4.8.3 Dual-registration mode

If both 5GMM and EMM are enabled, a UE, operating in the dual-registration mode shall maintain independent contexts for 5GMM and EMM and this includes independent lists of equivalent PLMNs. Coordination between 5GMM and EMM is not needed, except as specified in the present clause, clause 4.2A, 5.1.5 and 5.3.13A.

For dual-registration mode the following applies:

• a UE operating in the dual-registration mode may register to N1 mode only, S1 mode only, or to both N1 mode and S1 mode;
• when the UE decides to operate in dual-registration mode (see clause 5.5.1.2.4), NAS informs the lower layers about this;
• if a UE is registered in N1 mode only, then for registration in S1 mode the UE shall use:
• the same PLMN to which it is registered in N1 mode; or
• an equivalent PLMN; and
• if a UE is registered in S1 mode only, then for registration in N1 mode the UE shall use:
• the same PLMN to which it is registered in S1 mode; or
• an equivalent PLMN.

NOTE 1: It is up to UE implementation how to handle the case when the UE is registered in both N1 mode and S1 mode and the PLMNs to which the UE is registered, are not equivalent, e.g. search for a PLMN which is the same or equivalent to any of the registered ones.

When no PDU session is active and the UE has not registered to S1 mode yet, the UE may initiate the EPS attach procedure with PDN connection establishment if EMM-REGISTERED without PDN connection is not supported by the MME. If EMM-REGISTERED without PDN connection is supported by the MME, the UE may initiate either the EPS attach procedure without PDN connection establishment or the attach procedure with PDN connection establishment.

When at least one PDU session is active and the UE has not registered to S1 mode yet, the UE may initiate the EPS attach procedure. If necessary, the UE may transfer an active PDU session from N1 mode to S1 mode by initiating the EPS attach procedure with request type set to "handover" in the PDN CONNECTIVITY REQUEST message. After successfully attached in S1 mode, if necessary, the UE may transfer other active PDU sessions from N1 mode to S1 mode by initiating the PDN connectivity procedure with request type set to "handover" in the PDN CONNECTIVITY REQUEST message.

NOTE 2: It is up to UE implementation to determine which active PDU session is transferred from N1 mode to S1 mode.

When the UE has not registered to N1 mode, the UE may initiate the initial registration procedure. After successfully registered in N1 mode, if necessary, the UE may transfer one or more active PDN connections from S1 mode to N1 mode by initiating the PDU session establishment procedure with request type set to "existing PDU session".

NOTE 3: It is up to UE implementation to determine which active PDN connection is transferred from S1 mode to N1 mode.

If the MME supports EMM-REGISTERED without PDN connection, the UE that transferred all PDN connections to the 5GS, may stay in state EMM-REGISTERED. Otherwise, the UE shall enter state EMM-DEREGISTERED upon transferring all PDN connection to the 5GS.

NOTE 4: When the UE has registered in both N1 mode and S1 mode, it is up to UE implementation to maintain the registration update to date in both N1 mode and S1 mode.

See clause 6.1.4 for coordination between 5GSM and ESM.

See clause 4.8.2.3.2 for interworking between TNGF or N3IWF connected to 5GCN and E-UTRAN.