Functional Roles of the RNC

The RNC that controls a given Node B is known as the Controlling RNC
(CRNC). The CRNC is responsible for the management of radio resources
available at a Node B that it supports.
For a given connection between the UE and the core network, one RNC
is in control. This is called the Serving RNC (SRNC). For the user in question,
the SRNC controls the radio resources that the UE is using. In addition,
the SRNC terminates the Iu interface to or from the core network for the services being used by the UE. In many cases, though not all, the SRNC
is also the CRNC for a Node B that is serving the user.
As depicted in Figure 6-17, UTRAN supports soft handovers, which may
occur between Node Bs controlled by different RNCs. During and after a
soft handover between RNCs, one may find a situation where a UE is communicating
with a Node B that is controlled by an RNC that is not the
SRNC. Such an RNC is termed a Drift RNC (DRNC). The DRNC does not
perform any processing of user data (beyond what is required for correct
operation of the physical layer). Rather, data to or from the UE is controlled
by the SRNC and is passed transparently through the DRNC.
As a UE moves further and further away from any Node B controlled by
the SRNC, it will become clear that it is no longer appropriate for the same
RNC to continue to act as the SRNC. In that case, UTRAN may make the
decision to hand the control of the connection over to another RNC. This is
known as serving RNS (SRNS) relocation. This action is invoked under the
control of algorithms within the SRNC.