AudioCodes Mediant 3000, User Manual

Page: 411 / 872

Version 6.6 411 Mediant 3000
User's Manual 28. SBC Overview
28.4.1 Media Anchoring without Transcoding (Transparent)
To direct the RTP to flow through the device (for NAT traversal, firewall and security), all IP
address fields in the SDP are modified:

Origin: IP address, session and version id

Session connection attribute ('c=' field)

Media connection attribute ('c=' field)

Media port number

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.
Figure 28-3: SDP Offer/Answer Example
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 supported coders and are configured by using the parameter
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