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Back to IP Multimedia Subsystem (IMS)

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In one downstream forking scenario where RTP streams are generated by multiple early dialogs and SBC needs the

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 needs to cut through only one stream, the Downstream Forking employs “Early Dialog Media Selection Profile” to control SBC behavior

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 behavior. Support is based on the PEM value of the 18x or UPDATE or the first/last provisional response.

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• Receipt of RTP (Deferred)—The first/last RTP stream is cut-thru. A flag controls whether Local Ringback Tone generated by SBC counts the

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   counts as RTP stream for an early dialog. RTP Server Profile is also distinguishes between allowed/disallowed streams. Disallowed streams will not qualify for the decision process.
• Receipt of Provisional Response—RTP stream for the first/last sender of a provisional response is cut-thru. RTP Server Profile distinguishes between allowed/disallowed streams. Disallowed streams will not qualify for the decision process.
• Use P-Early-Media Header—P-Early-Media header value is used for selection the RTP streams according to the following rules:
• The last sent P-Early-Media value for each early dialog is used for comparison purposes.
• The precedence order of the values will, from highest to lowest, “sendrecv”, “sendonly”, “recvonly” and “inactive”.
• If two early dialogs have the same P-Early-Media value, media for the early dialog which last sent a reply with P-Early-Media header will cut through.
• A P-Early-Media value update on the early dialog with the currently active RTP stream will not trigger a new comparison but will impact the media cut-through behavior.

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