Policy — Policy Control in the 5GC (PCF)¶

Learning objectives¶

After reading this chapter, you will be able to:

• Explain that the PCF (Policy Control Function) is the center of policy control in the 5GC, and that it decides QoS, charging, traffic, access & mobility, and URSP.
• Diagram that the PCF's output is a PCC Rule (Policy and Charging Control Rule), distributed to the SMF over N7.
• Explain the flow in which the AF (Application Function) issues a QoS request to the PCF over N5, and a PCC Rule is generated.
• Explain that the URSP (UE Route Selection Policy) is the mechanism that distributes to the UE "which app to carry on which slice/DNN/PDU Session."
• Explain the evolution from 4G's PCRF/Gx to 5G policy control.

Prerequisites¶

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

• PDU Session (PDU Session Establishment) — policy is applied mainly per PDU Session
• QoS (the 5G QoS model) — a PCC Rule decides the QoS Flow (5QI/QoS). For the general explanation of 5QI and QoS Flow, see here
• Supplementary: PCF (details of the policy NF) / SMF (the target of PCC Rule application) / N7 (details of the PCF⇔SMF reference point)

Why — why policy control is needed¶

An operator cannot treat all traffic uniformly. Decisions such as the following are required.

• Give voice calls low-latency quality, video wide bandwidth, and background updates the bare minimum (QoS control).
• Decide which traffic to charge and how (charging control: Rating Group assignment).
• Block or rate-limit specific services according to the contract (traffic control: gating / rate limiting).
• Decide which app to carry on which slice/destination (URSP).

A role is needed to decide these consistently, per subscriber and per service. In the 5GC, the PCF (Policy Control Function) takes this central role.

Analogy: The PCF is the "rule-maker of traffic control." Looking at the subscriber's contract (policy data) and the app's request (AF request), it decides which vehicle (traffic) uses which lane (QoS/slice), where to collect tolls (charging), and where to close the road (gate), and hands the on-site officer (SMF) an instruction sheet (PCC Rule).

What — what the PCF decides, PCC Rules, and URSP¶

The areas the PCF controls are broadly as follows.

The outputs of QoS, charging, and traffic control take, collectively, the form of a PCC Rule (Policy and Charging Control Rule). A single PCC Rule contains "the target traffic (flow description) + the QoS to give (5QI) + the Rating Group + gate/bandwidth," and the PCF distributes it to the SMF over N7. The SMF reflects this into QoS Flow establishment and UPF metering/gate configuration.

URSP (UE Route Selection Policy)¶

URSP is the "traffic-sorting rule" on the UE side. It decides which slice (S-NSSAI)/DNN/PDU Session to carry the traffic of each app on the UE. The PCF generates the URSP from subscriber and operator policy and distributes it to the UE (TS 23.503).

This lets the UE itself perform sorting such as "business apps on slice A, video on slice B." For the general explanation of slices, see Network Slicing.

The diagram below shows how two kinds of policy generated by the same PCF are distributed in opposite directions. The PCC Rule goes to the network side (SMF→UPF/NG-RAN), and the URSP goes to the terminal side (UE).

flowchart LR
    subgraph NWside["Acts on the network side"]
        SMF["SMF/UPF/NG-RAN
(enforces QoS/charging/gate)"]
    end
    PCF["PCF
center of policy generation"]
    subgraph UEside["Acts on the UE side"]
        UE["UE
(app→slice/DNN/
PDU Session sorting)"]
    end

    PCF -->|"PCC Rule (downstream over N7)
what to handle, with which QoS/charging/gate"| SMF
    PCF -->|"URSP (distributed to the UE)
which app to carry where"| UE

    classDef pcf fill:#fde,stroke:#c69,stroke-width:2px;
    classDef nw fill:#eef,stroke:#66a;
    classDef ue fill:#efe,stroke:#3a3;
    class PCF pcf;
    class SMF nw;
    class UE ue;

How — where policy takes effect (Architecture)¶

The PCF retrieves subscriber policy data from the UDR (via N36), receives service requests from the AF over N5, and distributes PCC Rules to the SMF over N7. The Session Binding information (which PCF handles which PDU Session) is held by the BSF.

graph LR
    AF[AF
e.g. P-CSCF] -->|N5 QoS request| PCF[PCF
center of policy control]
    UDR[UDR
policy data] -->|N36| PCF
    PCF -->|N7 PCC Rule| SMF[SMF]
    PCF -.->|Session Binding registration| BSF[BSF]
    PCF -->|URSP| UE[UE]
    SMF -->|N4 gate/metering| UPF[UPF]
    SMF -->|QoS Flow establishment| RAN[NG-RAN]

The role of each Network Function¶

Main interfaces¶

Procedure — the Call Flow of policy application¶

(A) Policy application at PDU Session establishment (SMF-originated)¶

The flow in which, behind PDU Session establishment, the SMF requests policy from the PCF and receives a PCC Rule.

sequenceDiagram
    participant SMF
    participant PCF
    participant UDR
    participant UPF
    Note over SMF,PCF: Part of PDU Session establishment
    SMF->>PCF: N7 Npcf_SMPolicyControl_Create
(presents subscriber/DNN/S-NSSAI, etc.)
    PCF->>UDR: N36 retrieve policy data
    UDR-->>PCF: subscriber policy
    PCF->>PCF: generate PCC Rule
(5QI/Rating Group/gate)
    PCF-->>SMF: N7 return PCC Rule
    SMF->>SMF: decide QoS Flow
    SMF->>UPF: N4 set gate/metering (URR)
    Note over SMF: reflected into QoS Flow establishment

(B) AF-originated QoS request (N5)¶

The flow in which an AF (e.g. IMS's P-CSCF) requests the QoS of a specific service such as voice from the network. This is also behind the voice QoS establishment of VoNR.

sequenceDiagram
    participant AF
    participant PCF
    participant SMF
    participant UPF
    AF->>PCF: N5 Npcf_PolicyAuthorization_Create
(media info, requested QoS)
    PCF->>PCF: generate/update PCC Rule
    PCF->>SMF: N7 Npcf_SMPolicyControl_Update
(distribute PCC Rule)
    SMF->>SMF: establish dedicated QoS Flow
    SMF->>UPF: N4 set gate/metering
    SMF-->>PCF: response
    PCF-->>AF: N5 response (approval)

Signal Flow (main messages)¶

Comparison with EPC (4G)¶

The 5G changes are that the name/structure changed from PCRF→PCF and it went SBI (HTTP/2), and that URSP made it possible to control UE-side traffic sorting. The PCC Rule framework itself is inherited from 4G.

Release differences¶

Trouble Shooting¶

3GPP Specification¶

FAQ¶

Summary¶

• The PCF is the center of policy control in the 5GC. It decides QoS, charging, traffic, access & mobility, and URSP.
• The output is a PCC Rule (flow + 5QI + Rating Group + gate/bandwidth). It is distributed to the SMF over N7 (Npcf_SMPolicyControl).
• The AF issues a QoS request over N5 (Npcf_PolicyAuthorization) → the PCF generates a PCC Rule → over N7 to the SMF. This is behind VoNR's voice QoS.
• URSP is the route selection policy distributed to the UE (which app to carry on which slice/DNN/PDU Session).
• The PCF retrieves subscriber policy from the UDR (N36), and Session Binding information is held by the BSF.
• It evolved from 4G's PCRF/Gx/Rx to going SBI and adding URSP.

Comprehension check¶

Next Step¶

• On to Charging, which is closely linked with policy
• For the details of the policy NF, on to PCF; for the reference point details, N7
• Learn Network Slicing, the destination URSP sorts to