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[Linphone-developers] Receiving silent packets from ortp

From: Petr Kuba
Subject: [Linphone-developers] Receiving silent packets from ortp
Date: Thu, 13 Nov 2008 15:09:54 +0100
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv: Gecko/20070111 SeaMonkey/1.1

we have problems using ortp to receive rtp streams. The received streams often contain silent packets inserted by ortp. It looks like we do not understand several things in the ortp behavior. Let me describe our experiments.

First we used scheduling and blocking mode:
rtp_session_set_scheduling_mode (rtpSession,1);

However, it looks like rtp_session_recv_with_ts() is not blocking as it should be. If I understand it correctly this method should block until time given by the timestamp is reached. Am I right? In our case the method does not wait and immediately returns packet full of silence event if it is called too early. Does anyone know what is wrong?

Then we tried to use session_set_select() (with scheduling, of course) for waiting before calling rtp_session_recv_with_ts() but result was the same. This method was not blocking until given time. In this case, could someone explain us the following statement from the documentation of use session_set_select()?

"The first set is interpreted as a set of RtpSession waiting for receive events: a new buffer (perhaps empty) is availlable on one or more sessions of the set, or the last receive operation with rtp_session_recv_with_ts() would have finished if it were in blocking mode."

How can session_set_select() know when would have rtp_session_recv_with_ts() finished? It has no information about timestamp that will be provided to rtp_session_recv_with_ts().

Finally, we decided to use our own "scheduling": we wait until time for the next packet is reached. In this case we achieve the best behavior - we receive the smallest amount of silent packets. However, since we are testing on local network I would expect to receive no silent packets. And we receive some although we call rtp_session_recv_with_ts() after the timestamp passed.

So, my basic question is how can we convince rtp_session_recv_with_ts() not to return silent packets? Worse, it looks like that when ortp returns silent packet for some timestamp and then receives this packet it returns it in the next iteration. This makes the rtp stream "longer" and thus delayed. Could someone help us with this issue?

In summary, we would appreciate some more detailed description how receiving rtp streams works including description of the jitter algorithm behavior.

Thanks a lot for responses.
Petr Kuba

   Petr Kuba, Project Manager
   OptimSys, s.r.o
   Tel: +420 541 143 065
   Fax: +420 541 143 066

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