On Thu, May 10, 2018 at 11:44:14AM -0700, Steve Edwards wrote:
I receive an INVITE/SDP containing:
m=audio 11310 RTP/AVP 3 0 101
which I interpret as gsm, ulaw, rfc2833.
and I reply with an OK/SDP containing:
m=audio 15884 RTP/AVP 0 3 101
which I interpret as ulaw, gsm, rfc2833.
How can I tell which codec was actually used for the call?
On Fri, 11 May 2018, Daniel Tryba wrote:
AFAIK this is undetermined. The callee can send either ulaw or gsm,
unless the caller wants to narrow it down to 1 codec, see
https://tools.ietf.org/html/rfc4317#section-2.2
Most of the time the callee will pick the first (so in this case ulaw).
But there are media gateways out there that choose g711[au] above "more
complex" codecs regardless order in SDP. My prefer PSTN provider will
always prefer alaw if offered since that will prevent transcoding on
their side if the call goes to ISDN/POTS, but AMR if the call goes to
VoLTE.
So, without examining the RTP, you cannot tell which codec was actually
used?
In the above example, even though the INVITE/SDP says they prefer gsm over
ulaw and the OK/SDP says I prefer ulaw over gsm, they can choose to use
gsm or ulaw?
Can it be asymmetrical? They send gsm and I send ulaw?
--
Thanks in advance,
-------------------------------------------------------------------------
Steve Edwards sedwa...@sedwards.com Voice: +1-760-468-3867 PST
https://www.linkedin.com/in/steve-edwards-4244281
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