Text Telephone or TeletYpewriter

Description

Text Telephone (TTY), also known as TeletYpewriter or telecommunications device for the deaf (TDD), is a service that enables real-time text communication over traditional voice-band channels. In 3GPP systems, TTY support allows mobile devices to generate, transmit, and display text characters during a circuit-switched call. The text is converted into audio frequency tones (using Baudot or other protocols) that are carried within the voice channel. The receiving device decodes these tones back into text for display. This allows two TTY users, or a TTY user and a relay service, to conduct a typed conversation.

Architecturally, TTY functionality in a mobile network involves both the terminal and the network. The User Equipment (UE) must have a TTY-capable application or modem that can encode/decode the text into the specific audio modulation scheme (e.g., 45.45 baud Baudot code). During a CS call, the UE treats the TTY data as a special form of voice traffic. The network's role is primarily transparent; it must preserve the integrity of the audio-frequency tones as they traverse the voice path. This requires careful handling of voice codecs and avoidance of signal processing (like echo cancellation or voice activity detection) that could distort the TTY tones. The 3GPP specifications define specific modes of operation (e.g., TTY over GSM Full Rate, TTY over UMTS AMR) and requirements for the UE to indicate TTY capability and mode to the network.

Key components include the TTY terminal (which can be an external device connected to the mobile phone via an adapter or built-in software), the relay services used for communication between TTY users and voice users, and the network's codec and transmission planning. The service works in real-time, with characters sent as they are typed. A critical aspect is the negotiation and configuration at call setup. The UE signals its TTY capability to the MSC, which can influence the selection of a suitable speech codec and disable certain voice enhancements in the transmission path to ensure reliable TTY tone detection.

TTY's role is as a fundamental accessibility service. It is not a data service like SMS or instant messaging; it is a real-time, conversational service that uses the voice call infrastructure. This ensures it works even in areas without packet data coverage and provides a direct, connection-oriented session. With the evolution to all-IP networks (VoLTE, VoNR), 3GPP has also standardized Real-Time Text (RTT) as a more modern, IP-based successor, but TTY support remains crucial for backward compatibility and interoperability with legacy public switched telephone network (PSTN) TTY devices and services.

Purpose & Motivation

TTY support in mobile networks was introduced to ensure telecommunications accessibility for people with hearing or speech disabilities. Prior to its inclusion, mobile phones were largely inaccessible for real-time text conversation, creating a significant digital divide. The problem it solves is providing these users with functional equivalence to voice telephony, enabling them to use the primary mobile service—voice calls—for communication.

The historical context is that TTY devices have been used on the fixed-line PSTN for decades. As mobile telephony became ubiquitous, it was imperative to extend this accessibility feature to the mobile domain. Regulatory requirements in many countries, such as the Americans with Disabilities Act (ADA) in the U.S., drove the need for standardized implementation. Without standardized TTY support, manufacturers and operators would create incompatible solutions, harming interoperability and user experience.

3GPP standardization addressed the technical challenge of carrying the narrow-band TTY tones through digital voice codecs designed for human speech, which can distort non-speech signals. The purpose was to define a reliable method that works across different radio access technologies (GSM, UMTS) and core networks, ensuring that a TTY call can be handed over between cells and even between 2G/3G networks without service interruption. It also had to accommodate connections to traditional PSTN TTY devices and relay services. While newer technologies like RTT offer superior features, TTY remains a vital bridge technology, and its support is a testament to the industry's commitment to inclusive communication.