Detailed description of Supporting TTY Callers

Last Updated : Oct 30, 2012 |

Reliable transmission of TTY information complies with the requirements and guidelines that are outlined in United States accessibility-related laws. Those laws include:

  • Titles II, III, and IV of the Americans with Disabilities Act (ADA) of 1990.

  • Sections 251 and 255 of the Telecommunications Act of 1996.

  • Section 508 of the Workforce Investment Act of 1998.

Communication Manager TTY support is currently restricted to TTY devices that use either the:

  • US English standard TTY protocol, specified by ANSI/TIA/EIA 825 as: A 45.45 Baud FSK modem.

  • UK English standard TTY protocol, Baudot 50.

Important characteristics of the standards are:

  • TTYs are silent when not transmitting. Unlike fax machines and computer modems, TTYs have no handshake procedure at the start of a call, nor do they have a carrier tone during the call. This approach has the advantage of permitting TTY tones, DTMF, and voice to be intermixed on the same call.

  • The ability to intermix voice and typed TTY data on the same call. The most common usage is by people who are hard of hearing, but who can speak clearly. These people often prefer to receive text on a TTY device, and then speak in response. This process is referred to as Voice Carry Over (VCO).

  • Operation is half duplex. TTY users must take turns transmitting and typically cannot interrupt each other. If two people try to type at the same time, two TTY devices might show no text at all or show text that is unrecognizable. Also, no automatic mechanism exists to let TTY users know when a character that the user correctly typed was received incorrectly.

  • Each TTY character consists of a sequence of seven individual tones. The first tone is always a start tone at 1800 Hz. This tone is followed by a series of five tones, at either 1400 or 1800 Hz, which specify the character. The final tone in the sequence is always a stop tone at 1400 Hz. The stop tone is a border that separates this character from the next.

The following types of systems support TTY communication:

  • Analog telephones and trunks

  • Digital telephones and trunks

  • VoIP gateways

  • Messaging systems

  • Automated attendant systems

  • Interactive Voice Response (IVR) systems

  • Wireless systems in which a TTY-compatible coder is used