specification covers the gray shaded transmission and receiving parts of the Cellular Text Telephone Modem (CTM). The text telephone detector may act as specified in V.18 [2].

This section provides an overview of the CTM transmitter and the CTM receiver. The Cellular Text Telephone Modem is fully specified in the following sections. The ANSI C-Code for a bit-exact implementation is provided in [3].

Figure 1 – One Text Telephony architecture using CTM

(a) traditional text telephone on both subscribers’ sides, CTM acts as a
signal adaptation device between text telephone code and CTM signals

(b) alternative solution for the mobile subscriber’s part

7.1 Text Telephone devices

Text telephone devices are text-based terminals that allow the users to communicate by text, character by character via telephone networks. A text telephone device for PSTN consists of a keyboard, a text-oriented display and a modem, which transforms the text telephone characters into audio signals that can be transmitted via the telephone network. The protocols used by text telephone devices vary regionally. The major formats supported with text telephone devices are described in ITU-T V.18, reference [2].

7.2 Error resilient transmission in the speech path

The CTM transmitter transforms the text telephone characters into a signal that can be transmitted robustly via the speech codec and the radio transmission path of cellular phone systems. The corresponding CTM receiver decodes the signal back into text telephone characters. CTM signals are audio signals, which can be transmitted into the analogue domain or coded into PCM samples. Therefore, there is no requirement that the CTM and the speech encoder or decoder have to be physically integrated.

The text characters are coded in ISO 10 646-1 UTF-8 [6] according to ITU-T T.140 [5] for the transmission via the CTM link.

7.3 Interworking with traditional Text Telephone devices

Figure 1 (a) shows the situation that either side of the call has a traditional text telephone, which sends and receives speech and modem signals alternately in analog or PCM coded form. Therefore, an interworking function is needed to detect and to regenerate the modem signals for these traditional text telephones. This interworking function is called "text telephone detector/regenerator" here. For PSTN text telephone interworking, the text telephone detector/regenerator can be based on V.18 to cover all major text telephone systems worldwide or be made specific to the text telephone protocol supported. Within North America the text telephone detector/regenerator may be limited to detecting and regenerating Baudot code.

The text telephone signal can be adapted for use over the radio interface by a combination of a text telephone detector and CTM transmitter at one end and a corresponding CTM receiver and text telephone regenerator at the other end. On the mobile subscriber’s side, this adaptation might take place in a) an external adapter (analog in / analog out), which connects the text telephone to the mobile phone, or b) might be integrated into the mobile phone or c) into the text telephone device itself. For the PSTN subscriber’s side, the signal adaptation has to take place in the network in order to guarantee interworking with existing, traditional PSTN text telephone devices. Note that both signal adaptation devices – on the mobile subscriber’s side as well as on the network side – are functionally identical. In case of several CTM adapters in the speech path the unnecessary ones are negotiated into passive mode. In this rare case there exists a possibility of a few corrupted or lost characters during negotiation.

The signal adaptation devices in Figure 1 (a) remain in a passive (transparent) mode, if there are no text telephone characters to transmit. For the uplink at the mobile subscriber’s part, the CTM transmitter generates a zero‑valued output signal and both switches S1 and S3 are closed as long as the text telephone detector does not receive a traditional text telephone modem signal. For the downlink at the mobile subscriber’s part, the text telephone regenerator creates a zero‑valued output signal and both switches S2 and S4 are closed as long there is no valid CTM signal at the CTM receiver’s input. This guarantees that any other audio signal (e.g. speech) can pass in both directions without any modification. This allows the users to transmit speech and text alternately ("Voice Carry Over/Hearing Carry Over (VCO/HCO)").

An alternative implementation for the mobile subscriber’s side is shown in Figure 1 (b). Instead of a traditional text telephone, this implementation uses a text conversation user interface (e.g. keyboard and display) that is connected directly digitally to the CTM transmitter and receiver.

For interworking with PSTN text telephones on either side of the connection, the following functionality may be provided. The peripheral functions, which have to be added to the CTM functional blocks for supporting text telephone interworking, are described in ITU-T V.18. Reference [2] specifies general methods for detection of text telephone signals on the Modem side. ITU-T T.140 [5] specifies a common general text conversation format. Together these Recommendations should be used to provide translation between legacy modes and the common format..

For digital Text Telephone methods, such as IP text telephony, similar functionality can be arranged.

ITU-T H.248 Annex F [7] describes packages for addition to the general Gateway protocol H.248 for text telephony, text conversation and call discrimination. It describes the control of detection/translation mechanisms that may be applied..