Srtp-Rtp Transcoding - AudioCodes Mediant 800 MSBG User Manual

SIP User's Manual
If the Allowed Coders policy on SDP returns an empty coders list, the device (source leg)
rejects the call (SIP 488 or ACK and BYE). If both Coders Extension and Allowed Coders
policies on SDP (in this order) returns an empty coders list, the second leg rejects the call
(SIP 488, or ACK and BYE).
Below is an example, assuming that Allowed Coders list (ordered) includes G711A-law
(PCMA), G729, and G711U-law (PCMU), and Extended Coder is G729.
1. SDP offer - original offer:
m=audio 6050 RTP/AVP 0 8 4 96
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:4 G723/8000
a=fmtp:4 annexa=no
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
a=ptime:20
a=sendrecv
2. SDP offer - after manipulation:
m=audio 6010 RTP/AVP 8 0 96 18
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
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 - G711A-law ("8") and then G711U-law ("0") - and that the extended coder G729
("18") has been added. The G723 coder ("4) in the original offer was removed as it was not
defined in the Allowed Coders list (i.e., a restricted coder).
8.4.5.7

SRTP-RTP Transcoding

The device supports transcoding between SRTP and RTP. The device can also enforce
SBC legs to use SRTP\RTP, using the IP Profile parameter SBCMediaSecurityBehaviour:
As is (default): no special handling for RTP\SRTP is done.
SRTP: SBC legs negotiate only SRTP media lines, and RTP media lines are removed
from the incoming SDP offer\answer.
RTP: SBC legs negotiate only RTP media lines, and SRTP media lines are removed
from the incoming offer\answer.
Both: each offer\answer is extended (if not already) to two media lines - one RTP and
the other SRTP.
Version 6.2 501
8. IP Telephony Capabilities
February 2011