5G Standalone (5G SA) is the point at which 5G stops leaning on 4G. The radio network connects to a dedicated 5G Core, and the full set of 5G capabilities — network slicing, a flexible QoS model and a cloud-native, service-based core — becomes available. This article explains the main building blocks and the procedures engineers work with most often.

Non-standalone versus standalone

Early 5G launches mostly used non-standalone (NSA) operation, known in 3GPP as EN-DC (option 3x). The device keeps an LTE anchor connection for control signalling, the 5G NR radio adds capacity for user data, and everything is controlled by the existing 4G Evolved Packet Core (EPC). NSA was a quick way to deliver higher speeds, but it inherits the limits of the EPC.

In standalone (SA) operation, defined from 3GPP Release 15 onwards (option 2), the gNB connects directly to the 5G Core (5GC). Control and user plane both run over 5G, and the device registers with the 5GC rather than the EPC. This is what enables native slicing, the new QoS framework and voice over NR.

The 5G Core and its network functions

Simplified 5G Standalone architecture The UE connects over the radio to the gNB. The gNB connects to the AMF over N2 and to the UPF over N3. Control-plane functions AMF, SMF, AUSF, UDM, PCF, NRF and NSSF share a service-based interface. The SMF controls the UPF over N4, and the UPF connects to the data network over N6. Service-based interface (HTTP/2) AMF SMF AUSF UDM PCF NRF NSSF UE gNB UPF Data network NR radio N2 N3 N4 N6 Control plane above · user plane below
A simplified view of 5G SA. Many more network functions exist (for example UDR, NEF, SMSF and CHF); only the most commonly encountered are shown.

The 5GC separates control and user plane more cleanly than the EPC and breaks control functions into smaller network functions (NFs). The ones you will meet most often are:

  • AMF (Access and Mobility Management Function) — terminates N1 NAS signalling from the device and N2 (NGAP) from the gNB; handles registration, reachability and mobility.
  • SMF (Session Management Function) — establishes and manages PDU sessions, allocates IP addresses and controls the UPF over N4 using PFCP.
  • UPF (User Plane Function) — forwards user traffic between the gNB (N3, GTP-U) and the data network (N6), and applies the packet detection, forwarding and QoS rules installed by the SMF.
  • AUSF and UDM — together handle subscriber authentication (5G-AKA or EAP-AKA′) and subscription data; the UDM typically stores data in a UDR.
  • PCF (Policy Control Function) — provides access, mobility and session policy, including QoS decisions.
  • NRF (Network Repository Function) — the registry that NFs use to register themselves and discover one another.
  • NSSF (Network Slice Selection Function) — helps select the network slice instances that serve a device.

The service-based architecture

Instead of the fixed point-to-point Diameter interfaces of the EPC, most 5GC control-plane NFs expose services over HTTP/2 with JSON payloads, collectively called the service-based interface (SBI). An NF that needs something — the AMF asking the AUSF to authenticate a device, for example — discovers a suitable producer through the NRF and calls its API.

This design suits cloud-native deployment: NFs can scale, be upgraded and be replaced more independently. It also changes troubleshooting. Engineers who once read Diameter traces now inspect HTTP/2 requests and responses, often protected by TLS, and must confirm that NRF registration and discovery are working before blaming the NF itself.

Registration: getting a device onto the network

Before a device can send data, it must register with the 5GC. In simplified form:

  1. The device sends a Registration Request (NAS) via the gNB to an AMF, identifying itself with a concealed identifier (SUCI) or a temporary identifier (5G-GUTI).
  2. The AMF selects an AUSF, which obtains authentication vectors from the UDM. The device and network mutually authenticate.
  3. NAS security is established, and the AMF retrieves subscription and policy information from the UDM and PCF.
  4. The AMF sends Registration Accept, including the allowed network slices (NSSAI) and a new 5G-GUTI; the device replies with Registration Complete.

Failures at this stage usually show up as a reject cause in the NAS signalling. The cause code, together with the AMF and AUSF/UDM logs, is normally the fastest route to the problem — a subscription not provisioned, a slice not allowed, or an authentication mismatch.

PDU sessions and QoS flows

Registration gives the device a signalling relationship with the network; a PDU session gives it connectivity to a data network, identified by a DNN (the 5G equivalent of an APN) and a slice (S-NSSAI). The SMF selects a UPF, allocates an address, installs forwarding rules over N4 and instructs the gNB to set up radio resources.

Inside each PDU session, traffic is carried in QoS flows, each identified by a QFI and associated with a 5QI that defines its characteristics. This is more flexible than the EPS bearer model: several flows with different treatment can share one session, and the UPF and gNB map packets to flows using rules supplied by the SMF.

Network slicing

A network slice is a logical network built for a particular purpose — for example, enhanced mobile broadband, a private enterprise service or low-latency industrial traffic. Each slice is identified by an S-NSSAI made up of a Slice/Service Type (SST) and an optional Slice Differentiator (SD). The device requests slices during registration, the network returns the slices it is allowed to use, and each PDU session is associated with one of them.

Slicing touches almost every part of the system — subscription data, the NSSF, AMF and SMF selection, UPF placement and radio configuration — so slice-related problems often need investigation across several teams.

Voice in 5G Standalone

5G SA has no circuit-switched domain, so voice is delivered through IMS. Networks either support VoNR (voice over NR, with the voice media carried in a 5G QoS flow) or use EPS fallback, where the device moves to LTE to complete the call using VoLTE. Which method is used depends on network configuration and device support, and it is a common area for interoperability testing. Our IMS fundamentals article covers the IMS side of this in more detail.

What to check when 5G SA services fail

  • Start with the procedure. Establish whether the failure is in registration, PDU session establishment or user-plane forwarding — each points to different NFs.
  • Read the cause codes. NAS 5GMM and 5GSM cause values, NGAP causes and HTTP status codes on the SBI are all informative.
  • Confirm NF discovery. Check that the relevant NFs are registered with the NRF and that discovery returns the expected producer.
  • Check slice and DNN consistency. Subscription data, the device configuration, the AMF and SMF and the UPF must agree.
  • Verify N3, N4 and N6 paths. A session that is established in signalling but carries no traffic often points to PFCP rules, GTP-U routing or N6 connectivity.
  • Capture at the right points. Traces at the gNB–AMF, SMF–UPF and SBI interfaces, alongside NF logs, give a complete picture.

Summary

5G SA brings a genuinely new core: smaller, service-based network functions, a cleaner split between control and user plane, flexible QoS and native slicing. Understanding how registration, PDU sessions and slices fit together is the foundation for integrating, testing and operating it.

KL-UK supports 5G SA integration, troubleshooting and testing. Learn more about our 5G Standalone engineering services or our telecom testing services.

Published by KL-UK on . This article is for general information and does not replace vendor documentation or the relevant 3GPP specifications.