5.17.2.3.1 General

For interworking without the N26 interface, IP address preservation is provided to the UEs on inter-system mobility by storing and fetching SMF+PGW-C and corresponding APN/DNN information via the HSS+UDM. In such networks AMF also provides an indication that interworking without N26 is supported to UEs during Initial Registration in 5GC or MME may optionally provide an indication that interworking without N26 is supported in the Attach procedure in EPC as defined in TS 23.502 [3] and TS 23.401 [26]. The UE provides an indication that it supports Request Type flag "handover" for PDN connectivity request during the attach procedure as described in clause 5.3.2.1 of TS 23.401 [26] and during initial Registration and Mobility Registration Update in 5GC.

NOTE 1: The UE support of Request Type flag "handover" for PDN connectivity request during the attach procedure is needed for IP address preservation in the case of interworking without N26.

The indication that interworking without N26 is valid for the entire Registered PLMN and for PLMNs equivalent to the Registered PLMN that are available in the Registration Area. The same indication is provided to all UEs served by the same PLMN. UEs that operate in interworking without N26 may use this indication to decide whether to register early in the target system. UEs that only support single registration mode may use this indication as described in clause 5.17.2.3.2. UE that support dual registration mode uses this indication as described in clause 5.17.2.3.3.

Interworking procedures without N26 interface use the following two features:

1. When UE performs Initial Attach in EPC (with or without "Handover" indication in PDN CONNECTIVITY Request message) and indicates that it is moving from 5GC, the MME indicates to the HSS+UDM not to cancel the registration of AMF, if any.

2. When UE performs Initial Registration in 5GC and indicates that it is moving from EPC, the AMF indicates to the HSS+UDM not to cancel the registration of MME, if any.

To support mobility both for single and dual registration mode UEs, the following also are supported by the network:

3. When PDU Session are created in 5GC, the SMF+PGW-C which supports EPS interworking stores the SMF+PGW-C FQDN along with DNN in the HSS+UDM.

4. The HSS+UDM provides the information about dynamically allocated SMF+PGW-C and APN/DNN information to the target CN network. If there are multiple SMF+PGW-C serving the UE for the same DNN which support EPS interworking in 5GS, the HSS+UDM select one of them according to operator's policy and provides together with the associated APN to the MME.

5. When PDN connections are created in EPC, the MME stores the SMF+PGW-C and APN information in the HSS+UDM.

NOTE 2: Items 3, 4 and 5 are also supported in networks that support interworking with N26 procedures. This enables a VPLMN that does not deploy N26 interface to provide IP address preservation to roamed-in single-registration mode UEs from a HPLMN that only supports interworking with N26 procedures.

When the network serving the UE supports 5GS-EPS interworking procedures without N26 interface, the SMF shall not provide the UEs with mapped target system parameters of the target system when UE is in the source network.

A UE that operates in dual registration mode ignores any received mapped target system parameters (e.g. QoS parameters, bearer IDs/QFI, PDU Session ID, etc.).