AudioCodes Mediant 3000, User Manual

Page: 415 / 872

Version 6.6 415 Mediant 3000
User's Manual 28. SBC Overview
28.4.4 Transcoding Modes
The device supports the configuration of the voice transcoding mode (media negotiation)
between the two SBC legs. The device can be configured to perform transcoding only
when necessary. Typically, the SBC passes RTP packets transparently (RTP-to-RTP)
between the two user agents. If the device is configured to always perform transcoding,
then transcoding is performed on the outgoing SBC leg and the device's SBC application
interworks the media by implementing PSTN transcoding (since both legs have different
media capabilities).
In the SBC application, forced transcoding of voice in an SBC session allows the device to
receive capabilities that are not negotiated between the SBC legs. For example, if on the
SBC session you want to force Gain Control to use voice transcoding even though both
sides of the session have negotiated without SBC intervention (for example, coder
extension).
The transcoding mode can be configured using the parameters TranscodingMode and
IPProfile.
Note: 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.
28.4.5 Restricting Coders
The SBC Allowed Coders (coders restriction) feature determines the coders that can be
used for a specific SBC leg. This provides greater control over bandwidth by enforcing the
use of specific coders (
allowed coders groups ) while preventing the use of other coders.
This is done by defining a group of allowed coders for the SBC leg, as described below:
1. Configure a Coders Group for allowed coders, using the AllowedCodersGroup
parameter.
2. Select this Coders Group using the SBCAllowedCodersGroupID parameter of the IP
Profile table.
3. Enable this feature by setting the SBCAllowedCodersMode parameter of the IP Profile
table to
Restriction .
Coders that are not listed (including unknown coders) in the Allowed Coders Group are
removed from the SDP offer. Therefore, only coders common between the SDP offer and
Allowed Coders Group are used. If the SDP offer does not list any of the Allowed Coders,
the call is rejected, unless transcoding is configured (using Extension coders – see 'Coder
Transcoding' on page 416).
Notes:
•
For a list of supported coders, see 'Configuring Coders' on page 257 .
•
Allowed Coder Groups are applicable only to audio media.
The Allowed Coders process is as follows:
a. The device receives an incoming SIP message with SDP (offer) and checks the
offered coders.
b. The source (first) leg may have Allowed Coders (i.e. list of coders that can be used -
enforced).