Policy and Charging Control in a 5G Network

Policy and Charging Control in a 5G Network

Policy and Charging Control play a very critical role in the 5G ecosystem. It provides transparency and control over the consumption of Network resources during real-time service delivery.

PCF (Policy Charging Function) governs the Control plane functions via Policy rules defined and User plane functions via Policy enforcement. It works very closely with CHF (Charging Function) for Usage Monitoring. Through PCF, Operators can manage & govern network behavior.

(Click to enlarge) Policy Control in a 5G Network

Key aspects like QoS control, Traffic Steering/Routing, Application & its capabilities detection, Subscriber Spending/Usage Monitoring, Interworking with IMS Nodes, Enabling differentiated Services, Gating Control, Network slice enablement, Roaming support, etc. are supported by PCF.

Before we discuss more specifics about PCF, it will be a good idea to know about 5G Service Based Architecture. You can refer to my other article 5G Network Architecture - A Beginners Guide to gain a basic understanding.

Below is the simplified view of PCF and associated Network Functions in a typical 5G Network Architecture:-

(Click to enlarge) A simplified view of PCF in a 5G Network

AMF (Access and Mobility Management Function): It performs operations like Mobility Management, Registration Management, Connection Management, UE-based authentication, etc. Based on the Service requested by the Consumer, AMF selects the respective SMF for managing the user session context. When compared with 4G EPC, its functionalities resemble with MME of the 4G Network.

SMF (Session Management Function): Performs operations like Session Management, IP Address allocation & management for UE, User plane selection & Packets routing by working closely with UPF, QoS & Policy enforcement for Control Plane, used for Service registration/discovery/establishment, etc. When compared with 4G EPC, its functionalities resemble with MME, SGW-C (Control Plane), and PGW-C (Control Plane) of the 4G Network.

AF (Application Function): It performs operations like accessing the Network Exposure Function for retrieving resources, interaction with PCF for Policy Control, Applications Traffic Routing, Exposing services to End users, etc. It exposes the Application layer for interacting with 5G Network resources. When compared with 4G EPC, its functionalities resemble with AF of a 4G Network.

NEF (Network Exposure Function): It exposes services and resources over APIs within and outside the 5G Core. Services exposure by NEF is based on RESTful APIs over a Service-based interface bus. With the help of NEF, 3rd party applications can also access the 5G services. It acts as a Security layer when an outside application tries to connect with the 5G Core Network functions.

NWDAF (Network Data Analytics Function): NWDAF is used for data collection and analytics for Centralized as well as Edge computing resources. It provides network slice-specific data analytics to PCF and NSSF which in turn use this data for Policy decisions (PCF) and Slice selections (NSSF).

UDR (Unified Data Repository): It serves as a single repository of Subscription data, Application data, and Policy data by integrating with NF consumers (like NEF, AMF, PCF, etc.). We can store and retrieve the data via UDR. It also notifies the Subscription data changes.

UPF (User Plane Function): It performs User plane operations like maintaining PDU Session, Packet routing & forwarding, Packet inspection, Policy enforcement for User plane, QoS handling, etc. When compared with 4G EPC, its functionalities resemble SGW-U (Serving Gateway User Plane function) and PGW-U (PDN Gateway User Plane function) of 4G Network.

CHF (CHarging Function): CHF is an integral entity in CCS (Converged Charging System) which provides an Account Balance Management function, Rating Function, and Charging Gateway Function.

If compared with 4G EPC, CHF combines the functionality of OCF (Online Charging Function) and CDF (Charging Data Function). Hence, CHF enables Online and Offline Charging by closely interfacing with SMF.

CHF plays a critical role in monitoring the Subscriber's Usage consumption & Policy Counters by interacting with PCF. Together with PCF, it provides Policy and Charging Control during service delivery.

To learn more about Online Charging, please read Basics of Telecom Online Charging.

To learn more about Offline Charging, please read Basics of Telecom Offline Charging.

Interworking of PCF with associated 5G Network Functions is shown below:-

(Click to enlarge) 5G PCF Reference-based representation as per 3GPP

Let's drill down to the individual interfaces:-

PCF - AF Interface: Application-level session information is exchanged between AF and PCF which includes information like Bandwidth requirements for QoS, Identifying Application service providers & Applications, Traffic routing based on Applications access, Identifying Application traffic for Charging & Policy control.

PCF - UDR Interface: PCF retrieves the policy/subscription/application-specific data from UDR. Policy control-related subscription and application-specific data get provisioned into UDR. UDR can also generate notifications based on the changes in the subscription information, as per the Operator's pricing model.

PCF - NWDAF Interface: The PCF shall be able to collect directly slice specific network status analytic information from NWDAF. NWDAF provides network data analytics (i.e. load level information) to PCF on a network slice level and the NWDAF is not required to be aware of the current subscribers using the slice. PCF shall be able to use that data in its policy decisions.

PCF - NEF Interface: NEF exposes network function services and resources to the External world. In terms of interaction with PCF, it exposes the capabilities of network functions for supporting Policy and Charging.

PCF - CHF Interface: This interface behaves the same as between PCRF and OCS in the 4G Network. Through this integration, Operators can manage & control subscriber spending as well as usage control. CHF stores the policy counter information against the subscriber pricing plan and notifies PCF whenever the subscriber breaches the policy thresholds based on usage consumption. On receiving policy trigger information, PCF then applies the policy decision by interacting with SMF (which in turn informs UPF for policy enforcement). Check the following interface for more understanding.

PCF - AMF Interface: AMF acts as a single entry point for the UE connection. PCF provides Access and Mobility Management related policies for the AMF in order to trigger Policy rules on the UE or User-sessions.

PCF - SMF Interface: SMF receives Control plane info from NFs (like AMF, etc.) and User plane info from UPF. Information like Subscription details, QoS, and PDU Session level are present in SMF and it requests Usage-related information from UPF.

SMF triggers PCF to enforce Policy decisions once the Policy triggers related to Session Management are met. Similarly, PCF provisions the policy and charging control decision on SMF.

PCF - SMF - UPF Interface: PCF and UPF don't communicate directly with each other. They exchange policy actions/enforcements via SMF.

SMF provisions the policy & threshold rules on UPF for Usage Control based on the static/dynamic policy rules configured in PCF, pre-defined rules in SMF, and/or Credit control triggers received from CHF. This relationship is the same as between PCRF and PCEF in 4G/3G networks.

High-Level PCF Call Flow for a Session-based 5G Service:-

(Click to enlarge) PCF Call Flow for a Session-based 5G Service

A typical Policy & Charging Control flow is explained in the above figure. It explains how the Policy rules are first configured for Monitoring and later how Policy gets enforced at the bearer due to Policy rules trigger.

As Operators are focusing on new partnerships and business use cases based on 5G capabilities, PCF (or PCRF) continues to play a vital role in the Enablement, Control, and Monetization of advanced Digital Services.

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Glossary:  3GPP (3rd Generation Partnership Project), SBA (Service Based Architecture), UE (User Equipment), UPF (User Plane Function), AMF (Access & Mobility Management Function), SMF (Session Management Function), NEF (Network Exposure Function), PCF (Policy Control Function), CCS (Converged Charging System), AF (Application Function), UDR (Unified Data Repository), CHF (CHarging Function), OCF (Online Charging Function), CDF (Charging Data Function), NWDAF (Network Data Analytics Function), PCRF (Policy & Charging Rules Function), BSS (Business Support System), OSS (Operations Support System), IMS (IP Multimedia Subsystem), PCEF (Policy Control Enforcement Function), MME (Mobility Management Entity), SGW (Serving Gateway), PGW (PDN Gateway), EPC (Evolved Packet Core), CN (Core Network), NFs (Network Functions)

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