Monitoring key

Description

The monitoring key is a critical parameter in 3GPP policy and charging control (PCC) architecture, used for usage monitoring control. It serves as a reference to a set of service data flows or applications that share a common allowed usage on a per UE and DNN (Data Network Name) and S-NSSAI (Single Network Slice Selection Assistance Information) basis. In 5G, the monitoring key is used by the Session Management Function (SMF) and the Policy Control Function (PCF) to monitor and enforce usage limits for specific traffic aggregates. This allows operators to apply differentiated charging and policy rules based on usage thresholds.

The monitoring key is provisioned by the PCF to the SMF as part of the PCC rules or session management policy. When the SMF receives a monitoring key in a policy rule, it associates that rule with a usage monitoring context. The SMF then monitors the volume or time of traffic for the service data flows or applications that match the rule. When the accumulated usage reaches a threshold, the SMF reports the usage to the PCF, which may then take actions such as modifying the policy, terminating the session, or applying new rules. The monitoring key is unique within a PDU session and is used to correlate usage reports from the SMF to the PCF.

In the 5G core network, the monitoring key is included in the Policy and Charging Control (PCC) rules that are installed by the PCF into the SMF. These rules can be either dynamic (installed via the N7 interface) or predefined in the SMF. The monitoring key is also used in the N4 interface between the SMF and the User Plane Function (UPF) for usage reporting. The UPF measures the traffic and reports usage to the SMF, which then aggregates and reports to the PCF. The monitoring key allows the PCF to apply different usage limits for different sets of flows, enabling flexible and granular control.

The concept of monitoring key was introduced in Release 9 for 3GPP EPS (Evolved Packet System) and has evolved with the architecture. In EPS, the monitoring key was used by the PCEF (Policy and Charging Enforcement Function) and PCRF (Policy and Charging Rules Function) for usage monitoring control. In 5G, the roles are taken by the SMF and PCF, respectively. The monitoring key is defined in 3GPP specifications such as TS 23.203 (PCC architecture), TS 23.503 (5G policy framework), and TS 29.512 (N7 interface). It is an essential tool for operators to implement usage-based services, such as zero-rating, data caps, and fair usage policies.

The monitoring key is typically a string or integer value that is unique within the scope of a PDU session. It is included in the PCC rule along with other parameters such as the service data flow filter, charging key, and QoS parameters. When the SMF activates a PCC rule, it instructs the UPF to monitor the traffic for that rule and report usage. The UPF maintains counters for each monitoring key and reports when thresholds are reached or when the session ends. The SMF then sends a Usage Report to the PCF, which includes the monitoring key and the accumulated usage. The PCF can then decide whether to continue, modify, or terminate the service.

In summary, the monitoring key is a fundamental identifier for usage monitoring in 3GPP networks. It enables operators to track and control usage for specific sets of service data flows or applications, ensuring that policy and charging rules are enforced accurately. Its evolution from EPS to 5G reflects the ongoing need for flexible and granular usage control in mobile networks.

Purpose & Motivation

The monitoring key was created to address the need for granular usage monitoring and control in 3GPP networks. Before its introduction, usage monitoring was limited to aggregate levels per UE or per APN, which did not allow operators to differentiate between different types of traffic or applications. With the proliferation of diverse services (e.g., video streaming, VoIP, IoT), operators needed a way to apply different usage limits and charging rules for different service data flows. The monitoring key provides a mechanism to group service data flows or applications that share a common allowed usage, enabling operators to implement tiered services, zero-rating, and fair usage policies.

The monitoring key solves the problem of how to track usage for a specific set of flows across multiple network elements. In EPS, the PCEF and PCRF used the monitoring key to correlate usage reports from the gateway. In 5G, the SMF and PCF use it similarly, but with additional granularity per DNN and S-NSSAI. This allows operators to apply different usage limits for different network slices or data networks, which is essential for supporting diverse 5G use cases such as enhanced mobile broadband (eMBB), ultra-reliable low-latency communication (URLLC), and massive machine-type communication (mMTC).

The monitoring key also enables the implementation of usage-based policies that can be dynamically adjusted based on network conditions or subscriber behavior. For example, an operator might offer a promotional data package that allows unlimited usage for a specific application; the monitoring key would be used to identify that application's traffic and exempt it from the general data cap. Without the monitoring key, such differentiation would be difficult to achieve. The monitoring key is thus a key enabler for flexible and innovative service offerings in 3GPP networks.