SIP User's Manual 608 Document #: LTRT-12804
Mediant 800 MSBG
8.7 General
8.7.1 Transcoding using Third-Party Call Control
The device supports transcoding using a third-party call control Application server. This
support is provided by the following:
Using RFC 4117 (see ''Using RFC 4117'' on page 608)
Note: Transcoding can also be implemented using the IP-to-IP (IP2IP) application
and SBC application.
8.7.1.1 Using RFC 4117
The device supports RFC 4117 - Transcoding Services Invocation in the Session Initiation
Protocol (SIP) Using Third Party Call Control (3pcc) - providing transcoding services (i.e.,
acting as a transcoding server). This is used in scenarios where two SIP User Agents (UA)
would like to establish a session, but do not have a common coder or media type. When
such incompatibilities are found, the UAs need to invoke transcoding services to
successfully establish the session. Note that transcoding can also be performed using
NetAnn, according to RFC 4240.
To enable the RFC 4117 feature, the parameter EnableRFC4117Transcoding must be set
to 1 (and the device must be reset).
The 3pcc call flow is as follows: The device receives from one of the UAs, a single INVITE
with an SDP containing two media lines. Each media represents the capabilities of each of
the two UAs. The device needs to find a match for both of the media, and opens two
channels with two different media ports to the different UAs. The device performs
transcoding between the two voice calls.
In the example below, an Application Server sends a special INVITE that consists of two
media lines to perform transcoding between G.711 and G.729:
m=audio 20000 RTP/AVP 0
c=IN IP4 A.example.com
m=audio 40000 RTP/AVP 18
c=IN IP4 B.example.com
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