Text Telephony

Description

Text Telephony is a 3GPP service defined as an audiovisual conversation service that provides bi-directional real-time transfer of text and optionally audio between users in two locations. The audio may be transmitted alternating with text or simultaneously with text. This definition is sourced from ITU-T F.703, which specifies the general principles for audiovisual conversation services. In 3GPP, Text Telephony is specified in TS 21.905 (Vocabulary) and is closely related to Text Conversation (TS 22.226/23.226). It is designed to facilitate communication for individuals who are deaf, hard of hearing, or have speech disabilities, allowing them to use text as the primary medium while optionally including voice.

The service operates over a telecommunications network, typically using a real-time text protocol such as ITU-T T.140 for text transport. In a 3GPP context, Text Telephony can be implemented over circuit-switched networks (using GSM text telephony features) or over IMS (IP Multimedia Subsystem) using RTP and SIP. The key characteristic is the real-time, character-by-character transmission of text, which ensures that the conversation flows naturally. The optional audio component allows a hearing user to speak while the deaf user types, and vice versa, or both can use text simultaneously.

Architecturally, Text Telephony involves the establishment of a session between two terminals that support the service. The session setup includes negotiation of media types (text and audio) and their parameters. The text is carried in a dedicated channel or as part of a multimedia stream. The service may also support interworking with legacy text telephones (e.g., TTY devices) through gateways. 3GPP specifications define the procedures for call setup, text transmission, and interworking with other networks.

Text Telephony is a critical accessibility feature, ensuring that telecommunication services are inclusive. It is often used in conjunction with video telephony to provide total conversation, which combines voice, video, and text. The service requires low latency and high reliability to maintain the conversational nature. It also needs to handle mixed media synchronization when audio and text are used simultaneously.

In terms of evolution, Text Telephony has been part of 3GPP specifications since Release 5, although the concept existed earlier. The move to IMS and IP-based networks has enabled more flexible and feature-rich implementations. The service continues to be relevant in the 5G era, where real-time text can be integrated into enriched calling services. 3GPP continues to maintain the specifications to ensure interoperability and accessibility.

Purpose & Motivation

Text Telephony was created to provide a standardized telecommunication service for individuals who cannot use voice communication effectively. Before its standardization, text telephony was often implemented using proprietary or region-specific technologies, such as TTY in North America or minicom in the UK. These solutions were not interoperable and lacked the ability to combine voice and text seamlessly. 3GPP's adoption of the ITU-T F.703 definition brought a unified approach, enabling global interoperability and integration with modern networks.

The service solves the problem of accessibility by allowing real-time text communication with optional voice, which is essential for deaf, hard-of-hearing, and speech-impaired users. It also addresses the need for emergency communication, as text telephony can be used to contact emergency services. The inclusion of audio allows a hearing person to communicate with a deaf person without needing a relay service, as the deaf person can type while the hearing person speaks.

Historically, the introduction in Release 5 coincided with the evolution towards IMS and the need for multimedia services. Text Telephony leverages these advances to provide a richer communication experience. It also aligns with regulatory requirements for accessibility in many countries. The service continues to evolve, with ongoing work to support real-time text over 5G and to ensure interworking with emerging text-based services.