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Content for  TS 24.501  Word version:  17.0.0

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4.7  NAS over non-3GPP access

4.7.1  General

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.

4.7.2  5GS mobility management aspectsWord‑p. 84

4.7.2.1  General

The mobility management procedures defined over 3GPP access are re-used over non-3GPP access with the following exceptions:
  1. the registration status, and the 5GMM parameters of the UE's 3GPP access and non-3GPP access 5GMM state machine instances are independent in each of these accesses and can be different;
  2. single-registration mode and dual-registration mode do not apply for 5GMM over non-3GPP access;
  3. the RPLMN over non-3GPP access can be different from the RPLMN over 3GPP access. The MCC of the RPLMN over 3GPP access and the MCC of the RPLMN over the non-3GPP access can also be different;
  4. the registration for 3GPP access and for non-3GPP access are performed separately. Like for 3GPP access, an access stratum connection exists before the UE can perform the registration procedure for non-3GPP access. As over non-3GPP access the 5GS operates one single common registration area for an entire PLMN, which is associated with the operator-specific N3GPP TAI for the PLMN, list management of registration areas is not required, and registration updating due to registration area change with the registered PLMN is not performed. Furthermore, the periodic registration update procedure is also not performed. New registration at change of PLMN is required;
  5. the 5GMM over non-3GPP access in the UE considers that the N1 NAS signalling connection is established when the lower layers indicate that the access stratum connection is established succcessfully;
  6. the UE-initiated service request procedure via non-3GPP access is supported. Upon indication from the lower layers of non-3GPP access, that the access stratum connection is established between the UE and the network, the UE in 5GMM-REGISTERED state and in 5GMM-IDLE mode over non-3GPP access shall initiate the service request procedure via non-3GPP access. The UE may indicate with the service request message the PDU session(s) associated with non-3GPP access to re-establish user-plane resources for which the UE has pending user data to be sent;
  7. paging procedure is not performed via non-3GPP access;
  8. service area restrictions do not apply for non-3GPP access other than the wireline access;
  9. the establishment cause for non-3GPP access is determined according to subclause 4.7.2.2;
  10. eCall inactivity procedure is not performed via non-3GPP access;
  11. local area data network (LADN) does not apply for non-3GPP access;
  12. the Allowed PDU session IE shall not be included in the REGISTRATION REQUEST message or the SERVICE REQUEST message sent over non-3GPP access;
  13. DRX parameters do not apply for non-3GPP access;
  14. Mobile initiated connection only mode (MICO) does not apply for non-3GPP access;
  15. CIoT 5GS optimizations do not apply for non-3GPP access;
  16. unified access control does not apply for non-3GPP access;
  17. UE radio capability signalling optimisation (RACS) does not apply for non-3GPP access; and
  18. Closed access group (CAG) does not apply for non-3GPP access.
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4.7.2.2  Establishment cause for non-3GPP access

When establishment of an N1 NAS signalling connection over non-3GPP access is initiated, the UE shall:
  1. determine one or more access identities to be associated with the establishment of the N1 NAS signalling connection as specified in subclause 4.5.2 and table 4.5.2.1;
  2. select the establishment cause for non-3GPP access from the determined one or more access identities and the event which triggered initiation of the N1 NAS signalling connection over non-3GPP access by checking the rules specified in table 4.7.2.2.1; and
  3. provide the selected establishment cause for non-3GPP access to the lower layers.
If the access attempt matches more than one rule, the establishment cause for non-3GPP access of the lowest rule number shall be used.
Rule #
Access identities
Type of access attempt
Requirements to be met
Establishment cause for non-3GPP access

1
1
Any
Any
mps-PriorityAccess
2
2
Any
Any
mcs-PriorityAccess
3
11, 15
Any
Any
highPriorityAccess
4
12,13,14,
Any
Any
highPriorityAccess
5
0
Emergency
UE is attempting access for an emergency session (NOTE 1)
emergency
UE NAS initiated 5GMM specific procedures
Access attempt is for MO signalling
mo-Signalling
UE NAS initiated 5GMM connection management procedures or 5GMM NAS transport procedure
Access attempt is for MO data
mo-Data


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4.7.3  5GS session management aspectsWord‑p. 85
The session management procedures defined over 3GPP access are re-used over non-3GPP access with the following exceptions:
  • Serving PLMN rate control does not apply for non-3GPP access.
  • Small data rate control does not apply for non-3GPP access.
  • 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.

4.7.4  Limited service state over 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" response is received when registration or service request is performed (any USIM in the ME is then considered "invalid");
  3. a "5GS services not allowed" response is received when a registration or service request is performed;
  4. a "PLMN not allowed" response is received when registration or service request is performed;
  5. a "Tracking area not allowed" response is received when a registration or service request is performed;
  6. a "Roaming not allowed in this tracking area" response is received when a registration or service request is performed; or
  7. a "No suitable cells in tracking area" response 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 is considered as the selected PLMN for the duration the UE is registered for emergency services.
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4.7.5  NAS signalling using trusted WLAN access network |R16|Word‑p. 86
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 5.5.1.3 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 5GC 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.
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4.8  Interworking with E-UTRAN connected to EPC

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 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.
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4.8.2  Single-registration modeWord‑p. 87

4.8.2.1  General

If the UE receives the indication that "interworking without N26 interface not supported" (see TS 24.301), the UE operates as described in subclause 4.8.2.2.
If the UE receives the indication that "interworking without N26 interface supported" and
  1. the UE does not support dual-registration mode; or
  2. the UE supporting dual-registration mode determines to operate in single-registration mode,
the UE operates as described in subclause 4.8.2.3.
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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:
  1. the UE supports non-IP PDN type and at least one PDU session of Unstructured PDU session type is active;
  2. the UE supports IPv4 PDN type and at least one PDU session of IPv4 PDU session type is active;
  3. the UE supports IPv6 PDN type and at least one PDU session of IPv6 PDU session type is active;
  4. the UE supports IPv4v6 PDN type and at least one PDU session of IPv4v6 PDU session type is active; or
  5. 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:
  1. 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 UE is the 5G-RG and the selected PDU session is an MA PDU session established over 3GPP access, the 5G-RG shall include the ATSSS request PCO parameter in 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 TS 24.301) 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" in the PDN CONNECTIVITY REQUEST message; and
  2. otherwise, enter substates EMM-REGISTERED.NORMAL-SERVICE and 5GMM-REGISTERED.NO-CELL-AVAILABLE for 3GPP access and initiate a tracking area update procedure (see TS 24.301).
At inter-system change from N1 mode to S1 mode in EMM-IDLE mode when:
  1. the UE does not support non-IP PDN type or no PDU session of Unstructured PDU session type is active;
  2. the UE does not support IPv4 PDN type or no PDU session of IPv4 PDU session type is active;
  3. the UE does not support IPv6 PDN type or no PDU session of IPv6 PDU session type is active;
  4. the UE does not support IPv4v6 PDN type or no PDU session of IPv4v6 PDU session type is active; and
  5. 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:
  1. enter substate 5GMM-REGISTERED.NORMAL-SERVICE for 3GPP access and substate EMM-REGISTERED.NO-CELL-AVAILABLE;
  2. 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
  3. 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 TS 24.301) and:
  1. 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 UE is the 5G-RG and 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
  2. 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.
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4.8.2.3.2  Interworking between TNGF or N3IWF connected to 5GCN and E-UTRANWord‑p. 89
If:
  1. the UE has registered in neither N1 mode over 3GPP access nor S1 mode yet; and
  2. 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 TS 24.301). The request type is set as follows:
  • if all the PDU sessions which the UE intends to transfer are non-emergency PDU sessions, the request type is set to "handover"; and
  • if the PDU sessions which the UE intends to transfer include 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.
After successful completion of the EPS attach procedure, the UE shall initiate a UE requested PDN connectivity procedure with request type set to "handover" 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:
  1. the UE has not registered in N1 mode over non-3GPP access yet; and
  2. 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.
See subclause 6.1.4.2 for coordination between 5GSM and ESM.
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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-registation 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.
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4.8.4  Core Network selection for UEs not using CIoT 5GS optimizationsWord‑p. 90
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.
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4.8.4A  Core Network selection and redirection for UEs using CIoT optimizations |R16|Word‑p. 91

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 TS 23.501.
When selecting a PLMN as described in TS 23.122, the UE shall select a core network type (EPC or 5GCN) based on:
  1. indication from the lower layers about the CIoT EPS optimizations supported in EPC;
  2. indication from the lower layers about the CIoT 5GS optimizations supported in 5GCN;
  3. the CIoT EPS optimizations supported by the UE;
  4. the CIoT 5GS optimizations supported by the UE;
  5. the UE's preferred CIoT network behaviour for EPC; and
  6. 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.
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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 TS 23.501. 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.
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 TS 24.301.
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