13 Roles for media adding/deleting for access transfer

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

13.1 Introduction

This clause specifies the procedures for adding or deleting media to an existing multimedia session. Procedures are specified for the SC UE and the SCC AS.

13.2 SC UE

13.2.1 Adding or removing media through Gm

If the SC UE wants to add or remove media components to a session that was previously established using Gm reference point, the SC UE shall follow the procedures defined in 3GPP TS 24.229 [2] for adding/removing PS media.

If the SC UE wants to transfer media components from the source access leg to an existing target access leg (i.e. the access legs were previously established due to the partial session transfer) using Gm reference point, the SC UE shall:

1. add the media components to the target access leg; and

2. remove those media components from the source access leg,

by using procedures defined in 3GPP TS 24.229 [2] for adding/removing PS media.

If SC using ICS is enabled then if the SC UE wants to add or remove CS media components to a session, the SC UE shall follow the procedures defined in 3GPP TS 24.292 [4].

If the SC UE receives a SIP re-INVITE request or a SIP UPDATE request from the remote UE to add or remove media components to a session that was previously established using Gm, the SC UE shall:

– follow the procedures defined in 3GPP TS 24.229 [2] for adding or removing PS media; and

– if SC using ICS is enabled, follow the procedures defined in 3GPP TS 24.292 [4] for adding or removing CS media to the session.

13.2.2 Adding Gm control to existing CS session

The SC UE shall add Gm control to an existing CS session only when SC using ICS is enabled and when there is a single full-duplex session with speech media component over CS. If there is more than one full-duplex session with speech media component, the SC UE shall release all the ongoing sessions that are not currently active before attempting the procedures described in this section.

If SC using ICS is enabled and the SC UE wants to add Gm control to an existing CS session that was established without Gm, after registering with the IM CN subsystem, the SC UE shall send an initial SIP INVITE request over the PS access in accordance with 3GPP TS 24.229 [2]. The SC UE shall populate the SIP INVITE request as follows:

– set the Request-URI to the static STI;

– set the SDP payload, in accordance with the procedures defined in 3GPP TS 24.292 [4], proposing a speech media component over a circuit switched bearer. The SC UE can optionally include additional PS media to the SDP in accordance to the procedures defined in 3GPP TS 24.229 [2]; and

– signalling elements described in subclause 6A.2.2.2.

Upon receiving a SIP 200 (OK) response, the SC UE shall treat the ongoing CS call as established using Gm and shall follow the "ICS UE using Gm" procedures defined in 3GPP TS 24.292 [4] for controlling the CS call.

If SC using ICS is enabled and the SC UE receives a new SIP INVITE request containing a speech media component over a circuit-switched bearer in the SDP and the SCC AS PSI DN matches the B-party number of the ongoing CS call that was established without Gm, the SC UE shall:

– respond to the SIP INVITE request in accordance with the procedures defined in 3GPP TS 24.292 [4]; and

– treat the ongoing CS call as established using Gm and shall follow the "ICS UE using Gm" procedures defined in 3GPP TS 24.292 [4] for controlling the CS call.

13.3 SCC AS

13.3.1 Adding or removing media through Gm

If the SCC AS receives a SIP re-INVITE request or a SIP UPDATE request from the SC UE, in which already existing media components of the session are transferred from a source access leg to an already existing target access leg (i.e. the target access leg was already established due to partial session transfer), the SCC AS shall update the remote UE using the session transfer procedures defined in subclause 10.3.2.

NOTE: The SC UE indicates that media is switched from the source access leg to the target access leg by using the procedures defined in 3GPP TS 24.229 [2] for adding / removing PS media, i.e. the related connection and port information of the transferred media component within the SDP is changed from the source access leg to the target access leg.

If the SCC AS receives a SIP re-INVITE request or a SIP UPDATE request from the SC UE or remote UE to add/remove new media components, to an existing access leg of the session established using Gm, the SCC AS shall follow the procedures defined in 3GPP TS 24.229 [2] for adding or removing PS media and shall follow the procedures defined in 3GPP TS 24.292 [4] for adding or removing CS media to the session. If both a PS access leg and a CS access leg exist, the SCC AS shall offer on the PS access leg any new media components offered by the remote UE. If no PS access leg exists and a CS access leg exists, the SCC AS shall reject any new media components offered by the remote UE by setting the port to zero in an SDP answer sent on the remote leg. If a CS access leg exists, the SCC AS shall not offer any new media components offered by the remote UE via a CS access leg.

13.3.2 Adding Gm control to existing CS session

If the SCC AS receives a SIP INVITE request containing the static STI in the Request-URI or receives a SIP re-INVITE request containing one or more already existing media lines in the SDP for audio indicating a CS bearer as defined in IETF RFC 7195 [73] the SCC AS shall:

1) determine if this SIP INVITE request or SIP re-INVITE request is for an ongoing call by determining if the received contents of the Contact header field of the SIP request is bound to an ongoing CS call session identifier;

2) if the SC UE has an ongoing CS call:

– respond to the SIP INVITE request or SIP re-INVITE request in accordance with the procedures defined in 3GPP TS 24.292 [4]. The SCC AS shall populate the SIP INVITE request with signalling elements described in subclause 6A.4.3; and

– treat the ongoing CS call as established using Gm and shall follow the "SCC AS for service control over Gm" procedures defined in 3GPP TS 24.292 [4] for controlling the CS call; and

3) if the SIP INVITE request or SIP re-INVITE request contains additional PS media, the SCC AS shall send a SIP re-INVITE request towards the remote UE, including the newly added PS media, in accordance with the procedures defined in 3GPP TS 24.229 [2].

NOTE: When using the ICS controlled CS bearer, only one audio call can be active at a time. Nevertheless, several calls can be held in parallel. If the user decides to switch to another (previously held) call, the ICS controlled CS bearer is re-used for this call. Therefore no specific procedures for handling of held calls in the case of ICS controlled CS bearer are needed.

The SCC AS shall add Gm control to an existing CS session only when there is a single full-session with speech media component over CS. If the SCC AS wants to add Gm control to an existing CS session that was established without Gm, the SCC AS shall send a new SIP INVITE request over the PS access in accordance with 3GPP TS 24.229 [2]. The SCC AS shall populate the SIP INVITE request as follows:

– set the Request-URI to the public user identity of the UE; and

– set the SDP payload, in accordance with the procedures defined in 3GPP TS 24.292 [4], proposing speech media component over a circuit switched bearer.

Upon receiving a SIP 200 (OK) response, the SCC AS shall treat the ongoing CS call as established using Gm and shall follow the "SCC AS for service control over Gm" procedures defined in 3GPP TS 24.292 [4] for controlling the CS call.