Media Anchoring With Transcoding - AudioCodes Mediant 800B User Manual

User's Manual
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Session connection attribute ('c=' field)
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Media connection attribute ('c=' field)
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Media port number
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RTCP media attribute IP address and port
Each SBC leg allocates and uses the device's local ports (e.g., for RTP\RTCP\fax). The
local ports are allocated from a Media Realm associated with each leg. The legs are
associated with a Media Realm as follows: If the leg's IP Group is configured with a Media
Realm, then this is the associated Media Realm; otherwise, the leg's SRD Media Realm is
the associated one. The figure below illustrates an example of SDP handling for a call
between a LAN IP Phone 10.2.2.6 and a remote IP Phone 212.179.1.13 on the WAN.

28.4.2 Media Anchoring with Transcoding

The device performs transcoding when there are no common coders between the two user
agents (i.e., the SDP answer from one user agent doesn't include any coder included in the
offer previously sent by the other user agent). For transcoding, the device can be
configured to add media capabilities to user agents pertaining to a specific IP Group, and
then perform transcoding in cases where the selected coder in the answer SDP is not one
that appears in the original offer. The capabilities that can be added are one or more of the
device's
SBCExtensionCodersGroupID (points to a coders list) in the IP Profile table (which is
assigned to the IP Group). Therefore, to allow user agents of different IP Groups to
communicate with each other (regardless of their capabilities), an extended coders table
with at least one coder that is supported by each IP Groups' user agents needs to be
assigned to each IP Group. Therefore, each offer destined to specific IP Groups include
this coder.
In the scenario depicted in the figure below, the IP phone on the LAN side initiates a call to
the IP phone on the WAN. The initial SDP offer (from the LAN leg) includes codec G.711
as its supported codec. Since this is sent to a Destination IP Group that is configured with
an extended coder list, on the WAN leg the device adds another supported codec G.729 to
the SDP, which is now offered to the WAN IP phone. The WAN IP phone chooses the
extended codec (G.729) in its SDP answer to the device's WAN leg. Since this codec was
Version 6.8
Figure 28-3: SDP Offer/Answer Example
supported coders and are configured
493 Mediant 800B Gateway and E-SBC
28. SBC Overview
by using the parameter