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TTY Transport Over Packet-Switched Networks for Users

TTY Transport Over Packet-Switched Networks for Users. Norman Williams Gallaudet University RERC on Telecommunications Access December 6, 2011. Who uses the Baudot protocol?. TTYs Single-line captioned telephony (for calls to 9-1-1, where PSAP responses are transmitted as text via Baudot)

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TTY Transport Over Packet-Switched Networks for Users

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  1. TTY Transport Over Packet-Switched Networks for Users Norman Williams Gallaudet University RERC on Telecommunications Access December 6, 2011

  2. Who uses the Baudot protocol? • TTYs • Single-line captioned telephony (for calls to 9-1-1, where PSAP responses are transmitted as text via Baudot) • All-in-one phones combining voice, text, and amplification • Recent estimates based on NECA 2011 report and extrapolation for state TRS funds: 12% of all relay calls are via TTY

  3. Issues when transmitting as audio • Transmitting Baudot tones over VoIP is problematic: • Higher error rate than for voice calls due to bit errors, which can affect 8-bit based characters (5 data bits, 1 start bit, 2 stop bits) • Packet loss can garble TTY streams • No auto correction of Baudot tones • Too much compression of Baudot tones (e.g., via using something inferior to G.711 64kpbs audio codec)

  4. Issues when transmitting as audio • Echo cancellation can null valid tones. • Delay between packets. Also known as jitter. • Similar to problems with modem tones (e.g., alarm connections), which tend not to work. • VoIP networks have no easy way to make adjustments for TTY transmissions because TTYs have no handshake protocol that would allow networks to detect the case, in contrast to fax transmissions.

  5. Possible solutions: Equipment • Two options: • New SIP-based terminal for end users that: • Supports HCO & VCO, and voice calls • Supports RFC 4103 Real Time Text • Is easy to use just like TTYs today • New Analog adapter (ATA) that will: • Plug to a home router • Convert between Baudot and RFC 4103 Real Time Text • Carry voice calls as VoIP • Work with existing TTYs and captioned telephones

  6. Possible Solution: Custom Service Centrally funded national TTY-friendly VoIP service needs to be made available to TTY users • Carry normal voice calls like other VoIP providers • Free or low cost service to TTY users to give incentive for them to move to this solution • To keep costs low, service might be limited to registered TTY and captioned telephone users only • Supports RTT to RTT calls as well as RTT to TTY and captioned telephone calls

  7. Possible Solution: Custom Service • When carrying TTY signals to the other side, it will preserve signal quality by a combination of: • Avoiding IP-based voice calls on analog side (in contrast to some cheap long distance call providers that use IP). • Using traditional trunking calls. No IP-based call path to the other side for analog. • Another option: transparently transcoding TTY to RFC 4103 RTT on the IP segment, while supporting VCO and HCO. • Allows VCO & HCO calls for alternating text and voice calls • Until all TTY and captioned telephone users have switched to IP-based RTT terminals, this special VoIP service would be needed for analog TTY communications

  8. Acknowledgments The contents of this presentation were developed with funding from the National Institute on Disability and Rehabilitation Research, U.S. Department of Education, grant number H133E090001 (RERC on Telecommunications Access). However, those contents do not necessarily represent the policy of the Department of Education, and you should not assume endorsement by the Federal Government.

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