5G core network architecture and some basic concepts

The 5G wireless access network architecture mainly includes the 5G access network and the 5G core network, where NG-RAN represents the 5G access network and 5GC represents the 5G core network.

What does the 5G core network mainly include? Let’s first talk about the key AMF, SMF, and UPF.

• AMF: The full name is Access and Mobility Management Function. It is responsible for access and mobility management functions, terminal access rights and switching, etc.
• SMF: Session Management Function, which provides service continuity and uninterrupted user experience, including changes in IP addresses and/or anchor points.
• UPF: Full name User Plane Function, user plane management function, the area in which the PDU session associated with the UPF can be served by the (R)AN node through the N3 interface between the (R)AN and UPF without adding a new UPF in between or removing/reallocating the UPF.

Let's take a look at the 5G system architecture diagram:

AMFSMFUPF is in the main role.

AMF carries the following main functions:

The access and mobility management function (AMF) includes the following functions. Some or all of the AMF functions may be supported in a single instance of AMF:

• Terminate the RAN CP interface (N2).
• Termination of NAS(N1), NAS encryption and integrity protection.
• Registration management.
• Connection management.
• Accessibility management.
• Liquidity management.
• Lawful interception (applicable to AMF events and interfaces to LI systems).
• Provides transmission for SM messages between UE and SMF.
• Transparent proxy for routing SM messages.
• Access authentication.
• Access authorization.
• Provides transmission of SMS messages between the UE and the SMSF.
• Safety Anchor Function (SEAF).
• Location services management for regulatory services.
• Provides transport for location service messages between UE and LMF and between RAN and LMF.
• EPS bearer ID allocation for interworking with EPS.
• UE mobility event notification.

Regardless of the number of network functions, there shall be only one instance of the NAS interface per access network between the UE and the CN, terminating in at least one of the network functions implementing NAS security and mobility management.

In addition to the above AMF functions, AMF can also include the following functions to support non-3GPP access networks:

• The N2 interface and N3IWF are supported. On this interface, some information (e.g., 3GPP cell identity) and procedures (e.g., related to handover) defined by 3GPP access may not be applied, and non-3GPP access specific information not applicable to 3GPP access may be applied.
• NAS signaling is supported by UE over N3IWF. Some procedures supported by NAS signaling over 3GPP access may not be applicable to untrusted non-3GPP (eg paging) access.
• Supports authentication of UEs connected via N3IWF.
• Manages the mobility, authentication and individual security context state of UEs connected via non-3GPP access or via both 3GPP and non-3GPP connections.
• Support for coordinated RM management context that is valid for both 3GPP and non-3GPP accesses.
• Support for dedicated CM management context for UEs connecting over non-3GPP access.

Note: Not all features are necessarily supported in an instance of a network slice.

UPF carries the following main functions

The User Plane Function (UPF) includes the following functions. Some or all of the UPF functions can be supported in a single instance of UPF:

• Anchor point for intra-RAT/inter-RAT mobility (where applicable).
• The session point where the external PDU interconnects with the data network.
• Packet routing and forwarding (e.g., support for uplink classifiers to route traffic flows to instances of the data network, support for branching points to support multi-homed PDU sessions).
• Packet inspection (e.g., application detection based on the service data flow template and optional PFD received from the SMF).
• User plane partial policy rule implementation, such as gating, redirection, and traffic steering).
• Lawful Interception (UP Collection).
• Traffic usage report.
• QoS handling in user plane, e.g. UL/DL rate enforcement, reflective QoS marking in DL.
• Uplink traffic verification (SDF to QoS traffic mapping).
• Transport level packet marking in uplink and downlink.
• Downlink data packet buffering and downlink data notification triggering.
• Send and forward one or more “end markers” to the source NG-RAN node.
• ARP Proxying and/or IPv6 Neighbor Solicitation Proxying for Ethernet PDUs. The UPF responds to ARP and/or IPv6 Neighbor Solicitation requests by providing the MAC address that corresponds to the IP address sent in the request.

The Session Management Function (SMF) carries the following main functions

The Session Management Function (SMF) includes the following functions. Some or all of the SMF functions can be supported in a single instance of SMF:

• Session management, such as session establishment, modification and release, including tunnel maintenance between UPF and AN nodes.
• UE IP address allocation and management (including optional authorization).
• DHCPv4 (server and client) and DHCPv6 (server and client) functionality.
• ARP Proxying and/or IPv6 Neighbor Solicitation Proxying for Ethernet PDUs. The SMF responds to ARP and/or IPv6 Neighbor Solicitation requests by providing the MAC address that corresponds to the IP address sent in the request.
• Select and control UP functionality, including controlling the UPF to proxy ARP or IPv6 neighbor discovery, or forward all ARP/IPv6 neighbor solicitation traffic to the SMF, for Ethernet PDU sessions.
• Configure UPF traffic control to route traffic to the correct destination.
• Terminate the interface to the policy control function.
• Lawful Interception (interface for SM events and LI systems).
• Charge data collection and support billing interface.
• Control and coordinate UPF's collection of charging data.
• Terminates the SM part of an SM message.
• Downlink data notification.
• Originator of AN specific SM information, sent to AN via AMF via N2.
• Determines the SSC mode for the session.

Roaming function:

• Handles local implementation to apply QoS SLA (VPLMN).
• Billing data collection and billing interface (VPLMN).
• Lawful interception (interface of VPLMN and LI systems in the event of SM).
• Supports interaction with external DN to transmit signaling of PDU session authorization/authentication through external DN.