A mobile phone talks to the 5G core (5GC) network through the radio access network (RAN). The 5G RAN which uses New Radio (NR) technology has several base stations called gNB to which the mobile device connects.
The RAN connects to the core through two 5GC nodes, the AMF and the UPF.
Access & Mobility Management function
- All signaling messages between the device and the 5G core pass through the AMF.
- The AMF lets a mobile device register with the 5G network and then move within it.
- It also makes sure that the device remains reachable to the core network.
User Plane Function (UPF)
- The UPF is connected to the RAN and a data network (internet).
- All data traffic passes through it.
- It performs filtering (gating), traffic redirection, rate limit enforcement and Quality of Service (QoS) marking on the data packets.
The other important elements of the core are –
Session Management Function (SMF)
- The SMF connects to the AMF and manages the entire session life cycle.
- It allocates IP addresses and controls the behavior of UPF.
- It works with the PCF for policy and charging control of the sessions.
Unified Data Management Function (UDM)
- The UDM manages the user subscription data used for authorization.
- It also generates authentication data used to authenticate the device.
- It maintains the device to AMF/SMF mapping.
Unified Data Repository (UDR)
- The UDR is the database of the subscription data managed by the UDM.
- It also stores data of other network functions.
Authentication Server Function (AUSF)
- The AUSF utilizes the authentication credentials generated by the UDM and authenticates the devices through the AMF.
- It supplies encryption data to secure the communication with the devices.
Policy Control Function (PCF)
- The PCF allows creation of policies that affect QoS, charging and network slicing.
- Other network functions use these policies to make decisions.
Network Resource Function (NRF)
- NRF helps all 5GC network functions talk to each other.
- All network functions register with the NRF.
- Now, let’s say, the PCF wants to talk to the UDR. It asks the NRF for a list of UDRs.
- This way of discovering network functions through the NRF is called Service Discovery.
The 5G core has other network functions, but these are the ones that do the heavy lifting.
