What are Network Functions (NFs) in 5G?

In 5G, the network architecture is changed to ‘Service-based Architecture’ (SBA). SBA allows 5G core solution vendors to move to software-based platform. Hence, eliminating the need to be dependent on proprietary software and hardware vendors. Each software unit in 5G core network is called as the ‘Network Function’. Every NF is entitled to a particular job and acts as a producer as well as consumer for every other NF. The communication is usually done over a software-based stateless interface. Services exposed by these network functions are invoked using a standard API. 5G Core architecture has introduced the concept of CUPS (Control and User Plane Separation).

Architectural diagram of 5G core network functions and interfaces

Following are different Network Functions in 5G and their functionalities:
1. AUSF (Authentication Server Function): Performs the UE authentication. It relies on a backend service for computation and keys. UEs get authenticated only with AUSF in the home network. When the device roams in a serving network a Security Anchor Function acts as the authentication gateway between the serving network and AUSF in the home network.

2. AMF (Access and Mobility Management Function): AMF is a control plane function in 5G core that is solely responsible for registration management, mobility management, reachability management and connection management. It performs registration and de-registration of the UE with 5G core. AMF also performs NAS (Non-Access Stratum) signaling with the UE via gNodeB. Function of AMF is much similar to MME (Mobility Management Entity) from 4G core. It ensures that UE is always reachable.

3. UDM (Unified Data Manager): UDM acts as a centralized repository of the data for authorization, user registration and subscriber profiles. Function of UDM is much similar to HSS (Home Subscriber Server) from 4G. A stateless UDM can store its data in external entity called UDR (Unified Data Repository).

4. PCF (Policy Control Function): PFC maintains the unified policy framework that controls the UE’s behavior with the network. It provides policy rules to other network functions for their enforcement. PCF is similar to PCRF (Policy and Charging Rules Function) from 4G core.

5. UPF (User Plane Function): UPF is the crucial component of the 5G core. It is directly connected to the Data Networks (DN) like internet or IMS. It is responsible for packet routing, packet forwarding, QoS handling and PDU session management. It handles the downlink (DL) and uplink (UL) rate enforcement. It also performs the downlink packet buffering for the UE.

6. NSSF (Network Slice Selection Function): As discussed in the previous post(s) earlier, 5G introduces the concept of ‘network slicing’, where a piece of 5G network is dedicated to a specific use case. NSSF assists AMF with selection of a network slice to serve a particular device. It determines the NS-SAI (Network Slice Selection Assistance Information) for the device.

7. NRF (Network Repository Function): NRF is the key network function in 5G core. It acts as an internal broker for all core network functions. It maintains an updated repository of all network functions along with services provided by them with NF discovery information in an entity called ‘NF Profile’. It allows consumer NFs to discover provider NFs and keep track of other NF instances.

8. NEF (Network Exposure Function): One of the biggest advantages of 5G SBA is that it emphasizes the use of HTTP/2 based stateless APIs for communication. NEF facilitates a third-party application function (AF) by securely exposing some of the services offered by 5G core network functions. It acts as an ‘external broker’ for third-party applications having access to 5G core network information. For example, an external application may try to request information such as UE reachability from AMF. NEF does the job of retrieving this information from AMF and provide it to the external application.

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