Access and Mobility Management Function (AMF) | 5G Service Based Architecture

It is a control plane function within 5G Core Network. Its functions include:

• Registration Management
• Reachability Management
• Connection Management
• Mobility management

Registration Management allows a UE to register/deregister with the 5G system. Registration is a must for a UE to use 5G services. UE moves to RM-Registered state after registration (from RM-Deregistered state). The AMF interacts with other network functions during the registration procedure (e.g. forwarding the Permanent Equipment Identifier (PEI) to the SMF, UDM and PCF.

Connection Management is used to establish and release the control plane signalling connection between the UE and the AMF, i.e. across the N1 interface. Establishing an N1 connection moves the UE from CM-Idle to CM-Connected. The N1 signalling connection allows NAS messages to be exchanged between the UE and the AMF.

NAS signalling procedures managed by the AMF include Registration, Authentication, Service Request and Identity Request.

PDU Session Establishment, Modification and Release procedures are NAS signalling procedures managed by the SMF. The AMF acts as a transparent router for these procedures and forwards messages between the UE and the AMF.

Reachability Management is used to ensure that a UE is reachable, i.e. it can be paged. The AMF supports Reachability Management by storing location information as part of the UE Context. The UE context includes the registration area (a Tracking Area, or a list of Tracking Areas) within which the UE is registered. It may also include more specific information regarding the UE location, i.e. recommended cells and nodes for paging.

MICO (mobile Initiated Connection Only) devices are categorized as being unreachable. They cannot be paged and are only able to establish mobile initiated connection from the CM-Idle state.

Mobility Management is used to maintain knowledge of UE’s location within the network. The UE is required to complete periodic registration updates after it has completed initial registration. These periodic updates ensure that the UE is still present within the network and has not moved out of coverage or became unavailable due to any other reason, e.g. battery has drained. The UE is also required to complete updates due to mobility, e.g. if it moves out of the Tracking Area or list of Tracking Areas with which it is currently registered.

Other Functions of AMF:

The AMF is responsible for selecting an appropriate Authentication Server Function (AUSF) during the registration procedure. The AUSF allows the UE to authenticate itself with the 5G Core Network, i.e. it verifies that the subscriber is genuine and is authorized to access the network. The AMF may be configured to use a specific AUSF, or it may use the NRF to discover a suitable AUSF within the 5G Core Network.

The AMF is responsible for selecting an appropriate Unified Data Management (UDM) for the registration procedure. The UDM manages the user’s subscription information. The AMF may be configured to use a specific UDM, or it may use the NRF to discover a suitable UDM within the 5G Core Network.

The AMF is also responsible for selecting an appropriate Policy Control Function (PCF) and Session Management Function (SMF).

The AMF provides support for SMS. MT-SMS are received from the SMS Function (SMSF) and are packaged within a NAS message before being transferred to the UE. AMF receives the MO-SMS within uplink NAS messages and forwards them to the SMSF. The SMSF is an optional network function.