This clause describes the main IP-Connectivity Access Network specific concepts that are used for the provisioning of IMS services over untrusted WLAN access to EPC.
Following specific considerations apply to the case of untrusted WLAN access:
The Mobility related procedures for untrusted WLAN access to EPC are described in TS 23.402.
For untrusted WLAN access, ePDG is used to interface the access network and to control relevant PDN connectivity (over S2b) towards a P-GW.
For untrusted WLAN access, the way the notification of the loss of IP-CAN session for a UE is triggered within an untrusted WLAN is out of scope of 3GPP specifications.
For untrusted WLAN access, the Reporting of User location information (i.e. UE local IP address and Network Provided WLAN Location Information) in the EPS is defined in clauses 126.96.36.199.8, 188.8.131.52, 7.9.2, 7.10 and 7.11.14 of TS 23.402 and the related PCC Access Network Information reporting procedure is defined in clause H.4 of TS 23.203. Information flows on how user location information can be further distributed within IMS can be found in Annex R.
In order to fulfil regulatory requirements, IMS requires in addition to the UE local IP address, the identity of the ePDG to which the UE is connected, and the UE source port (TCP port or UDP port) used by the UE to establish the IKEv2 tunnel with the ePDG.
A UE accessing IMS via untrusted WLAN shall support the following:
If available, provide the identity of the WLAN AP the UE is currently associated with, and used for IMS signalling at IMS registration, IMS emergency registration, IMS session initiation, SMS over IP, and in any procedure defined in TS 24.229, where access network information is provided.
If available, provide geographical location coordinates during IMS emergency session setup.
Provide the cell information (cell-ID) for the most recent seen cell at IMS registration, IMS emergency registration, IMS emergency session initiation, and in any procedure defined in TS 24.229 where access network information is provided. The cell-ID access information shall include additional information describing when the information about the cell-ID was collected and that can be used to calculate the "age" of the information. The information about the cell-ID shall be included in an appropriate field of the SIP request that is distinguished from the information about the access network currently used to transport SIP signalling (i.e. Untrusted WLAN).
This clause describes the functions that are used to support IMS services for roaming users in deployments without IMS-level roaming interfaces. This annex is applicable to UTRAN and E-UTRAN using EPC access and to 5GC. In this roaming model the PGW is located in the home PLMN and therefore UE IMS signalling and user plane are routed to home PLMN.
The architecture to support IMS services for roaming users, including Voice over IMS, in deployments without IMS-level roaming interfaces is shown in Figure W.2-1
The following architecture requirements apply:
P-CSCF (at HPLMN) identifies the serving network (VPLMN) where the UE is located using the procedure defined in clause W.3.
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Figure W.2-1: IMS traffic home routed
The corresponding architecture for 5GS is defined in clause Y.9.2.
In LBO roaming model where P-CSCF is located in VPLMN (see Annex M.1), the home network determines the serving PLMN of the UE from the location of the P-CSCF during initial IMS Registration, using the P-CSCF network identifier.
In deployments without IMS-level roaming interfaces, the home network determines the serving PLMN of the UE using procedure defined in TS 23.203 or TS 23.503, where 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 corresponding architecture for 5GS is defined in Annex Y. In this case the P-CSCF interfaces with a PCF instead of interfacing with a PCRF.
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Figure W.3-1: Subscription by P-CSCF to changes in PLMN ID during initial IMS Registration
3GPP PS Data Off is an optional feature. When activated by the user and when the network supports the feature, this feature allows control of the IMS services and more broadly SIP based services using the IMS framework that the user is allowed to access, for both originating and terminating sessions.
The list of 3GPP PS Data Off Exempted Services is configured by the HPLMN in the UE and in the network for enforcement. The list of SIP based service can include any one or any combination of the following services:
SMS over IMS;
Particular IMS Services not defined by 3GPP, where each such IMS service is identified by an IMS communication service identifier.
The UE shall include an indication that depicts the 3GPP PS Data Off status (active/inactive) at initial IMS registration, and subsequent to that, any time the end user changes the 3GPP PS Data Off status in a (re-)REGISTER request. In all these registration requests the UE shall register the SIP based services that are configured in the UE.
The UE and the network shall ensure that the proper services are enforced in accordance with the 3GPP PS Data Off status.
The UE may be provisoned by HPLMN with up to two enumerated lists of SIP-based services that are 3GPP PS Data Off exempted either via Device Management or in the UICC, one list is valid for the UEs camping in the home PLMN and the other list is valid for any VPLMN the UE is roaming in. When the UE is configured only with a single list, without an indication to which PLMNs the list is applicable, then this list is valid for the home PLMN and any PLMN the UE is roaming in.
A UE provisioned with an updated list shall enforce the updated list immediately.
When the UE changes its 3GPP PS Data Off status from inactive to active, the UE shall ensure that only the 3GPP PS Data Off Exempted Services in the provisioned list are allowed to be transported, and the corresponding IP uplink packets shall be sent accordingly as follows:
The UE shall prevent sending of UE-originating SIP requests which are for services other than the 3GPP PS Data Off Exempted Services configured in the UE via device management or in the UICC.
The UE shall prevent sending of SDP offers and SDP answers with media streams for the media types other than those related to the 3GPP PS Data Off Exempted Services configured via device management or in the UICC.
A UE shall immediately stop sending any media packets and shall terminate all ongoing sessions for all SIP-based services that are not 3GPP PS Data Off Exempt.
A UE provisioned with an updated list shall enforce the updated list immediately. To that effect, the UE shall immediately stop sending any media packets and terminate all ongoing SIP-based sessions handling the 3GPP PS Data Off Exempt Service(s) removed from the list.
The HPLMN network shall provision in the UE either via Device Management or in the UICC the lists of 3GPP PS Data Off Exempt Services. Up to two lists are provisioned in the UE for that purpose: one list is used when the UE is at home, while the second list is used when the UE is roaming in any visited PLMN.
The information shall be provisioned in the UE prior to enabling the 3GPP PS Data Off Service. Any change to either one or both lists shall immediately be sent to the UE for enforcement.
Application Servers implementing the SIP-based services shall enforce the SIP based 3GPP PS Data Off Exempted services for all UEs.
Each Application Server shall be configured with up to two lists of 3GPP PS Data Off Exempt Services, one list for non-roaming users, and the other list for users roaming in the various VPLMNs with whom roaming agreements exist.
The AS shall become aware of the UE 3GPP Data Off status (active/inactive) at IMS (re-)Registration through third party registration. If the UE has changed its 3GPP PS Data Off status from inactive to active, the AS shall ensure that only SIP-based services which are part of the SIP-based 3GPP PS Data Off Exempt Services are permitted.
If the UE has changed its 3GPP PS Data Off status from active to inactive, the AS shall also let through the terminating requests to the UE for services that were not Data Off exempt.