In the LTE case, on the left side, we can see the bearer‐oriented approach with a strict one‐to‐one mapping between EPS‐bearers, EPS radio access bearers (E‐RAB), S1 and radio bearers.
In the 5G case, on the right side, we see the flow‐based approach which allows to decouple the assignment of packets to flows (controlled by the CN) from the assignment of flows to DRBs (being in the responsibility of the RAN).
The particular example of three different QoS flows mapped to two different DRBs is here to be seen as an example only. Note that in the context of introducing the new QoS architecture, 3GPP has also agreed to introduce a new UP protocol layer in the RAN above Packet Data Convergence Protocol (PDCP), called Service Data Adaptation Protocol (SDAP). This is responsible to map QoS flows to radio bearers and mark the QoS flow in uplink (UL) and downlink (DL) packets.
Reference :
5G System Design
Architectural and Functional Considerations and Long Term Research
