Coder Transcoding - AudioCodes E-SBC User Manual

CHAPTER 30 SBC Overview
If you assign the SIP entity an Allowed Audio Coders Group for coder restriction and a
Coders Group for extension coders (i.e., voice transcoding), the allowed coders take
precedence over the extension coders. In other words, if an extension coder is not listed
as an allowed coder, the device does not add the extension coder to the SDP offer.

Coder Transcoding

By default, the device forwards media packets transparently (i.e., no media negotiation) between
the SIP endpoints. However, when there are no common coders between two SIP entities that
need to establish voice communication (i.e., the SDP answer from one SIP entity doesn't include
any coder included in the SDP offer previously sent by the other), you can configure the device to
perform audio coder transcoding between the inbound and outbound legs in order to enable media
flow between them.
Transcoding may also be performed in scenarios where the same coder has been chosen between
the legs, but where coder transrating is required. For example, the coders may use different coder
settings such as rate and packetization time (G.729 at 20 ms to G.729 at 30 ms).
The coders that the device adds to the SDP offer on the outbound leg is referred to as extension
coders. The extension coders are configured using Coder Groups (see
which you need to then assign to the IP Profile associated with the SIP entity.
The figure below illustrates transcoding between two SIP entities (IP Groups) where one uses
G.711 (LAN IP phone) and the other G.729 (WAN IP phone). The initial SDP offer received on the
inbound leg from the LAN IP phone includes coder G.711 as the supported coder. In the outgoing
SDP offer on the outbound leg to the WAN IP phone, the device adds extension coder G.729 to the
SDP, which is supported by the WAN IP phone. The subsequent incoming SDP answer from the
WAN IP phone includes the G.729 coder as the chosen coder. Since this coder was not included in
the original incoming SDP offer from the LAN IP phone, the device performs G.729- G.711
transcoding between the inbound and outbound legs.
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Configuring Coder Groups),