Not just in IMS but in general media proxies usually expect traffic from both ends before they start relaying it from one endpoint to another. This is basically how media proxies get aware of as from where the RTP would come from and where it is suppose to go.

During the call setup, the media proxy advertises its address to both caller (by changing SDP in 200 OK) and the callee (by changing SDP in initial invite), so that they both know where to send RTP to. However on the other hand, media proxy is NOT aware of actual address from where it would receive media from (the address in original SDPs which it receives from caller or callee may have private address or some other NAT issue associated with them, e.g. symmetric NAT etc.). Therefore, as soon as call establishes, the media proxy waits for incoming RTP from caller side and the callee side. When caller sends RTP, media proxy learns the actual address of caller from where it has received the audio and when callee sends RTP, media proxy learns the actual address of callee to where it is suppose to send RTP that it had received from caller and vice versa.

Thank you.

On Tue, Feb 24, 2015 at 4:21 PM, Badri Ranganathan badri@arcatech.com wrote:

Hi all,

I'll make this question a little clearer.

User 1 ---------------- SBC ----+ other IMS network components ---------------------- User 2 Same scenario

User 1 Making the call + plays RTP Traffic

User 2 Answering the call + records the RTP Traffic coming from User 1 and does a PESQ voice quality analysis

If user 2, after answering the call, did not play RTP as well, the SBC will not forward the RTP from User 1 to User 2. [The media gateways responsible for the RTP traffic is also a part of the IMS network right?]

So why this sort of bi-directional traffic imposition in an IMS Network ?

Thanks, Badri.

-----Original Message----- From: Badri Ranganathan Sent: 24 February 2015 15:13 To: sr-users@lists.sip-router.org Subject: Regarding RTP in IMS Networks.

Hi all,

I have a doubt w.r.t media traffic in IMS Networks. If I have a setting as follows -

User 1 Making the call + plays RTP Traffic

User 2 Answering the call + records the RTP Traffic coming from User 1 and does a PESQ voice quality analysis

I see that User 2 has to play RTP Traffic before it can start recording the RTP coming from user 1.

i.e., User 2 Answering the call + (Play RTP Traffic ) + records the RTP Traffic coming from User 1 and does a PESQ voice quality analysis.

Why am I seeing this behaviour. Is it just a setting in the IMS network that can be changed or is it a standard behaviour in IMS Networks ? If yes, then could anyone please let me know the logic behind such a decision?

Please do let me know if you have any ideas. Any responses really appreciated !

Thanks, Badri.

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