In 5G networks, Quality of Service (QoS) is implemented using QoS flows, which represent the finest granularity of QoS differentiation. Each QoS flow is identified by a unique QoS flow ID, and the NG-RAN (Next-Generation Radio Access Network) can map multiple QoS flow IDs to a single radio bearer or establish a one-to-one relationship between them, depending on the implementation.
To understand the evolution of QoS implementation, let’s revisit the 4G architecture. In 4G, the end-to-end service between the UE (User Equipment) and the external network was represented by an EPS (Evolved Packet System) bearer, which comprised the radio bearer between the UE and the eNB (evolved NodeB), the S1 bearer between the eNB and the S-GW (Serving Gateway), and the S5/S8 bearer between the S-GW and the P-GW (Packet Gateway). Each EPS bearer could be either dedicated (e.g., for applications like VoLTE) or default (e.g., for internet traffic).
4G QoS was managed using Quality of Service Class Indicators (QCI) with values ranging from 1 to 9. Each EPS bearer was assigned a specific QCI, and there was a one-to-one mapping between the QCI and the radio bearer, S1 bearer, and S5/S8 bearer. This disjoint approach, however, was not considered scalable for 5G.
In 5G, QoS flows provide a more flexible and scalable approach to QoS management. Rather than having dedicated radio bearers for different QoS requirements, 5G allows multiplexing of QoS flows within a single radio bearer. The GNB (gNodeB) is responsible for managing the QoS flows and ensuring that each flow receives the appropriate treatment.
To determine how packets are mapped to specific QoS flows, Packet Detection Rules (PDRs) are installed in the UPF (User Plane Function) by the SMF (Session Management Function). These PDRs are based on a five-tuple (source IP, destination IP, protocol, source port, destination port) and specify the QoS flow ID to which matching packets should be assigned.
The same PDRs can be installed on the UE for uplink traffic, allowing the UE to mark packets with the appropriate QoS flow ID before sending them to the network.
Each QoS flow ID is associated with a set of performance requirements defined by 3GPP, including priority level, packet delay budget, and maximum allowable packet error rate. These requirements guide the treatment of packets within the network to ensure that the desired QoS is achieved.
In summary, 5G QoS is implemented using QoS flows, which provide a more scalable and flexible approach to QoS management compared to the QCI-based approach used in 4G. By multiplexing QoS flows within radio bearers and using PDRs to classify packets, 5G networks can efficiently deliver services with varying QoS requirements.