AudioCodes Mediant 3000, User Manual

Page: 417 / 872

Version 6.6 417 Mediant 3000
User's Manual 28. SBC Overview
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
a=ptime:20
a=sendrecv
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
In the SDP, the "m=audio 6010 RTP/AVP 8 0 96 18" line shows that the coder priority has
changed - G.711A-law ("8") and then G.711U-law ("0") - and that the Extension coder
G.729 ("18") has been added. The G.723 coder ("4) in the original offer was removed as it
was not defined in the Allowed Coders list (i.e., a restricted coder).

To configure Extension coders:
1. In the Coders Group table (see 'Configuring Coder Groups' on page 260 ), configure a
Coders Group for extension coders.
2. In the IP Profile table, select this Coders Group in the 'Extension Coders Group ID'
parameter.
3. In the IP Profile table, enable this feature by setting the 'Allowed Coders Mode'
parameter to
Restriction or Restriction and Preference .
28.4.7 Prioritizing Coder List in SDP Offer
In addition to restricting the use of coders with Allowed coders, the device can prioritize the
coders listed in the SDP offer. This feature is referred to as
Coder Preference . This is done
on both SBC legs:

Incoming SDP offer: The device arranges the coder list according to the order in the
Allowed Coders Group table. The coders listed higher up in the table take preference
over ones listed lower down in the table. This feature is enabled by setting the
'Allowed Coders Mode' parameter in the IP Profile table to
Preference or Restriction
and Preference . If set to Preference , in addition to the Allowed coders that are listed
first in the SDP offer, the original coders received in the SDP are retained and listed
after the Allowed coders. Thus, this mode does not necessarily restrict coder use to
Allowed coders, but uses (prefers) the Allowed coders whenever possible.

Outgoing SDP offer: If only Allowed coders are used, the coders are arranged in the
SDP offer as described above. However, if Extension coders are also used, then the
coder list is arranged according to the 'SBC Preferences Mode' parameter in the IP
Profile table. This parameter can be configured to add the Extension coders after the
Allowed coders (i.e., at the end of the list - default) according to their order in the
Coders Group table, or arrange Allowed and Extension coders according to their
position in the Coders Group table.
28.4.8 SRTP-RTP and SRTP-SRTP Transcoding
The device supports transcoding between SRTP and RTP. The device can also enforce
specific SBC legs to use SRTP and/or RTP. The device’s handling of SRTP/RTP is
configured using the IP Profile parameter, SBCMediaSecurityBehaviour, which provides
the following options:

SBC passes the media as is, regardless of whether it’s RTP or SRTP (default).

SBC legs negotiate only SRTP media lines (m=); RTP media lines are removed from
the incoming SDP offer\answer.

SBC legs negotiate only RTP media lines; SRTP media lines are removed from the
incoming offer\answer.

Each SDP offer\answer is extended (if not already) to two media lines for RTP and
SRTP.