5G NR MAC Overview
5G NR MAC is the Medium Access Control sublayer of the NR radio protocol stack. It sits between RLC and PHY in Layer 2 and provides the immediate control needed to move data and control information efficiently over the air interface.
In NR, MAC maps logical channels to transport channels and handles multiplexing and demultiplexing, scheduling-related reporting through Scheduling Request, Buffer Status Report, and Power Headroom Report, HARQ, logical channel prioritization, and random access-related behavior. It matters because many practical radio behaviors visible in logs, throughput results, access success, and uplink responsiveness depend on MAC operation.
Quick facts
| Technology | 5G NR |
|---|---|
| Layer | Layer 2 MAC sublayer |
| Above MAC | RLC |
| Below MAC | PHY |
| Main spec | 3GPP TS 38.321 |
| Release | Release 18 |
| Core topics | Logical/transport channel mapping, multiplexing, demultiplexing, HARQ, SR, BSR, PHR, DRX, random access, MAC PDUs, MAC CEs |
| Related pages | Random Access, HARQ, Scheduling Request, Buffer Status Report, Power Headroom Report, DRX, MAC PDU Format, MAC Control Elements, Sidelink MAC, MBS MAC |
MAC topics
Logical Channels and Transport Channels | Random Access | HARQ | Scheduling Request | Buffer Status Report | Power Headroom Report | DRX | MAC PDU Format | MAC Control Elements | Sidelink MAC | MBS MAC
Contents
Overview
The NR MAC layer is the execution layer for radio data handling decisions that must be applied per transmission opportunity, per transport block, and per UE activity state. It is narrower than RRC in configuration scope and lower than RLC in segmentation and retransmission architecture, but it is where radio resource use becomes operational.
MAC is the layer to study when investigating uplink access, grant usage, retransmission behavior, scheduling responsiveness, control element signaling, and the practical movement of logical-channel traffic onto transport channels.
Position in the NR stack
MAC is part of Layer 2. In operational terms, RRC configures behavior, RLC prepares data units and buffering behavior, MAC decides how data and control fit into actual transport opportunities, and PHY transmits and receives the radio waveform.
+------------------------------+
| RRC |
| Radio resource control |
+------------------------------+
| SDAP / PDCP / RLC |
| Higher-layer data handling |
+------------------------------+
| MAC |
| Mapping, multiplexing, HARQ, |
| SR/BSR/PHR, RA support |
+------------------------------+
| PHY |
| Physical transmission |
+------------------------------+| Layer | Relation to MAC |
|---|---|
| RRC | Configures logical channels, reporting behavior, DRX, random access parameters, and feature context. |
| RLC | Supplies and receives SDUs while MAC decides how those SDUs fit into transport opportunities. |
| PHY | Executes transmission and reception for the transport blocks and access resources MAC relies on. |
Main functions
| Function | Meaning | Why it matters |
|---|---|---|
| Mapping between logical channels and transport channels | MAC places data from logical channels onto appropriate transport channels. | It determines how signaling, user data, paging, broadcast, and sidelink data are actually carried. |
| Multiplexing | MAC combines multiple MAC SDUs and control elements into one MAC PDU. | It affects grant efficiency and the composition of actual MAC transmissions. |
| Demultiplexing | MAC parses received MAC PDUs and delivers content to the correct destination. | It is required for correct receive-side interpretation of data and control. |
| Scheduling information reporting | MAC reports uplink need and radio context through SR, BSR, and PHR. | It helps the gNB allocate uplink resources and interpret UE capability to transmit. |
| HARQ | MAC manages transport-block acknowledgment and retransmission processes. | It directly affects throughput, latency, and radio-link reliability. |
| Logical channel prioritization | MAC resolves which logical-channel data should be served first under limited grants. | It protects important signaling and enforces intended traffic behavior. |
| Random access support | MAC handles behavior associated with RA progression, Msg3 use, and access-related state. | It is critical for initial access, recovery, uplink synchronization, and mobility-related entry. |
Mapping between logical channels and transport channels
Logical channels represent information type. Transport channels represent the transport method. MAC joins the two and therefore turns upper-layer data intent into radio-carried content. Use Logical Channels and Transport Channels for the full mapping view.
Multiplexing
MAC builds transmission units by combining one or more MAC SDUs, control elements, and padding as needed. This is central to efficient grant use in both uplink and downlink. Use MAC PDU Format and MAC Control Elements for the carried-content view.
Demultiplexing
On reception, MAC interprets headers, separates control elements from SDUs, and passes content to the right function. Correct demultiplexing is required for protocol operation and for meaningful log decoding. Use Multiplexing and Demultiplexing for the full receive-side parsing path.
Scheduling information reporting
MAC participates in uplink scheduling using mechanisms such as Scheduling Request, Buffer Status Report, and Power Headroom Report. These functions expose demand and transmission constraints to the scheduler.
HARQ
Hybrid ARQ belongs operationally to MAC because it governs acknowledgment, process tracking, and retransmission behavior close to real transmission timing.
Logical channel prioritization
When available grant size is insufficient for all pending traffic, MAC applies prioritization rules so that critical signaling and intended QoS behavior are preserved. Use Logical Channel Prioritization for the selection rules.
Random access support
Random access includes PHY and higher-layer aspects, but MAC is central in the use of Msg3 on UL-SCH, access progression handling, temporary identifier use, and related control behavior. Use it together with PHY initial access and the RRC connection setup flow.
Logical channels and transport channels
Logical channels are defined by the type of information carried. Transport channels define how that information is transported over the radio interface. MAC maps logical channels onto transport channels according to procedure, direction, and service context.
The full topic belongs on its dedicated page: Logical Channels and Transport Channels.
| Channel type | Defined by | Examples | Practical meaning |
|---|---|---|---|
| Logical channels | Information type | BCCH, PCCH, CCCH, DCCH, DTCH, MCCH, MTCH, SBCCH, SCCH, STCH | What kind of information is being carried |
| Transport channels | Transport method over radio | BCH, DL-SCH, PCH, UL-SCH, RACH, SL-BCH, SL-SCH | How information is carried between MAC and PHY |
| Physical channels | Actual radio-level structures | PBCH, PDSCH, PUSCH, PRACH, PDCCH, PSCCH, PSSCH | Where transmission and reception occur in PHY |
Note: Logical, transport, and physical channels are different abstractions. MAC is the layer where logical-channel intent is turned into transport-channel use.
Core MAC procedures
Detailed behavior belongs in child pages. This hub page identifies the main procedural areas used most often.
| Procedure | Purpose | Reference page |
|---|---|---|
| Random Access | Establish uplink access, synchronization context, or recovery entry. | Random Access |
| Uplink Operation | Handle uplink data movement, grant use, and control reporting. | Uplink Operation |
| Downlink Operation | Handle downlink transport reception, demultiplexing, and HARQ feedback interaction. | Downlink Operation |
| Scheduling Request | Ask for uplink resources. | Scheduling Request |
| Buffer Status Report | Report pending uplink data volume. | Buffer Status Report |
| Power Headroom Report | Report uplink power margin context. | Power Headroom Report |
| DRX | Control discontinuous reception behavior. | DRX |
| Timing Advance | Maintain uplink timing alignment. | Timing Advance |
| Reconfiguration and Reset | Apply MAC-affecting changes and reset-related behavior. | Reconfiguration and Reset |
HARQ overview
HARQ in NR MAC is the mechanism used to improve reliability and efficiency through retransmission control tied to transport block delivery. It operates close to actual transmission timing and therefore belongs to MAC rather than to upper-layer retransmission logic.
HARQ directly affects throughput, latency, BLER response, scheduler efficiency, and air-interface robustness. In trace analysis, HARQ behavior often explains why data is delayed, why throughput collapses under radio stress, or why link quality changes produce unstable user experience. Use it together with PHY HARQ when retransmission behavior is the main question.
Reference page: HARQ
MAC PDU and MAC Control Elements
A. MAC PDU
A MAC PDU is the unit MAC exchanges with PHY over a transport channel. It contains one or more MAC subPDUs. A subPDU may carry a MAC SDU, a MAC Control Element, or padding.
| Term | Meaning |
|---|---|
| MAC PDU | Complete MAC-layer transmission unit. |
| MAC subPDU | Header plus payload segment inside the MAC PDU. |
| MAC SDU | Service data unit delivered to MAC from an upper layer such as RLC. |
| Padding | Filler added when grant size exceeds useful payload size. |
Reference page: MAC PDU Format
B. MAC Control Elements
MAC Control Elements are compact MAC-layer signaling structures carried inside MAC PDUs. They support operational control that must be exchanged quickly and efficiently without requiring full RRC signaling.
Important CE families include buffer and scheduling related CEs, power reporting related CEs, timing and synchronization related CEs, DRX-related control, contention resolution related control, sidelink-related control, and MBS or newer feature support elements where applicable.
They matter because they are heavily used in live procedures, appear directly in protocol logs and decodes, and often explain missing or unexpected behavior during trace analysis.
Reference pages: MAC Control Elements, MAC Control Elements Index
How MAC interacts with RRC, RLC, and PHY
| Layer | Relation to MAC |
|---|---|
| RRC | Configures MAC-relevant behavior such as logical channels, SR/BSR/PHR settings, DRX, random access parameters, and feature activation context. |
| RLC | Supplies SDUs to MAC, receives SDUs from MAC after demultiplexing, and handles segmentation, reassembly, and ARQ where applicable. |
| MAC | Performs transport-channel mapping, multiplexing, demultiplexing, prioritization, HARQ handling, control-element exchange, and access-related behavior. |
| PHY | Executes actual transmission and reception for the transport blocks and random access resources MAC relies on. |
Cross-layer reading is often needed because isolated MAC analysis is usually insufficient. A delayed uplink event may be caused by RRC configuration, RLC buffering, MAC prioritization, or PHY radio conditions. Good troubleshooting usually moves across these boundaries rather than staying within one layer.
Note: In many field logs, the visible symptom appears in MAC first, but the root cause may be in RRC configuration or PHY limitation.
MAC in real call flows
| Procedure area | Why MAC matters |
|---|---|
| Initial access | MAC is involved in random access behavior, Msg3 handling, and grant-driven progression after access response. |
| System information | MAC maps broadcast and scheduling-related content onto the correct transport path for delivery. |
| Paging | MAC participates in paging transport handling and related receive-side interpretation. |
| Dedicated signaling | MAC prioritization and grant use affect control-plane responsiveness. |
| User-plane transfer | Throughput, delay, grant use, HARQ behavior, and multiplexing efficiency depend heavily on MAC. |
| Sidelink | MAC governs sidelink transport handling, control/data multiplexing, and channel use in sidelink procedures. |
| MBS | MAC supports multicast and broadcast service transport behavior and associated control/data handling paths. |
Release 18 scope
This page uses Release 18. Modern NR MAC is broader than a basic uplink and downlink unicast view, so the page should reflect current scope rather than only early NR behavior.
| Traditional MAC focus | Modern Release 18 MAC scope |
|---|---|
| Basic UL and DL scheduling behavior | Expanded support across advanced NR service types and feature sets. |
| Random access, SR, BSR, PHR, HARQ | Core mechanisms remain central but now sit inside a broader feature context. |
| Unicast-centered traffic view | Sidelink MAC and MBS-related MAC handling are part of the broader NR MAC picture. |
| Basic control elements | Expanded MAC Control Element families support newer behaviors and reporting needs. |
| Conventional mobile broadband focus | Positioning-related MAC behavior and specialized service modes are more visible. |
| Standard data transmission | Small data transmission and feature-efficient operation are part of modern system design. |
Release 18-relevant areas to track on MAC pages include sidelink MAC, MBS-related channels and behavior, positioning-related MAC behavior, small data transmission, and expanded MAC Control Element coverage.
Release note: The core MAC architecture remains anchored in TS 38.321, but the practical scope of NR MAC in Release 18 is broader than in early NR deployments.
Troubleshooting
| Symptom | MAC area to inspect | Why |
|---|---|---|
| Access failure | Random Access, Msg3 handling, contention resolution, timing alignment | Initial entry often fails through RA progression issues rather than only PHY detection problems. |
| Delayed uplink data | SR, BSR, grant availability, logical channel prioritization | Data may exist in buffers but not receive resources quickly enough. |
| Poor uplink throughput | HARQ, BSR, PHR, grant sizing, prioritization | Throughput degradation often reflects retransmissions, insufficient grants, or power limitation. |
| Poor downlink throughput | HARQ behavior, scheduling outcome visibility, MAC PDU composition | Downlink performance is affected by retransmissions and constrained delivery behavior. |
| Paging misunderstanding | PCCH/PCH handling, DRX context, paging-related reception assumptions | Paging issues are often confusion between higher-layer intent and MAC/PHY delivery conditions. |
| Reconfiguration side effects | MAC reset or reconfiguration impact, logical channel changes, DRX changes | Behavior may change immediately after RRC updates that alter MAC rules or state. |
| Sidelink issue | SL channel handling, sidelink MAC control, transport mapping | Sidelink failures often require dedicated MAC interpretation rather than standard cellular-only assumptions. |
| Newer feature issue | Release-specific CE support, MBS or small-data behavior, positioning context | Modern features may fail because a MAC feature family is misunderstood or incompletely decoded. |
Practical MAC troubleshooting usually starts with four questions: was there data or control pending at MAC, did MAC have an opportunity to transmit or receive it, did prioritization or HARQ affect that opportunity, and did RRC configuration or PHY conditions make the observed behavior expected rather than faulty.
References
- 3GPP TS 38.321, NR; Medium Access Control (MAC) protocol specification
- 3GPP TS 38.300, NR; NR and NG-RAN Overall Description; Stage-2
- 3GPP TS 38.331, NR; Radio Resource Control (RRC) protocol specification
- 3GPP TS 38.213, NR; Physical layer procedures for control
- 3GPP TS 38.214, NR; Physical layer procedures for data
FAQ
What is 5G NR MAC?
5G NR MAC is the Medium Access Control sublayer in Layer 2 of the NR stack. It sits between RLC and PHY and handles channel mapping, multiplexing, demultiplexing, HARQ, scheduling-related reporting, logical channel prioritization, and random access-related behavior.
Is MAC part of Layer 2?
Yes. In NR, MAC is a Layer 2 sublayer positioned below RLC and above PHY.
Which spec defines 5G NR MAC?
The main MAC protocol specification is 3GPP TS 38.321. Architectural context is provided by 3GPP TS 38.300.
What are the most important MAC topics to study first?
Start with logical and transport channels, random access, HARQ, Scheduling Request, Buffer Status Report, Power Headroom Report, DRX, and MAC PDU structure.
Why is MAC important for troubleshooting?
Many operational issues appear first at MAC level, including access failure, delayed uplink, throughput instability, retransmission behavior, prioritization effects, and control-element signaling problems.
Does Release 18 MAC include sidelink and advanced features?
Yes. A Release 18 MAC view should include sidelink MAC, MBS-related areas, broader control-element coverage, and newer feature contexts such as positioning-related behavior and small data transmission.
What is the difference between MAC and RLC?
RLC focuses on segmentation, reassembly, buffering behavior, and ARQ where applicable. MAC focuses on transport-channel mapping, multiplexing, prioritization, reporting, HARQ, and radio-access execution behavior.
What is the difference between MAC and RRC?
RRC configures and controls radio behavior at a higher layer. MAC executes operational radio data handling using those configurations during actual transport opportunities.
SEO meta
SEO title: 5G NR MAC Overview: Complete 38.321 Release 18 Reference
SEO description: Complete 5G NR MAC overview based on 3GPP TS 38.321 Release 18. Learn MAC functions, logical and transport channels, random access, HARQ, SR, BSR, PHR, DRX, MAC PDUs, control elements, sidelink, MBS, and modern NR MAC behavior.