SRNC and DRNC are concepts for a connected UE. The SRNC handles the connection to one UE, and may borrow radio resources of a certain cell from the DRNC.
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
RAB: The service that the access stratum provides to the non-access stratum for transfer of user data between User Equipment and CN.
RB: The service provided by the layer2 for transfer of user data between User Equipment and Serving RNC.
RL: A “radio link” is a logical association between single User Equipment and a single UTRAN access point. Its physical realization comprises one or more radio bearer transmissions.