SRB0 (grey): CCCH – RRC messages before connection (e.g., RRCSetupRequest). Always present.
SRB1 (blue): DCCH – RRC + NAS, priority 1 (e.g., RRCReconfiguration, initial NAS).
SRB2 (orange): DCCH – NAS only, after security (e.g., authentication, registration).
SRB3 (green): DCCH – EN‑DC/NR‑DC signalling between UE and secondary node.
SRB4 (pink): DCCH – application layer measurement reports (e.g., QoE). After security.
SRB5 (purple): DCCH – NR‑DC SCG signalling (secondary cell group). After security.