While the PSTN provides inband progress information to signal early media (such as a ring tone or a busy signal), the same does not hold true for SIP. The originating party includes Session Description Protocol (SDP) information, such as codec usage, IP address, and port number, in the outgoing INVITE message. In response, the terminating party sends its codec, IP address, and port number in a 183 Session Progress message to indicate possible early media.
The 183 Session Progress response indicates that the message body contains information about the media session. Both 180 Alerting and 183 Session Progress messages may contain SDP, which allows an early media session to be established prior to the call being answered.
When early media needs to be delivered to SIP endpoints prior to connection, Cisco CallManager always sends a 183 Session Progress message with SDP. While Cisco CallManager does not generate a 180 Alerting message with SDP, it does support the 180 Alerting message with SDP when it receives one.
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