11 Roles for PS-PS access transfer in conjunction with PS-CS access transfer

24.2373GPPIP Multimedia (IM) Core Network (CN) subsystem IP Multimedia Subsystem (IMS) service continuityRelease 17Stage 3TS

11.1 Introduction

This clause specifies the procedures for PS-PS access transfer in conjunction with PS-CS access transfer. Procedures are specified for the SC UE and the SCC AS. For SC UE or SCC AS not supporting ICS procedures, PS-PS access transfer with a remote end in conjunction with PS-CS access transfer with the same remote end is only possible when the UE is active in a single CS call with the remote end i.e. support of session transfer with more than one CS call is not provided.

11.2 SC UE

11.2.1 SC UE procedures for PS to PS+CS access transfer

11.2.1.1 General

The SC UE may be engaged in one or more ongoing sessions before performing access transfer. By an ongoing session, it is meant a session for which the SIP 2xx response for the initial SIP INVITE request to establish this session has been sent or received.

11.2.1.2 SC UE procedures for PS to PS+CS access transfer using ICS

This subclause applies for IMS sessions containing not only speech media component, otherwise subclause 9.2.1 applies.

If SC using ICS is enabled then if the SC UE is using Gm, then for each session with speech media component to be transferred and starting with the full-duplex session with active speech media component, the SC UE shall send a SIP INVITE request to the SCC AS as specified for call origination for ICS UE using Gm in 3GPP TS 24.292 [4]. The SC UE shall populate the SIP INVITE request as specified for PS-PS access transfer with full media transfer in subclause 10.2.1 with the following exceptions:

– The SC UE shall indicate in the SIP INVITE request that the speech media component is using CS bearer with its corresponding media description.

– When sending the SIP INVITE request for the full-duplex sessions with inactive speech media component and if precondition is used, the SC UE shall indicate that the related local preconditions for the speech media component are met.

– For the full-duplex session with active speech media component, upon receiving the PSI DN from the SCC AS, the SC UE shall follow the procedures for call origination for ICS UE using Gm in 3GPP TS 24.292 [4] to set up the CS bearer.

If service control over Gm for the CS bearer is retained on the source access leg, the SC UE shall:

– send an SIP INVITE request as specified for partial session transfer in subclause 10.2.2. indicating transfer of non-speech media to the target access leg; and

– send a SIP re-INVITE request over the source access leg indicating that the speech media component is to be transferred to a CS bearer as described in 3GPP TS 24.292 [4] subclause 8.2.2.2. If other media components are retained or added on the source access leg, then these are included in the SDP offer.

For the full-duplex session with active speech media component, upon receiving the SCC AS PSI DN from the SCC AS, the SC UE shall follow the procedures for call origination for ICS UE using Gm in 3GPP TS 24.292 [4] to set up the CS bearer.

11.2.1.3 SC UE procedures for PS to PS+CS access transfer not using ICS

If the SC UE is not using ICS capabilities and if the SC UE does not apply the MSC server assisted mid-call feature as specified in subclause 9.2.1A, then access transfer is only possible when the UE is active in a single full-duplex session with active speech media component.

For the non-speech components to be transferred to the PS Target Access Leg, the SC UE shall send a SIP INVITE request to the SCC AS as specified for PS-PS access transfer with partial media transfer in subclause 10.2.1. For the speech media component to be transferred to the CS Target Access leg, the SC UE shall send to the SCC AS a CC SETUP message as specified in 3GPP TS 24.008 [8]. When sending the CC SETUP message, the SC UE shall populate the CC SETUP message as follows:

1) the called party BCD number information element set to the STN; and

2) Type Of Number set to "International" and Numbering Plan Indicator set to "E.164".in the Called Party BCD Number information element.

Upon receiving the SIP 2xx response from the SCC AS for the PS Target Access Leg and sending SIP ACK request and upon receiving CS call setup confirmation message, e.g. CC CONNECT message, for the CS Target Access Leg, the SC UE shall send a SIP BYE request to terminate the Source Access Leg, following the procedures specified in 3GPP TS 24.229 [2].

If the SC UE receives any SIP 4xx – 6xx response to the SIP INVITE request for the PS Target Access leg and receives CS call setup failure message for the CS Target Access Leg, then session transfer has not occurred and the call will continue in the original domains.

If the SC UE receives any SIP 4xx – 6xx response to the SIP INVITE request for the PS Target Access Leg and receives CS call setup confirmation message for the CS Target Access Leg, then the session transfer is only successful for part of the media components. The SC UE shall update the Source Access leg by following the procedures specified for PS-PS access transfer with partial media transfer in subclause 10.2.2 to indicate that all media components other than the speech media component are still maintained on the Source Access Leg.

If the SC UE receives CS call setup failure message for the CS Target Access Leg but receives a SIP 2xx response for the PS Target Access Leg, then the session transfer is only successful for part of the media components. Upon sending SIP ACK request, the SC UE shall update the Source Access leg by following the procedures specified for PS-PS access transfer with partial media transfer in subclause 10.2.2 to indicate that the speech media component is still maintained on the Source Access Leg.

11.2.1.4 SC UE procedures for PS to PS+CS access transfer not using ICS with MSC server assisted mid-call feature

In addition to the procedures described in subclause 11.2.1.3 the SC UE shall:

– act as described in subclause 9.2.1A; and

– if the MSC server assisted mid-call feature is applied, transfer the non-speech media components of the additional transferred session to the PS Target Access Leg as specified for PS-PS access transfer with partial media transfer in subclause 10.2.2.

11.2.2 SC UE procedures for PS+CS to PS access transfer

11.2.2.1 General

The SC UE may be engaged in one or more ongoing sessions before performing access transfer. By an ongoing session, it is meant a CS call for which the CC CONNECT message has been sent or received or a call for which the SIP 2xx response for the initial SIP INVITE request to establish this session has been sent or received.

If not already registered over the PS Target Access Leg, the SC UE shall follow the procedures specified in subclause 6.2 to perform IM CN subsystem registration over the Target Access Leg before performing PS/CS to PS access transfer.

11.2.2.2 SC UE procedures for PS+CS to PS access transfer using ICS

If SC using ICS is enabled then if the original sessions are established using ICS capabilities as defined in 3GPP TS 24.292 [4], then for each full-duplex session with speech media component to be transferred and starting with the session with active speech media component, the SC UE shall send a SIP INVITE request to the SCC AS in accordance with the UE procedures specified in 3GPP TS 24.229 [2]. The SC UE shall populate the SIP INVITE request as specified for PS-PS access transfer with full media transfer in subclause 10.2.1. The SC UE shall indicate in the SIP INVITE request that the speech media component is using PS media.

Upon receiving SIP BYE request for the Source Access Leg, the SC UE shall follow the ICS using Gm procedures specified in 3GPP TS 24.292 [4] to release the session. The SC UE also releases the associated CS bearer if no other sessions depend on the CS bearer.

11.2.2.3 SC UE procedures for PS+CS to PS access transfer not using ICS

If the original sessions are not established using ICS capabilities, then access transfer is only possible when the SC UE has a single session with active full-duplex speech media component. The SC UE shall send a SIP INVITE request to the SCC AS in accordance with the UE procedures specified in 3GPP TS 24.229 [2].

The SC UE shall populate the SIP INVITE request as follows:

– the Request-URI set to static STI;

– the Require header field including "replaces" option tag;

– the Replaces header field populated as specified in IETF RFC 3891 [10], containing the dialog identifier of the session to be transferred on the PS Source Access Leg; and

– the SDP payload set for the media component(s) to be transferred, in accordance the UE SDP origination procedures specified in 3GPP TS 24.229 [2]. The SC UE shall create an SDP offer that contains media components in the following order:

1) the same number of media lines, each corresponding to one of the media components in the session on the PS Source Access Leg; For each media line the SC UE shall indicate the same media type as its corresponding media component in the original session and indicate at least one codec that was negotiated during the original session. If the SC UE determines to remove a media component during the transfer, then the SC UE shall set the media line for this media component to include a port number with value zero;

2) one speech media component to be transferred, corresponding to the speech media component in the session on the CS Source Access Leg; and

3) if the SC UE determines to add new media component(s) during the transfer, then one additional media line with the desired media type and codecs each new media component.

If the SC UE receives any SIP 4xx – 6xx response to the SIP INVITE request, then session transfer has not occurred and the call will continue in the original domains.

11.3 SCC AS

11.3.1 Distinction of requests sent to the SCC AS

The SCC AS needs to distinguish between the following initial SIP INVITE requests to provide specific functionality relating to access transfer:

– SIP INVITE requests routed to the SCC AS containing a STI belonging to the subscribed user in the Replaces header field or Target-Dialog header field and not containing Inter UE Transfer SCC AS URI in the Request-URI. In the procedures below, such requests are known as "SIP INVITE requests due to STI".

– SIP INVITE requests routed to the SCC AS containing either a dynamic STN, a static STN or an IMRN in the Request-URI. In the procedures below, such requests are known as "SIP INVITE requests due to PS to CS STN".

– SIP INVITE requests routed to the SCC AS containing a static STI in the Request-URI and a STI in the Replaces or Target-Dialog header field. In the procedures below, such requests are known as "SIP INVITE requests due to two STIs".

NOTE: The media streams that need to be transferred are identified using information described in the subsequent subclauses 11.3.2 and 11.3.3.

Other SIP initial requests for a dialog and requests for a SIP standalone transaction can be dealt with in any manner conformant with 3GPP TS 24.229 [2].

11.3.2 SCC AS procedures for PS to PS+CS access transfer

This subclause does not apply to reception of a SIP INVITE request due to STI with a CS media.

When the SCC AS receives a SIP INVITE request due to STI with PS and CS media on the Target Access Leg, the SCC AS shall follow the PS-PS Access Transfer procedures specified in subclause 10.3.2. with the following exceptions:

If the SIP INVITE request includes an active speech media component using CS bearer, then the SCC AS shall follow the procedures for SCC AS for service control over Gm in 3GPP TS 24.292 [4] to send the PSI DN to the SC UE and wait for the SC UE to set up CS bearer before sending a SIP re-INVITE request to the remote UE.

– The SCC AS shall correlate the STI with the allocated PSI DN in order to identify the remote leg to be updated.

– If service control over Gm is retained on the source access leg, and the SCC AS receives a SIP re-INVITE request indicating CS bearer on an existing session, the SCC AS shall follow procedures as described in 3GPP TS 24.292 [4] subclause 8.4.2 to send the PSI DN to the SC UE and wait for the SC UE to set up CS bearer before sending a SIP re-INVITE request to the remote end.

– The SCC AS shall include a new SDP offer in the SIP re-INVITE request, following the rules specified in 3GPP TS 24.229 [2], containing the following media information:

– the media characteristics as received in the SIP INVITE request due to STI with PS+CS media received on the Target Access Leg for media streams whose port is not set to zero; and

– the media characteristics as received in the SIP re-INVITE request for media streams whose port is not set to zero.

When the SCC AS receives a SIP INVITE request due to PS to CS STN on the Target Access Leg, the SCC AS shall follow the PS-CS Access Transfer procedures specified in subclause 9.3.2. However, as the Source Access Leg contains media components other than speech media component, the SCC AS does not initiate release for Source Access Leg.

11.3.3 SCC AS procedures for PS+CS to PS access transfer

This subclause applies to reception of a SIP INVITE request due to STI with a PS media only.

When the SCC AS receives a SIP INVITE request due to STI on the Target Access Leg, the SCC AS shall follow the PS-PS access transfer procedures specified in subclause 10.3.2.

When the SCC AS receives a SIP INVITE request due to two STIs on the Target Access Leg, the SCC AS shall:

– associate the SIP INVITE request received on the Target Access Leg with two ongoing sessions:

a) an ongoing SIP dialog on the PS Source Access Leg: This is done by matching the dialog ID present in the Replaces header field (see IETF RFC 3891 [10]) or Target-Dialog header field (see IETF RFC 4538 [11]) of the SIP INVITE request with an ongoing dialog. By an ongoing SIP dialog, it is meant a dialog for which a SIP 2xx response to the initial SIP INVITE request has been sent or received; and

b) a different ongoing SIP dialog with active speech media component;

– if the SCC AS is unable to associate the SIP INVITE request with either one of the above two dialogs, send a SIP 480 (Temporarily Unavailable) response to reject the SIP INVITE request relating to the access transfer and not process the remaining steps; and

– if the session transfer is possible:

a) follow the procedures defined in IETF RFC 3891 [10] for replacing the two sessions on the Source Access Legs with the SIP request received on the Target Access Leg, including terminating the two Source Access Legs by sending a SIP BYE request on each session towards the SC UE in accordance with 3GPP TS 24.229 [2]; and

b) send a SIP re-INVITE request towards the remote UE using the existing established dialog. The SCC AS shall populate the SIP re-INVITE request as follows:

1) set the Request-URI to the URI contained in the Contact header field returned at the creation of the dialog with the remote UE; and

2) a new SDP offer, including the media characteristics as received in the SIP INVITE request due to two STIs received on the Target Access Leg, by following the rules of 3GPP TS 24.229 [2].