Real time, character by character text conversation is a component that increases usability in a conversational session. Global Text Telephony is a feature that adds the capability to use a text conversation component in a session. It is called Global Text or GTT here. Interworking with corresponding features in other networks is an important part of Global Text. Specifically, the different kinds of PSTN text telephone systems supported by the international text telephone modem standard ITU-T V.18 are included in the modes for interworking. One important reason to offer the Global Text feature is to enable emergency service access to people who are depending on a written dialogue. A more elaborated background is found in Annex A, and requirements in Annex B. Figure 1 gives an overview of the user environment for Global Text.

This Technical Specification defines the stage one description of the Global Text Telephone Feature, GTT.. Stage one is the set of requirements which shall be supported for the provision of the real-time text conversation feature, seen primarily from the subscriber's and service providers' points of view. This TS includes information applicable to network operators, service providers, terminal and network manufacturers. This TS contains the core requirements for the Global Text Telephony feature, which are sufficient to provide a complete feature to incorporate in conversational services. This TS defines the requirements for GTT to be understood as a framework to enable real-time transmission of text, for the purpose of a text based or text supported conversation between users. Text may be transported alone or in combination with other media in the session, especially video and voice. Thus the GTT enables text conversation to be included in any mobile conversational service. Interworking with existing text telephony in PSTN as well as emerging forms of standardised text conversation in all networks is within the scope of this document. Interoperation with Multimedia Messaging Services is also within scope of this feature.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

The following list gives the high level requirements of the GTT. These are requirements which are independent of the user's perception of the feature:-

• Service definition
  Global Text shall provide a real time conversational text feature. A general definition is found in Recommendation ITU-T F.700 [5].
• Global Text Telephony host environments
  A standardised method for Global Text Telephony shall at least be specified for each host environment in the mobile networks that can carry voice.
• Standards compliant and forward compatible text conversation
  Global Text Telephone mechanisms shall provide the capability to support current and evolving text telephony and text conversation features by re-using existing standards as far as possible and proposing extensions (as necessary) to existing standards (i.e. the global text telephony feature shall support the evolution of Total Conversation technologies in all networks).
• Consistent text conversation
  Regardless of the selected host environment, the far end terminal, the interworking facilities involved, GTT shall be capable of providing a consistent way of handling the conversation.
• Global Text Telephone access
  Within the capabilities of networks and terminals, the user shall be provided consistent access to the GTT regardless of the access point.
  For example: the user shall be capable of accessing the text conversation features through a number of different access points, including networks based on 3GPP specifications, access through dedicated multimedia terminals, and access through combinations of mobile phones combined with text user interface devices.
• Interoperability
  Global Text Telephony shall support interoperability with existing and emerging text telephone systems and text conversation features.
  Global Text Telephony shall support a minimum set of environments where text conversation is supported to ensure full interoperability between different terminals and networks.
• Emergency calls
  The implementation of Global Text Telephony shall enable a user to make emergency calls to, and receive calls from, an emergency call centre via a text telephony device used in conjunction with GTT enabled user equipment.

Network operators have many differing requirements, and GTT shall be supported in the network in a manner which allows network operators to consider different configurations depending on their network and commercial requirements. Thus, an identified set of functionalities and protocols shall be standardised to ensure interoperability across networks and terminals to support GTT. The following general requirements shall be supported.

The user shall be able to use and access GTT in a secure manner. Within the limitations enforced by the host environments, the following apply.

• It shall be possible for the contents of GTT sessions to be read only by the intended recipient(s).
• The "Security Threats and Requirements" specified in TS 21.133 shall not be compromised.

It is foreseen that users will not be expected to pay a premium for access to the GTT service. Hence the requirement to capture detailed information relating to the use of GTT is seen as optional. Various charging mechanisms may be supported in addition to what is defined for the host environment, the following charging characteristics may be considered:-- Text was used in the call.- The duration for which text was used in a call.

It shall be possible to use GTT to communicate with similar features in other networks as specified in ITU-T H.248 Annex F, in the Text Conversation and Text Telephone packages [3]. This includes:

• PSTN text telephones, using voice telephony as a carrier according ITU-T V.18 [6].
• IP text telephones, using IP multimedia protocols and text according to ITU-T T.140 [4].
• IP conversational multimedia terminals using IP multimedia protocols with text according to ITU-T T.140 [4].
• CS multimedia with text according to ITU-T T.140 [4].

There is no requirement that the host environment combinations for interworking shall be extended for GTT support beyond the host environment interworking established for other media. For interworking with text conversation in mobile networks outside the scope of the GTT specifications, interworking shall be specified as for other external networks. For mobile services within scope of the GTT specification, interworking shall be possible between different forms of GTT, within the limits of interworking between the different host environments. GTT shall permit use of the supplementary services defined for the host environment. Users of both GTT and Multimedia Messaging [7], [8], should be provided the following interoperation opportunities:

• If a call with a text component fails, it should be possible to offer the user to send a message to a MultiMedia Messaging facility such that the message may later be delivered in non-real time to the destination.
• External User Equipment interface specifications should enable both GTT and MMS text handling functions to be placed in the same device.

The GTT feature may be implemented in a number of ways. The GTT specifications shall allow cost effective implementations both for networks with a small part of the subscribers using GTT as well as networks with relatively many subscribers using GTT.

Text telephone systems were implemented mainly for distant conversation with deaf, hard-of-hearing, speech-impaired and deaf-blind users. The text telephone systems offer a real time, character by character, conversation in text, optionally combinable with voice. With proper implementation in mobile systems, the feature is expected to be of interest for any user. The general user needs are described in ETSI ETR 333 Human Factors, Text Telephony; User requirements and Recommendations [2]. In PSTN, seven different, openly specified systems for text telephony exist, and are used in different regions. Some proprietary modes are also used. The open specifications are called Baudot, DTMF, EDT, V.21, Bell103, Minitel and V.18. They all use different modem technologies and character coding for the transmission of text. They are briefly described in the annexes of ITU-T V.18 [6]. ITU-T V.18 [6] is an automoding mechanism that enables communication with all the legacy modes on the modem level. It also detects when both parties have V.18, and invokes an internationally useful character set, defined in ITU-T T.140, and optionally transmission of text and voice simultaneously. Native modulation for V.18 is V.21, while V.61 is used when simultaneous voice and text is wanted. Other modulations can be negotiated. V.18 and T.140 are intended for use in new PSTN text telephones. They are also intended for use in gateways, bridging from the fragmented situation in the PSTN, into new services, designed according to the Total Conversation concept.

This section is informative and specifies the initial requirements that formed the base of definition of GTT. The 3GPP GTT specifications together define what requirements will be met. Most requirements in this chapter are derived from the requirements expressed to be urgent by the FCC and the CTIA TTY forum in USA. The relation with these requirements are documented for each requirement. Many of these requirements are written with the view that a PSTN text telephone is attached to the mobile station for implementation of the basic GTT functionality.