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RRC is Radio Resource Control. It is a layer 3 protocol used between the UE and the Base station. This protocol is specified by the 3GPP. RRC messages are transported via PDCP (Packet Data Convergence Protocol). By means of the signaling functions, the RRC configures the user and the control planes according to the network status and it allows for Radio Resource Management strategies that are required to be implemented. RRC parameters should be understood by the network and the UE which can communicate via radio channel. RRC is a layer within the 5G NR protocol stack. RRC (Radio Resource Control) protocol is used on the Air interface. The major functions of the RRC protocol include connection establishment and release functions, broadcast of system information, the establishment of radio bearers, reconfiguration, and release of RRC, paging notification, and releases.  RRC exists only in the control plane, in the UE, and in the gNB. The RRC idle mode is known as no connection mode, has the lowest energy consumption. The states in the RRC connected mode, are in order of decreasing power control. The transitions to lower energy consumption states occur when inactivity timers trigger. Different operators have different configurations for the inactivity timers, which leads to differences in energy consumption. 

In the 5G NR, RRC has three distinct stages: 


RRC state 

It refers to various different phases in which UE/Network be after RRC connection setup and before RRC release. There are three states that occur after Radio Bearer setup. RRC state change refers to the process of switching between these states. 

RRC Services and Functions 

  • Broadcast of System information related to AS (Access Stratum) and NAS (Non-Access Stratum). 
  • Paging services are initiated by 5GC and NG-RAN.
  • Establishment, maintenance, and release of an RRC connection between the UE and the NG-RAN. 
  • Security functions including the key management services.
  • Establishment, configuration, maintenance, and release of Signaling Radio Bearers (SRBs) and Data Radio Bearers (DRBs). 
  • Mobility management functions like handover and context transfer services. 
  • UE 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 Idle mode operations 

  • PLMN selection
  • Broadcast of System Information
  • Cell re-selection mobility 
  • Paging for mobile terminated data is initiated by 5GC
  • Paging for mobile terminated data is managed by 5GC
  • DRX for CN paging configured by NAS

RRC Inactive Mode Operation 

  • Broadcast of system information
  • Cell re-selection mobility
  • Paging is initiated by NG-RAN
  • RAN based notification area is managed by NG-RAN
  • DRx for RAN paging configured by NG-RAN
  • The UE AS context is stored in NG-RAN and the UE
  • NG-RAN knows the RNA which the UE belongs to 

RRC Connected Mode Operation 

  • 5GC - NG-RAN connection is established for UE
  • The UE AS context is stored in NG-RAN and UE
  • NG-RAN knows the cell which the UE belongs to
  • Transfer of unicast data to/from the UE
  • Network controlled mobility including measurements

RRC Model required in 5G NR 

The RRC state is a solution to System access, power savings, and mobility optimization. 5G has to support eMBB, URLLC, and Massive IoT services at the same cost and energy dissipation. It is to support:

  • To support URLLC services that transmit small packets that require ultra-low latency and high reliability.
  • Power savings mode to transmit and receive packets.
  • Devices need periodic DL (Downlink) small packet transmission.

RRC states in 5g nr