The AS provides the NAS with a service of information transfer between UE and the CN in what is named a Radio Access Bearer (RAB).
RAB consists of two:
1. Radio bearer (RB) between the UE and RNC
2. Iu Bearer between RNC and CN
RB is only for user data either in CS or PS mode.
RB is for both user data and signaling.
Especially for signaling purpose RB, we call it SRB: In the control plane, the signalling information is transfered through the Signaling Radio Bearer (SRB).
However, RAB consists of Iu bearer and user data RB (not SRB).
