The Problem with 4G
In 4G (LTE), the core network was designed like a chain.
Each component had fixed connections to specific other components. Want to add a new feature? You’d have to rewire everything.
This made 4G inflexible and hard to upgrade.
The 5G Solution: Service-Based Architecture
5G redesigns the core as a service marketplace.
Instead of fixed connections, components offer services on a common bus. Anyone who needs a service can request it.
Why is this better?
| 4G Approach | 5G Approach |
|---|---|
| Fixed point-to-point links | Flexible service bus |
| Hard to add features | Easy to extend |
| Monolithic design | Cloud-native, containerized |
| Hardware-dependent | Software-defined |
Think of it like going from dedicated phone lines to the internet.
The Three Main Components
The 5G core has many components, but three do the heavy lifting:
| Component | Name | One-Line Summary |
|---|---|---|
| AMF | Access and Mobility Management | “Who are you and where are you?” |
| SMF | Session Management Function | “Set up your data connection” |
| UPF | User Plane Function | “Actually move your data” |
AMF: The Gatekeeper
AMF handles everything about you connecting to the network.
When you turn on your phone:
- Phone says “Hello, I’m here” → AMF receives it
- AMF checks your identity → Are you a valid subscriber?
- AMF registers your location → Which cell are you in?
AMF’s responsibilities:
| Task | What It Means |
|---|---|
| Registration | Phone joins the network |
| Authentication | Verify you’re legitimate |
| Mobility | Track which cell you’re in |
| Security | Encryption setup |
AMF is like a receptionist - handles check-in and knows where everyone is.
SMF: The Planner
SMF sets up the path for your data.
When you want to browse the web:
- AMF tells SMF “this user wants data”
- SMF creates a PDU session (data pipe)
- SMF assigns you an IP address
- SMF tells UPF how to route your traffic
SMF’s responsibilities:
| Task | What It Means |
|---|---|
| Session setup | Create the data connection |
| IP allocation | Give you an address |
| QoS control | Set speed/priority rules |
| UPF selection | Choose which UPF to use |
SMF is like a network planner - designs the pipes for your data to flow.
UPF: The Workhorse
UPF is where your data actually flows.
AMF and SMF just make decisions. UPF moves packets.
When you load a webpage:
- Your phone sends a request → UPF receives it
- UPF checks the rules → Where should this go?
- UPF forwards to internet → Packet leaves the network
- Response comes back → UPF sends it to your phone
UPF’s responsibilities:
| Task | What It Means |
|---|---|
| Packet forwarding | Move data in/out |
| Traffic inspection | Check what’s flowing |
| QoS enforcement | Apply speed limits |
| Usage reporting | Track how much you use |
UPF is like the postal service - doesn’t make decisions, just delivers packages.
How They Work Together
The flow when you connect:
| Step | What Happens | Component |
|---|---|---|
| 1 | Phone requests connection | → AMF |
| 2 | AMF authenticates you | AMF |
| 3 | AMF requests session setup | AMF → SMF |
| 4 | SMF configures UPF | SMF → UPF |
| 5 | Data starts flowing | Phone ↔ UPF ↔ Internet |
Control plane (AMF, SMF) makes decisions. User plane (UPF) moves data.
Control Plane vs User Plane
5G separates these two planes completely.
| Plane | Purpose | Components | Traffic |
|---|---|---|---|
| Control | Decisions, signaling | AMF, SMF, etc. | Small, occasional |
| User | Actual data | UPF | Large, constant |
Why separate them?
- Scale independently - Need more data capacity? Add UPFs. More users connecting? Add AMFs.
- Place differently - UPF can be at network edge (low latency). Control plane can be centralized.
- Secure differently - Control plane is highly protected. User plane is optimized for speed.
The Other Components
The core has more functions for specific tasks:
| Component | Full Name | What It Does |
|---|---|---|
| UDM | Unified Data Management | Stores subscriber info |
| AUSF | Authentication Server | Security verification |
| PCF | Policy Control Function | Rules and policies |
| NRF | Network Repository Function | Service discovery |
| NSSF | Network Slice Selection | Picks the right slice |
| NEF | Network Exposure Function | APIs for external apps |
Service-Based Interfaces
In SBA, components communicate using HTTP/REST APIs.
Each component exposes services that others can call:
| Component | Service Name | What It Offers |
|---|---|---|
| AMF | Namf | Mobility, registration |
| SMF | Nsmf | Session management |
| UDM | Nudm | Subscriber data |
| PCF | Npcf | Policy decisions |
| NRF | Nnrf | Service discovery |
The “N” prefix means “Network function”. Namf = AMF’s network services.
Network Slicing
One powerful feature of SBA: network slicing.
The same physical network can be divided into virtual networks, each optimized for different uses.
| Slice | Optimized For | Example Use |
|---|---|---|
| eMBB slice | High bandwidth | Video streaming |
| URLLC slice | Low latency | Remote surgery |
| mMTC slice | Many devices | IoT sensors |
Each slice gets its own AMF, SMF, UPF instances. Same hardware, different virtual networks.