Drift RNCs support the Serving RNC by providing radio resources A UE in connection state has at least one and only one SRNC, but can has 0 or multiple DRNCs
Inside the UTRAN, the RNCs of the Radio Network Subsystems (RNC) can be interconnected together through the Iur. Iu(s) and Iur are logical interfaces. Iur can be conveyed over direct physical connection between RNCs or virtual networks using any suitable transport network .
For each connection between User Equipment UE and the UTRAN, One RNC is the Serving RNC. When required, Drift RNCs support the Serving RNC by providing radio resources. The role of an RNC (Serving or Drift) is on a per connection basis between a UE and the UTRAN (SRNC and DRNC overview completed).
RAB, RB and RL in WCDMA
RB: The service provided by the layer2 for transfer of user data between User Equipment and Serving RNC.