The shift to Service-Based Architecture (SBA) is the most fundamental change in core network design since GPRS. Here’s the complete technical deep dive:
What Is SBA? (3GPP TS 23.501):
→ All NFs expose and consume services via HTTP/2 REST APIs
→ NFs register in NRF (Network Repository Function) — service discovery
→ Any NF can call any other NF directly (no fixed topology)
→ Fully cloud-native: containerised (K8s), stateless where possible
→ Inter-NF interface naming: N{provider} — e.g., Namf, Nsmf, Nudm
Complete 5G Core NF Reference Map:
→ AMF (Access & Mobility Mgmt Function): NAS, mobility, paging
→ SMF (Session Mgmt Function): PDU session, IPAM, UPF selection
→ UPF (User Plane Function): data forwarding, QoS, traffic detection
→ UDM (Unified Data Mgmt): subscriber profile, key storage
→ AUSF (Auth Server Function): AKA authentication, KAUSF derivation
→ PCF (Policy Control Function): policy rules, QoS authorisation
→ NRF (Network Repository Function): NF registration & discovery
→ NSSF (Network Slice Selection): NSSAI → AMF/SMF mapping
→ NEF (Network Exposure Function): external API exposure (CAMARA/NEF)
→ NWDAF (Network Data Analytics): AI/ML analytics, load prediction
→ SMSF (SMS Function): SMS over NAS delivery
→ AF (Application Function): enterprise/OTT app interface to PCF/NEF
Key 5G Core Reference Points vs SBA:
→ N1: UE ↔ AMF (NAS — Non-Access Stratum signalling)
→ N2: gNB ↔ AMF (NGAP — NG Application Protocol)
→ N3: gNB ↔ UPF (GTP-U — user plane tunnel)
→ N4: SMF ↔ UPF (PFCP — Packet Forwarding Control Protocol)
→ N6: UPF ↔ Data Network (Internet / enterprise LAN)
→ N9: UPF ↔ UPF (for UPF chaining in slicing)
→ N11: AMF ↔ SMF (SBA: Nsmf_PDUSession)
→ N8/N10: AMF/SMF ↔ UDM (subscriber data retrieval)
PDU Session Establishment — Key Message Flow:
UE → AMF: NAS PDU Session Establishment Request
AMF → SMF: Nsmf_PDUSession_CreateSMContext
SMF → UDM: Nudm_SDM_Get (subscription data)
SMF → PCF: Npcf_SMPolicyControl_Create (policy)
SMF → UPF: N4 Session Establishment (PFCP)
SMF → AMF: N1N2MessageTransfer
AMF → gNB: N2 PDU Session Resource Setup
gNB → UE: RRC Reconfiguration + NAS PDU Session Accept
5G Core SBA is not just architecture elegance. It’s what enables slicing, CAMARA APIs, AI analytics, and cloud-native operations at carrier scale.
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