Associated QoS Flow

Description

The Associated QoS Flow is a concept introduced in 3GPP Release 19 to support the 5G Multicast-Broadcast Services (MBS) architecture. In 5G MBS, a multicast session can be delivered to multiple UEs simultaneously using either a shared multicast delivery method or an individual unicast delivery method. The Associated QoS Flow is specifically used for the 5GC Individual MBS traffic delivery method, where the multicast data is delivered to a UE via a unicast PDU Session. This flow belongs to the associated PDU Session of the UE and is mapped from a multicast QoS Flow in the multicast MBS session. The mapping ensures that the QoS characteristics (e.g., 5QI, GBR, MBR) of the multicast flow are preserved when the data is delivered individually. The Associated QoS Flow is established when the UE joins a multicast MBS session and the network decides to use individual delivery, for example, due to the UE being in a cell that does not support multicast broadcast or due to policy decisions. The flow is managed by the SMF and enforced by the UPF and RAN. The concept is defined in 3GPP TS 23.247 (Architectural enhancements for 5G multicast-broadcast services) and TS 29.244 (Interface between the Control Plane and the User Plane nodes). The Associated QoS Flow enables service continuity and consistent QoS for MBS users regardless of the delivery method, and it is a key enabler for flexible MBS deployment.

Purpose & Motivation

The Associated QoS Flow exists to solve the problem of delivering multicast MBS traffic to UEs that cannot receive multicast broadcast (e.g., UEs in cells without MBS support or UEs that prefer unicast) while maintaining the same QoS treatment as the multicast flow. Before its introduction, delivering multicast content individually would require separate QoS flows with potentially different characteristics, leading to inconsistent user experience. The Associated QoS Flow ensures that the QoS parameters of the multicast flow are mapped to a unicast flow, providing seamless service continuity. It also simplifies the charging and policy control by allowing the network to apply the same QoS rules as for the multicast session. This concept was motivated by the need for a unified MBS architecture that supports both multicast and unicast delivery methods without duplicating QoS management. It addresses the limitations of previous approaches where multicast and unicast were handled by separate systems, causing complexity and potential QoS mismatches.