5G Architecture

Service-based architecture and cloud-native network functions.

1. A Paradigm Shift: From Physical Boxes to Virtual Services

The architecture of a mobile network is its fundamental blueprint. It defines how all the different pieces of hardware and software work together to deliver connectivity. For generations, this architecture was based on a collection of large, specialized, and expensive hardware boxes, each performing a specific function. The 4G Evolved Packet Core (EPC) was the pinnacle of this design philosophy, with distinct physical nodes like the MME, SGW, and PGW connected by fixed, point-to-point interfaces.

The 5G architecture represents a complete break from this legacy. It is not just an evolution of the 4G core; it is a revolution built on the principles of modern cloud computing and software development. The central idea behind the 5G network is to move away from monolithic hardware boxes and towards a flexible, programmable, and highly efficient system where network functions are virtualized software components.

This new design is known as the Service-Based Architecture (SBA). In the SBA, the core network is no longer a rigid hierarchy of interconnected devices. Instead, it is a fluid collection of software applications called Network Functions (NFs) that can run on standard, off-the-shelf servers, whether in a large data center or at the edge of the network. These NFs communicate with each other like modern web services, discovering each other and interacting through standardized Application Programming Interfaces (APIs). This cloud-native approach is what gives 5G its unprecedented flexibility and power, enabling it to deliver not just faster internet but a vast array of new services, from the massive Internet of Things to mission-critical industrial automation.

2. Core Principles of the 5G Architecture

The power and flexibility of the 5G architecture are rooted in several fundamental design principles derived from the IT and cloud computing worlds.

Service-Based Architecture (SBA)

This is the most critical philosophical change from 4G. Instead of defining the network by its physical nodes and the interfaces between them (like the S1, S11, SGi interfaces in 4G), the 5G architecture defines the network by its software functions and the services they provide.

In this model:

A Network Function (NF) is a modular software component responsible for a specific task (e.g., managing user sessions, authenticating users).
Each NF can act as a service producer, offering its capabilities to other NFs through a well-defined API.
Each NF can also act as a service consumer, discovering and using the services offered by other NFs to perform its own tasks.

This communication happens over a common service bus or message bus. This decouples the NFs from each other, allowing an operator to easily add new functions, update existing ones, or scale them independently without redesigning the entire network. It is analogous to how modern web applications are built using microservices that communicate via APIs.

Control and User Plane Separation (CUPS)

While CUPS was introduced as an enhancement for 4G, it is a native and central principle in the 5G architecture. It dictates that the functions responsible for processing user data traffic (the User Plane) must be separate from the functions that handle network signaling and control (the Control Plane).

Independent Scaling: An operator can scale data handling capacity (e.g., adding more User Plane Functions) without having to scale the control functions, and vice versa.
Flexible Placement: The User Plane Function (UPF), which is the workhorse of data forwarding, can be placed anywhere in the network. It can be centralized in a data center or distributed to the network edge, closer to the users. This is the key enabler for , which is critical for low-latency applications.

Cloud-Native and Virtualized (NFV)

The 5G architecture is designed to be cloud-native. This means its Network Functions are not tied to specific hardware. They are implemented as software that can run on any generic, commercial off-the-shelf (COTS) server hardware. This concept is called Network Function Virtualization (NFV). This brings several cloud computing advantages to the telecom world:

Cost Reduction: Operators are no longer locked into buying expensive, proprietary hardware from a single vendor. They can use standard IT servers, which are cheaper and more flexible.
Rapid Deployment: New network functions and services can be deployed as software updates in minutes or hours, rather than the months or years required for physical hardware rollouts.
Scalability on Demand: If a specific area experiences a surge in traffic, an operator can automatically spin up new virtual instances of a Network Function to handle the load and then shut them down when they are no longer needed, optimizing resource usage.

Stateless Network Functions

A key cloud-native principle adopted in 5G is that most Network Functions should be stateless. This means the NF that processes a transaction does not store any long-term information about the user's session or state. Instead, this crucial context is stored in a centralized, dedicated database (the UDR - Unified Data Repository).

This approach provides massive benefits for reliability and scalability. If a software instance of a specific NF fails, another instance can be spun up instantly and continue processing transactions for the user, because all the necessary information can be retrieved from the central UDR. There is no loss of service. It also makes it easy to scale a function by simply adding more identical, stateless instances.

3. The 5G System Components: An Overview

The end-to-end 5G system architecture is comprised of the user device, a new radio network, and the new 5G core network.

UE (User Equipment): This is your 5G-capable device, such as a smartphone, a tablet, or an IoT sensor.
NG-RAN (Next-Generation Radio Access Network): This is the 5G radio network. Its primary component is the gNodeB (gNB), which is the 5G equivalent of the LTE eNodeB. The gNB handles the 5G New Radio (NR) connection with the UE. The NG-RAN architecture is also designed to be highly flexible, supporting options like a split between a Centralized Unit (CU) and a Distributed Unit (DU), which allows for more efficient radio resource management.
5GC (5G Core Network): This is the new, service-based core network that acts as the brain of the system. It connects the radio network to external data networks and provides all the advanced features of 5G.

4. Key Functions of the 5G Core Network (5GC)

The 5G Core is not made of monolithic blocks like the 4G EPC. Instead, its functionality is distributed across a set of specialized, interoperable Network Functions (NFs). Here are the most important ones:

Core Control Plane Functions

: This function is the single entry point for all connection and mobility-related signaling from the UE. It handles device registration with the network, authentication handshaking, and tracking the device's location when it's idle or moving. It is essentially the evolution of the control part of the 4G MME.
: The SMF is responsible for everything related to a user's data session. When you want to connect to the internet, the SMF is responsible for establishing a session, assigning your device an IP address (from the UPF), selecting the UPF that will handle your data traffic, and applying the necessary QoS rules to your data bearers.
: This function acts as the authentication server. It manages the security procedures to verify the identity of a user when they connect to the network.
: The PCF is the evolution of the 4G PCRF. It is the brain behind network policy, providing rules to the other NFs to ensure that the correct QoS is applied to different data flows and that charging is handled correctly.

Core User Plane Function

: The UPF is where the rubber meets the road for your data. It combines the data-plane functions of the 4G SGW and PGW into a single, streamlined entity. Its job is to route your IP packets at high speed. It also acts as the interconnection point to external data networks (like the internet), inspects packets to enforce QoS policies received from the SMF, and gathers usage statistics for billing. The flexibility to place UPFs at the edge of the network is critical for 5G's low-latency promise.

Core Data and Discovery Functions

: The UDM is the master database for subscriber information in 5G, evolving the role of the 4G HSS. It stores the user's profile, their security credentials, and their subscription details.
: This is the physical database where the UDM and PCF store their data. Separating the data logic (UDM) from the physical storage (UDR) is a key cloud-native principle that enhances scalability and resilience.
: The NRF is the linchpin of the Service-Based Architecture. It acts as a dynamic service discovery engine. Every NF, when it comes online, registers its services with the NRF. When another NF needs to use a particular service (e.g., the AMF needs to find an AUSF), it simply queries the NRF, which returns a list of available and suitable NF instances.