What is Layers 2/3?
Layers 2 and 3 in the OSI model correspond to the Data Link Layer and the Network Layer respectively. In 5G NR, Layer 2 comprises MAC (Medium Access Control), RLC (Radio Link Control), PDCP (Packet Data Convergence Protocol), and SDAP (Service Data Adaptation Protocol). Layer 3 is the RRC (Radio Resource Control) protocol. Together, these layers handle scheduling, segmentation, retransmission, ciphering, QoS flow mapping, connection management, and mobility — forming the intelligence layer of the radio protocol stack.
How Does Layers 2/3 Work?
In 5G NR, MAC handles scheduling, multiplexing, HARQ retransmission control, and random access. RLC handles segmentation/reassembly with three modes: TM (transparent), UM (unacknowledged), and AM (acknowledged with ARQ). PDCP handles header compression (ROHC), ciphering, integrity protection, duplicate detection, and in-order delivery — critical for dual connectivity handovers. SDAP (new in 5G) maps QoS flows from the core network to Data Radio Bearers (DRBs). Layer 3 RRC handles connection establishment/release, mobility (measurement configuration and handover), system information broadcasting, and security activation. In the CU/DU split architecture, PDCP/SDAP/RRC reside in the CU while MAC/RLC reside in the DU.
Use Cases
5G NR protocol stack implementation, CU/DU functional split design, QoS-aware scheduling and resource management, mobility and handover optimization, and dual connectivity management for EN-DC and NR-DC.
3GPP / Standards Reference
3GPP TS 38.321 (MAC), TS 38.322 (RLC), TS 38.323 (PDCP), TS 38.324 (SDAP), TS 38.331 (RRC)
Related Terms
Layer 1 | Layers 4-7 | gNB | RAN | QoS
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