4.8 Interworking with E-UTRAN connected to EPC

24.5013GPPNon-Access-Stratum (NAS) protocol for 5G System (5GS)Release 17Stage 3TS

4.8.1 General

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 3GPP TS 23.501 [8]). Support of single-registration mode is mandatory for UEs supporting both S1 mode and N1 mode.

During the EPS attach procedure (see 3GPP TS 24.301 [15]) 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 3GPP TS 24.301 [15]) 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.

4.8.2 Single-registration mode

4.8.2.1 General

If the UE receives the indication that "interworking without N26 interface not supported" (see 3GPP TS 24.301 [15]), the UE operates as described in subclause 4.8.2.2.

If the UE receives the indication that "interworking without N26 interface supported" and

a) the UE does not support dual-registration mode; or

b) the UE supporting dual-registration mode determines to operate in single-registration mode,

the UE operates as described in subclause 4.8.2.3.

4.8.2.2 Single-registration mode with N26 interface

See subclause 5.1.4.2 for coordination between 5GMM and EMM and subclause 6.1.4.1 for coordination between 5GSM and ESM.

4.8.2.3 Single-registration mode without N26 interface

4.8.2.3.1 Interworking between NG-RAN and E-UTRAN

At inter-system change from N1 mode to S1 mode in EMM-IDLE mode when:

a) the UE supports non-IP PDN type and at least one PDU session of Unstructured PDU session type is active;

b) the UE supports IPv4 PDN type and at least one PDU session of IPv4 PDU session type is active;

c) the UE supports IPv6 PDN type and at least one PDU session of IPv6 PDU session type is active;

d) the UE supports IPv4v6 PDN type and at least one PDU session of IPv4v6 PDU session type is active; or

e) at least one PDU session of Ethernet PDU session type is active and:

1) the UE supports non-IP PDN type; or

2) the UE and the network support Ethernet PDN type in S1 mode;

the UE shall proceed as follows:

a) if the UE supports sending an ATTACH REQUEST message containing a PDN CONNECTIVITY REQUEST message with request type set to "handover" or "handover of emergency bearer services" to transfer a PDU session from N1 mode to S1 mode and the UE has received an "interworking without N26 interface supported" indication from the network, the UE shall:

1) enter substates EMM-DEREGISTERED.NORMAL-SERVICE and 5GMM-REGISTERED.NO-CELL-AVAILABLE for 3GPP access;

2) map the PDU session(s) which the UE intends to transfer to EPS to the default EPS bearer context of the corresponding PDN connection(s) as specified in subclause 6.1.4.2; and

3) initiate an EPS attach procedure and include in the ATTACH REQUEST message a PDN CONNECTIVITY REQUEST message with:

– the request type set to "handover of emergency bearer services" to activate a default EPS bearer context for an active emergency PDU session, if the session to be transferred is an emergency PDU session; or

– the request type set to "handover" message to activate a default EPS bearer context for an active non-emergency PDU session, if the session to be transferred is a non-emergency PDU session. If the selected PDU session is an MA PDU session established over 3GPP access, the UE shall include the ATSSS request parameter in the Extended protocol configuration options IE of the ESM INFORMATION RESPONSE message.

After successful completion of the EPS attach procedure, the UE shall reset the registration attempt counter for 3GPP access and the attach attempt counter (see 3GPP TS 24.301 [15]) and attempt to activate each of the other default EPS bearer contexts, if any, by initiating a stand-alone PDN connectivity procedure with request type set to "handover" for non-emergency PDU session or "handover of emergency bearer services" for emergency PDU session in the PDN CONNECTIVITY REQUEST message; and

b) otherwise, enter substates EMM-REGISTERED.NORMAL-SERVICE and 5GMM-REGISTERED.NO-CELL-AVAILABLE for 3GPP access and initiate a tracking area update procedure (see 3GPP TS 24.301 [15]).

At inter-system change from N1 mode to S1 mode in EMM-IDLE mode when:

a) the UE does not support non-IP PDN type or no PDU session of Unstructured PDU session type is active;

b) the UE does not support IPv4 PDN type or no PDU session of IPv4 PDU session type is active;

c) the UE does not support IPv6 PDN type or no PDU session of IPv6 PDU session type is active;

d) the UE does not support IPv4v6 PDN type or no PDU session of IPv4v6 PDU session type is active; and

e) no PDU session of Ethernet PDU session type is active or:

1) the UE does not support non-IP PDN type; and

2) the UE, the network or both do not support Ethernet PDN type in S1 mode;

the UE shall enter substates EMM-DEREGISTERED.NORMAL-SERVICE and 5GMM-DEREGISTERED.NO-CELL-AVAILABLE for 3GPP access, and initiate an attach procedure.

At inter-system change from S1 mode to N1 mode in 5GMM-IDLE mode, the UE shall:

a) enter substate 5GMM-REGISTERED.NORMAL-SERVICE for 3GPP access and substate EMM-REGISTERED.NO-CELL-AVAILABLE;

b) map the default EPS bearer context(s) of the PDN connection(s) which the UE intends to transfer to 5GS, if any, to the corresponding PDU session(s) as specified in subclause 6.1.4.2; and

c) initiate the registration procedure for mobility and periodic registration update over 3GPP access indicating "mobility registration updating" in the 5GS registration type IE of the REGISTRATION REQUEST message (see subclause 5.5.1.3).

After having successfully registered in N1 mode over 3GPP access, the UE shall reset the registration attempt counter for 3GPP access, and the attach attempt counter (see 3GPP TS 24.301 [15]) and:

a) if the UE supports the PDU session establishment procedure with request type set to "existing PDU session" or "existing emergency PDU session" to transfer a PDN connection from S1 mode to N1 mode and the UE has received an "interworking without N26 interface supported" indication from the network, attempt to transfer the PDN connection(s) which the UE intends to transfer to 5GS, if any, from S1 mode to N1 mode by:

– if the PDN connection which the UE intends to transfer is a PDN connection for emergency bearer services, initiating the PDU session establishment procedure with request type set to "existing emergency PDU session" to transfer the PDN connection for emergency bearer services; and

– if the PDN connection which the UE intends to transfer is a non-emergency PDN connection, initiating the PDU session establishment procedure with request type set to:

1) "MA PDU request", if the PDN connection to be transferred is a user-plane resource of an MA PDU session; or

2) "existing PDU session" to transfer the non-emergency PDN connection; and

b) otherwise, establish PDU session(s) corresponding to the PDN connection(s) which the UE intends to transfer to 5GS, if any, by initiating the PDU session establishment procedure with request type set to "initial request".

See subclause 5.1.4.3 for coordination between 5GMM and EMM and subclause 6.1.4.2 for coordination between 5GSM and ESM.

If:

a) the UE has registered in neither N1 mode over 3GPP access nor S1 mode yet; and

b) the UE has at least one active PDU session associated with non-3GPP access which the UE intends to transfer to EPS,

the UE shall initiate an EPS attach procedure and include a PDN CONNECTIVITY REQUEST message with a request type in the ATTACH REQUEST message to activate a default EPS bearer context for one of the active PDU sessions which the UE intends to transfer to EPS (see 3GPP TS 24.301 [15]). The request type is set as follows:

– if the PDU session which the UE intends to transfer is a non-emergency PDU session, the request type is set to "handover"; and

– if the PDU session which the UE intends to transfer is an emergency PDU session, the request type is set to "handover of emergency bearer services" and the default bearer to be activated is the default EPS bearer context for the emergency PDU session.

NOTE 1: It is necessary for the UE to support sending an ATTACH REQUEST message containing a PDN CONNECTIVITY REQUEST message with request type set to "handover" or "handover of emergency bearer services" to transfer a PDU session from N1 mode to S1 mode for interworking between TNGF or N3IWF connected to 5GCN and E-UTRAN.

NOTE 2: The order of PDU sessions to be tranferred to EPS is up to UE implementation.

After successful completion of the EPS attach procedure where the activated default EPS bearer context is not for emergency service, the UE shall initiate a UE requested PDN connectivity procedure with request type set to "handover" for non-emergency PDU session or "handover of emergency bearer services" for emergency PDU session in the PDN CONNECTIVITY REQUEST message to transfer each of the other PDU sessions which the UE intends to transfer to EPS, if any.

If:

a) the UE has not registered in N1 mode over non-3GPP access yet; and

b) the UE has at least one active PDN connection which the UE intends to transfer to TNGF or N3IWF connected to 5GCN,

the UE shall initiate an initial registration procedure over non-3GPP access (see subclause 5.5.1.2).

After successful completion of the 5GS initial registration in N1 mode over non-3GPP access, the UE shall initiate a UE-requested PDU session establishment procedure with a request type to transfer each of the PDN connections which the UE intends to transfer to TNGF or N3IWF connected to 5GCN, if any. The request type is set as follows:

– if the PDN connection which the UE intends to transfer is a PDN connection for emergency bearer services, the request type is set to "existing emergency PDU session" to transfer the PDN connection for emergency bearer services; and

– if the PDN connection which the UE intends to transfer is a non-emergency PDN connection, the request type is set to "existing PDU session" to transfer the non-emergency PDN connection.

NOTE 3: If the UE has no active PDU session associated with non-3GPP access which the UE in N1 mode intends to transfer to EPS or no active PDN connection associated with 3GPP access which the UE in S1 mode intends to transfer to TNGF or N3IWF connected to 5GCN, the interworking between TNGF or N3IWF connected to 5GCN and E-UTRAN is not supported.

See subclause 6.1.4.2 for coordination between 5GSM and ESM.

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 subclause, subclause 5.1.5 and 5.3.13A.

For dual-registration mode the following applies:

a) 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;

b) when the UE decides to operate in dual-registration mode (see subclause 5.5.1.2.4), NAS informs the lower layers about this;

c) 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

d) 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.

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 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.

4.8.4 Core Network selection for UEs not using CIoT 5GS optimizations

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 3GPP TS 23.122 [5] 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 3GPP TS 36.331 [25A]), the UE selects a core network type (EPC or 5GCN) as specified in 3GPP TS 23.167 [6] annex H.2 for initiating emergency calls when in the state 5GMM-DEREGISTERED.LIMITED-SERVICE or EMM-DEREGISTERED.LIMITED-SERVICE.

NOTE 1: If the PLMN selection information provisioned in the USIM does not contain any prioritization between E-UTRAN and NG-RAN for a PLMN, which core network type to select for that PLMN is up to UE implementation.

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 3GPP TS 24.301 [15]; 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.

NOTE 2: The NG-RAN can provide a target core network type to the UE during RRC connection release with redirection (see 3GPP TS 36.331 [25A] and 3GPP TS 38.331 [30]).

4.8.4A Core Network selection and redirection for UEs using CIoT optimizations

4.8.4A.1 Core network selection

A UE that supports CIoT optimizations performs core network selection (i.e. it selects EPC or 5GCN) if the lower layers have provided an indication that the current E-UTRA cell is connected to both EPC and 5GCN as specified in 3GPP TS 23.501 [8].

When selecting a PLMN as described in 3GPP TS 23.122 [5], the UE shall select a core network type (EPC or 5GCN) based on:

a) indication from the lower layers about the CIoT EPS optimizations supported in EPC;

b) indication from the lower layers about the CIoT 5GS optimizations supported in 5GCN;

c) the CIoT EPS optimizations supported by the UE;

d) the CIoT 5GS optimizations supported by the UE;

e) the UE’s preferred CIoT network behaviour for EPC; and

f) the UE’s preferred CIoT network behaviour for 5GCN.

The UE shall provide the selected core network type information to the lower layer during the initial registration procedure.

4.8.4A.2 Redirection of the UE by the core network

The network that supports CIoT optimizations can redirect a UE between EPC and 5GCN as specified in subclause 5.31.3 of 3GPP TS 23.501 [8]. The network can take into account the UE’s N1 mode capability or S1 mode capability, the CIoT network behaviour supported and preferred by the UE or the CIoT network behaviour supported by the network to determine the redirection.

NOTE: It is assumed that the network would avoid redirecting the UE back and forth between EPC and 5GCN.

The network redirects the UE to EPC by rejecting the registration request with the 5GMM cause #31 "Redirection to EPC required" as specified in subclause 5.5.1.2.5 and 5.5.1.3.5. Upon receipt of reject message, the UE disables the N1 mode capability for 3GPP access as specified in subclause 4.9.2 and enables the E-UTRA capability if it was disabled in order to move to EPC.

When there is no ongoing registration procedure for a UE in 5GMM-CONNECTED mode, if the AMF determines to redirect the UE to EPC, the AMF shall initiate the generic UE configuration update procedure to indicate registration requested and release of the N1 NAS signalling connection not requested as described in subclause 5.4.4. The network then redirects the UE to EPC by rejecting the registration request as specified in subclause 5.5.1.3.5.

The network that supports CIoT optimizations can also redirect a UE from EPC to 5GCN as specified in subclause 5.3.19.2 of 3GPP TS 24.301 [15].