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.