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COMMITTEE CONTRIBUTION Technical Committee TR-30 Meetings

COMMITTEE CONTRIBUTION Technical Committee TR-30 Meetings. ABSTRACT

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COMMITTEE CONTRIBUTION Technical Committee TR-30 Meetings

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  1. COMMITTEE CONTRIBUTION Technical Committee TR-30 Meetings ABSTRACT This contribution provides a copy of a presentation made to the August 2002 Signaling for VoIP Summit by Jim Turner, ATIS. It describes a problem with the transport of TTY Signals over VoIP. The presentation is provided to TR-30.1 for information.

  2. Jim Turner Technical Coordinator TTY, TTSI, & IITC (202)662-8662 Wk (630)972-1454 Wk @ Hm jturner@atis.org

  3. TTY & VoIP GAP Analysis • All Public Safety Answering Points must support TTY (The American for Disabilities Act) • All wireless digital phones must support TTY (FCC 94-102) • Under 1% Total Character Error Rate (TCER) is the accepted performance standard

  4. TTY & VoIP GAP Analysis • VoIP May not be able to support under 1% TCER for TTY calls. Reasons: • TTYs are silent when not transmitting • TTY Operation is “half Duplex” • Each TTY character is made from seven individual tones and is several VoIP packets in length

  5. TTY & VoIP GAP Analysis Baudot - 5 bit character code A 1400 Hz = 1 0 0 0 1 1 Start bit 1800 Hz = 0 Start bit Stop Bit 22 MS +/- 0.4MS Pulse Width

  6. TTY & VoIP GAP Analysis Baudot Continued St 0 0 0 1 1 stp St 1 1 0 1 1 Stp St 0 0 0 1 1 stp A Figures Dash - Mark Hold With 5 bits there are 32 possible characters. Figures and letters extend this to 64 characters. Line Feed, Space, Carriage Return, Figures, and letters have the meaning in Figures and Letters.

  7. TTY & VoIP GAP Analysis • All techniques used for digitizing voice are able to digitize TTY tones (1400 & 1800 HZ are in the audible range) • The issue is not caused by the sampling standards (G.711, IS 825, etc.) • The issue is caused by VoIP latency • There are QoS standards addressing latency (Diffserv & RSVP)

  8. Mobile to Land Circuit Switched MSC • V Cell Send Letter A PSAP Central Office Receive Letter A

  9. TTY & VoIP GAP Analysis • Typical VoIP packet is 20ms of the digitized audio sample. • A TTY Character is at least 154 ms long. • Packet loss of 0.2% is considered very good while packet loss of 2.0% is typical. • Statistically a 4% TCER is expected with a packet loss of 0.5% assuming a typical 20 ms packet size. • IP phones have ways of making up for these loses so they are not detected by the human ear.

  10. MSC Cell Send Letter A Mobile to land VoIP PSAP Central Office Delayed Packet Receive

  11. TTY over VoIP Sample Send: your checking account balance is $146.45 as of 6pm Missed part of the figures packet for a packet loss of 2% Receive: YOUR CHECKING ACCOUNT BALANCE IS DBRYNBT AS OF 6 PM 7 characters missed out of 50 for a TCER of 14%

  12. What is being done about it? • Issue is being submitted to NRIC VI Focus Group 3 from the Voice & Data Interoperability working group • Contact Jim Turner with Proposed Solutions & will add to the submission

  13. Possible Solutions • Move from Baudot to some other form of lifeline communication. • Come up with a Standards Solution. • V.18 Sup[port Baudot for a defined period of time & then transition to ASCCII • As areas convert to VoIP users in the area would transition. Visitors would suffer errors. • Some other out of the box solution • Replace all Baudot devices with computers

  14. SUMMARY • TTY is the Hearing & Speech Impaired community’s current means of communication for Lifeline Services. • VoIP may not support error rates of under 1% TCER for TTY users. • Need to insure this issue is resolved before VoIP can be deployed. • NRIC VI FG 3 is working issue.

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