Telecommunications Industry Association TR-30.1/02-09-117

(TIA)McLean, VA September 10 - 12, 2002

COMMITTEE CONTRIBUTIONTechnical Committee TR-30 Meetings

SOURCE: TR-30 Chair

CONTACT: Fred LucasFAL Associates

TITLE: TTY Presentation at Signaling for VoIP Summit

PROJECT: PN-4628

DISTRIBUTION: Members of TR-30.1

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.

August 13-14, 2002Washington, DC

Jim Turner

Technical Coordinator

TTY, TTSI, & IITC

(202)662-8662 Wk

(630)972-1454 Wk @ Hm

jturner@atis.org

August 13-14, 2002Washington, DC

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

August 13-14, 2002Washington, DC

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

August 13-14, 2002Washington, DC

TTY & VoIP GAP AnalysisBaudot - 5 bit character code

Start bit

1400 Hz = 1

1800 Hz = 0

Start bit Stop Bit

22 MS +/- 0.4MS Pulse Width

0 0 0 1 1

A

August 13-14, 2002Washington, DC

TTY & VoIP GAP AnalysisBaudot Continued

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.

St 0 0 0 1 1 stp St 1 1 0 1 1 Stp St 0 0 0 1 1 stp

August 13-14, 2002Washington, DC

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)

August 13-14, 2002Washington, DC

Mobile to Land Circuit Switched

•V

Send Letter A

Receive Letter A

Cell

MSC

PSAP

Central Office

August 13-14, 2002Washington, DC

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.

August 13-14, 2002Washington, DC

Mobile to land VoIP

Send Letter ACell

MSC

Receive

PSAP

Central Office

Delayed Packet

August 13-14, 2002Washington, DC

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%

August 13-14, 2002Washington, DC

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

August 13-14, 2002Washington, DC

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

August 13-14, 2002Washington, DC

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.