Cisco Cisco IOS Software Release 12.4(6)T

© 2006 Cisco Systems, Inc. All rights reserved.

The Cisco Text Relay for Baudot Text Phones (Cisco Text Relay) feature implements a mechanism for 
reliable transport of teletype text phone (TTY) signals over VoIP calls. TTY text phones, also known as 
telecommunication devices for the deaf (TDD), are specialized phones that enable people who are deaf, 
hard of hearing, or speech-impaired to communicate over time-division multiplex (TDM) or IP networks 
by allowing them to type messages to one another instead of, or in addition to, talking and listening. 

Cisco Text Relay is based on the proposed ITU V.151 (V.ToIP) standard. Cisco Text Relay leverages 
Audio/T.140, which means text characters are carried over the same Real-Time Transport Protocol (RTP) 
stream as voice (similar to the way dual tone multifrequency (DTMF) characters are carried in the RTP 
stream [RFC 2833]). Cisco Text Relay has minimal impact on bandwidth because the text characters are 
transported efficiently in the RTP stream. There is also a configurable redundancy option enabling TTY 
to run reliably in demanding network conditions.

Your Cisco IOS software release may not support all of the features documented in this module. To reach 
links to specific feature documentation in this module and to see a list of the releases in which each feature is 
supported, use the 

.

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