5G Core Network, System Architecture & Registration Procedure

The 5G System architecture (based on 3GPP TS 23.501: System Architecture for the 5G System; Stage 2) consists of the following network functions (NF). The functional description of these network functions is specified in clause 6.
- Authentication Server Function (AUSF)
- Core Access and Mobility Management Function (AMF)
- Data network (DN), e.g. operator services, Internet access or 3rd party services
- Structured Data Storage network function (SDSF)
- Unstructured Data Storage network function (UDSF)
- Network Exposure Function (NEF)
- NF Repository Function (NRF)
- Network Slice Selection Function (NSSF)
- Policy Control function (PCF)
- Session Management Function (SMF)
- Unified Data Management (UDM)
- Unified Data Repository (UDR)
- User plane Function (UPF)
- Application Function (AF)
- User Equipment (UE)
- (Radio) Access Network ((R)AN)

As you can see, this is slightly more complex than the 2G/3G/4G Core Network Architecture.

Alan Carlton, Vice President, InterDigital and Head of InterDigital International Labs Organization spanning Europe and Asia provided a concise summary of the changes in 5G core network in ComputerWorld:

Session management is all about the establishment, maintenance and tear down of data connections. In 2G and 3G this manifested as the standalone General Packet Radio Service (GPRS). 4G introduced a fully integrated data only system optimized for mobile broadband inside which basic telephony is supported as just one profile.

Mobility management as the name suggests deals with everything that needs doing to support the movement of users in a mobile network. This encompasses such functions as system registration, location tracking and handover. The principles of these functions have changed relatively little through the generations beyond optimizations to reduce the heavy signaling load they impose on the system.

The 4G core network’s main function today is to deliver an efficient data pipe. The existence of the service management function as a dedicated entity has been largely surrendered to the “applications” new world order. Session management and mobility management are now the two main functions that provide the raison d’etre for the core network.

Session management in 4G is all about enabling data connectivity and opening up a tunnel to the world of applications in the internet as quickly as possible. This is enabled by two core network functions, the Serving Gateway (SGW) and Packet Data Gateway (PGW). Mobility management ensures that these data sessions can be maintained as the user moves about the network. Mobility management functions are centralized within a network node referred to as Mobility Management Entity (MME). Services, including voice, are provided as an “app” running on top of this 4G data pipe. The keyword in this mix, however, is “function”. It is useful to highlight that the distinctive nature of the session and mobility management functions enables modularization of these software functions in a manner that they can be easily deployed on any Commercial-Off-The-Shelf (COTS) hardware.

The biggest change in 5G is perhaps that services will actually be making a bit of a return...the plan is now to deliver the whole Network as a Service. The approach to this being taken in 3GPP is to re-architect the whole core based on a service-oriented architecture approach. This entails breaking everything down into even more detailed functions and sub-functions. The MME is gone but not forgotten. Its former functionality has been redistributed into precise families of mobility and session management network functions. As such, registration, reachability, mobility management and connection management are all now new services offered by a new general network function dubbed Access and Mobility Management Function (AMF). Session establishment and session management, also formerly part of the MME, will now be new services offered by a new network function called the Session Management Function (SMF). Furthermore, packet routing and forwarding functions, currently performed by the SGW and PGW in 4G, will now be realized as services rendered through a new network function called the User Plane Function (UPF).

The whole point of this new architectural approach is to enable a flexible Network as a Service solution. By standardizing a modularized set of services, this enables deployment on the fly in centralized, distributed or mixed configurations to enable target network configurations for different users. This very act of dynamically chaining together different services is what lies at the very heart of creating the magical network slices that will be so important in 5G to satisfy the diverse user demands expected. The bottom line in all this is that the emphasis is now entirely on software. The physical boxes where these software services are instantiated could be in the cloud or on any targeted COTS hardware in the system. It is this intangibility of physicality that is behind the notion that the core network might disappear in 5G.

3GPP TS 23.502: Procedures for the 5G System; Stage 2, provides examples of signalling for different scenarios. The MSC above shows the example of registration procedure. If you want a quick refresher of LTE registration procedure, see here.

I dont plan to expand on this procedure here. Checkout section "4.2.2 Registration Management procedures" in 23.502 for details. There are still a lot of FFS (For further studies 😉) in the specs that will get updated in the coming months.


Further Reading:

• 3GPP TR 23.799: Study on Architecture for Next Generation System
• 3GPP TS 23.501: System Architecture for the 5G System; Stage 2
• 3GPP TS 23.502: Procedures for the 5G System; Stage 2