5 General Requirements

22.2263GPPGlobal Text Telephony (GTT)Release 17Stage 1TS

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.

5.1 Text conversation host environments

The protocol environments for text conversation shall be the same as the ones used for other multimedia conversation calls and voice calls. The selected environment for a session is called the host environment.

Supported host environments are:

– Packet switched multimedia

– Circuit switched multimedia.

– The text conversation carried by an data transmission procedure, possible to combine with a voice session.

– Circuit switched voice telephony, with the text conversation carried in-band in the voice channel.

5.2 Text conversation management

It shall be possible to allow GTT use without GTT specific subscription.

Note: There is no requirement for GTT feature specific subscription, but for "normal GTT calls" a subscription for telephony is required. For "emergency GTT call" no subscription of any kind is required.

Session start up delay and loss of initial characters should be minimised as far as practicably possible.

The text transport method should remain transparent to the network, unless specific functionality is required to provide a satisfactory quality of service.

5.3 Registration of text conversation capabilities

The availability of text conversation capabilities in the user terminal may need to be communicated to the network at call establishment in order to access the GTT service. However, a user is expected to be able to access the GTT service without explicit GTT registration with the operator.

5.4 Session control

The session control (call control ) functions shall use the same procedures as the selected host environment.

Additional operations needed to invoke the text component may be automated by the text conversation user interface.

It shall be possible to establish text-only calls as well as calls with other media components.

5.5 Invoking text conversation

Calls with terminals where text conversation is used shall bemonitored for requests to start the text feature.

When either party in a call begins usage of the text feature, an effort shall be made to establish text conversation. It shall be possible to start text conversation during a call already established in voice or video mode by adding the text component.

The mode selected for text conversation ( IP Multimedia, CS Multimedia, Voice path etc. ) shall be determined according to a prioritised procedure.

Modes allowing simultaneous voice and text shall by default have higher priority, while the user shall be given opportunities to prioritise text-only modes. The default order shall be: IP Multimedia, CS Multimedia, data path text plus voice, voice path text.

The user shall be given call progress information and text invocation progress information in text or other non-audible media.

5.6 Text conversation handling during calls

Text transmission shall be done character by character as entered, or in small groups transmitted so, that no character is delayed before transmission more than 0.5 seconds. (as stated in ITU-T T.140 [4])

The text transmission shall allow a rate of at least 10 characters per second so that human typing speed as well as speech to text methods of generating conversation text can be supported.

The end-to-end delay of characters shall be less than two seconds, measured between two mobile users, or between one mobile user and any interworking fixed network user, assuming that the fixed network does not contribute with more than one second to this figure.

The character corruption rate should be less than 1% in conditions where users experience the voice transmission quality to be low but useful.

The transmission of the text conversation shall be made according to the character set and operations defined in ITU-T T.140. [4]. This requirement enables smooth interworking with minimal loss of functionality between different text conversation environments. The allowed limitations in character set support specified in ITU-T T.140 shall be obeyed, so that two terminals always have a minimum common character set available for conversation.

Figure 2 gives an example of a possible layout of a real time text conversation according to ITU-T T.140.



Hi, this is Anne.

Yes, have you heard that I will come to Paris in November?

Oh, hello Anne, I am glad you are calling!
It was long since we met!

No, that was new to me. What brings you here?

Figure 2: A possible way to display a conversation with one window each

Display of the conversation shall be done according to the principles specified in T.140, unless the display is provided through interworking with a legacy mode text telephone, when the limitations of that device may govern.

NOTE: It shall be noted that support for any written language is possible with T.140, and that many languages require other writing directions than left-to-right.

5.7 Alerting

Since many GTT users are deaf or hard-of-hearing, the terminals used for GTT shall provide an interface that can be used to activate alternative alerting modes e.g. flash or vibration.

5.8 Addressing

GTT shall support the addressing formats of the host environments. to identify the different kinds of endpoints of the call used in the different host environments (see section 9, interworking.)

5.9 Roaming

Access to the GTT service while roaming is desirable.

Note: Operators may consider using a Virtual Home Environment toolkit to provide GTT services to roaming subscribers. It can be expected that the subscriber will need to specifically subscribe to this service and no access to the GTT service will be possible if the visited network does not support the appropriate VHE toolkit. It should also be noted that emergency calls bypass most VHE toolkits, therefore emergency calls will require GTT support in the local network.