Textphone

A textphone is a device used by the deaf and/or hard-of-hearing in order to communicate over PSTN or IP networks. The first working textphone was demonstrated on June 23, 1964 at the Alexander Graham Bell Association for the Deaf in Salt Lake City, Utah. There are three men attributed to bringing this device to the market: James C. Marsters, a deaf dentist from Pasadena, Robert Weitbrecht, a deaf physicist at Stanford, and Andrew Saks, a deaf businessman from the Bay area.

There are a variety of PSTN textphone devices used around the world with several standards. The table below summarizes the standards in use:

Protocol	Rate	Countries
Baudot (TIA 825-A)	45.45bps	USA, Ireland, South Africa
Baudot (TIA 825-A)	50bps	Australia
Bell 103	300bps	USA
EDT	110bps	Germany, Switzerland, Austria, Italy, Spain
V.23	1200bps	France, Belgium
V.21	300bps	United Kingdom, Iceland, Norway, Denmark, Sweden
V.18 (native)	300bps	United Kingdom, Australia (planned)
DTMF	n/a	Denmark, Holland

The ITU-T and IETF created standards for transporting text over IP networks. ITU-T Recommendation V.151 define how to transport TTY signals over IP networks between two PSTN gateway devices. It utilizes RFC 4351 as the transport, interleaving voice and text packets to align with what is received from the PSTN circuit. The IETF also approved RFC 4103, which defines how to transport text as its own RTP session.

To enable communication for users of TTY devices, gateways must support ITU-T Recommendation V.151 (RFC 4351). Native IP devices (including softphones and hardware IP phones) should implement RFC 4103, as the preferred mode of communication for text. Additionally, such IP devices should also implement RFC 4351 to allow proper interconnection with legacy TTY devices.

Resources: ToIP Information Site