The UMTS uses a layered protocol architecture at different interface points, each layer performing a set of specific functions. These architectures are usually described in terms of the control plane and user plane protocols. The control plane protocols are concerned with the signaling and control required to establish a connection between a UE and the network, or request specific services or resources from the network. The user plane protocols, on the other hand, specify how the user data is to be transferred across an interface after a connection has been established between the UE and the network.

Figure 6-3 shows the protocol architecture of the UTRAN. It is also the lower-layer protocols on UE. On the UTRAN side, the physical layer, which is responsible for carrying the information bits, is provided by the BTS while the other layers reside in an RNC. The Radio Resource Control (RRC) is a layer 3 protocol in the control plane that interfaces with the radio link control (RLC) sublayer of layer 2 and terminates in the UTRAN. It deals with two types of control and signaling messages-those that are generated at the higher layers and others that are generated in the RRC itself. The first type is nonaccess stratum (NAS) messages, which originate at a UE and terminate in the core network or vice versa. The RRC layer simply passes these NAS messages to the higher layers or to the RLC layer below en route to...