5G NR RRC (Radio Resource Control) Protocol

The main services and functions of the RRC sub-layer include:

  • Broadcast of System Information related to AS and NAS
  • Paging initiated by 5GC or NG-RAN
  • Establishment, maintenance and release of an RRC connection between the UE and NG-RAN including
  • Addition, modification and release of carrier aggregation;
  • Addition, modification and release of Dual Connectivity in NR or between E-UTRA and NR.
  • Security functions including key management
  • Establishment, configuration, maintenance and release of Signalling Radio Bearers (SRBs) and Data Radio
    Bearers (DRBs)
  • Mobility functions including
  • Handover and context transfer
  • UE cell selection and reselection and control of cell selection and reselection
  • Inter-RAT mobility
  • QoS management functions
  • UE measurement reporting and control of the reporting
  • Detection of and recovery from radio link failure
  • NAS message transfer to/from NAS from/to UE

RRC supports the following states which can be characterised as follows:

  1. RRC_IDLE
  2. RRC_INACTIVE
  3. RRC_CONNECTED

RRC_IDLE:

  • PLMN selection;
  • Broadcast of system information;
  • Cell re-selection mobility;
  • Paging for mobile terminated data is initiated by 5GC;
  • Paging for mobile terminated data area is managed by 5GC;
  • DRX for CN paging configured by NAS.

RRC_INACTIVE:

  • PLMN selection;
  • Broadcast of system information;
  • Cell re-selection mobility;
  • Paging is initiated by NG-RAN (RAN paging);
  • RAN-based notification area (RNA) is managed by NG- RAN;
  • DRX for RAN paging configured by NG-RAN;
  • 5GC – NG-RAN connection (both C/U-planes) is established for UE;
  • The UE AS context is stored in NG-RAN and the UE;
  • NG-RAN knows the RNA which the UE belongs to.

RRC_CONNECTED:

  • 5GC – NG-RAN connection (both C/U-planes) is established for UE;
  • The UE AS context is stored in NG-RAN and the UE;
  • NG-RAN knows the cell which the UE belongs to;
  • Transfer of unicast data to/from the UE;
  • Network controlled mobility including measurements.

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Reference :

3GPP TS 38.300

3GPP TS 38.331

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