UMTS Networks : Architecture, Mobility and Services

UMTS Protocols 325
Figure 10.25 Logical structure of an RRC protocol entity in WCDMA-FDD
also used for frequently repeated messages, like paging, to avoid unnecessary overheads.
Acknowledged mode AM-SAP is used for control signalling that is specific
to one UE whenever reliability of message exchange is required. Unacknowledged
mode UM-SAP, on the other hand, is used to avoid the potential delay present in
acknowledged-mode signalling (e.g., when releasing an RRC connection the release
message is repeated many times—quick repeat function—via UM-SAP to increase
the probability of the UE receiving it).
Figure 10.25 also shows the logical structure of an RRC protocol entity. The Dedicated
Control Function Entity (DCFE) is used to handle all signalling specific to one
UE. In the SRNC there is one DCFE entity for each UE having an RRC connection
with this RNC. The Paging and Notification Function Entity (PNFE) handles paging
messages sent to idle-mode UEs. In a CRNC there is at least one PNFE entity for each
cell to be controlled. The Broadcast Control Function Entity (BCFE) handles system
information broadcasting on BCCH and FACH logical channels. There is at least one
BCFE entity for each cell in a CRNC. Besides these functional entities a special
Routing Function Entity (RFE) is also modelled on top of Figure 10.25. Its task is
to route non-access stratum messages to different MM/CM entities at the UE end and
to different CN domains at the RNC end.
As key executors of radio resource allocation, the RRC entities in the UE and RNC
have control over the L1, MAC and RLC entities at their end. In order to execute
control commands on these lower layer protocol entities, special control SAPs are
available to the RRC protocol entity as shown in Figure 10.25. These control SAPs
are also used for reporting measurements and exceptional conditions detected by the
lower layers.
The major function of the RRC protocol is to control the radio bearers, transport
channels and physical channels. This is done by set-up, reconfiguration and release of
different kinds of radio bearers. Before such actions can take place, RRC protocol
communication must itself be initiated by using a number of (minimum four) signalling
radio bearers. The resulting signalling connection, together with any other subsequently
established bearers, is called RRC connection. During RRC connection, set-up,