Description
RAT/Frequency Selection Priority (RFSP) is a core network parameter, defined as an index (RFSP Index), that is mapped to UE-specific behavior for idle mode and connected mode mobility. The core network, specifically the Mobility Management Entity (MME) in EPS or the Access and Mobility Management Function (AMF) in 5GC, assigns an RFSP Index to a UE based on its subscription profile (from the Unified Data Management, UDM), the requested network slice, or local operator policies. This index is then provided to the radio access network (E-UTRAN or NG-RAN) via the S1 or N2 interface. The RAN uses the RFSP Index to select and apply a specific set of mobility policies, known as RFSP in Mobility Control (RFSP-in-MC), which are pre-configured in the RAN nodes (e.g., eNBs, gNBs).
These RFSP-in-MC policies dictate how the RAN should handle the UE in terms of cell reselection priorities (for idle/inactive UEs) and handover parameters (for connected UEs). For idle mode, the RFSP Index influences the absolute priorities broadcast in System Information Blocks (SIBs) for different frequency layers and RATs (e.g., giving NR a higher priority than LTE for a premium subscriber). For connected mode, it can affect the thresholds and hysteresis used in measurement reporting and handover decisions, effectively biasing the UE towards or away from certain cells. The RAN translates the abstract RFSP Index into concrete radio resource control (RRC) parameters sent to the UE.
The architecture involves close interaction between the core network's subscriber database, the control plane network functions (MME/AMF), and the RAN. The UDM/HSS stores the Subscribed RFSP Index as part of the user's subscription data. During attachment or session establishment, the MME/AMF may derive a new RFSP Index based on the subscribed index, the network slice selection, and local policy. This dynamic derivation allows for real-time traffic steering. The index itself is an integer (typically in a range like 1-256), where each value corresponds to a specific policy set configured by the operator in both the core and the RAN.
How it works in practice involves several steps. First, at UE attachment, the AMF/MME determines the RFSP Index. It then includes this index in the UE context established with the gNB/eNB. The gNB/eNB, which has a locally configured table mapping RFSP Index values to specific mobility control parameters, applies the corresponding policy. For example, RFSP Index 5 might map to a policy that sets the priority of 5G NR band n78 to '7' (high) and LTE band 3 to '3' (low) for idle mode reselection. This steers UEs with that index to prefer 5G. The network can also update the RFSP Index for a connected UE via a UE Context Modification procedure, allowing for dynamic policy changes based on changing network conditions or service requirements.
Purpose & Motivation
RFSP was created to provide the network with a powerful and standardized tool for intelligent traffic steering and mobility management. Before mechanisms like RFSP, network control over UE cell selection was largely limited to broadcasting static priorities in SIBs, which applied uniformly to all UEs in a cell. This was insufficient for implementing subscriber-differentiated services, efficient load balancing, or service-aware network slicing. RFSP solves this by introducing a UE-specific, network-assigned policy index that can be tailored to individual subscribers or service flows.
The primary problems RFSP addresses are inefficient resource utilization and the inability to align mobility with business or service logic. For instance, an operator might want to steer machine-type communication (MTC) devices to robust but lower-capacity 2G/3G layers, while steering premium smartphone users to high-capacity 4G/5G layers. Without RFSP, this would require complex and non-standard workarounds. RFSP provides a clean, standardized interface between the core network's subscriber/service awareness and the RAN's radio resource control.
Historically introduced in 3GPP Release 9 for LTE/EPS, RFSP's importance grew with the advent of network slicing and 5G's service-based architecture. It enables the core network to communicate the 'context' of a UE (e.g., that it is part of a massive IoT slice or an enhanced mobile broadband slice) to the RAN, so the RAN can apply appropriate mobility behaviors. This allows the network to optimize for different, often conflicting, goals simultaneously—such as maximizing throughput for some users while maximizing battery life or coverage for others—all within a single, shared RAN infrastructure.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (93 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-9, normative work from Rel-15.
In Release 15, the RFSP function was enhanced to allow the MME to consider the UE's support for dual connectivity with NR and the absence of Access Restriction for NR when selecting the Serving and PDN Gateways (SGW and PGW). Furthermore, the specification clarified that the RFSP Index, used by the E-UTRAN to control idle mode camping and active mode redirection, is forwarded during procedures involving a secondary RAN node for Dual Connectivity.
- PDN GW selection for 5GC UE TS 23.401CR3234
- Per EPS bearer, RAN selection of DC (or non-DC) usage TS 23.401CR3258
- UE configuration for NAS signalling low priority via OMA-DM or USIM not applicable in 5GS TS 24.501CR0084
- Support of Priority Services TS 29.514CR0023
- Comparison of Encoding Proposals for Npcf_PolicyAuthorization service and selection of Encoding Proposal 1. TS 29.890CR0002
- Comparison of Encoding Proposals for Npcf_SMPolicyControl Service and selection of Encoding Proposal. TS 29.890CR0004
+ 19 more changes
In Release 16, the RFSP function was enhanced to incorporate a "Subscriber RRM Group" as an additional parameter to the SPID/RFSP index. This provides the network with a more granular mechanism for applying radio resource management strategies, such as controlling idle mode camping or redirecting active mode UEs, based on subscriber grouping. Furthermore, the release introduced mechanisms for "Priority Sharing Indication" and the "Use of ARP value for Priority Paging," integrating core network priority metrics into the RAN's RFSP-driven policies.
- Subscriber RRM Group as additional parameter to SPID/RFSP TS 23.401CR3499
- Introduction to UE selection of CN for 5G CIoT (for 24.501) TS 24.501CR1046
- Priority Sharing Indication TS 29.514CR0126
- TSN AF selection by PCF TS 29.514CR0236
- Removal of update status dependency for sub-state selection TS 24.501CR1493
- Defenition of CAG cell, CAG ID and CAG selection TS 24.501CR1718
+ 4 more changes
In Release 17, the key enhancement for the RFSP function was the introduction of a mapping between the RAT type and frequency and the RFSP Index, providing a more granular mechanism for the network to apply specific RRM strategies. Furthermore, the release clarified the procedure for the MME to select the RFSP Index in use based on operator configuration and UE context, such as during Tracking Area Updates or inter-MME mobility. These updates refined how the RFSP Index controls UE-specific cell reselection priorities and decisions on redirecting UEs between frequency layers or RATs.
- Multimedia Priority Service (MPS) Phase 2 support for Data Transport Service TS 23.401CR3620
- Enabling selection of an SNPN other than the subscribed SNPN TS 24.501CR3130
- RFSP Index associated with the Target NSSAI TS 29.507CR0186
- The exception in Network Slice Admission Control for Emergency and Priority Services TS 24.501CR3415
- Paging priority correction for MPS TS 23.401CR3689
- High priority access before pass the NSSAA TS 24.501CR2513
+ 12 more changes
In Release 18, the RFSP function was enhanced to incorporate the user's **Partially Allowed NSSAI** as an input for RFSP derivation. This allows the network's radio resource management strategy, which controls idle mode camping and active mode redirection, to be more precisely aligned with the slice access permissions granted to the UE. Additionally, the release introduced mechanisms for **Slice-based N3IWF or TNGF selection**, extending slice-aware network selection principles to non-3GPP access.
- Equivalent SNPN usage for mobile identity selection TS 24.501CR4840
- UE to indicate its support for Slice-based N3IWF selection to the network TS 24.501CR4961
- Enhanced CAG selection - enforcement in successful cases TS 24.501CR4977
- Enhanced CAG selection - enforcement in unsuccessful cases TS 24.501CR4978
- UE to indicate its support for Slice-based TNGF selection to the network TS 24.501CR5121
- Enhanced CAG selection - providing additional information TS 24.501CR4976
+ 22 more changes
In Release 19, enhancements to the RFSP function focused on improved handling for high priority access, particularly for services like MPS for Messaging and MT SMS over NAS. This included clarifications on exemptions for high priority access and refinements to procedures like timer T3540 handling for cell or PLMN selection when such priority is active. The updates also provided further specification on PDU session selection for QoS differentiation and introduced the concept of PDU Session Priority.
- MPS for Messaging Paging Priority TS 24.501CR6570
- Initial paging with priority in 5GS TS 24.501CR6948
- CHF selection aspects TS 29.507CR0316
- MPS priority paging for MT SMS TS 23.401CR3833
- Timer T3540 handling for causes triggering cell or PLMN selection TS 24.501CR6238
- Missing NOTE for T3540 for a UE with high priority access in selected PLMN or SNPN TS 24.501CR6546
+ 6 more changes
Explore further
Broader topics and technologies where RFSP plays a role.
Defining Specifications
3GPP specifications that define or reference RFSP, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.401 vj50 | Evolved Packet System (EPS) Stage 2 Description | Rel-19 |
| TS 23.700 vk00 | XR Services Application Enablement Layer | Rel-20 |
| TS 23.725 vg20 | Study on URLLC Architecture Enhancements | Rel-16 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 29.507 vj40 | 5G Access & Mobility Policy Control Service | Rel-19 |
| TS 29.514 vj40 | 5G System; Policy Authorization Service; Stage 3 | Rel-19 |
| TS 29.518 vj50 | AMF Service Based Interface Protocol | Rel-19 |
| TS 29.523 vj20 | 5G Policy Control Event Exposure Service | Rel-19 |
| TS 29.534 vj20 | 5G Access & Mobility Policy Authorization Service | Rel-19 |
| TS 29.890 vg00 | CT3 5G System Technical Report | Rel-16 |