QoS Notification Control

Description

QoS Notification Control (QNC) is a mechanism defined within the 5G System (5GS) that enables the network to inform a User Equipment (UE) about modifications to its authorized Quality of Service (QoS) parameters for an existing Protocol Data Unit (PDU) Session or QoS Flow. Unlike static QoS provisioning, QNC allows for dynamic adjustments after a session is established. This notification is crucial for aligning the UE's understanding of its available resources with the network's current policy decisions or operational state.

The QNC procedure is primarily driven by the Policy Control Function (PCF). When the PCF decides that a change in authorized QoS is required—for reasons such as policy updates, network congestion, slice-specific management, or application request—it sends a policy update to the Session Management Function (SMF). The SMF, which is responsible for PDU session management, then initiates the notification process towards the UE. This involves the SMF communicating with the Access and Mobility Management Function (AMF) and ultimately the Radio Access Network (RAN) to deliver the QoS notification message to the UE.

Upon receiving the notification, the UE is expected to adapt its uplink traffic behavior accordingly. For example, if the authorized bitrate for a QoS Flow is reduced, the UE should limit its transmission rate to the new value. The notification typically contains updated QoS parameters such as the 5G QoS Identifier (5QI), Aggregate Bit Rate (ABR), Guaranteed Flow Bit Rate (GFBR), or Maximum Flow Bit Rate (MFBR). QNC operates within the framework of the UE policy framework and is distinct from QoS rule provisioning; it does not install new rules but modifies the authorization associated with existing rules or flows. This mechanism is integral to achieving closed-loop QoS management, where the network can respond in near real-time to changing conditions and maintain service-level agreements (SLAs), especially in complex environments like network slicing.

Purpose & Motivation

QNC was introduced in 5G (Rel-15) to address the limitations of static QoS management prevalent in previous generations. In 4G/LTE, QoS parameters were largely established during bearer setup and could only be modified through complex bearer modification procedures, which were not optimized for rapid, policy-driven changes. The advent of 5G brought about diverse service requirements, network slicing, and dynamic policy control, necessitating a more agile method to communicate QoS changes to the end device.

The primary problem QNC solves is the synchronization of QoS state between the network's policy decision point (PCF) and the UE. Without QNC, a UE might continue transmitting at a rate that the network can no longer support due to congestion, slice re-prioritization, or a change in user subscription, leading to packet drops, inefficiency, or violation of SLAs. QNC enables efficient network resource utilization by allowing the network to downgrade (or occasionally upgrade) QoS authorizations on-the-fly. This is particularly important for network slicing, where resources are shared and must be allocated elastically, and for service continuity when a user moves between different slice instances or when application requirements change mid-session. It provides the granular control needed for advanced 5G use cases like industrial IoT, ultra-reliable low-latency communication (URLLC), and enhanced mobile broadband (eMBB).