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<div class="div">
<h1>Text Telephony (TTY) </h1>

<p>Hearing impaired and speech-impaired persons have been
using specific Text Telephone (referred to as TTY) equipment in the fixed
network for many years to transmit text and speech through ordinary speech
traffic channels. Modern digital cellular systems, however, do not provide
satisfactory character error rates for text transmitted in the speech channel
with the traditional modulation developed for the fixed network.</p>


<p>There are two types of text telephones: TTY (TeleType)
and TDD (Telecommunications Device for the Deaf). TTYs are basically mechanical
teleprinters and TDDs are their electronic counterparts. Virtually all text
telephones support the ‘5-bit operational mode’ using the Baudot code.</p>


<p>The GSM1900 Standards Body, T1P1, has selected and recommended
the Cellular Text Telephone Modem (CTM) solution to improve TTY/TDD signalling.
This solution employs a modulation technique that passes through the speech
codec with less distortion and includes error protection, interleaving, and
synchronisation.</p>


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<img alt="urn:mars:dn0470267:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0470267&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: Functionality is located in TCSM in Nokia's solution</p>
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<img alt="urn:mars:dn0470279:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0470279&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: TTY Functionality</p>
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<p>In Nokia's implementation, TTY functionality is provided
for the TCSM2 by software update.  </p>
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