Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Figure 17.8 Example for relay service invocation by a caller

In the example shown in Figure 17.8, the caller may be on a 2G mobile net-
work and the mobile/2G-Internet gateway is not shown here for simplicity.
The called party is deaf and decides to invoke a relay service. SIP third-party
call control is used on the IP side of the gateway. The call flow messages in Fig-
ure 17.8 is:

1. Caller A sends send an INVITEto B and the SDP A data payload indi-
   cates its connection data (where it expects the media streams) and its
   media capabilities.
   (1) INVITE SDP A

m=audio 20000 RTP/AVP 0

c=IN IP4 A.example.com

2. Callee B sends an INVITEto T with SDP information of both SDP A
   and its own SDP B (SDP A+B).


3. Transcoder service T replies with 200 OKand includes the SDP data it
   wants to use with A and with B (SDP TA+TB).
   200 OK SDP TA+TB

m=audio 30000 RTP/AVP 0

c=IN IP4 T.example.com

m=text 30002 RTP/AVP 96

c=IN IP4 T.example.com

a=rtpmap:96 t140/1000

1. INVITE SDP A

A T B

Hearing and

speech impaired

2. INVITE SDP A + B


3. ACK


4. ACK


5. 200 OK SDP TA + TB


6. 200 OK SDP TA

7A Media (voice) 7B Media (text)

Accessibility for the Disabled 297