IMG 1010 - SIP Codec Negotiation

Overview

When an inbound SIP call is received, the IMG will negotiate what Codec to use when connecting the call. When negotiating a Codec selection the IMG must first know whether to use the Codec selections of the remote SIP gateway (Selections are in the SIP INVITE message.) or to use the Codec selections from the local IMG. Below is a screen capture of the SIP Profile Object Pane in ClientView. Note that either Local or Remote can be selected from a drop down menu in the Codec Priority Field.

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• If the remote gateway sends a preferred payload in the incoming SDP (ptime) and the payload size is valid and within the IMG's preferred payload Min/Max settings in the Supported Vocoders Object pane for the codec selected/negotiated, then the IMG will transmit that payload size in its RTP stream. See screen capture below of the SIP Profile Object Pane.
  

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• If the remote gateway does not send a preferred payload size or the payload size sent is not valid or not within our Min/Max range, the IMG will use its own preferred payload size for the transmitted RTP stream.

• The IMG will include ptime for SDP Answers and for SDP Offers that include a single codec. When the IMG includes ptime in its SDP:

  • For SDP Offers, it will be based on the Preferred Payload Size.

  • For SDP Answers, it will depend on the setting for Ptime Source for SDP Answer. Refer to the SIP Signaling Gateway Profile (SGP) in the SIP Profile (see IMG 1010 - SIP Profile - 10.5.3) .