What does each layer in the 5G protocol stack do?

PHY: modulation/demodulation, channel coding (LDPC/polar), OFDM (IFFT/FFT), beamforming, MIMO processing. MAC: scheduling (dynamic/semi-persistent/configured grants), HARQ with soft combining, multiplexing/demultiplexing of logical channels, BSR, PHR. RLC: segmentation and reassembly, ARQ (acknowledged mode), duplicate detection, reordering. PDCP: ciphering and integrity protection, header compression (ROHC), reordering and duplicate elimination, split bearer handling for dual connectivity. SDAP (new in 5G): maps QoS flows to data radio bearers. This layer was added because 5G supports fine-grained QoS per flow rather than per bearer as in LTE. RRC: connection setup/release, measurement configuration and reporting, mobility (handover), security activation, system information broadcasting.

What is new in the 5G protocol stack vs. LTE?

Key differences: (1) SDAP layer: new layer between PDCP and application for QoS flow mapping. LTE had no equivalent. (2) RLC: simplified, no concatenation function (moved to MAC). Only three modes: TM, UM, AM. (3) MAC: added configured grants (Type 1 and Type 2) for URLLC without scheduling request overhead. Added BWP (bandwidth part) management. (4) PDCP: added integrity protection for user plane (optional), improved split bearer for EN-DC and NR-DC. (5) PHY: LDPC for data (replacing turbo codes), polar codes for control (replacing TBCC), flexible numerology (15/30/60/120/240 kHz SCS), slot-based and mini-slot scheduling. (6) RRC: added INACTIVE state (between IDLE and CONNECTED) for IoT power saving.

mmWave & 5G

5G Protocol Stack

Q: How does SDAP enable per-flow QoS?

In LTE, QoS was managed per EPS bearer: all packets in a bearer got the same treatment. This was coarse-grained. In 5G, QoS operates per flow: each application flow (video, voice, gaming) can have different latency, reliability, and priority requirements identified by a QFI (QoS Flow Identifier). SDAP maps these QoS flows to appropriate data radio bearers (DRBs). Multiple flows can share a DRB or each can have its own. SDAP adds a 1-byte header containing the QFI to each packet. This enables fine-grained QoS without the signaling overhead of creating separate bearers for each flow, which was a scalability problem in LTE.

The layered radio protocol architecture for 5G NR, consisting of six layers: PHY (physical), MAC (medium access control), RLC (radio link control), PDCP (packet data convergence protocol), SDAP (service data adaptation protocol, new in 5G), and RRC (radio resource control). Each layer encapsulates specific functions from OFDM modulation and MIMO processing at PHY to QoS flow mapping at SDAP to connection management at RRC.

Category: mmWave & 5G

Layers: PHY, MAC, RLC, PDCP, SDAP, RRC

New vs. LTE: SDAP layer, LDPC/Polar, RRC_INACTIVE

Understanding the 5G Protocol Stack

The protocol stack is the software architecture that transforms IP packets into over-the-air OFDM symbols and back. Each layer adds specific processing: SDAP maps QoS flows, PDCP encrypts and compresses headers, RLC segments for the radio link, MAC schedules and multiplexes, and PHY modulates and transmits. The stack runs on both the UE and gNB, with peer-to-peer communication at each layer.

The biggest architectural change from LTE is the addition of the SDAP layer for per-flow QoS management and the introduction of the RRC_INACTIVE state for IoT power saving. At PHY, LDPC codes replace turbo codes for data, polar codes replace tail-biting convolutional codes for control, and flexible numerology (15-240 kHz SCS) replaces the fixed 15 kHz of LTE.

5G NR Protocol Layer Functions

SDAP: QoS flow ↔ DRB mapping (QFI header)
PDCP: Ciphering, integrity, ROHC, reordering, DC split
RLC: Segmentation, ARQ (AM), reordering (UM/AM)
MAC: Scheduling, HARQ (16 processes), MUX, BSR, PHR
PHY: LDPC/Polar, OFDM, MIMO, beamforming
RRC: Connection, mobility, measurements, SIB

Data flow (DL):
IP → SDAP (add QFI) → PDCP (encrypt, compress) →
RLC (segment) → MAC (schedule, HARQ) →
PHY (encode, modulate, OFDM, beam) → antenna

5G NR vs. LTE Protocol Differences

Layer	LTE	5G NR
QoS mapping	None (per-bearer QoS)	SDAP (per-flow QoS)
Data FEC	Turbo codes	LDPC
Control FEC	TBCC	Polar (CA-SCL)
SCS	15 kHz fixed	15/30/60/120/240 kHz
HARQ processes	8 (FDD), 15 (TDD)	16
RRC states	IDLE, CONNECTED	IDLE, INACTIVE, CONNECTED

Common Questions

Frequently Asked Questions

What does each layer do?

PHY: modulation, coding, OFDM, MIMO. MAC: scheduling, HARQ, multiplexing. RLC: segmentation, ARQ. PDCP: encryption, header compression. SDAP: QoS flow mapping. RRC: connection, mobility, measurements.

What changed from LTE?

SDAP layer added for per-flow QoS. LDPC replaces turbo codes. Polar replaces TBCC for control. Flexible SCS (15-240 kHz). RRC_INACTIVE state. Configured grants for URLLC. No RLC concatenation.

How does SDAP enable per-flow QoS?

Maps QoS flows (identified by QFI) to data radio bearers. Multiple flows can share a DRB. Adds 1-byte QFI header. Enables fine-grained QoS without creating separate bearers per flow (LTE scalability problem).

Related Terms

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