Let's start with simple definitions for these two type of architectures
Reference Point architecture consist of a set of Network Elements (NE) which use point to point interfaces to inter-connect those Network Elements.
- Signalling procedures are specified for each point to point interface.
- This architecture type can lead to repetition within the specifications if same signalling procedure is used across multiple interfaces.
Service Based architecture replaces the set of Network Elements with a set of Network Functions (NF). Each Network Function is a service provider to other Network Function(s).
- The point to point interfaces are replaced by a common bus which inter-connects all Network Functions.
- Services are specified for the Network Function providing them, rather than for each pair of providing and consuming Network Functions
5G Reference Point System Architecture
5G System Reference point architecture is shown in Figure below. It lists a set of Network elements and set of point to point interfaces which interconnect those Network Elements (functionality of these network elements is not listed in this article)
- 5GS Reference point architecture describes the separation of control plane and user plane functions.
- Uplink data is transferred from UE to 5G Access Network. It is transferred through user plane functions (UPF) before forwarded into an external network ( internet etc.) Same is the case for down-link data but in opposite direction.
- Of-course there are exceptions to the separation of user plane and control plane functions. For example user plane data belonging to the Short Message Service (SMS) can be transferred using Non-Access Stratum (NAS) signalling between the UE and Access and Mobility Management Function (AMF).
- Point to Point interfaces connecting the Network Elements are labelled as N1, N2, N3, etc. known as Reference points according to 3GPP TS 23.501
- N3 is the Reference Point between the 5G Access Network and the User Plane Function (UPF)
- N1 is the Reference Point between UE and the Access and Mobility Management Function (AMF). It is usually shown as a direct logical connection between the UE and AMF.
- N9 Reference point is used to inter-connect User Plane Functions (UPF). First UPF can be used to provide connectivity to the 5G Access Network while a second UPF can be used to provide connectivity to external data network. N9 Reference Point defines the connection between two UPF.
- N26 Reference point can be used to connect an AMF within the 5G Core Network, with a Mobility Management Entity (MME) within 4G Evolved Packet Core (EPC). This reference point is used for inter-working between 4G and 5G handovers.
- 3GPP Next Generation Radio Access Network (NG RAN), user plane data and control plane signalling are transferred between the UE and NG RAN using the Uu air-interface.
5G System Service Based System Architecture
5G System service based architecture is shown below. It specifies a set of Network Function (NF) and a common bus which connects these Network Functions.
Service based architecture is applicable to the control plane section of 5G Core Network only.
Network Function Repository Function (NRF) plays a central role within the Service based architecture. Network functions register the services they offer with the NRF. They use NRFas a database to discover the services offered by other Network Functions. Following three mechanisms are used.
- service registration: allows a Network Function to register with the NRF and information the NRF which services are offered
- service discovery: allows a Network Function to discover other Network Functions and the services they offer
- service authorization: ensures that a Network Function is authorized to use the services provided by other Network Functions
Services Offered by a Network Function
Example of services offered by a Network Function is presented in the Table below. This table represents the services offered by Access and Mobility Management Function (AMF).
The list of Network Functions which are described in 3GPP standards are listed in the table below.
- 3GPP TS 29.518
- 3GPP TS 23.501
- 3GPP TS 23.502
- 3GPP TS 29.500
- 5G NR in Bullets by Chris Johnson