Use Cases for SIP and SDP Offer/Answer

This page contains a list of use cases or call scenarios for SIP and SDP Offer/Answer.

There are a few call scenarios that we expect to see when dealing with more telephone-like side of SIP:

• calling to existing PSTN networks (early session)
• doing resource reservations
• calling to 3G IMS
• call hunting
• having external party setting up your call, etc.

Case #1: Basic Call

This is the basic SIP call model with the most simple offer-answer exchange.

	A		        B
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	|----INVITE (offer)---->|
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	|< - - 180 Ringing - - -|
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Case #2: Early Media

Another case, slightly more complex. The SDP answer is sent with 180 Ringing in order to establish an "early session". B might not be a SIP phone, but a gateway to PSTN, for instance. Using this "early session" B can play tones like "burr-burr" or "thesubscriber you tried to reach is not in the coverage...":

	A		        B
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	|----INVITE (offer)---->|
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Case #4: UPDATE with Offer

This is a call model with two rounds of offer/answer and 100rel. It can be used, for instance, when the endpoints have to make sure that there are enough network capacity for the call to succeed. They can establish a new radio bearer, for instance, before progressing with the call. The initial offer would contain SDP attribute "inactive", the second "sendrev":

	A		        B
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	|----INVITE (offer)---->|
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Case #7: 3GPP Call Model

Here is a third alternative, know as "3GPP call model", where there is not 2 but 3 offer-answer rounds, allowing A and B to do precise resource reservations after they have agreed on media, codecs and transport addresses used during the call:

        	A		        B
        	|			|
        	|----INVITE (offer)---->|
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        	||
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        	|			|
        	|----UPDATE (offer3)--->|
        	||		|		|
	|			|-INVITE (off)->|		|
	|			|-----------INVITE (off)------->|
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