A 5G System (5GS) is consist of User Equipment (UE), 5G Base station (gNB), and a 5G Core Network (5GC) as shown in following picture. In 5GS architecture, the 5GC is located at the heart and enables the 5G RAN to support 3GPP defined three use cases i.e. Enhanced Mobile Broadband (eMBB), Ultra Reliable Low Latency Communication (URLCC) and Massive Machine Type Communication (mMTC).
3GPP has defined 5G Core to utilize cloud virtualisation, service-based architecture (SBA) across all 5G functions and procedure including authentication, security, session management and aggregation of traffic from end devices. The 5G core network supports virtualization of software functions to use NFVI in design 5G Networks including MEC infrastructure. The new architecture has adopted separation of user plane and control plane functions which enables independent scaling of the control and user plane functions e.g. a Mobile Network Operator (MNO) can add more user plane functions without adding more control plane and vice versa depending on deployment strategies. User plane functions could be distributed and geographically close to 5G RAN to minimise user plane latency, while control plane functions could be centralised to get benefits of Virtualization.
Reference and Service Based Architecture
3GPP has defined ‘Reference Point Architecture’ and ‘Service Based Architecture’. In “Reference Point Architecture” the Network Elements (NEs) use point-to-point interfaces to inter-connect each other. Each point-to-point interface has specified certain signalling protocols and procedures.
In ‘Service based‘ architecture, the set of Network Elements (NEs) are replaced with a set of Network Functions (NFs). Each Network Function can provide services to other Network Functions, i.e. each Network Function is a service provider. The point-to-point interfaces are replaced by a common bus which inter-connects all Network Functions. The services are specified for the Network Function providing them, instead of each pair of providing and consuming Network.
Service Based 5G System Architecture
5G System Service Based architecture is illustrated below. This architecture includes a set of Network Functions (NFs) and a common bus know as service bus to inter-connects the Network Functions. The Service Based architecture is applicable to the control plane and Reference Point architecture is used for the user plane of the 5G Core Network.
The interface naming for Reference point is given as number with letter “N” like N2, N3 or N4. N2 is the logical reference point between gNB and AMF, whereas N3 is reference point between gNB and UPF. The naming for service based interface include network function name with letter “N” e.g. Namf is the interface between AMF network function and Common Service Bus.
The Network Repository Function (NRF) is the main network function in service based architecture and plays a key role. All network functions register the services they offer with the NRF using service registration procedure and later they use the NRF as a database to discover the services offered by other Network with service discovery and service authorization procedure.
Within Service Based Architecture, Network Functions communicates to each other using http protocols defined methods like POST, GET, PUT, PATCH and DELETE.
The Service Based architecture represents a move towards a cloud implementation of the Core Network. Legacy Core Networks have been deployed using separate hardware for each Network Element. Newer 5G Core Networks have been deployed using virtualised Network Elements to allow multiple Network Elements to operate using a shared underlying hardware platform. Virtualised can provide fault tolerance by allowing the software to switch between hardware platforms in case of a failure in 5G System.
- 3GPP TS 23.501 System architecture for the 5G System (5GS)