The layer 2 of NR is split into the following sublayers: Medium Access Control (MAC), Radio Link Control (RLC), Packet Data Convergence Protocol (PDCP) and Service Data Adaptation Protocol (SDAP). The two figures below depict the Layer 2 architecture for downlink and uplink, where:
The physical layer offers to the MAC sublayer transport channels;
The MAC sublayer offers to the RLC sublayer logical channels;
The RLC sublayer offers to the PDCP sublayer RLC channels;
The PDCP sublayer offers to the SDAP sublayer radio bearers;
The SDAP sublayer offers to 5GC QoS flows;
Comp. refers to header compression and segm. to segmentation;
Control channels (BCCH, PCCH are not depicted for clarity).
Radio bearers are categorized into two groups: data radio bearers (DRB) for user plane data and signalling radio bearers (SRB) for control plane data.
For IAB, the Layer 2 of NR also includes: Backhaul Adaptation Protocol (BAP).
The BAP sublayer supports routing across the IAB topology and traffic mapping to BH RLC channels for enforcement of traffic prioritization and QoS.
Figure 6.1-3 below depicts the Layer-2 architecture for downlink on the IAB-donor. Figures 6.1-4 and 6.1-5 depict the Layer-2 architecture for downlink and uplink on the IAB-node, where the BAP sublayer offers routing functionality and mapping to BH RLC channels.