Content for  TS 24.501  Word version:  19.0.0

From the UE's NAS perspective, in general the procedures and messages defined for 5GMM and 5GSM are used over non-3GPP access as over 3GPP access. However, a number of aspects are different as described in the following subclauses.

The session management procedures defined over 3GPP access are re-used over non-3GPP access with the following exceptions:

1. Serving PLMN rate control does not apply for non-3GPP access;
2. Small data rate control does not apply for non-3GPP access;
3. Handling of 5GSM cause value #82 "maximum data rate per UE for user-plane integrity protection is too low" does not apply for non-3GPP access;
4. MBS does not apply for non-3GPP access;
5. Support of redundant PDU sessions does not apply for non-3GPP access; and
6. PDU set handling does not apply for non-3GPP access.

There are a number of situations in which the UE is unable to obtain normal service from a PLMN over non-3GPP access and the UE enters the limited service state over non-3GPP access. These include:

1. no USIM in the ME;
2. an "illegal UE" or "illegal ME" is received when registration, network-initiated de-registration or service request is performed (any USIM in the ME is then considered "invalid");
3. a "5GS services not allowed" is received when a registration, network-initiated de-registration or service request is performed;
4. a "PLMN not allowed" is received when registration, network-initiated de-registration or service request is performed;
5. a "Tracking area not allowed" is received when a registration, network-initiated de-registration or service request is performed;
6. a "Roaming not allowed in this tracking area" is received when a registration, network-initiated de-registration or service request is performed;
7. void; or
8. a "Serving network not authorized" is received when a registration or service request is performed.

In limited service state with a valid USIM in the UE, the network selection is performed as defined in TS 24.502. With the exception of performing initial registration for emergency services, no registration requests are made until a valid USIM is present. For registration for emergency services, the PLMN of the current N3IWF or TNGF is considered as the selected PLMN for the duration the UE is registered for emergency services.

A trusted WLAN interworking function (TWIF) provides functionalities for a non-5G capable over WLAN (N5CW) device to access 5GCN, including:

1. NAS signalling over N1 NAS signalling connection with AMF; and
2. PDU session establishment, modification and release on behalf of the N5CW device, over N2 connection with the AMF.

The TWIF registers on behalf of the N5CW device to an AMF according to subclause 5.5.1 by populating the parameters for the registration by using implementation specific default values which are the same for N5CW devices. The TWIF may request to establish a PDU session as specified in subclause 6.4.1.2 on behalf of the N5CW device upon receipt of an IP configuration request from the N5CW device by populating either all the required parameters or part of the required parameters for the PDU session establishment by using implementation specific default values from the TWIF's configuration. Only one PDU session is supported when N5CW device accessing 5GCN via the TWIF. Upon loss of the IP address of the N5CW device, the TWIF acting on behalf of the N5CW device shall initiate the UE-requested PDU session release procedure as defined in subclause 6.4.3.

In order to interwork with E-UTRAN connected to EPC, the UE supporting both S1 mode and N1 mode can operate in single-registration mode or dual-registration mode (see TS 23.501). Support of single-registration mode is mandatory for UEs supporting both S1 mode and N1 mode. During the EPS attach procedure (see TS 24.301) or initial registration procedure (see subclause 5.5.1.2), the mode for interworking is selected if the UE supports both S1 mode and N1 mode, and the network supports interworking. The mode for interworking may also be selected during the EPS tracking area updating procedure (see TS 24.301) or registration procedure for mobility and periodic registration update (see subclause 5.5.1.3). For interworking between E-UTRAN connected to EPC and TNGF or N3IWF connected to 5GCN, the UE shall operate as specified in either subclause 4.8.2.3 or subclause 4.8.3. Which subclause the UE follows is chosen by the UE irrespective of the interworking without N26 interface indicator.

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 subclause, subclause 5.1.5 and 5.3.13A. For dual-registration mode the following applies:

1. 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;
2. when the UE decides to operate in dual-registration mode (see subclause 5.5.1.2.4), NAS informs the lower layers about this;
3. if a UE is registered in N1 mode only, then for registration in S1 mode the UE shall use:
   1. the same PLMN to which it is registered in N1 mode; or
   2. an equivalent PLMN; and
4. if a UE is registered in S1 mode only, then for registration in N1 mode the UE shall use:
   1. the same PLMN to which it is registered in S1 mode; or
   2. an equivalent PLMN.

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. 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". 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. See subclause 6.1.4 for coordination between 5GSM and ESM. See subclause 4.8.2.3.2 for interworking between TNGF or N3IWF connected to 5GCN and E-UTRAN.

If the UE is capable of both N1 mode and S1 mode, when the UE needs to use one or more functionalities not supported in 5GS but supported in EPS and the UE is in 5GMM-IDLE mode, the UE may disable the N1 mode capability for 3GPP access (see subclause 4.9.2). If the UE is capable of both N1 mode and S1 mode and lower layers provide an indication that the current E-UTRA cell is connected to both EPC and 5GCN without also providing an indication that a target core network type was received from the NG-RAN, the UE shall select a core network type (EPC or 5GCN) based on the PLMN selection procedures as specified in TS 23.122 and provide the selected core network type information to the lower layer during the initial registration procedure. If the UE is capable of both N1 mode and S1 mode and the lower layers have provided an indication that the current E-UTRA cell is connected to both EPC and 5GCN and an indication of whether the network supports IMS emergency services via either EPC or 5GCN or both (see TS 36.331), the UE selects a core network type (EPC or 5GCN) as specified in TS 23.167 Annex H.2 for initiating emergency calls when in the state 5GMM-DEREGISTERED.LIMITED-SERVICE or EMM-DEREGISTERED.LIMITED-SERVICE. If the UE is capable of both N1 mode and S1 mode and lower layers provide an indication that the current E-UTRA cell is connected to both EPC and 5GCN with:

1. an indication that target core network type EPC was received from the NG-RAN, the UE shall select the EPC and proceed with the appropriate EMM procedure as specified in TS 24.301; or
2. an indication that target core network type 5GCN was received from the NG-RAN, the UE shall select the 5GCN and proceed with the appropriate 5GMM procedure.