5G Protocol Stack: Layer 1, Layer 2, and Layer 3 Explained

This page provides an overview of the 5G Protocol Stack, detailing the functions of Layer 1, Layer 2, and Layer 3. In 5G, Layer 1 is the PHYSICAL Layer, Layer 2 includes MAC, RLC, and PDCP, and Layer 3 is the RRC layer.

Figure 1 illustrates the 5G Protocol Stack, showcasing both the User Plane and Control Plane protocol layers.

5G Layer 1 | 5G PHYSICAL Layer

The 5G Layer 1, also known as the PHYSICAL (PHY) Layer, is responsible for the following functions:

• Error detection on the transport channel and indication to higher layers
• FEC encoding/decoding of the transport channel
• Hybrid ARQ soft-combining
• Rate matching of the coded transport channel to physical channels
• Mapping of the coded transport channel onto physical channels
• Power weighting of physical channels
• Modulation and demodulation of physical channels
• Frequency and time synchronization
• Radio characteristics measurements and indication to higher layers
• Multiple Input Multiple Output (MIMO) antenna processing
• Transmit Diversity (TX diversity)
• Digital and Analog Beamforming
• RF processing

The 38.200 series of documents define processing of PDSCH and PUSCH channels through the PHYSICAL layer modules.

5G Layer 2 | 5G MAC, RLC, PDCP Layer

The 5G Layer 2 encompasses the MAC, RLC, and PDCP sublayers.

MAC Sublayer Functions:

• Beam management
• Random access procedure
• Mapping between logical channels and transport channels
• Concatenation of multiple MAC SDUs belonging to one logical channel into transport block (TB)
• Multiplexing/demultiplexing of 5G-MAC SDUs belonging to one or different logical channels into/from transport blocks (TB) delivered to/from the physical layer on transport channels
• Scheduling information reporting
• Error correction through HARQ
• Priority handling between logical channels of one UE
• Priority handling between UEs by means of dynamic scheduling
• Transport format selection
• Padding

RLC Sublayer Functions:

• Transfer of upper layer PDUs
• Error Correction through ARQ (only for AM data transfer)
• Reordering of 5G-RLC data PDUs (only for UM and AM data transfer)
• Duplicate detection (only for UM and AM data transfer)
• Protocol error detection (only for AM data transfer)
• 5G-RLC SDU discard (only for UM and AM data transfer)
• Segmentation (only for UM and AM data transfer)
• Resegmentation (only for AM data transfer)
• 5G-RLC re-establishment

PDCP Sublayer Functions (User Plane):

• Transfer of user data
• In-sequence delivery of upper layer PDUs at 5G-PDCP re-establishment procedure for 5G-RLC AM
• Duplicate detection of lower layer SDUs at 5G-PDCP re-establishment procedure for 5G-RLC AM
• Retransmission of 5G-PDCP SDUs at mobility in connected mode for 5G-RLC AM
• Ciphering and deciphering (Note: Only AES shall be mandatory)
• Timer-based SDU discard in uplink

PDCP Sublayer Functions (Control Plane):

• Ciphering and Integrity Protection (Note: Only AES shall be mandatory)
• Transfer of control plane data

5G Layer 3 | 5G RRC Layer

The 5G Layer 3, known as the RRC Layer, has the following responsibilities:

• Broadcasting of system information to NAS and AS.
• Establishment, maintenance, and release of RRC connection.
• Security, including key management.
• Establishment, configuration, maintenance, and release of point-to-point radio bearers.
• Mobility functions along with cell addition and cell release.
• UE measurement reporting, control of UE reporting, UE-based mobility.
• NAS direct message transfer to/from NAS from/to UE.

References:

• TS V5G.300
• TS V5G.201
• 3GPP TS 38.212 (Multiplexing and channel coding), TS 38.321 (MAC specifications), TS 38.322 (RLC specifications)
• Website: https://5gtf.net