User's Manual 562 Document #: LTRT-89730
Mediant 3000
Note: If you assign the SIP entity an Allowed 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.
28.5.4 Coder Transcoding
By default, the device forwards media packets transparently (i.e., no media negotiation)
between 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
''Configuring Coder Groups'' on page 385), 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.
Figure 28-5: Transcoding using Extended Coders (Example)
To Next Page
Loading...
Need help?
General
