Service Area Restriction (TS 23.501 §5.3.4 — AMF + PCF)

Mirrored documentation

Mirrored read-only from claudia-5gc @ v2.3.1. Edit upstream, not here.

Purpose

The AMF enforces Service Area Restriction (SAR) to prevent UEs from registering in, or being handed over to, Tracking Areas that are not allowed by the PCF’s AM policy. When a UE attempts to register from a restricted TA, the AMF rejects the registration with 5GMM cause #73 (Serving network not authorized). The allowed-TA list may be included in the Registration Accept to help the UE avoid re-registering unnecessarily.

Specifications

Topic	Reference
Architecture	TS 23.501 §5.3.4
Procedure flow	TS 23.502 §4.2.2.2.2 step 14c
Stage 3 AM policy data model	TS 29.507 §6.1.1.2.5 (ServiceAreaRestriction)
Registration Reject cause	TS 24.501 §6.1.3.2 cause #73
Registration Accept Allowed NSSAI / TAI list	TS 24.501 §8.2.7

Sequence Diagram

sequenceDiagram
    participant UE
    participant gNB
    participant AMF
    participant PCF

    gNB->>AMF: NGAP InitialUEMessage (NAS Registration Request, TAI)
    AMF->>PCF: POST /npcf-ampolicycontrol/v1/policies\n{supi, accessType, plmnId}
    PCF-->>AMF: 201 {polAssoId, rfsp, servAreaRes:{restrictionType:"ALLOWED_AREAS",areas:[{tacs:[…]}]}}
    AMF->>AMF: Check UE's TAI against servAreaRes.areas

    alt UE is in an ALLOWED area
        AMF-->>UE: Registration Accept (5GMM, [Allowed TAI List])
    else UE is in a RESTRICTED area
        AMF-->>UE: Registration Reject\n  5GMM Cause #73 (Serving network not authorized)
    end

Information Elements

ServiceAreaRestriction (PCF → AMF, inside PolicyAssociation)

IE	Type	Description
restrictionType	enum	ALLOWED_AREAS or NOT_ALLOWED_AREAS
areas	array	List of Area objects
areas[*].tacs	array<string>	TAC values in hex (e.g., "000001")
areas[*].areaCode	integer	Optional area identifier
maxNumOfTAs	integer	Max number of TAs per area (optional)

5GMM Cause for Reject (TS 24.501 §8.2.8.2 IEI 0x58)

Cause	Value	Description
Serving network not authorized	73 (0x49)	UE registering from a non-allowed TA

Error Cases

Condition	AMF action
UE TAI not in allowed areas	Send Registration Reject, cause #73
PCF returns servAreaRes with empty areas	Treat as unrestricted (no restriction enforced)
PCF unavailable (AM policy call failed)	No restriction enforced (non-fatal)
Handover to restricted TA	Reject HandoverRequest/PathSwitchRequest

Implementation Notes

• ServiceAreaRestriction is an optional field in the PCF PolicyAssociation response. When absent, no restriction is enforced.
• The current dev PCF returns no servAreaRes (unrestricted) — add it to per-subscriber configuration when needed (future UDR integration point).
• AMF checks the UE’s TAI (from NGAP UserLocationInformation) against the PCF-returned servAreaRes before sending Registration Accept. The check happens after Phase3 PCF call.
• On rejection: AMF sends a NAS Registration Reject (cause #73) and does NOT assign a GUTI.
• The allowed-TA list in Registration Accept (IEI 0x45) is optional; if PCF provides ALLOWED_AREAS, AMF should include them so the UE avoids re-registering.
• Cause 0x49 = 73 decimal per TS 24.501 Table 9.11.3.2.1.1 (5GMM cause values).