Asterisk bridge - ptime set to 20 but frames arriving in 10ms chunks

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Amitoj Cheema

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Jul 18, 2017, 7:40:42 AM7/18/17

to UniMRCP

Hi all,

While testing out asterisk-unimrcp bridge, I realized that the packets sent from asterisk client are arriving after intervals of 10ms. As per the docs, this should be 20ms, since in mrcp.conf, ptime= 20ms. Even the size of the frames is 160, which corresponds to 10ms window.

How is packetization time recalculated?

Arsen Chaloyan

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Jul 19, 2017, 11:10:51 PM7/19/17

to UniMRCP

The Asterisk/UniMRCP client may send RTP packets every 10 ms, if the ptime attribute is set to 10 in the SDP answer from the MRCP server.

See https://tools.ietf.org/html/rfc3264

   If the ptime attribute is present for a stream, it indicates the
   desired packetization interval that the offerer would like to
   receive.  The ptime attribute MUST be greater than zero.

The above means that the ptime parameter set in mrcp.conf indicates the client rx capabilities/preferences and should be reflected in the RTP stream sent from server.

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