9 Roles for call modifcation initiated towards an ICS UE
24.2923GPPIP Multimedia (IM) Core Network (CN) subsystem Centralized Services (ICS)Release 17Stage 3TS
9.1 Introduction
This clause specifies call modification procedures initiated towards an ICS UE for a call using a service control signalling path over Gm. The associated procedures for the ICS UE and the SCC AS are also specified in this clause; specifically, the procedure when a CS bearer is added or removed is specified. If a CS bearer is already used for the session, the ICS UE can add or remove video media to/from the existing CS bearer by applying the procedure in accordance with 3GPP TS 24.229 [11] and the CS SDP as described in IETF RFC 7195 [36].
9.2 ICS UE
9.2.1 General
This clause specifies the procedures for a UE when the call modification is initiated towards an ICS UE.
9.2.2 ICS UE using Gm
9.2.2.1 General
This subclause specifies call modification procedure when a session is modified towards an ICS UE using a service control signalling path over Gm; specifically, the procedure when a CS bearer is added or removed is specified.
9.2.2.2 ICS UE is offered a CS bearer
When the ICS UE receives a SIP INVITE request on an existing session containing a request for establishment of a CS bearer as described in by IETF RFC 7195 [36], the ICS UE shall:
1) send a reliable SIP 183 (Session Progress) response towards the IM CN subsystems as specified in 3GPP TS 24.229 [11]. The ICS UE shall populate the SIP 183 (Session Progress) response as follows:
i) include in the Contact header field:
– a public GRUU or temporary GRUU as specified in 3GPP TS 24.229 [11] if a GRUU was received at registration;
– the feature tag g.3gpp.ics set to "principal"; and
ii) the SDP answer accepting an audio stream over a circuit-switched bearer as described by IETF RFC 7195 [36], as follows:
– a "c=" line with the nettype portion set to "PSTN" and the addrtype portion and connection-address portions both set to "-";
– a CS media "m=" line with the media portion set to "audio", port portion set to "9", proto portion set to "PSTN" and fmt portion set to "-";
– an a=setup attribute set to "active";
– an a=cs-correlation attribute set to "callerid" along with the MSISDN, of ICS UE in the CS domain, in international E.164 number format;
– an a=connection attribute set to "new"; and
– an indication that the related local preconditions for QoS are not met as specified in 3GPP TS 24.229 [11];
b) shall setup a CS call by sending a CC SETUP message in accordance with 3GPP TS 24.008 [7] for 3GPP systems. The UE shall populate the CC SETUP message for 3GPP systems as follows:
i) the called party BCD number information element set to the SCC AS PSI DN received in the SDP body of the SIP INVITE request, in the connection-address portion of the "c=" line, appended to the "PSTN" nettype portion as described in IETF RFC 7195 [36]; and
c) when the CS resources are available to the UE, the ICS UE shall send an SDP offer including an indication that the related local preconditions for QoS for audio as met as specified in 3GPP TS 24.229 [11].
When the ICS UE originates a non-CS bearer call using Gm reference point, the ICS UE shall act in accordance with 3GPP TS 24.229 [11].
9.2.2.3 ICS UE is offered PS media
If the ICS UE is offered media in the PS domain using Gm reference point, the ICS UE shall act as specified in 3GPP TS 24.229 [11].
9.2.2.4 ICS UE is offered media both in CS and PS domain
If the ICS UE is offered media both in the PS domain using Gm reference point and the CS domain using a service control signalling path over Gm, the ICS UE shall act as specified in subclause 10.2.2.4.
9.2.2.5 SCC AS removes a CS bearer
When the ICS UE receives a SIP INVITE request from the network containing SDP with an indication that the CS bearer will not be used any longer with the port number set to zero for the CS media "m=" line with nettype portion set to "PSTN" as described in IETF RFC 7195 [36], the ICS UE shall release the CS bearer resources in accordance with 3GPP TS 24.008 [8].
9.2.2.6 SCC AS removes PS media
If the ICS UE is instructed to remove a media stream in the PS domain using Gm reference point, the ICS UE shall remove the media stream as specified in 3GPP TS 24.229 [11].
9.3 MSC server enhanced for ICS
No special procedure is required for call modification procedure at the MSC server enhanced for ICS.
9.4 SCC AS
9.4.1 Terminating Access domain selection
When the SCC AS receives on an existing session a SIP re-INVITE request or a SIP UPDATE request to add media stream(s) towards the ICS UE the SCC AS shall
– perform Terminating Access Domain Selection (T-ADS) based upon criteria described in 3GPP TS 23.292 [6];
– if T-ADS results in choosing to deliver all media in the PS domain, skip the following steps and continue with call termination in the IM CN subsystem as specified in subclause 9.4.3; and
– if T-ADS results in choosing to deliver media in the CS domain, and using Gm for service control, acting as a B2BUA, the SCC AS shall act in accordance with subclause 9.4.2.
9.4.2 SCC AS adds a CS bearer
When the SCC AS receives on an existing session a SIP re-INVITE request to add media stream(s) towards the ICS UE the SCC AS shall:
1) allocate an SCC AS PSI DN associated with the SCC AS and the INVITE request from the originating UE;
2) create a SIP INVITE request based upon the request from the originating UE and include the following:
i) the Request URI set to the earlier received contact address
ii) an SDP offer based on the received SDP offer from the originator and including the following:
– in the SDP offer include additional CS media "m=" line with media portion set to audio, port portion set to "9", proto portion set to "PSTN" and fmt portion set to"-" as described in IETF RFC 7195 [36];
– in the SDP offer include, a "c=" line with the nettype portion set to "PSTN" and with the addrtype portion set to "E164" and the connection-address portion set to the SCC AS PSI DN allocated in step 1), in international E.164 number format, in accordance with IETF RFC 7195 [36];
an a=setup attribute set to "passive";
– an a=connection attribute set to "new";
an a=cs-correlation attribute set to "callerid";
– an indication that preconditions are not met; and
3) route the created SIP INVITE request towards the terminating ICS UE.
When the SCC AS has received the 18x response containing the SDP answer accepting an audio stream over a circuit-switched bearer from the ICS UE and receives the initial SIP INVITE request with the Request-URI containing a valid SCC AS PSI DN encoded as a tel URI (see IETF RFC 3966 [42]) or SIP URI with the "user" SIP URI parameter set to "phone", in accordance with 3GPP TS 23.003 [4], subclause 13.5 as allocated in the above step 1 and with the P-Asserted-Identity header field containing the E.164 number received in the a=cs-correlation attribute contained in the SDP answer in the response to the SIP INVITE request received from the ICS UE then, the SCC AS shall:
NOTE: The SCC AS could receive the SIP INVITE request before the SIP 18x response. IETF RFC 7195 [36] provides guidance on handling this situation.
1) use the SCC AS PSI DN and E.164 number received in the a=cs-correlation attribute contained in the SDP answer from the response to the SIP INVITE request received from the ICS UE to correlate the SCC AS PSI DN against the incoming SIP INVITE request from the originating UE.
2) send a response to the initial SIP INVITE request with the Request-URI containing the SCC AS PSI DNin accordance with 3GGP TS 24.229 [11], indicating local preconditions met.
When the SCC AS has received the 18x response containing the SDP answer accepting an audio stream over a circuit-switched bearer from the ICS UE and the SIP INVITE request with the Request-URI containing the SCC AS PSI DN number, the SCC AS shall prepare an SDP answer to be sent to the remote UE. The SDP answer shall be based on the SDP answer received from the ICS UE and the SDP offer received from the CS domain. If the "m=" line of the SDP offer from the CS domain includes more than one codecs the SCC AS shall delete the lowest priority codecs. The status line in the response sent to the remote UE shall be the same as received in the 18x response from the ICS UE. The SDP answer sent to the remote UE shall be in accordance with IETF RFC 3264 [31].
When the SCC AS gets aware that the remote precondition is fulfilled on the leg towards the remote UE and on the leg with the CS domain, the SCC AS shall send a SIP UPDATE request to the ICS UE indicating that precondition is met.
When the SCC AS receives precondition is met onthe leg with the CS domain the SCC AS shall:
1) send a SIP 200 (OK) for the SIP UPDATE request on the leg with the CS domain; and
2) send a SIP 200 (OK) for the SIP INVITE request on the leg with the CS domain. The SCC AS may, based on operator policy, include a Feature-Caps header field defined in IETF RFC 6809 [48] with a "+g.3gpp.ics" header field parameter as described in clause B.4.
9.4.3 SCC AS adds PS media
When the SCC AS receives an SDP offer on an existing session a request to add or modify a non-voice stream towards the ICS UE the SCC AS shall act in routing AS in accordance with 3GPP TS 24.229 [11].
9.4.4 SCC AS removes a CS bearer
If the SCC AS receives an SDP offer from the originating UE that indicates that all the CS media streams are removed, the SCC AS shall send a SIP INVITE request towards the IM CN subsystems as specified in 3GPP TS 24.229 [11]. The SCC AS shall populate the SIP INVITE request as follows:
– the Request URI set to the contact address of the terminating ICS UE; and
– an indication that the CS bearer will not be used any longer by setting the port number to zero for the CS media "m=" line with nettype portion set to "PSTN" as described in IETF RFC 7195 [36].
9.4.5 SCC AS add/remove CS video media
In case of a CS bearer with video media already used for the session and the SCC AS decides to add or remove CS video media to/from the existing CS bearer, the SCC AS shall apply the procedure in accordance with 3GPP TS 24.229 [11] and the CS SDP as described in IETF RFC 7195 [36].