RRC Message specification and Function

RRC is one of the important layers in LTE towards Air interface (Uu).I would like to explain here RRC w.r.t to eNodeB. RRC controls all the lower layers like PHY, MAC, RLC, and PDCP. RRC communicates to lower layers for cell specific configuration/reconfiguration and UE specific configuration/reconfiguration. RRC performs various operations which would be covered here. RRC protocol layer exists in UE & eNodeB; it is part of LTE air interface control plane. The main services and functions of the RRC sub layer include:

• Broadcast of System Information related to the non-access stratum (NAS);
• Broadcast of System Information related to the access stratum (AS);
• Paging;
• Establishment, maintenance and release of an RRC connection between the UE and E-UTRAN
• Security functions including key management;
• Establishment, configuration, maintenance and release of point to point Radio Bearers;
• Mobility functions
• QoS management functions;
• UE measurement reporting and control of the reporting;
• NAS direct message transfer to/from NAS from/to UE.

Same thing of above we can see functionality of RRC Protocol layer (L3 layer)   in details.

• Network Broadcast of system information message which contains information about
  • Including NAS common information;
  • Information applicable for UEs in RRC_IDLE, e.g. cell (re-)selection parameters, neighboring cell information and information (also) applicable for UEs in RRC_CONNECTED, e.g. common channel configuration information.
  • Including ETWS notification, CMAS notification;
• After broadcast the system information other important functionality of RRC is ‘’RRC connection control’’ through different procedure of connection.
  • Paging;
  • Establishment/ modification/ release of RRC connection, including e.g. assignment/ modification of UE identity (C-RNTI), establishment/ modification/ release of SRB1 and SRB2, access class barring;
  • Initial security activation, i.e. initial configuration of AS integrity protection (SRBs) and AS ciphering (SRBs, DRBs);
  • For RNs, configuration of AS integrity protection for DRBs;
  • RRC connection mobility including e.g. intra-frequency and inter-frequency handover, associated security handling, i.e. key/ algorithm change, specification of RRC context information transferred between network nodes;
  • Establishment/ modification/ release of RBs carrying user data (DRBs);
  • Radio configuration control including e.g. assignment/ modification of ARQ configuration, HARQ configuration, DRX configuration;
  • For RNs, RN-specific radio configuration control for the radio interface between RN and E-UTRAN;
  • In case of CA, cell management including e.g. change of PCell and addition/ modification/ release of SCell(s);
  • QoS control including assignment/ modification of semi-persistent scheduling (SPS) configuration information for DL and UL, assignment/ modification of parameters for UL rate control in the UE, i.e. allocation of a priority and a prioritized bit rate (PBR) for each RB;
  • Recovery from radio link failure;
• In Mobility functions , Inter-RAT mobility including e.g. security activation, transfer of RRC context information;
• Measurement configuration and reporting:
  • Establishment/ modification/ release of measurements (e.g. intra-frequency, inter-frequency and inter- RAT measurements);
  • Setup and release of measurement gaps;
  • Measurement reporting;
• Other functions including e.g. transfer of dedicated NAS information and non-3GPP dedicated information, transfer of UE radio access capability information, support for E-UTRAN sharing (multiple PLMN identities);
• Generic protocol error handling;
• Support of self-configuration and self-optimization;
• Support of measurement logging and reporting for network performance optimization ;