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 at either 45.45 bps or 50 bps using the Baudot code, a 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, 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 cdma2000 [7] wireless networks using 3GPP2 13K Vocoder [2], EVRC [1], SMV [8], EVRC-B, EVRC-WB, and EVRC-NW [1], corresponding to Service Options 17, 3, 56, 68, 70, and 73, respectively. An electronic package 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 C simulation of the vocoder with the most recent TTY extension, which is provided in the accompanying electronic package.