5G MAC Protocol

Medium Access Control (MAC) protocol specification written in (3GPP TS 38.321).

  • MAC is a protocol layer above the physical layer and below the RLC layer.
  • The ‘Medium Access Control’ (MAC) layer receives both the user plane and control plane data from the set of logical channels provided by the RLC layer.
  • Multiplexes and Demultiplexes MAC SDUs belonging to one or different logical channels into from transport blocks delivered to from the physical layer on transport channels
  • Performs error correction through the Hybrid Automatic Repeat Request (HARQ) mechanism
  • Handles priority between different UEs and different types of data (logical channels) within a single UE.
  • It also handles scheduling functions.
  • Add padding bits.
MAC structure overview
MAC structure overview
  • The MAC provides services to the RLC in the form of logical channels. 
  • Set of Logical channel is specified for NR( BCCH, CCCH, PCCH, DCCH, DTCH,)
  1. Broadcast Control Channel(BCCH): This is used to broadcast system information from gNB to all the devices in a cell. RRC messages for MIB, SIB1, and SystemInformation
  2. Paging Control Channel (PCCH): This is used for the paging of devices whose location on a cell level is not known to the network.
  3. Common Control Channel (CCCH): This is used for transmission of control information in conjunction with random access. Signalling on SRB0
  4. Dedicated Control Channel (DCCH): This used for transmission of control information to/from a device. Signalling on SRB1 or SRB2,SRB3
  5. The Dedicated Traffic Channel (DTCH): This used for the transmission of user data from a device.( User Plane data)

Set of Transport  Channel specified for NR(DL-SCH,BCH,PCH,UL-SCH,RACH)

  1. Broadcast Channel (BCH) : BCH is used to deliver master information block(MIB) to the UE.
  2. Paging Channel (PCH): PCH is used for delivering paging messages to the UE.
  3. Uplink Shared Channel (UL-SCH) : UL-SCH is used to carry user data in uplink.
  4. Random Access Channel (RACH): RACH channel is used for random access procedure.

MAC Entities:

  • The MAC entity of the UE handles the following transport channels:
    Broadcast Channel (BCH)
    Downlink Shared Channel(s) (DL-SCH)
    Paging Channel (PCH)
    Uplink Shared Channel(s) (UL-SCH)
    Random Access Channel(s) (RACH)
MAC structure overview with two MAC entities
MAC structure overview with two MAC entities