Support of TTY Service Over IP Using Global Text Telephony
|Publication Date:||1 August 2017|
This Technical Report (TR) describes the means by which the Teletypewriter (TTY) service can be provided over IP between operators' networks through the use of the Global Text Telephony (GTT) capability which enables simultaneous audio and/or video with text media stream.
TTY service allows real time conversation in text between two persons having a baudot-capable device. This service is supported through the Circuit Switched (CS) public network. Although new Internet technologies have reduced the need for this service, it still plays an important role, especially for emergency 9-1-1 calls.
Real-Time Text (RTT) is a term used to define the ability to instantly communicate text as it is typed, as opposed to after a sentence or thought is completed, in the manner of instant messaging. This term has now been replaced with Fast Text. RTT can now be signalled over IP networks. RTT can be optionally combined in any combination with voice and/or video. When this combined service is provided by an IP Multimedia Subsystem (IMS) network, it is referred to as GTT. GTT is supported in IMS networks via RTT capability using IETF SIP/SDP for the negotiation of the text media stream and IETF RFC 4103 [RFC 4103] RTP-text for transport, with text coded according to ITU-T Recommendation T.140 [T.140].
Figure 1.1 shows a generalized view of the GTT feature architecture. It combines different networks and network types and integrates text conversation systems already existing within these networks.
Throughout the remainder of this document the following terms will be used as described:
• TTY - this will be used in reference to the actual service. The means to support the transport of text for this service is technology dependent: i.e., Baudot for circuit and RFC 4103 [RFC 4103] text for packet.
• GTT - this will be used to refer to the ability to support audio and/or video streams simultaneously with text (RTT) in an IP network.
• RTT - this is used to refer to the text stream in an IP network as encoded per RFC 4103 [RFC 4103].