 usual AAL protocol for IP traffic transport. But in the case of the Iu Circuit-Switched interface, AAL2 is a good option for the efficient transport of circuit-switched data. In the framework of this thesis, the lub user plane is the major concern for dimensioning of the UMTS radio access network. A more detailed introduction of the lub interface will be presented in section 3.3.3. 3.3.2 Channels in the UTRAN When a user requests to establish a connection to transport data across the radio access network, the UTRAN need to allocate radio and transport resources for that connection. The resource allocations are handled by setting up channels. In UMTS, there are three layers of channels, as shown in Figure 3.6. Node B Logical Channels Transport Channels Physical Channels Figure 3.6: Channel types in the UTRAN (UMTS Rel99) On the highest layer, the MAC layer provides data transfer services to the RLC layer by means of Logical Channels. A set of Logical Channel types is defined for different kinds of data transfer services as offered by MAC. Each Logical Channel type is defined by what type of information is transferred. It shall be noticed that Logical Channels are not actually channels. They can be interpreted as different logical tasks that the network and the terminal should carry out. Once the data is on the Logical Channel, the physical layer in turn offers information transfer services to the MAC layer via Transport Channels that are described by 3.3 UMTS Release 99 how and with what characteristics data is transferred over the radio interface. The Transport Channels performs actual information transfer between the UE and the access domain. Over the air interface, the Transport Channels are then mapped in the physical layer to different Physical Channels. As can be seen from Figure 3.6, Physical Channels are only present in the air interface whereas Logical Channels and Transport Channels are maintained between the UE and the RNC. More information about the defined Logical Channels, Transport Channels, Physic