[SIPForum-discussion] Codec Prefrence Option (RTP Profile selectionin multiple Codec)

Herve Jourdain herve.jourdain at mstarsemi.com
Wed May 21 02:20:41 UTC 2008



Rtpmap is not mandatory for all codecs in the m line. It’s only mandatory
for dynamic codecs. For the static ones, it’s encouraged to declare a
rtpmap, but it’s not mandatory.

So it does it for G.729, but not for G.711 µLaw, which is the codec
identified as “0” in your m line.

It’s possible to do so, even if declaring it also as a rtpmap is strongly
encouraged by the RTP RFCs.

Static codecs are described in the RTP/AVP RFC, and so should be supported
by all VoIP SW even without rtpmap.


As for the precedence, it comes from the order in the m line, regardless of
rtpmap being present or not. Rtpmap just describes the codec by its MIME
type, and links it to a Payload Type used to transport it, no more no less.







From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Shakil, Rashid
Sent: mardi 20 mai 2008 17:18
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Codec Prefrence Option (RTP Profile
selectionin multiple Codec)





Quick question I was analyzing the SDP which my softswtich is sending in
response to an INVITE for an inbound call I received from my SIP peer. I
noticed that I can see two codec choices in media description (m=) line but
the rtpmap attribute of the SDP just showing one. Can any one tell what does
it means and which one has precedence if I have media description and rtpmap
present in an SDP? 


c=IN IP4

t=0 0

m=audio 21092 RTP/AVP 18 0 100

a=rtpmap:18 G729/8000

a=rtpmap:100 telephone-event/8000

a=fmtp:100 0-11



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