E.1 Mobility related concepts

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

E.1.0 General

The Mobility related procedures for GPRS and EPS are described in TS 23.060 [23] and TS 23.401 [70] respectively and the IP address management principles are described in TS 23.221 [7]. As specified by the GPRS/EPS procedures, the UE shall acquire the necessary IP address(es) as part of the PDP Context activation procedures for GERAN/UTRAN access and Attach (or PDN connectivity)/EPS bearer activation procedure(s) for E-UTRAN.

If the UE changes its IP address due to changes triggered by the GPRS/EPS procedures or according to TS 23.221 [7], then the UE shall re- register in the IMS.

If the UE acquires an additional IP address, then the UE may perform an IMS registration using this additional IP address as the contact address. If IMS registration is performed, this IMS registration may co-exist with the previous IMS registration from this UE and the UE shall be notified that this IMS registration results in multiple simultaneous registrations.

NOTE: The UE can acquire an additional IP address that can be used for registration to the IMS only outside of the EPS.

When a routeing area update or tracking area update is not performed due to the Idle mode Signalling Reduction feature being active (see TS 23.401 [70] for more information), then the UE shall also not perform an IMS re registration because of such routeing area and tracking area change.

Similarly, the UE shall not perform an IMS re registration because the UE has moved between GERAN and UTRAN cells that share the same RAI.

When the PLMN changes, and the attempt to perform an inter-PLMN routeing area update or tracking area update is unsuccessful, then the UE should attempt to re-attach to the network using GPRS/EPS procedures and register for IMS services. Typically this will involve a different GGSN/P‑GW.

In Dual Connectivity with EPC case, the UE shall use the access network information based on the primary cell of the Master RAN node that is serving the UE for network location information when the UE interacts with IMS, regardless whether the IMS traffic is routed via the Master RAN node or the Secondary RAN node or both.

E.1.1 Procedures for P‑CSCF discovery

E.1.1.0 General

This clause describes the P‑CSCF discovery procedures applicable for GERAN/UTRAN access. All the procedures described in clause 5.1.1 apply with the following additions:

P‑CSCF discovery shall take place after GPRS/EPS attach and after or as part of a successful activation of a PDP context (in the case of GERAN/UTRAN access) and EPS bearer (in the case of E-UTRAN access) for IMS signalling using the following mechanisms:

a. For GERAN/UTRAN access: Transfer a Proxy‑CSCF address within the PDP Context Activation signalling to the UE, as described in clause E.1.1.1. The UE shall request the P‑CSCF address(es) when activating the PDP context. The GGSN/P‑GW shall send the P‑CSCF address(es) to the UE when accepting the PDP context activation. Both the P‑CSCF address(es) request and the P‑CSCF address(es) shall be sent transparently through the SGSN/S‑GW for GERAN/UTRAN.

b. For E-UTRAN access: Transfer a P‑CSCF address within the EPS Attach or PDN Connectivity Procedures to the UE, as described in clause E.1.1.1. The UE shall request the P‑CSCF address(es) in the EPS Attach or PDN Connectivity request. The P‑GW shall send the P‑CSCF address(es) to the UE when accepting the EPS Default bearer activation. Both the P‑CSCF address(es) request and the P‑CSCF address(es) shall be sent transparently through the intermediate network entities (e.g. MME/S‑GW).

When using DHCP/DNS procedure for P‑CSCF discovery (according to the mechanisms described in clause 5.1.1.1) with GPRS/EPS, the GGSN/P‑GW acts as DHCP Relay agent relaying DHCP messages between UE and the DHCP server.

E.1.1.1 GPRS/EPS procedure for P‑CSCF discovery

This alternative shall be used for UE(s) not supporting DHCP. This may also be used for UE(s) supporting DHCP.

Figure E.1: P‑CSCF discovery using PDP Context Activation signalling

1. The UE requests establishment of a PDP context according to clause 4.2.6 (QoS requirements for IM CN subsystem signalling). The UE indicates that it requests a P‑CSCF IP address(es). The indication is forwarded transparently by the SGSN to the GGSN in the Create PDP Context Request or to the S‑GW/P‑GW in the Create Default Bearer Request.

2. The GGSN/P‑GW gets the IP address(es) of the P‑CSCF(s). The mechanism to do this is a matter of internal configuration and is an implementation choice.

3. If requested by the UE, the GGSN/P‑GW includes the IP address(es) of the P‑CSCF(s) in the Create PDP Context Response or in the Create Default bearer response. The P‑CSCF address(es) is forwarded transparently by the SGSN to the UE.

After reception of the IP address of a P‑CSCF the UE may initiate communication towards the IM CN Subsystem.

NOTE: This request of a P‑CSCF IP address(es) and response is not transparent for pre-R5 SGSN when using the Secondary PDP Context Activation Procedure as defined in TS 23.060 [23].

E-UTRAN access only:

The procedure for E-UTRAN access applies to both Initial E-UTRAN Attach and PDN Connectivity Request.

Figure E.2: P‑CSCF discovery using EPS bearer activation signalling

1. During Initial Attach/PDN Connection Request, the UE indicates that it requests a P‑CSCF IP address(es).

2. The MME sends a Create Default Bearer Request to the S‑GW.

3. The S‑GW forwards the request to the P‑GW and the P‑GW gets the IP address(es) of the P‑CSCF(s). The mechanism to do this is a matter of internal configuration and is an implementation choice.

4. If requested by the UE, the P‑GW includes the IP address(es) of the P‑CSCF(s) in the Create Default Bearer Response.

5. The S‑GW forwards the response to the MME

6. Completion of procedures, as described in TS 23.401 [70].

After reception of the IP address of a P‑CSCF the UE may initiate communication towards the IM CN Subsystem.

E.1.2 Support for Enhanced Coverage for data centric UEs

Support for Enhanced Coverage (CE) for data centric UE is specified in TS 23.401 [70].

If the UE’s usage setting is set to "data centric" as defined in TS 23.221 [7] and it is operating in CE mode B then IMS PS voice/video services are not to be used in this IP-CAN.

If the UE’s usage setting is set to "data centric" and it is operating in CE mode B, then the UE shall reject any SIP invite for IMS PS voice/video services as per the existing IMS procedures. In addition, the UE may deregister from IMS PS voice/video services.

When the UE determines that the radio conditions are suitable for IMS PS voice/video services (e.g. UE is in normal coverage or in CE mode A) then UE may (re-)register for IMS PS voice/video services.

NOTE: How UE determines that the radio conditions are suitable for voice/video services is left up to the UE implementation.