[Standards] UPDATED: XEP-0301 (In-Band Real Time Text)

Edward Tie famtie at xs4all.nl
Mon Jul 9 06:09:31 UTC 2012

Hi Mark,

I want to add on history of textphone and XMPP to old telephones that 
uses older protocols:


There are many different textphone standards.

      Baudot code

The original standard used by TTYs is the Baudot code 
<http://en.wikipedia.org/wiki/Baudot_code> implemented asynchronously at 
either 45.5 or 50 baud, 1 start bit, 5 data bits, and 1.5 stop bits. 
Baudot is a common protocol in the US.

      Turbo Code

In addition to regular Baudot, the UltraTec 
company implements another protocol known as Enhanced TTY, which it 
calls "Turbo Code," in its products. Turbo Code has some advantages over 
Baudot protocols, such as a higher data rate, full ASCII 
<http://en.wikipedia.org/wiki/ASCII> compliance, and full-duplex 
capability. However, Turbo Code is proprietary, and UltraTec only gives 
its specifications to parties who are willing to license it.

      Other legacy protocols

Other protocols used for text telephony are European Deaf Telephone 
(EDT) and Dual-tone multi-frequency signaling 
<http://en.wikipedia.org/wiki/Dual-tone_multi-frequency_signaling> (DTMF).

The ITU V series recommendations are a collection of early modem 
standards approved by the ITU <http://en.wikipedia.org/wiki/ITU> in 1988.

  * ITU V.21 <http://en.wikipedia.org/wiki/ITU_V.21> [1]
    specifies 300 bits per second duplex mode.
  * ITU V.23 <http://en.wikipedia.org/wiki/ITU_V.23> [2]
    specifies audio frequency-shift keying
    modulation to encode and transfer data at 600/1200 bits per second.


In 1994 the ITU 
approved the V.18 
standard [3] <http://www.itu.int/rec/T-REC-V.18/en>. V.18 is a dual 
standard. It is both an umbrella protocol that allows recognition and 
interoperability of some of the most commonly used textphone protocols, 
as well as offering a native V.18 mode, which is an ASCII 
<http://en.wikipedia.org/wiki/ASCII> full- or half-duplex modulation method.

Computers can, with appropriate software and modem, emulate a V.18 TTY. 
Some voice modems, coupled with appropriate software, can now be 
converted to TTY modems by using a software-based decoder for TTY tones. 
Same can be done with such software using a computer's sound card, when 
coupled to the telephone line.

In the UK, a virtual V.18 network, called TextDirect, exists as part of 
the Public Switched Telephone Network 
thereby offering interoperability between textphones using different 
protocols. The platform also offers additional functionality like call 
progress and status information in text and automatic invocation of a 
relay service for speech-to-text calls.

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