PCF — Policy Control Function¶

Learning objectives¶

After reading this page, you will be able to:

• Explain in one sentence the role the PCF plays in the 5GC (the hub of policy control).
• Explain the differences among the three kinds of policy (AM Policy / SM Policy / UE Policy).
• Explain what a PCC rule (QoS, charging, gating) and URSP (UE Route Selection Policy) are.
• Diagram which interfaces (N5/N7/N15 and others) the PCF uses to cooperate with which NFs and AFs.
• Explain at which point the PCF appears within PDU Session establishment.
• Explain the differences from the 4G (EPC) PCRF (Gx/Rx) (SBI-ification, the new addition of URSP).

Prerequisites¶

It will go more smoothly if you understand the following first:

• Basics of SBA (Service Based Architecture), NFs, and SBI → Curriculum
• The concept of PDU Session and QoS (5QI/QFI) → PDU Session
• The roles of the AMF/SMF (the sides that query the PCF for policy) → AMF
• The basic concepts of QoS and sessions (if not yet) → Curriculum

What you will learn in this chapter¶

We first enter from Why (what goes wrong without the PCF), then organize What (the three policy types, PCC rule, URSP, role, and APIs) with tables and Mermaid diagrams, and finally move to How (its appearance in the PDU Session, comparison with EPC, and reference specs).

Note that the details of the PDU Session establishment procedure itself are consolidated in PDU Session. This page focuses on the PCF as an NF (its role, policy types, and cooperation partners), and directs you there for the procedural specifics.

Why — why the PCF is needed¶

The network must decide, according to the contract plan and usage conditions, "at what quality (QoS) to carry this communication," "how to charge for it," and "which slice/DNN (Data Network Name = the identifier of the destination network) this app should be routed to." If this decision were implemented separately in each NF, you would have to modify every NF each time the operator's service terms change.

Therefore, a department that consolidates and decides the rules — QoS, charging, access control, and route selection — in one place is needed. This is the PCF (Policy Control Function). With the PCF, the operator can centrally manage operator policies such as "gold members get bandwidth priority," "video is flat-rate," and "this business app goes to a dedicated slice," and each NF (AMF/SMF/UE) simply follows the rules the PCF decides.

Without the PCF (or without cooperating with it), each NF can operate only with preconfigured static defaults, and fine-grained control per subscriber and per app (dynamic PCC) is impossible.

Analogy: The PCF is the terms-and-agreements department that decides service terms per membership rank. Reception (the AMF) and the field (SMF/UPF) receive the terms document (policy) defined by this department and, following it, guide visitors and decide line quality. The terms department obtains the member rank from HR (UDR), which holds the member roster, also accepts requests from external partner stores (AF) such as "give this customer VIP treatment," and finalizes the terms.

Overview¶

The PCF (Policy Control Function) is the hub of policy control in the 5GC. It provides the following three kinds of policy, each to the responsible NF.

• AM Policy (Access and Mobility Policy) … to the AMF (N15, Npcf_AMPolicyControl). Access and mobility policies such as service area restrictions, RFSP (RAT/Frequency Selection Priority), and UE-AMBR (the maximum bandwidth per device).
• SM Policy (Session Management Policy) … to the SMF (N7, Npcf_SMPolicyControl). Session/QoS/charging policy including the PCC rule (QoS, charging, gating).
• UE Policy … to the UE (delivered via the AMF). UE-side route selection policy such as URSP (UE Route Selection Policy).

The PCF retrieves per-subscriber policy data from the UDR (Unified Data Repository) (Nudr), and accepts requests from external applications from the AF (Application Function) (N5, Npcf_PolicyAuthorization). The substance of persistent policy data is stored in the UDR, and the PCF retrieves it to make individual policy decisions — a division of labor.

The PCF corresponds to the 4G PCRF (Policy and Charging Rules Function), but its interfaces have been SBI-ified, and a 5G-new function, UE Policy (URSP), has been added.

Basic Concept — explanation for beginners¶

First, a "policy" is a rule of the form "when the conditions are like this, handle it like this." The PCF is the role that decides and distributes these rules. It divides into three kinds by where the rules are distributed.

• AM Policy (access and mobility rules) … rules about how to connect, such as "this subscriber cannot use this area" or "the priority while idle is this." Handed to the reception-role AMF.
• SM Policy (session/QoS rules = PCC rule) … rules about the quality and charging of communication, such as "this communication gets bandwidth ◯, latency priority, and this is flat-rate charging." Handed to the session-responsible SMF. The substance of this is the PCC rule (described later).
• UE Policy (device-side rules = URSP) … rules that tell the device itself which route to choose, such as "route this app's communication to this slice/destination." Delivered to the UE via the AMF.

What is a PCC rule (simply)¶

A PCC rule (Policy and Charging Control rule) is the core of the SM Policy, a rule that summarizes on a single sheet "how to handle" a given communication flow. Roughly, it includes the following:

• Which communication (service data flow: identified by destination IP, port, etc.)
• QoS (5QI/QoS characteristics, guaranteed bandwidth GBR, etc. — specification of quality)
• Charging (online/offline, charging key — how to calculate the fee)
• Gating (pass/stop that flow — admission control)

In other words, a PCC rule is an instruction sheet saying "this communication is passed (or stopped) at this quality, with this charging."

What is URSP (simply)¶

URSP (UE Route Selection Policy) is a rule that instructs the UE (device) on "which PDU Session (which DNN/slice) to use per app." For example, it enables the device itself to judge distributions such as "business apps to the internal-facing slice, video to the general-facing DNN." URSP is decided by the PCF as part of the UE Policy and delivered to the UE via the AMF. URSP is a mechanism newly developed in 5G.

Network Function details¶

About the BSF (Binding Support Function): In configurations where multiple PCFs may be involved in a single PDU Session, a mapping is needed that binds a session to the PCF responsible for it. What manages this "session ⇔ responsible PCF" binding information is the BSF (Nbsf). The AF or NEF refers to the BSF when finding "the PCF that handles this session's policy." Whether to deploy a BSF, and how to resolve binding information, is implementation/configuration-dependent (see the NF dictionary for details).

Architecture¶

The PCF functions as the hub of policy decisions in the 5GC control plane, cooperating with the AMF, SMF, AF, and UDR (and the NEF/BSF).

flowchart LR
  UE(("UE"))
  AMF["AMF"]
  SMF["SMF"]
  PCF["PCF"]
  AF["AF (application)"]
  NEF["NEF"]
  UDR[("UDR (policy data)")]

  AMF -- "N15 (Npcf_AMPolicyControl)" --> PCF
  SMF -- "N7 (Npcf_SMPolicyControl)" --> PCF
  AF -- "N5 (Npcf_PolicyAuthorization)" --> PCF
  NEF -. "external AF may go via NEF" .-> PCF
  PCF -- "Nudr (policy retrieval)" --> UDR
  PCF -. "UE Policy (URSP) delivered via AMF" .-> AMF
  AMF -. "URSP" .-> UE

How to read the diagram: The AMF queries the PCF for AM Policy over N15, the SMF for SM Policy (PCC rule) over N7, and the AF for application requests (PolicyAuthorization) over N5. The PCF retrieves subscriber policy from the UDR over Nudr and uses it for decisions. UE Policy (URSP) is decided by the PCF and delivered to the UE via the AMF. There are also configurations where an external AF does not connect directly to the PCF but goes via the NEF; whether it is a direct AF-PCF connection or via the NEF is implementation/configuration-dependent.

Note (implementation/configuration-dependent): The dashed line via the NEF in the diagram above, and binding via the BSF, depend on the operator/vendor configuration. AFs within the trust domain may connect directly to the PCF over N5, but third-party AFs typically go via the NEF (configuration-dependent).

Interface¶

The main interfaces the PCF has and their roles.

Treatment of N36 / Nudr (to be confirmed, implementation-dependent): Data retrieval between the PCF and UDR is represented as an SBI by Nudr (the DataRepository service). The reference point name N36 (UDR ⇔ PCF) is sometimes used, but in the SBI-ified 5GC it is common to describe it by the service name Nudr. Because the correspondence between reference-point notation (Nxx) and service notation (Nxxx) varies by context, the strict reference-point number is to be confirmed.

You can also refer to the individual interface explanations (N5 / N7 / N15). For the list, please see the Interface dictionary.

Appearance in procedures¶

In PDU Session establishment (UE-requested PDU Session Establishment), as the SMF assembles the session, it establishes an SM Policy Association between the SMF and PCF as needed and retrieves the PCC rule (authorized QoS, charging) (N7, Npcf_SMPolicyControl_Create). Based on the retrieved PCC rule, the SMF decides the UPF's forwarding rules and QoS Flow (QFI/5QI).

This PCF cooperation is optional/conditional, performed when in a dynamic PCC configuration. In a configuration that does not cooperate with the PCF (static PCC), the session continues with the SMF's default QoS (configuration-dependent).

On the other hand, AM Policy is handled between the AMF and PCF (N15) in the context of mobility management such as registration (Registration), and UE Policy (URSP) is decided by the PCF and delivered to the UE via the AMF.

The detailed flow of the procedure (message sequences, conditions, IEs, etc.) is consolidated in PDU Session. In particular, for the positioning of SM Policy Association establishment, refer to the "SM Policy Association (PCF cooperation)" description there.

Comparison with EPC¶

• In 4G: The PCRF (Policy and Charging Rules Function) decided policy and charging rules. It cooperated with the SMF/PGW side over Gx and with the application side (AF/P-CSCF, etc.) over Rx, dedicated interfaces (Diameter-based). The PCC framework (QoS, charging, and gating control by PCC rules) has existed since 4G.
• In 5G: The PCRF's role was taken over by the PCF, and the interfaces were SBI-ified. The Gx equivalent becomes N7 (Npcf_SMPolicyControl), and the Rx equivalent becomes N5 (Npcf_PolicyAuthorization). Furthermore, AM Policy (N15) for access and mobility and UE Policy (URSP) that controls the device's route selection were developed in 5G.

The PCC framework itself (the control philosophy via PCC rules) continues from 4G. The main changes in 5G are the SBI-ification and the addition of AM Policy / UE Policy (URSP).

Release differences¶

• Rel-15: Defines the PCF's basic functions (AM/SM Policy, PCC rule, UE Policy (URSP), each Npcf service).
• Rel-16: Functional extensions (e.g., TSC/time-synchronization-related policy, network-slicing-related extensions, etc.). The specific section numbers and items are to be confirmed.
• Rel-17: Further extensions. Specific items and section numbers are to be confirmed.

3GPP Specification¶

• 3GPP TS 23.501 §6.2.4 — PCF functional definition (role of the Network Function)
• 3GPP TS 23.503 — policy and charging control framework (PCC framework, PCC rule, policy models such as URSP)
• 3GPP TS 29.507 — Npcf_AMPolicyControl service (AM Policy, N15). Individual section numbers are to be confirmed
• 3GPP TS 29.512 — Npcf_SMPolicyControl service (SM Policy = PCC rule, N7). Individual section numbers are to be confirmed
• 3GPP TS 29.514 — Npcf_PolicyAuthorization service (AF cooperation, N5). Individual section numbers are to be confirmed
• 3GPP TS 29.525 — Npcf_UEPolicyControl service (UE Policy delivery). To be confirmed (the URSP policy model itself is defined in TS 23.503)

Note (to be confirmed): Each Npcf service and its corresponding TS number are as above, but the detailed section numbers differ by Release and are individually to be confirmed. The organization that the content model of URSP is TS 23.503 and the service spec of UE Policy delivery is the TS 29.525 series is common (the strict correspondence is to be confirmed).

Summary¶

• The PCF is the hub of policy control in the 5GC, providing three kinds — AM Policy, SM Policy, and UE Policy — to their respective responsible parties.
• AM Policy goes to the AMF (N15), SM Policy (PCC rule = QoS/charging/gating) goes to the SMF (N7), and UE Policy (URSP) is delivered to the UE via the AMF.
• Application requests from the AF are accepted over N5 (Npcf_PolicyAuthorization), and subscriber policy is retrieved from the UDR (Nudr).
• The PCC framework continues from 4G; the main changes in 5G are the SBI-ification and the addition of UE Policy (URSP).
• It corresponds to the 4G PCRF (Gx/Rx). Whether it is a direct AF-PCF connection or via the NEF, and binding via the BSF, are implementation/configuration-dependent.

Next Step¶

• PDU Session — the procedure in which the SMF retrieves the PCC rule from the PCF (details)
• AMF — the party that queries the PCF for AM Policy
• Registration — the starting point of mobility management to which AM Policy relates
• Related NFs: SMF / AF / NEF / UDR / BSF — for the list of terms see the NF dictionary
• Interface dictionary — for checking N5/N7/N15
• Network slicing — Network Slicing chapter (the positioning of slice route selection via URSP)