- B sends an ACKto T indicating the SDP data sent by T are OK.
- Only now will B reply with a 200 OKmessage to A and include the SDP
data for T (SDP T).
200 OK SDP TA
m=audio 30000 RTP/AVP 0
c=IN IP4 T.example.com- A will send an ACKmessage back to B. A is ready to send media to T.
- The endpoints A and B are both exchanging media streams with T.
There are two bidirectional media streams (a total of four):
■■ Audio from A to T
■■ Text from T to B
■■ Text from B to T
■■ Audio from T to A.
Notice in this example that caller A does not need to be aware that callee B
is deaf and is using a relay service to receive text. The UA for caller A also does
not need to be enabled for SIP third-party call control. Only the relay service
and the deaf user need to support it.
The detailed messages for this and other scenarios for network-based
transcoding are illustrated in RFC 4117. Call scenarios for relay services with
human operators sometimes can support two human agents—one for each
sense of transmission so as to provide some degree of privacy since. Each
agent is included in only one direction of the conversation. By contrast, using
text-to-speech conversion and/or avatars in the endpoints does not expose
any private information to other humans in the call.Summary
Accessibility to communications for deaf, hearing-impaired, and speech-
impaired people is a social imperative for human and economic reasons. Exist-
ing relay services can support only audio and text. Implementing international
communications for impaired users is practically impossible on the PSTN
because of the many national and regional incompatible signaling and data
formats.
Text over IP can support interactive text conversation and is a media type
specifically designed for impaired users.
Video and avatars can support sign languages, finger spelling, and lip read-
ing on low-cost consumer PCs, laptops, and mobile devices for impaired users.298 Chapter 17