5G NR Protocol stack is divided into 2 planes: User Plane and Control Plane.
User Plane is used to transfer user data and control plane is used to transfer system control signalling transmission.
5G NR User Plane Protocol Stack
5G NR Control Plane Protocol Stack
MAC stands for Medium Access Control
An RCL PDU(Protocol Data Unit) consist of RLC header and a RLC payload.
At MAC layer, an RLC PDU is called as MAC SDU (Service Data Unit). Then MAC adds a Subheader to this and will become MAC subPDU.
Multiple MAC SDU and CEs can be part of single MAC PDU.
Functions Of MAC Layer
1. Mapping between logical and transport channel.
2. Combine MAC SDU from different logical channel into one transport block and deliver to lower layer.
3. Decompose MAC SDU received from lower layer and deliver to different channels.
5. LCP: Logical channel priority
6. BSR: Buffer Status Report
7. RACH: Random access
8. Uplink timing maintenance
9. PHR: Power headroom report
Services provided by MAC
Services provided by MAC layer to upper layer:
1. data transfer
Services expected by MAC layer from lower PHY layer:
1. Data transmission.
2. HARQ feedback indication.
3. Scheduling request signaling.
Below table shows the MAC Layer Procedures
Channel Mapping at MAC Layer
MAC layer Downlink channel mapping
MAC layer Uplink channels mapping
BCCH: Broadcast Control Channel: Downlink channel to broadcast system information.
PCCH: Paging Control Channel: Dwnlink channel to broadcast paging messages and system information changes.
CCCH: Common Control Channel: When UE and gNB(network) have not established an RRC Connection, this channel is used to transmit control information.
DCCH: Dedicated Control Channel: When UE and gNB(network) have established an RRC Connection, this channel is used to transmit control information.
DTCH: Dedicated Traffic Channel: User to transmit UE data in both uplink and downlink.
Below shows complete diagram of MAC Entity
Image and Text Source TS 38.321