UMTS Networks : Architecture, Mobility and Services

UMTS Radio Access Network 137
Figure 5.35 Relation between SRB, RB, signalling connection, RAB and the role of RNC
the UE may store the identity of the found PLMN for possible future use and restart
the cell search mechanism.
The first cell search for a PLMN is normally the most difficult for the UE, since it has
to scan through a number of scrambling codes to find the correct one. Once the UE
obtains the necessary information to capture the BS controlled by the corresponding
RNC it can request initial access to the UTRAN, resulting in a transition from idle
mode to connected mode.
Up to this point the RNC has been involved in the controlling function in terms of
system information broadcasting and controlling the related BS to which the UE was
trying to get radio connection. Once this has been carried out, the RNC participates
actively in controlling how access is provided by considering the context of the RRC
connection set-up message requested by the UE via the Random Access Channel
(RACH). In this regard, the RNC plays a central role in controlling radio connection
by checking the UE identity, the reason for the requested RRC connection and UE
capability, which helps it in allocating the initial signalling RB for the UE. Based on
this information the RNC decides whether or not to allocate the signalling RB to the
UE, which is used to carry the rest of the signalling for initial access and to provide all
the services thereafter. In case the RNC does not accept the access request the UE can
restart the initial access within a predefined time span.
Irrespective of the direction from which the higher layer service is requested, the
RRC connection requested for creating the SRB is always started by the UE, as
shown in Figure 5.36. When the RNC receives the RRC connection set-up request, it
sets up a radio link over the Iub interface to the BS with which the UE is going to have
radio connection. In this context, radio link is defined as the logical association between
a single UE and a single UTRAN access point, and the physical act of making it a
reality comprises one or more RB transmissions. If this step is successful, then the RNC
informs the UE that the RRC connection is set up and the UE responds to the RNC
concerning completion of RRC connection.
The SRB is now ready between the RNC and the UE and, hence, the RNC can
convert and transfer signalling connections and RABs between the UE and the CN.