Checking that codec is negotiated properly by both parties
Normally both parties should agree on same codec to be used for the session (call), but sometimes one or both party/parties get it wrong and the call ends up using different codec to encode/decode the packets.
Potential Problems with Codec Negotiation
Different iLBC mode used by either party
iLBC has two framing modes: 20ms or 30ms. Each party specifies which framing mode it wants to receive by specifying the following line in the SDP:
a=fmtp:117 mode=20
PJMEDIA SHOULD allow different modes to be used for TX and RX, and it obeys the fmtp mode that the remote party wants to receive and that it indicates in the SDP that it sends. However, some user agents might not be able to do this.
To verify whether this is or isn’t the case, experiment with
changing the iLBC mode that is used by pjsua with using
--ilbc-mode=20
or --ilbc-mode=30
command line argument.
This will change the mode preference that is advertised by PJSIP in the
outgoing SDP.
Wrong codec negotiation
In a worse case, it is also possible that codec negotiation has gone totally wrong in either party, and both parties end up with completely different codec for the call. If PCMA/PCMU codec negotiation mismatches, both may end up with a noisy audio.
More than one codecs are active in remote
While it is okay, according to the standard, to answer an SDP offer with more than one active codecs in a single media (m=) line, currently PJMEDIA does not support having two codecs active in the same stream. Thus when it sees such SDP answer, it will create an updated offer with just one codec from the answer.
While we don’t anticipate this to create the noisy problem, just be aware of this mechanism and keep an eye on potential side effects.
Checking which codec is being used by pjsua
Use pjsua’s dq
(dump quality) command from pjsua menu to check
which codec is being used for the call:
>>> dq
19:01:38.878 pjsua.c
[CONFIRMED ] To: sip:localhost;tag=213e15bcf98b4c0394a402881e885431
Call time: 00h:01m:44s, 1st res in 1452 ms, conn in 1682ms
#0 iLBC @8KHz, sendrecv, peer=192.168.0.66:4000
RX pt=117, stat last update: 00h:00m:01.943s ago
total 4.3Kpkt 164.0KB (302.2KB +IP hdr) @avg=12.5Kbps
pkt loss=0 (0.0%), dup=0 (0.0%), reorder=0 (0.0%)
(msec) min avg max last
loss period: 0.000 0.000 0.000 0.000
jitter : 0.125 15.779 1695.000 1.250
TX pt=117, ptime=20ms, stat last update: 00h:00m:09.304s ago
total 5.1Kpkt 197.2KB (363.4KB +IP hdr) @avg 15.0Kbps
pkt loss=0 (0.0%), dup=0 (0.0%), reorder=0 (0.0%)
(msec) min avg max last
loss period: 0.000 0.000 0.000 0.000
jitter : 14.750 15.570 18.875 18.875
RTT msec : 0.854 24.516 125.000 18.783
The important bit of above output is this line:
#0 iLBC @8KHz, sendrecv, peer=192.168.0.66:4000
which tells us that iLBC is being used for the call (although unfortunately it doesn’t tell the mode).
Try to use other codec
If you know what codec is likely to be used by remote party, you can
force pjsua to prefer certain codec to be used, by using
--add-codec NAME
command. The NAME is the shortest string that uniquely
identifies the codec, such as:
pcma
pcmu
speex/8000
speex/16000
speex/32000
ilbc
etc.
For iLBC, you can change the RX mode used by pjsua with using
--ilbc-mode=20
or --ilbc-mode=30
command line argument.