AudioCodes Mediant 4000 SBC, User Manual

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CHAPTER 22 SBC Overview
Mediant 4000 SBC | User's Manual
a. Assign the Coders Group to the IP Profile, using the 'Extension Coders Group' parameter
(SBCExtensionCodersGroupName).
b. Enable extension coders by configuring the 'Allowed Coders Mode' parameter to
Restriction or Restriction and Preference .
● To implement transcoding, you must configure the number of required DSP
channels for transcoding (for example, MediaChannels = 120). Each transcoding
session uses two DSP resources.
● The transcoding mode can be configured globally, using the TranscodingMode
parameter or for specific calls, using the IP Profiles table.
Transcoding Mode
By default, the device performs transcoding only when necessary. This refers to all types of
transcoding (interworking) that require the use of the device's DSPs, for example, voice coder
transcoding, DTMF negotiations, and fax negotiations. Transcoding is required, for example, when
two SIP entities use different coders. In such a scenario, the device can be configured to use a
different coder for each leg (inbound and outbound), using IP Profiles. If the SIP entities use the
same coder, the device does not perform transcoding.
Alternatively, you can configure the device to always perform transcoding, regardless of whether it
is required or not. This is referred to as
forced transcoding. For example, if the SIP entities use the
same coder, the device performs transcoding of the same coder (e.g., G.711 and G.711) between
the two legs.
To configure the transcoding mode, use the global parameter TranscodingMode or the IP Profile
parameter IpProfile_TranscodingMode parameter.
If the transcoding mode is configured to Force Transcoding (i.e., always perform
transcoding) for an IP Profile associated with a specific SIP entity, the device also
applies forced transcoding for the SIP entity communicating with this SIP entity,
regardless of its IP Profile settings.
Prioritizing Coder List in SDP Offer
In addition to restricting the use of coders using Allowed Audio Coders Groups (see Configuring
Allowed Audio Coder Groups ), you can also prioritize the coders listed in the SDP offer. This
feature is referred to as Coder Preference and applies to both SBC legs:
■ Incoming SDP offer: The device arranges the coder list in the incoming SDP offer according
to the order of appearance of the Allowed Audio Coders Group that is associated with the
incoming dialog. The coders listed higher up in the group take preference over ones listed lower
down. To configure this, configure the 'Allowed Coders Mode' parameter (IpProfile_
SBCAllowedCodersMode) in the associated IP Profile to
Preference or Restriction and
Preference . If you configure the parameter to to Preference , the coders in the SDP offer that
also appear in the Allowed Audio Coders Group are listed first in the SDP offer, and the coders
in the SDP offer that do not appear in the Allowed Audio Coders Group are listed after the
Allowed coders in the SDP offer. Therefore, this setting does not restrict coder use to Allowed
coders, but uses (prefers) the Allowed coders whenever possible.
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