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The RRC (Radio Resource Control) protocol in 5G NR provides six essential services to upper layers: Broadcast of common control information – Delivers system information (MIB/SIBs) to all UEs for cell access and configuration. Notification of UEs in RRC_IDLE – Enables paging for mobile terminating calls and SMS. ETWS and CMAS notifications – Delivers emergency alerts (earthquake, tsunami, public warnings) to UEs. Transfer of dedicated signalling – Handles UE-specific RRC messages like handover, reconfiguration, and NAS transport. Broadcast of positioning assistance data – Provides GNSS/OTDOA assistance for accurate UE positioning. Transfer of application layer measurement configuration and reporting – Supports QoE/QoS measurement collection for network optimization. In turn, RRC expects four key services from lower layers (PDCP, RLC, MAC, PHY): integrity protection, ciphering, loss-less in-sequence delivery, and no duplication — ensuring secure, reliable, and ordered signalling message transfer.
Understand the NR UE States, State Transitions and Inter-RAT Mobility (3GPP Rel-17/18).
5G NR defines seven SRBs (SRB0–SRB6) for RRC and NAS signalling. SRB0 uses CCCH for initial access. SRB1–SRB6 use DCCH for dedicated signalling. SRB1 handles connection setup and early NAS. SRB2 carries NAS/logged measurements post-security, with lower priority. SRB3 is for Dual Connectivity. SRB4/5/6 carry application/logged measurement reports after AS security activation. NAS piggybacking is limited: downlink for bearer procedures; uplink only during setup/resume. Once AS security activates, all SRB1–SRB6 messages are PDCP-protected. Split SRB supported only for SRB1/SRB2 in MR-DC & MP. For shared spectrum (FR1) , SRB0/1/3 use highest CAPC = 1; SRB2 CAPC is configurable. For L2 U2N relay, SRB0–SRB2 use PC5 and Uu Relay RLC channels, not Uu CCCH/DCCH.
Explore 3GPP Release 19 standardized network slices and SST values 1–7, including their key characteristics, use cases, and role in 5G network slicing.
