5G NR MAC Logical Channels and Transport Channels
Logical channels define what type of information MAC handles. Transport channels define how that information is carried between MAC and PHY.
This page explains the NR MAC channel model used in 3GPP TS 38.321 with Release 18. Use it to separate logical channels, transport channels, and physical channels correctly and to understand how MAC maps signaling, traffic, paging, broadcast, MBS, and sidelink content onto actual transport paths.
| Technology | 5G NR |
|---|---|
| Area | MAC channel model |
| Main specification | 3GPP TS 38.321 |
| Architecture context | 3GPP TS 38.300 |
| Release baseline | Release 18 |
| Main use | Channel mapping, decode interpretation, procedure analysis, and troubleshooting |
| Key distinction | Logical channels describe information type; transport channels describe carriage method |
| Best follow-up | MAC overview, multiplexing and demultiplexing, logical channel prioritization, random access, MAC PDU format |
Definition and purpose
Logical channels are defined by the type of information carried. Transport channels are defined by how information is carried over the radio interface. MAC is the layer that maps logical channels onto transport channels.
This distinction matters because the same procedure can involve several layers of channel naming. Logical channels describe control, traffic, broadcast, paging, MBS, or sidelink information. Transport channels describe the MAC-to-PHY carriage path. Physical channels are a separate PHY concept and should not be mixed into MAC-level channel mapping.
| Channel view | Main question | Typical examples | Why engineers care |
|---|---|---|---|
| Logical channel | What kind of information is this? | BCCH, PCCH, CCCH, DCCH, DTCH, MCCH, MTCH, SBCCH, SCCH, STCH | Shows traffic meaning and service context |
| Transport channel | How is the information carried between MAC and PHY? | BCH, DL-SCH, PCH, UL-SCH, RACH, SL-BCH, SL-SCH | Shows delivery path and procedure context |
| Physical channel | Where is it transmitted on air? | PBCH, PDSCH, PUSCH, PRACH, PSCCH, PSSCH | Needed for PHY correlation, but not a replacement for MAC mapping |
Where it fits
Logical channels sit at the MAC and RLC boundary.
Transport channels sit at the MAC and PHY boundary.
Physical channels are a different concept and should not be merged with logical or transport channel names.
Applications / NAS
↓
RRC / SDAP / PDCP / RLC
↓
Logical channels
↓
MAC mapping / multiplexing
↓
Transport channels
↓
PHY / physical channelsLogical, transport, and physical channels
| Category | Defined by | Interface | What it represents | Examples | Why it matters |
|---|---|---|---|---|---|
| Logical channels | Type of information | MAC and RLC | Service intent and traffic category | BCCH, PCCH, CCCH, DCCH, DTCH, MCCH, MTCH, SBCCH, SCCH, STCH | Shows what information MAC is handling |
| Transport channels | How information is transported | MAC and PHY | Radio-side delivery path | BCH, DL-SCH, UL-SCH, PCH, RACH, SL-BCH, SL-SCH | Shows how MAC hands traffic toward Layer 1 |
| Physical channels | Actual radio resources and signaling | PHY and RF resource usage | The on-air transmission structure | PDSCH, PUSCH, PBCH, PRACH, PSCCH, PSSCH, PSBCH | Shows what is actually transmitted over the air |
Logical channels in 5G NR
Logical channels tell MAC what the information is before it is mapped to a transport channel. In protocol reading and log analysis, the logical channel is the first strong indicator of whether the content is broadcast, paging, common control, dedicated control, user traffic, MBS-related content, or sidelink traffic.
| Logical channel | Full name | Type | Direction / context | Main purpose | Transport mapping | Engineering note |
|---|---|---|---|---|---|---|
| BCCH | Broadcast Control Channel | Control | Downlink broadcast | Cell-wide system information and broadcast control | BCH, DL-SCH | Most often tied to MIB and SIB-related reads |
| PCCH | Paging Control Channel | Control | Downlink paging | Paging indication and paging-triggered activity | PCH | Useful when checking paging occasions and UE reachability |
| CCCH | Common Control Channel | Control | Access and setup | Common control before dedicated resources are established | UL-SCH, RACH, DL-SCH | Often seen in initial access and connection setup |
| DCCH | Dedicated Control Channel | Control | Dedicated signaling | Connection-specific control signaling | UL-SCH, DL-SCH | Common in connected-state signaling |
| DTCH | Dedicated Traffic Channel | Traffic | Dedicated user traffic | User-plane data and dedicated traffic | UL-SCH, DL-SCH | Most common path for connected user payload |
| MCCH | MBS Control Channel | Control | MBS downlink | MBS control and configuration | DL-SCH | Modern NR reference coverage should include it |
| MTCH | MBS Traffic Channel | Traffic | MBS downlink | MBS payload delivery | DL-SCH | Pairs with MCCH for MBS operation |
| SBCCH | Sidelink Broadcast Control Channel | Control | Sidelink | Broadcast control in sidelink mode | SL-BCH | Used in PC5-side broadcast contexts |
| SCCH | Sidelink Control Channel | Control | Sidelink | Sidelink control signaling | SL-SCH | Important for direct UE-to-UE operation |
| STCH | Sidelink Traffic Channel | Traffic | Sidelink | Sidelink traffic between UEs | SL-SCH | Use this when traffic is carried over PC5 |
Transport channels in 5G NR
Transport channels describe how MAC hands information to PHY. They are the MAC-side transport abstractions used for broadcast, paging, scheduled downlink, scheduled uplink, random access, and sidelink transport.
| Transport channel | Full name | Direction | Main purpose | Typical carried information | Logical channels mapped | Procedure relevance | Engineering note |
|---|---|---|---|---|---|---|---|
| BCH | Broadcast Channel | DL | Broadcast delivery | Broadcast system information | BCCH | Broadcast acquisition | Used for broadcast-path delivery |
| DL-SCH | Downlink Shared Channel | DL | Scheduled downlink transport | Control, traffic, broadcast on shared path, MBS payloads | BCCH, CCCH, DCCH, DTCH, MCCH, MTCH | General scheduled downlink | Most flexible downlink MAC path |
| PCH | Paging Channel | DL | Paging transport | Paging information | PCCH | Paging and UE notification | Separate paging path from regular DL-SCH data |
| UL-SCH | Uplink Shared Channel | UL | Scheduled uplink transport | Common control, dedicated control, dedicated traffic | CCCH, DCCH, DTCH | Main uplink signaling and payload path | Used heavily after access completes |
| RACH | Random Access Channel | UL | Access transport | Random access-related uplink transmission | CCCH | Initial access and recovery | Do not treat it as a general payload path |
| SL-BCH | Sidelink Broadcast Channel | Sidelink | Sidelink broadcast transport | Sidelink broadcast control | SBCCH | PC5 broadcast delivery | Sidelink-specific path |
| SL-SCH | Sidelink Shared Channel | Sidelink | Sidelink shared transport | Sidelink control and traffic | SCCH, STCH | Direct UE-to-UE sidelink transport | Carries both sidelink control and traffic |
Mapping rules
The MAC entity maps logical channels onto transport channels. The available mapping depends on direction, procedure, and RRC-configured behavior.
Keep the mapping separate by uplink, downlink, and sidelink. Most misreads happen when a reader mixes these contexts or assumes that one logical channel always has one fixed transport path.
Uplink channel mapping
| Logical channel | UL-SCH | RACH |
|---|---|---|
| CCCH | X | X |
| DCCH | X | |
| DTCH | X |
Downlink channel mapping
| Logical channel | BCH | PCH | DL-SCH |
|---|---|---|---|
| BCCH | X | X | |
| PCCH | X | ||
| CCCH | X | ||
| DCCH | X | ||
| DTCH | X | ||
| MCCH | X | ||
| MTCH | X |
Sidelink channel mapping
| Logical channel | SL-BCH | SL-SCH |
|---|---|---|
| SBCCH | X | |
| SCCH | X | |
| STCH | X |
Common mapping patterns
Use this section as the quick lookup layer. The exact mapping still depends on procedure and configuration, but these are the combinations that matter most in everyday engineering work.
If the transport side does not match the logical channel you expected, first check whether the procedure context was interpreted correctly before assuming payload or decoder failure.
| Logical channel | Uplink path | Downlink path | Sidelink path | Practical note |
|---|---|---|---|---|
| BCCH | BCH, DL-SCH | Broadcast and system-information traffic | ||
| PCCH | PCH | Paging traffic | ||
| CCCH | UL-SCH, RACH | DL-SCH | Common control during access and setup | |
| DCCH | UL-SCH | DL-SCH | Dedicated signaling after connection setup | |
| DTCH | UL-SCH | DL-SCH | User-plane data in connected mode | |
| MCCH | DL-SCH | MBS control signaling | ||
| MTCH | DL-SCH | MBS traffic delivery | ||
| SBCCH | SL-BCH | Sidelink broadcast control | ||
| SCCH | SL-SCH | Sidelink control signaling | ||
| STCH | SL-SCH | Sidelink traffic |
Where channel mapping is used
| Procedure area | Typical logical channels | Typical transport channels | Why it matters |
|---|---|---|---|
| System information delivery | BCCH | BCH, DL-SCH | Separates broadcast-path delivery from scheduled downlink delivery |
| Paging | PCCH | PCH | Explains paging transport and idle-state notification |
| Initial access and setup | CCCH | RACH, UL-SCH, DL-SCH | Tracks common control before dedicated signaling is established |
| Dedicated signaling | DCCH | UL-SCH, DL-SCH | Shows the connected-state control path |
| User-plane transfer | DTCH | UL-SCH, DL-SCH | Shows the main dedicated payload path |
| MBS operation | MCCH, MTCH | DL-SCH | Tracks multicast control and multicast traffic delivery |
| Sidelink communication | SBCCH, SCCH, STCH | SL-BCH, SL-SCH | Separates PC5 channel use from cellular UL/DL assumptions |
Relation to RRC, RLC, MAC, and PHY
RLC delivers data into logical channels, so logical channel identity begins above the MAC PDU level. MAC maps those logical channels onto transport channels and multiplexes the resulting content for delivery. RRC configures the multiplexing and channel context. PHY then carries the transport-channel content through physical channels.
This is why channel reading can fail if only one layer is examined. The logical channel may be correct, but the expected transport channel can still change because of procedure stage, configuration, or feature context.
| Layer | Relation to channel mapping |
|---|---|
| RRC | Configures logical channels, access behavior, MBS context, sidelink context, and the conditions under which channel mapping is relevant. |
| RLC | Supplies the SDUs associated with logical-channel identity. |
| MAC | Maps logical channels to transport channels, applies multiplexing, and constructs MAC PDUs. |
| PHY | Implements the physical channels that actually carry the transport-channel content on air. |
Troubleshooting and log-analysis relevance
| Symptom | MAC area to inspect | Why it matters |
|---|---|---|
| Paging visible but UE does not respond | PCCH and PCH interpretation | Paging problems often come from mixing logical and transport views or ignoring paging context. |
| Broadcast content appears in the wrong place | BCCH mapping to BCH or DL-SCH | BCCH does not imply one single transport path, so procedure context matters. |
| Access starts but setup interpretation is wrong | CCCH use on RACH, UL-SCH, and DL-SCH | Common control moves across multiple transport contexts during access and setup. |
| Dedicated signaling seems misclassified | DCCH versus CCCH, plus UL-SCH and DL-SCH context | Access-stage control and dedicated control are often confused in traces. |
| Sidelink logs look incomplete or wrong | SBCCH, SCCH, STCH with SL-BCH and SL-SCH | Sidelink should not be read using only normal cellular uplink and downlink assumptions. |
| MBS traffic is hard to classify | MCCH and MTCH on DL-SCH | Modern NR channel analysis should include MBS, not only classic unicast channels. |
Common mistakes
- Treating logical channels as if they were physical channels.
- Assuming BCCH maps only to BCH and forgetting BCCH can also use DL-SCH.
- Confusing PCCH with PCH instead of keeping logical and transport sides separate.
- Confusing CCCH with DCCH during access and connected-state signaling.
- Treating RACH like a general-purpose uplink data path instead of an access-related transport channel.
- Ignoring MCCH and MTCH in modern NR multicast and broadcast analysis.
- Ignoring SBCCH, SCCH, and STCH when the procedure is actually sidelink-based.
Release 18 scope note
- Release 18 coverage should include classic cellular channels and the newer MBS and sidelink-related channels used in modern NR.
- MCCH and MTCH matter because multicast and broadcast service traffic is part of the NR MAC model.
- SBCCH, SCCH, and STCH matter because sidelink has its own logical and transport channel structure.
- A modern 5G NR channel page should be broader than older simplified explanations that only listed BCCH, PCCH, CCCH, DCCH, and DTCH.
Related pages
| Page | Why open next |
|---|---|
| 5G NR MAC Overview | Return to the MAC hub for stack position, functions, and section-wide navigation. |
| 5G MAC multiplexing and prioritization | Study how logical-channel data is packed and prioritized inside the MAC PDU. |
| 5G MAC Random Access | See how CCCH uses RACH and UL-SCH during initial access and setup. |
| 5G MAC PDU and subheader format | Follow channel mapping into actual MAC PDU structure. |
| 5G MAC Control Elements | Understand the control signaling that travels alongside mapped SDUs inside MAC PDUs. |
| 5G sidelink MAC | Go deeper into the sidelink-specific channel model. |
| 5G MBS MAC | Go deeper into MCCH and MTCH context in multicast and broadcast operation. |
FAQ
What is the difference between logical channels and transport channels in 5G NR?
Logical channels identify the type of information carried, while transport channels identify how MAC carries that information toward PHY.
Is this topic defined by 3GPP TS 38.321?
Yes. The MAC-side channel model and mapping behavior are defined primarily in 3GPP TS 38.321, with broader architecture context from 3GPP TS 38.300.
Does MAC define logical channels or physical channels?
MAC works with logical channels and transport channels. Physical channels belong to PHY and should be treated separately.
Can BCCH map to more than one transport channel?
Yes. In NR, BCCH can map to BCH or DL-SCH depending on the broadcast content and procedure context.
What is the difference between PCCH and PCH?
PCCH is a logical channel that identifies paging information type. PCH is the transport channel used to carry paging at the MAC-to-PHY boundary.
What is the difference between CCCH and DCCH?
CCCH is common control used around access and setup before dedicated signaling is established. DCCH is dedicated control used once the UE has dedicated signaling context.
Which transport channel carries dedicated uplink traffic in NR?
UL-SCH carries dedicated uplink control and traffic such as DCCH and DTCH, and it can also carry CCCH in uplink mapping.
What are MCCH and MTCH?
MCCH is the MBS Control Channel and MTCH is the MBS Traffic Channel. In this MAC view, both are carried on DL-SCH.
What are sidelink logical channels in NR?
The sidelink logical channels are SBCCH, SCCH, and STCH. They support sidelink broadcast control, sidelink control, and sidelink traffic.
Why is channel mapping important for troubleshooting?
Channel mapping tells you what kind of information MAC is handling and which transport path it uses, so it is essential for procedure analysis, decoder interpretation, and symptom-led troubleshooting.