Registration flow, mobility, N1/N2 interfaces, paging, GUAMI/GUTI assignment, AMF pool design, handover types, N26 interworking
1. What Is the AMF — The Simple Version
The AMF (Access and Mobility Management Function) is the entry point of the 5G Core for every UE. All NAS (Non-Access Stratum) signalling from a UE terminates at the AMF. It handles the two fundamental questions for any mobile connection: who is this UE (authentication coordination with AUSF/UDM) and where is it (tracking area management and handover coordination).
The AMF does not forward user data — that is the UPF. It does not manage PDU sessions — that is the SMF. What it does is orchestrate the access and mobility procedures that make all the rest possible. In a large GCC operator deployment, the AMF handles millions of registration events per day, hundreds of thousands of handovers per hour, and paging for every idle UE that receives downlink data.
| 3GPP Reference |
| 3GPP TS 23.502 Section 4.2 — Registration procedures |
| 3GPP TS 23.502 Section 4.9 — Handover procedures (Xn, N2, 5GS-to-EPS) |
| 3GPP TS 29.518 — AMF Services: Namf_Communication, Namf_EventExposure |
| 3GPP TS 24.501 — NAS protocol for 5GS (AMF NAS layer reference) |
2. Architecture — AMF Functions and Interfaces
| AMF Function | Description | Key Interface |
| NAS termination | Terminates NAS layer 3 messages from UE. Relays session management NAS to SMF. | N1 (UE via gNB), N11 (SMF) |
| NGAP termination | Terminates N2 interface with gNB. Receives UE context from RAN. | N2 (gNB) — SCTP multi-homed |
| Authentication coordination | Invokes AUSF for 5G-AKA or EAP-AKA’. Manages key derivation chain. | N12 (AUSF), derived KAMF from KSEAF |
| Registration management | Initial, mobility, and periodic registration. Assigns 5G-GUTI. | N1/N2, N8 (UDM), N22 (NSSF), N15 (PCF) |
| Mobility management | Tracks UE location at TA granularity. Coordinates Xn/N2/N26 handovers. | N2 (source/target gNB), N26 (MME for 5G↔4G) |
| Session management relay | Relays PDU session NAS between UE and SMF. Does not interpret SM messages. | N11 (SMF), N1 (UE) |
| Paging | Determines paging area, triggers gNB paging for idle UE downlink data. | N2 (all gNBs in paging area) |
| GUAMI/GUTI assignment | Assigns 5G-GUTI to UE after successful registration. | N1 (UE) — in Registration Accept |
Table 1 — AMF functional responsibilities. The AMF is the signalling hub — it coordinates with every other NF but processes no user data.
3. Step-by-Step — Initial Registration
Here is the complete initial registration flow for a UE connecting for the first time to an SA 5G network:
Step 1 — UE sends RRC Setup + NAS Registration Request to gNB. Registration Type = Initial Registration. Identity = SUCI (first time) or 5G-GUTI (if previously registered). Requested NSSAI included if UE has configured S-NSSAIs.
Step 2 — gNB sends N2: NGAP Initial UE Message to AMF. Contains: NAS Registration Request, User Location Info (NR CGI, TAI), Access Type (3GPP), RAN UE NGAP ID. AMF is selected by gNB based on GUAMI from the UE’s 5G-GUTI (if present) or load-balanced selection from configured AMF set.
Step 3 — AMF queries NRF for AUSF serving this UE’s home PLMN. Sends N12: Nausf_UEAuthentication_Authenticate Request with SUCI. 5G-AKA exchange follows — see Post 05 for full detail. Result: UE authenticated, KAMF established.
Step 4 — AMF sends N8: Nudm_UECM_Registration to register itself as serving AMF for this SUPI. Sends N8: Nudm_SDM_Get to retrieve Access and Mobility subscription data: subscribed S-NSSAIs, UE-AMBR, subscribed DNN list, roaming restrictions, RFSP index.
Step 5 — AMF queries NSSF (N22: Nnssf_NSSelection_Get) with UE Requested NSSAI, subscription, and current TA. NSSF returns Allowed NSSAI for this UE in this TA. If Allowed NSSAI requires a different AMF set, NSSF returns the target AMF set and AMF redirects the UE.
Step 6 — AMF invokes PCF (N15: Npcf_AMPolicyControl_Create) to get AM Policy for this UE: RFSP index (RAN frequency/RAT selection priority), UE-AMBR, background data transfer policy.
Step 7 — AMF assigns 5G-GUTI to UE (GUAMI + new 5G-TMSI). Sends N2: NGAP Initial Context Setup Request to gNB — includes NAS Security Mode Command (activates NAS security), UE security capabilities, GUTI, and optionally PDU Session Resource Setup (if default PDU session to be established). gNB responds with N2: Initial Context Setup Response.
Step 8 — AMF sends NAS: Registration Accept to UE containing: 5G-GUTI, TAI List (the TAs where this GUTI is valid), Allowed NSSAI, T3512 (periodic registration timer, default 3400s = ~57 minutes), and optionally Network Slicing Indication.
4. Key Parameters and Technical Terms
| Term | Definition | Operational Significance |
| GUAMI | Globally Unique AMF Identifier: MCC+MNC+AMF Region ID (8-bit)+AMF Set ID (10-bit)+AMF Pointer (6-bit) | Used by gNB to route Initial UE Messages to the correct AMF in a pool. Embedded in 5G-GUTI. |
| 5G-GUTI | 5G Globally Unique Temporary Identifier: GUAMI + 5G-TMSI (32-bit). UE’s working temporary identity. | Refreshed regularly by AMF. Old 5G-GUTI in Registration Request: AMF recovers context via UDM UECM. |
| T3512 (Periodic Registration Timer) | Timer for periodic registration update. Default: 3400s (~57 min). UE re-registers before expiry. | If T3512 expires before UE re-registers: AMF marks UE as implicitly deregistered. Sessions released. |
| AMF Pool | Set of AMF instances sharing the same AMF Set ID. UEs can be served by any AMF in the pool. | Load balancing within pool. UE context must be accessible across pool members (shared session store or context transfer). |
| AMF Pointer | 6-bit field in GUAMI that identifies a specific AMF instance within a Set. | gNB uses AMF Pointer to route to specific AMF for a UE with existing 5G-GUTI. Determines which AMF gets the session. |
| N2 SCTP Multi-homing | N2 uses SCTP which supports multiple IP paths (primary + secondary). Association uses both for fast failover. | gNB configures primary and secondary AMF IP endpoints. SCTP heartbeat detects path failure in < 1s. |
| Xn Handover | gNB-to-gNB handover via direct Xn interface. AMF involvement: only path switch notification. | Fastest handover type (< 50ms). Source gNB prepares target gNB directly. AMF receives N2 Path Switch Request after radio handover. |
| N2 Handover | AMF-coordinated handover — AMF relays context between source and target gNB via N2. | Used when Xn is not available between gNBs. Slower (~100–200ms). More AMF signalling load. |
| N26 Interface | AMF-to-MME interface for session context transfer during 5G↔4G handover. | Without N26: 5G→4G handover causes brief session interruption (re-establishment needed). With N26: seamless. |
| NGAP Paging | AMF sends NGAP Paging messages to all gNBs in UE’s last known TA when downlink data arrives. | Paging storm risk: if many UEs simultaneously have downlink data while idle, AMF generates large paging burst. |
Table 2 — AMF key parameters. T3512, GUAMI pool design, and N26 configuration are the three AMF parameters most commonly misconfigured in initial SA deployments.
5. Common Issues in the Field
| Field Note: Paging Storm After AMF Restart — gNB N2 Queue Overflow |
| AMF pod restarted after software upgrade. Context store not preserved (StatefulSet volume mount missing). |
| 150,000 UEs implicitly deregistered. When they received downlink data in idle mode: AMF pageed all of them. |
| 12,000 NGAP Paging messages/second to gNBs (normal: ~200/second). Several gNBs dropped paging. |
| UEs not responding to paging — their sessions were already gone. Cascading re-registration storm. |
| Fix: AMF StatefulSet with PersistentVolume for UE context. Test context recovery after pod restart. |
| Also: AMF paging rate limiter — cap at 1000 paging/second, queue overflow to next cycle. |
| Field Note: N26 Not Configured — VoLTE Drops on 5G→4G Handover |
| SA deployment in Muscat. UEs on 5G making VoLTE calls. When walking to basement (4G-only coverage): call dropped. |
| Without N26: 5G→4G handover = no context transfer. UE re-registers on 4G. IMS call not maintained. |
| VoNR not yet deployed — operator still using VoLTE fallback. N26 was never configured in AMF/MME. |
| Fix: configure N26 on AMF (AMF N26 endpoint = MME S10 IP) and MME (S10 peer = AMF N26 IP). |
| Test: drive test from 5G to 4G coverage during active VoLTE call. Verify SRVCC or session continuity. |
6. Troubleshooting
| Symptom | Root Cause | Check | Fix |
| Registration success rate drops | AMF overloaded or NRF/AUSF/UDM unreachable | AMF: N12 auth latency, N8 UDM response time; NRF pod health | Scale AMF; check AUSF/UDM pod health; verify NRF caching |
| UE stuck in registration loop | NSSF returning empty Allowed NSSAI — UE slice not available in TA | NSSF policy: TA list for UE S-NSSAI; UDM: subscriber S-NSSAI subscription | Add TA to NSSF policy; check UDM subscription |
| Calls drop on 5G→4G handover | N26 not configured or MME N26 endpoint unreachable | AMF: N26 interface config; MME: S10 peer AMF; ICMP ping across N26 | Configure N26 on both AMF and MME; verify routing |
| Mass UE deregistration after AMF restart | Context not persisted — AMF using ephemeral pod storage | AMF StatefulSet: PersistentVolume mounted for context store | Mount PersistentVolume for AMF UE context; test recovery after pod restart |
| gNB cannot reach AMF — N2 association fails | AMF N2 endpoint IP/port misconfigured or firewall blocking SCTP 38412 | gNB: N2 SCTP association attempt logs; AMF: NGAP endpoint config | Verify AMF N2 IP in gNB config; open SCTP 38412 in firewall |
Table 3 — AMF troubleshooting. N26 and context persistence are the two most common AMF configuration gaps in initial SA deployments.
7. Summary — Key Takeaways
| Topic | Key Takeaway |
| AMF role | NAS termination + mobility management. No user data, no session management. Orchestrates auth (AUSF), subscription (UDM), policy (PCF), slice selection (NSSF). |
| Registration flow | 8-step flow. NSSF slice selection at Step 5 is the most common failure point — missing TA in NSSF policy = UE cannot access enterprise slices. |
| GUAMI pool design | AMF pool = same Set ID. gNB routes to AMF Pointer within Set. Load balance within pool. Context must be accessible across pool members. |
| N2 SCTP | Multi-homed SCTP for N2. Configure primary + secondary AMF IP on each gNB. Test SCTP failover before go-live. |
| N26 | Configure before SA launch if 4G coverage exists. Without N26: 5G→4G handover drops sessions. Critical for VoNR and enterprise users. |
| AMF context persistence | AMF must persist UE context to PersistentVolume (StatefulSet). Pod restart without context = mass deregistration = re-registration storm. |
| Paging rate limiting | Set AMF paging rate limit to protect gNB N2 queue. Recovery after AMF restart without rate limiting = paging storm = gNB NGAP queue overflow. |
Table 4 — Post 10 summary. AMF is the UE lifecycle hub. Context persistence and N26 are the two most critical operational requirements.
Next: Post 11 — UPF Architecture & User Plane