Reference Point System Architecture vs Service Based Architecture


​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)

​5G System Reference Point Architecture

  • ​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.

​5G System Service Based Architecture

​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.

  1. service registration: allows a Network Function to register with the NRF and information the NRF which services are offered
  2. service discovery: allows a Network Function to discover other Network Functions and the services they offer
  3. 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).

​Services provided by the Access & Mobility Management Function (AMF)

​The list of Network Functions which are described in 3GPP standards are listed in the table below. 


  1. ​3GPP TS 29.518
  2. 3GPP TS 23.501
  3. ​3GPP TS 23.502
  4. 3GPP TS 29.500​​​
  5. 5G NR in Bullets by Chris Johnson