Persons with hearing impairments or speech impediments are able to communicate over the telephone using TTY/TDD (teletype/text telephone devices for the deaf), hereafter referred to as TTY devices. These devices typically have a keyboard and a 2-line display, allowing a TTY user to type messages over the phone lines to another TTY user. TTY devices transmit the characters using the Baudot code, a 45.45 bps binary frequency shift keyed scheme that is carrierless, half duplex, and without error protection.
In June 1996, the FCC adopted Section 20.18(c) of the Commission’s Rules. It requires that, as of October 1, 1997, all covered wireless carriers must be capable of transmitting
911 calls from individuals with speech or hearing disabilities through means other than mobile radio handsets, e.g., through the use of TTY devices. The pre-existing digital wireless networks have rendered TTY conversations unintelligible. The frame and bit errors that occur over the air, as well as the variable rate speech coding and noise suppression, all conspired to create unacceptably high character error rates.
The purpose of this standard is to establish minimum performance requirements and a test procedure for verifying solutions for transporting the Baudot code over IS-2000/IS-95 wireless networks. A CDROM containing tools and test vectors for the purpose of performing these minimum performance tests accompanies this standard.
For the remainder of this document, the term test vocoder refers to the speech coder with TTY extension for which compliance with this document is being verified. The term reference vocoder refers to the floating-point standard of that same vocoder with the most recent TTY extension.
This specification uses the following verbal forms: “Shall” and “shall not” identify requirements to be followed strictly to conform to the standard and from which no deviation is permitted. “Should” and “should not” indicate that one of several possibilities is recommended as particularly suitable, without mentioning or excluding others; that a certain course of action is preferred but not necessarily required; or that (in the negative form) a certain possibility or course of action is discouraged but not prohibited. “May” and “need not” indicate a course of action permissible within the limits of the standard. “Can” and “cannot” are used for statements of possibility and capability, whether material, physical, or causal.
