I.1 Overview of GTT interworking between the IMS and Circuit Switched (CS) networks

29.1633GPPInterworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networksTS

The support of Global Text Telephony (GTT) is optional, but may be required by national regulatory requirements. If GTT is supported, the procedures described within this Annex shall be applied.

Global Text Telephony (GTT) offers real time conversation in text, optionally combined with voice. GTT is mainly used for distant conversation with hearing or speech impaired users.

GTT is supported in IMS via the Real-Time Text protocol over RTP, using IETF SIP/SDP for the negotiation of the text media and IETF RFC 4103 [124] RTP-text for transport, with text coded according to ITU‑T Recommendation T.140 [123]. See 3GPP TS 23.226 [122] and 3GPP TS 26.114 [104].

In PSTN, different specified systems for text telephony exist and are used in different regions, e.g. Baudot (in US), EDT, V.21, Bell103, Minitel and V.18. They all use different modem technologies within PCM and different character coding for the transmission of text. They are described in the annexes of ITU-T Recommendation V.18 [125]. ITU-T Recommendation V.18 [125] is an international text telephone modem standard with an automoding mechanism that enables communication with all the different kinds of PSTN text telephone systems. Any party of a GTT call may at any time initiate text or send voice. Speech and text may be used in an alternating manner during a conversation on the PSTN. It is also possible that speech is transferred in one direction and text in the opposite direction. However, speech and text can not be used in the same direction at the same time in most sub-modes of V.18.

In the 3GPP CS radio interface, a dedicated CTM modem is used (see 3GPP TS 26.226 [126]), which is terminated within the CS domain and interworked to PTSN inband text telephony format.

Interworking between Real-Time Text over RTP within IMS and PSTN text telephony is provided at the MGCF / IM-MGW by the MGCF triggering the insertion of an Interworking (conversion) function within the IM-MGW which then behaves in accordance with ITU‑T Recommendation V.18 [125] or any of its specific sub-modes.

The support of this Interworking function between IP-based Real-Time Text over RTP and modem based transmission of real-time text is optional both at the MGCF and IM-MGW, but can be required by national regulatory bodies. If this Interworking function is supported, the procedures described within this Annex shall be applied.

The Interworking function in the IM-MGW shall support the detection of text modem signals on the CS side and the conversion between text/modem signals and Real-Time Text over RTP.

The IM_MGW shall support at least one of the sub-modes listed in ITU-T Recommendation V.18 [125] (e.g. Baudot). The support of any of the sub-modes is optional.

The procedures to detect and convert text/modem involve expensive MGW resources. The present GTT interworking procedures intend to allow cost effective implementations by avoiding additional load or resources in MGW for calls not using text telephony (which represent most of the calls).

It is assumed that IMS terminals supporting text media will not automatically offer text media, but that this will be instead governed by terminal configuration options and user interactions to suit the communication preferences and abilities of the user. However, an IMS terminal desiring to set up a GTT call will offer Real-Time Text media, possibly in parallel to voice media. The IMS-MGW shall then provide the conversion between Real-Time Text over RTP and text/modem signals. On the contrary, if the mobile does not request Real-Time Text support, no Interworking function is necessary. An IMS Multimedia terminal configured to use Real-Time Text Telephony but receiving an SDP offer for voice-only media will accept this offer and then send an own subsequent SDP offer adding text media. When receiving such a subsequent offer for text media, the IMS-MGW shall provide the conversion between Real-Time Text over RTP and text/modem signals at the CS interface. On the contrary, if the mobile does not offer Real-Time Text, no Interworking function is necessary.