6.2.1 AMF

The Access and Mobility Management function (AMF) includes the following functionality. Some or all of the AMF functionalities may be supported in a single instance of an AMF:

• Termination of RAN CP interface (N2).
• Termination of NAS (N1), NAS ciphering and integrity protection.
• Registration management.
• Connection management.
• Reachability management.
• Mobility Management.
• Lawful intercept (for AMF events and interface to LI System).
• Provide transport for SM messages between UE and SMF.
• Transparent proxy for routing SM messages.
• Access Authentication.
• Access Authorization.
• Provide transport for SMS messages between UE and SMSF.
• Security Anchor Functionality (SEAF) as specified in TS 33.501 [29].
• Location Services management for regulatory services.
• Provide transport for Location Services messages between UE and LMF as well as between RAN and LMF.
• EPS Bearer ID allocation for interworking with EPS.
• UE mobility event notification.
• S-NSSAIs per TA mapping notification.
• Support for Control Plane CIoT 5GS Optimisation.
• Support for User Plane CIoT 5GS Optimisation.
• Support for restriction of use of Enhanced Coverage.
• Provisioning of external parameters (Expected UE Behaviour parameters or Network Configuration parameters).
• Support for Network Slice-Specific Authentication and Authorization as in clause 5.15.10 and Annex L for TS 23.502 [3].
• Support for charging.
• Controlling the 5G access stratum-based time distribution based on UE's subscription data.
• Controlling the gNB's time synchronization status reporting and subscription.

NOTE 1: Regardless of the number of Network functions, there is only one NAS interface instance per access network between the UE and the CN, terminated at one of the Network functions that implements at least NAS security and Mobility Management.

In addition to the functionalities of the AMF described above, the AMF may include the following functionality to support non-3GPP access networks:

• Support of N2 interface with N3IWF/TNGF. Over this interface, some information (e.g. 3GPP Cell Identification) and procedures (e.g. Handover related) defined over 3GPP access may not apply and non-3GPP access specific information may be applied that do not apply to 3GPP accesses.
• Support of NAS signalling with a UE over N3IWF/TNGF. Some procedures supported by NAS signalling over 3GPP access may be not applicable to untrusted non-3GPP (e.g. Paging) access.
• Support of authentication of UEs connected over N3IWF/TNGF.
• Management of mobility, authentication and separate security context state(s) of a UE connected via a non-3GPP access or connected via a 3GPP access and a non-3GPP access simultaneously.
• Support as described in clause 5.3.2.3 a co-ordinated RM management context valid over a 3GPP access and a Non 3GPP access.
• Support as described in clause 5.3.3.4 dedicated CM management contexts for the UE for connectivity over non-3GPP access.
• Determine whether the serving N3IWF/TNGF is appropriate based on the slices supported by the N3IWFs/TNGFs as specified in clause 6.3.6 and clause 6.3.12 respectively.

NOTE 2: Not all of the functionalities are required to be supported in an instance of a Network Slice.

In addition to the functionalities of the AMF described above, the AMF may include policy related functionalities as described in clause 6.2.8 of TS 23.503 [45].

The AMF uses the N14 interface for AMF re-allocation and AMF to AMF information transfer. This interface may be either intra-PLMN or inter-PLMN (e.g. in the case of inter-PLMN mobility).

In addition to the functionality of the AMF described above, the AMF may include the following functionality to support monitoring in roaming scenarios:

• Normalization of reports according to roaming agreements between VPLMN and HPLMN (e.g. change the location granularity in a report from cell level to a level that is appropriate for the HPLMN); and
• Generation of charging/accounting information for Monitoring Event Reports that are sent to the HPLMN.

In addition to the functionality of the AMF described above, the AMF may provide support for AMF overload control, Network Slice restriction and Network Slice instance restriction based on NWDAF analytics.

In addition to the functionalities of the AMF described above, the AMF may provide support for the Disaster Roaming as described in clause 5.40.

In addition to the functionalities of the AMF described above, the AMF may also include following functionalities to support Network Slice Admission Control:

• Support of NSAC for maximum number of UEs as defined in clauses 5.15.11.1 and 5.15.11.3.

In addition to the functionality of the AMF described above, the AMF may include the following functionality to support SNPNs:

• Support for Onboarding of UEs for SNPNs.

In addition to the functionalities of the AMF described above, the AMF may also include following functionalities to support satellite backhaul:

• Support for reporting satellite backhaul category and its modification based on AMF local configuration to SMF as defined in clause 5.43.4.

In addition to the functionalities of the AMF described above, the AMF may also include following functionalities for regenerative-based satellite access:

• Support for NG Removal procedure defined in TS 38.413 [34].

In addition to the functionalities of the AMF described above, the AMF may also include following functionalities to support UE-Satellite-UE communication:

• Support for reporting to SMF the UE's serving satellite identifier and its change as defined in clause 5.4.14.

In addition to the functionalities of the AMF described above, the AMF may provide support for Network Slice instance change for PDU sessions as defined in clause 5.15.5.3.

In addition to the functionalities of the AMF described above, the AMF may also support functionalities for Partial Network Slice support in a Registration Area as described in clause 5.15.17.

In addition to the functionalities of the AMF described above, the AMF may also include functionalities to support NS-AoS not matching deployed Tracking Areas as described in clause 5.15.18.

In addition to the functionalities of the AMF described above, the AMF may also include functionalities to support Network Slice Replacement as described in clause 5.15.19.

In addition to the functionalities of the AMF described above, the AMF may also include functionalities to enforce the LADN Service Area per LADN DNN and S-NSSAI for a UE as described in clause 5.6.5a, as well as to enforce the LADN Service Area per LADN DNN for a UE in clause 5.6.5.

In addition to the functionalities of the AMF described above, the AMF may also include following functionalities to support Indirect Network Sharing:

• Support for selecting the SMF of participating operator (H-SMF) possibly considering the relevant UE location information as specified in clause 6.3.2.
• Support for Network Slicing handling considering what is described in clause 5.18.5.

NOTE 3: The AMF of hosting operator can identify the UE's participating operator (i.e. in the case of Indirect Network Sharing) during the registration procedure based on internal implementation and then the H-SMF selection can be performed considering the relevant UE location information.

In addition to the functionalities of the AMF described above, the AMF may also include following functionalities to support Local Offloading Management as specified in clause 5.34.11:

• Support to decide the insertion or removal of I-SMF to support Local Offloading Management based on SMF selection subscription data, the current UE location, SMF service area and local offloading management service area.

In addition to the functionalities of the AMF described above, the AMF may provide support for energy related features as described in clause 5.51.