 the three modes of operation, transparent, unacknowledged and acknowledged. BMC functions only in the transparent and unacknowledged modes, providing broadcast/multicast scheduling and transmission to the user data. Layer 3 is also contains sub-layers, but the radio resource control (RRC) sub-layer is the one that interacts with layer 2. It handles the control plane signalling between the UE and network in connected mode. It is also responsible for bearer functions like establishment, release, maintenance and reconfiguration in the user plane and of radio resources in the control plane. Functions of the RRC include radio resource management and mobility management, as well as power control, ciphering, routing (of PDUs) and paging. 7.2.2 Protocol Structure for Universal Terrestrial Radio Access Network (UTRAN) The protocol structure for UTRAN is based on the model described above and can be used further to study the protocol structure for different interfaces in detail. As shown in Figure 7.3(b), there are two main layers, the radio network and the transport network, and two planes, the user plane and the control plane. The visible part of the network is the radio layer, while the transport layer elements (or equipments/technology) can vary without making any changes to the radio layer character- istics. Transportation of all user-specific data CS or PS is done through the user plane, while the transport plane is responsible for all signalling activities in the network. The control plane protocol present in the radio network layer is known as the 'application protocol'. It includes protocols such as RANAP, RNSAP and NBAP (see below). The transport network layer has another control plane known as the transport network control plane that is respon- sible for all the signalling within it. The protocol here is known as ALCAP. The transport layer has its own user plane also for the data bearers in the transport layer. RADIO INTERFACE PROTOCOL ARCHITECTURE Radio Network Application Protocol Transport Network Control Pl