[Standards] UPDATED: XEP-0301 (In-Band Real Time Text)
famtie at xs4all.nl
Mon Jul 9 06:09:31 UTC 2012
I want to add on history of textphone and XMPP to old telephones that
uses older protocols:
There are many different textphone standards.
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.
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
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> 
specifies 300 bits per second duplex mode.
* ITU V.23 <http://en.wikipedia.org/wiki/ITU_V.23> 
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  <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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