Which protocol layer In the gNB is responsible for QoS mapping?

The Service Data Adaptation Protocol (SDAP) is a new layer introduced in the 5G New Radio (NR) user plane protocol stack. Its primary function is to map Quality of Service (QoS) Flows, which are established by the 5G Core (5GC), to specific Data Radio Bearers (DRBs). This mapping is based on the QoS Flow ID (QFI) present in the header of the downlink packets. This function allows the 5G system to decouple core network QoS handling from the radio interface, providing greater flexibility in managing data services.

A. RLC: The Radio Link Control (RLC) layer is responsible for segmentation, reassembly, and error correction (ARQ), not the initial mapping of QoS flows to radio bearers.
B. PDCP: The Packet Data Convergence Protocol (PDCP) layer handles header compression, ciphering, and integrity protection. It operates on data that has already been mapped to a radio bearer.
C. MAC: The Medium Access Control (MAC) layer is responsible for scheduling data on the physical channels and managing Hybrid ARQ (HARQ). It multiplexes data from different logical channels.

1. 3GPP TS 37.324 V17.0.0 (2021-12), Evolved Universal Terrestrial Radio Access (E-UTRA) and NR; Service Data Adaptation Protocol (SDAP) specification (Release 17).

Section 4.1, "SDAP functions": This section explicitly states, "The SDAP layer is configured by RRC. The main services and functions of the SDAP sublayer include: a) transfer of user plane data; b) mapping between a QoS flow and a data radio bearer (DRB) for the DL and the UL; c) marking QoS flow ID (QFI) in both DL and UL packets."

2. 3GPP TS 38.300 V17.4.0 (2023-03), NR; NR and NG-RAN Overall description; Stage-2 (Release 17).

Section 8.2, "SDAP": "The SDAP sublayer is configured by RRC. For a DRB, the SDAP sublayer maps QoS flows to that DRB in both DL and UL." This document provides the high-level architectural description confirming SDAP's role.

3. Dahlman, E., Parkvall, S., & Sköld, J. (2020). 5G NR: The Next Generation Wireless Access Technology (2nd ed.). Academic Press.

Chapter 10, "User-Plane Protocols", Section 10.4 "SDAP": "The main purpose of the SDAP layer is to map QoS flows to radio bearers... In the downlink, the SDAP layer receives packets from the 5G core network. Each packet belongs to a certain QoS flow and the SDAP layer maps each QoS flow to a specific radio bearer."