W.4 Procedures related to PLMN ID change

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

W.4.1 Subscription to changes of PLMN ID at IMS Initial Registration

In deployments without IMS-level roaming interfaces, the home network determines the serving PLMN of the UE using procedure defined in TS 23.203 [54], TS 23.503 [95] or TS 29.214 [11], where the P-CSCF requests the PCRF/PCF to report the PLMN identifier where the UE is currently located. The received PLMN ID information is then forwarded in the SIP REGISTER request.

This procedure shall be applied by the P-CSCF at initial UE IMS registration.

The below procedure relates to EPS, the corresponding procedure for 5GS is defined in Annex Y.

Figure W.4.1-1: Subscription by P-CSCF to changes in PLMN ID during initial IMS Registration

1. The UE sends a SIP REGISTER request to the P-CSCF.

2. If this is the initial IMS registration then the P-CSCF subscribes to the PCRF/PCF to be notified of the PLMN ID where the UE is currently attached. The subscription to PLMN changes is active as long as the UE is IMS registered.

3. The PCRF/PCF forwards the PLMN ID to the P-CSCF. The P-CSCF stores the PLMN ID.

4. The P-CSCF includes the received PLMN ID in the SIP REGISTER request before forwarding the request to the I-CSCF.

5. Normal IMS registration procedure is then completed.

W.4.2 UE is not active in an IMS voice session

The following procedure shall be applied by IMS at reception of PLMN change when the UE is not active in an IMS voice call, and when the PDN connection/PDU session is kept at PLMN change.

NOTE 1: This clause assumes that the UE has not established an IMS session for other services. Additional functionality covering non-voice IMS sessions is up to future enhancements.

Figure W.4.2-1: Procedure for PLMN change

1. EPC/5GC detects PLMN change.

2. H-PCRF/PCF notifies P-CSCF that PLMN change has occurred and indicates the new PLMN ID.

3. P-CSCF sends SIP MESSAGE with new PLMN ID to S-CSCF.

4. Based on regulatory requirements (see NOTE 2), S-CSCF sends SIP NOTIFY with re-authentication request to the UE.

NOTE 2: Regulatory requirements in the country of the new PLMN could require the possibility to get access to IMS signalling information. If the source PLMN uses encryption for IMS signalling, the Re-Registration procedure can trigger the HPLMN to change encryption to use null encryption which would enable the new PLMN to get access to future IMS signalling information. Also, the Re-registration can trigger to turn on encryption in the new PLMN if the HPLMN requires it.

5. UE initiates re-registration procedure. During the re-registration procedure, the P-CSCF may update the SIP signalling encryption depending on roaming agreements, e.g., moving from HPLMN to VPLMN can result in turning off IMS encryption while retaining integrity protection of IMS signalling.

W.4.3 UE is active in an IMS voice session

The following procedure shall be applied by IMS at reception of PLMN change when the UE is active in an IMS voice call, and when the PDN connection/PDU session is kept at PLMN change.

Figure W.4.3-1: PLMN change detected by EPC/5GC

0. UE is active in an IMS voice call.

1. EPC/5GC detects PLMN change.

2. H-PCRF/PCF notifies the P-CSCF that PLMN change has occurred and indicates the new PLMN ID.

3. P-CSCF sends SIP MESSAGE with the new PLMN ID to the S-CSCF.

3a. P-CSCF sends SIP MESSAGE with new PLMN ID to the TAS so that the TAS may update any charging records with the new PLMN ID.

4. Based on regulatory requirements (see NOTE 1), S-CSCF sends SIP NOTIFY with re-authentication request to the UE.

5. UE initiates re-registration procedure. During the re-registration procedure, the P-CSCF may update the SIP signalling encryption depending on roaming agreements, e.g., moving from HPLMN to VPLMN can result in turning off IMS encryption while retaining integrity protection of IMS signalling.

6. Once the re-registration procedure is completed and based on regulatory requirements (see NOTE), the P-CSCF may send a SIP MESSAGE to the TAS indicating that TAS should send a re-INVITE request to the UE.

7. TAS sends SIP Re-INVITE to UE including, subject to the privacy settings associated with the original call, call related information (e.g. calling and called party information, indication whether UE was calling party or called party in the call, redirecting party information (if present), media endpoints and used codecs). Codec endpoints are translated in the P-CSCF if needed. The UE should not reveal this information to the user.

8. UE sends SIP 200 OK to TAS.

NOTE: Whether the P-CSCF executes step 4 and/or step 6 depends on regulatory requirements of the new PLMN. Regulatory requirements in the country of the new PLMN could require the possibility to get access to IMS signalling information. If the source PLMN uses encryption for IMS signalling, the IMS Re-Registration procedure can trigger the HPLMN to change encryption to use null encryption which would enable the new PLMN to get access to future IMS signalling information. Also, the IMS Re-registration can trigger to turn on encryption in the new PLMN if required by the HPLMN. Sending SIP Re-Invite request to the UE enables the new PLMN to get access to call related information.

Annex X (normative):
IMS 3GPP PS Data Off Service Accessibility