AudioCodes Mediant 4000 SBC User Manual page 541

CHAPTER 22 SBC Overview
●
●
●
●
●
●
As an example for using allowed and extension coders, assume the following:
■ Inbound leg:
● Incoming SDP offer includes the G.729, G.711, and G.723 coders.
m=audio 6050 RTP/AVP 18 0 8 4 96
a=rtpmap:18 G729/8000
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
The SDP "m=audio 6010 RTP/AVP 18 0 8 4 96" line shows the coder priority, where "18"
(G.729) is highest and "4" (G.723) is lowest.
● Allowed Audio Coders Group for coder restriction includes the G.711 and G.729 coders
(listed in order of appearance).
■ Outbound leg:
● Allowed Audio Coders Group for coder restriction includes the G.723, G.726, and G.729
coders (listed in order of appearance).
● Allowed Audio Coders Group for coder extension (transcoding) includes the G.726 coder.
If you assign a SIP entity an Allowed Audio Coders Group for coder restriction
(allowed coders) and a Coders Group for extension coders, 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.
If none of the coders in the incoming SDP offer on the inbound leg appear in the
associated Allowed Audio Coders Group for coder restriction, the device rejects the
call (sends a SIP 488 to the SIP entity that initiated the SDP offer).
If none of the coders (including extension coders) in the outgoing SDP offer on the
outbound leg appear in the associated Allowed Audio Coders Group for coder
restriction, the device rejects the call (sends a SIP 488 to the SIP entity that
initiated the SDP offer).
For coder transcoding, the following prerequisites must be met (otherwise, the
extension coders are not added to the SDP offer):
✔
The device must support at least one of the coders listed in the incoming SDP
offer.
✔
The device must have available DSPs for both legs (inbound and outbound).
✔
The incoming SDP offer must have at least one media line that is audio
('m=audio').
The device adds the extension coders below the coder list received in the original
SDP offer. This increases the chance of media flow without requiring transcoding.
The device does not add extension coders that also appear in the original SDP
offer.
- 508 -
Mediant 4000 SBC | User's Manual