Media Anchoring Without Transcoding (Transparent); Media Anchoring With Transcoding - AudioCodes Mediant 3000 User Manual

Enterprise session border controller / voip digital media gateway

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User's Manual

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.

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

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

Figure 28-3: SDP Offer/Answer Example

28. SBC Overview

Mediant 3000