With the functionality of the 4G Mobility Management Entity (MME) now decomposed, the 5G Core Access and Mobility Management Function (AMF) receives all connection and session related information from the User Equipment (UE) (N1/N2) but is responsible only for handling connection and mobility management tasks. All messages related to session management are forwarded over the N11 reference interface to the Session Management Function (SMF).
As a mobile network comprises many AMF instances, a Globally Unique AMF Identifier (GUAMI) is employed. The UE specifies this in the first Non-Access Stratum (NAS) message it sends, which is routed to the required AMF by the Radio Access Network (RAN). Applicable to both 3GPP access and non-3GPP access, the GUAMI also ensures that messages from a UE, registered through both access networks, get forwarded to the same AMF. The Non-3GPP Interworking Function (N3IWF) is responsible for routing messages outside the 5G RAN.
Performing the role of access point to the 5G core, thereby terminating RAN control plane and UE traffic originating on either the N1 or N2 reference interface, the AMF implements NAS ciphering and integrity protection algorithms. Following the initial NAS message, the AMF sends an Authentication and Key Agreement (AKA) request in an equivalent manner to the MME in Evolved Packet Core (EPC) infrastructures. This precedes the UE authorization process, in which the 5G Core Service Based Architecture (SBA) Unified Data Management (UDM) function supersedes the Home Subscriber Server (HSS). This is performed over the N8 reference interface and therefore employs the HTTP/2 Service Based Interface (SBI) message bus. Lastly, in order to send applicable and appropriate event information, the AMF connects to Lawful Intercept (LI) systems.
While the information could be stored in another manner, the AMF typically queries the 5G Service-Based Architecture’s (SBA’s) Network Repository Function (NRF) to discover and select available SMF instances. These can be identified by IP address or Frequently Qualified Domain Name (FQDN). This process is outlined in further detail here.
Assuming the responsible for mobility management, the AMF is in charge for managing handovers between gNodeB’s (gNB’s), within the Next Generation Radio Access Network (NG-RAN). Formally referred to as an X2 handover, in 5G it is termed an Xn handover. This represents the updated reference interface application protocol (XnAP) employed between gNB base stations, which is defined within the 3GPP’s Technical Specification (TS) 38.423.
UE handover between source and target gNB’s within 5G infrastructures
With the data flowing from a UE via a source gNB to the data plane of the 5G Core (User Plane Function / UPF), Radio Resource Control (RRC) signaling is employed to continuously measure and report on signal quality. When the source node detects that a handover is required, it connects with the target gNB to commence the switching process. Once the tunnels have been moved across to the target gNB, the UE performs a handover and connects to that same target node. A path switch request is made from the target gNB to the AMF and, once acknowledged, the data can flow from the UE through that target node and on to the prescribed UPF.