A.2 Introduction

24.2063GPPStage 3TSVoice call continuity between Circuit Switched (CS) and IP Multimedia Subsystem (IMS)

A.2.1 General

The signalling flows provided in this annex follow the methodology developed in 3GPP TS 24.228 [7]. The following additional considerations apply:

a) 3GPP TS 24.228 [7] shows separate signalling flows with no configuration hiding between networks, and with configuration hiding between networks. There is no voice call continuity specific functionality associated with this hiding, and therefore such separate signalling flows are not show in the present document;

b) 3GPP TS 24.228 [7] does not show the functionality between the S-CSCF and the AS. As voice call continuity depends on the functionality provided by various AS, the signalling flows between S-CSCF and AS are shown in the present document;

c) 3GPP TS 24.228 [7] breaks down the functionality of the various CSCFs. Such intervening activity in the CSCFs is in general not relevant to showing the functionality within voice call continuity, and therefore the CSCFs are collapsed into a single entity labelled "Intermediate IM CN subsystem entities";

d) where entities are combined as in c) above, and the signalling flow is directed to such a combined entity, the contents of the signalling flow represent the contents of the sending entity;

e) where entities are combined as in c) above and the signalling flow originates at such a combined entity, the contents of the signalling flow represent the contents of the receiving entity; and

f) within the CS domain, both ISUP and BICC can be used. The signalling flows represent only the ISUP signalling flows, and the BICC signalling flows (which can be assumed to be similar with no additional VCC specific content) are not shown.

A.2.2 Key required to interpret signalling flows

The key to interpret signalling flows specified in 3GPP TS 24.228 [7] subclauses 4.1 and 4.2 applies with the additions specified below:

– tel:+1-241-555-3333 represents the IMRN associated with a call made by UE#1.

– sip:dtf1.home1.net represents the address within the originating IM CN subsystem of the AS providing the DTF.

– sip:dtf2.home2.net represent the address within the terminating IM CN subsystem of the AS providing the DTF.

– tel:+1-241-555-4444 represents the CSRN associated with a call made by UE#1.

– tel:+1-212-555-5555 represents the VDN associated with UE#1.

– sip:domain.xfer@dtf1.home1.net represents the VDI associated with UE#1.

– sip:csaf1.home1.net represents the address of the AS providing the CSAF and therefore the address stored against the PSI.

Each signalling flow table contains descriptions for headers where the content of the header is new to that signalling flow, as is already performed in 3GPP TS 24.228 [7].

However, 3GPP TS 24.228 [7] includes extensive descriptions for the contents of various headers following each of the tables representing the contents of the signalling flows. Where the operation of the header is identical to that shown in 3GPP TS 24.228 [7], then such text is not reproduced in the present document.

Additional text can also be found on the contents of headers within 3GPP TS 24.228 [7] in addition to the material shown in the present document.

In order to differentiate between messages for SIP and media, the notation in figure A.2-1 is used.

Figure A.2-1: Signalling flow notation