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Content for  TS 23.231  Word version:  17.0.0

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4.4.3  Deferred MGW selectionp. 14

To initiate the optional "Deferred MGW Selection", a (G)MSC Server shall indicate to the succeeding node in the initial SIP-I SDP offer that the MGW has not been seized by including an unspecified connection address and shall indicate that "preconditions are not met". The (G)MSC server may additionally include a "proposed" MGW identity within the SDP offer. The (G)MSC server may additionally include a list of "additionally proposed" MGW identities within the initial SDP offer.
Upon reception of an INVITE with unspecified connection address, the succeeding node may seize a MGW (MGW Bypass may be supported in which case the MGW would not be selected, see clause 4.4.4).
If the succeeding note seizes a MGW, it shall insert its own connection address in the SDP offer to the further succeeding 3GPP Node and in the subsequent SDP answer back to the preceding node.
If the succeeding node supports the deferred MGW selection, and it received a "proposed" MGW Identity and a list of "additionally proposed" MGW identities, it may take this into account when selecting the MGW. If the succeeding node supports the deferred MGW selection, it should include the identity of the selected MGW in the SDP answer.
Upon reception of the SDP answer, the (G)MSC Server that initiated the "deferred MGW" selection shall seize a MGW. It should use the received MGW identity to select the MGW, preferably using the same MGW to save resources, if it has a Mc H.248 gateway control protocol interface to that MGW. The (G)MSC Server shall then send a new SDP offer in a PRACK request or an UPDATE request including the connection address and an indication that preconditions are met (provided upstream continuity is also achieved).
Upon reception of the new SDP offer, the succeeding node will update the MGW configuration with the new connection address.
In the example sequence chart in Figure 4.4.3.1, which assumes the network model in Figure 4.4.2.1, the offerer is an originating MSC Server, which does not signal a MGW identity and at the same time indicates that the user plane is not connected. The answerer is an MSC-IW-Server, which seizes the MGW at the network border and returns the MGW identity to the originating MSC Server. The originating MSC Server is able to select the same MGW and seizes a bearer termination and indicates in the second SDP offer to the MSC-IW Server that the user plane is connected.
Copy of original 3GPP image for 3GPP TS 23.231, Fig. 4.4.3.1: Deferred MGW selection (message sequence chart)
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