e AAL 2 Physical Layer Figure 3.8: lub interface - user plane protocol stack As also shown in Figure 3.8, at the lub user plane AAL2 and ATM protocols are used at the Transport Network Layer to transport MAC frames between Node B and its associated 3 UMTS Terrestrial Radio Access Network (UTRAN) RNC. Thereby, each Frame protocol stream is carried by an AAL2 connection over the ATM across the lub. A detailed introduction of the ATM layer is presented in section 3.3.3.2, and the AAL2 protocol is given in section 3.3.3.3. Higher layer PDUs (e.g. IP packets) as RLC SDUs (Service Data Unit) are segmented into a number of RLC blocks. After segmentation, RLC headers are added to form RLC PDUs. Then the RLC sends these RLC PDUs to the MAC layer over logical channels. In the MAC layer, each RLC PDU is put into a MAC PDU also called Transport Block. Here the MAC is transparent, i.e. without adding MAC headers. The MAC, in turn, performs scheduling and Transport Format selection for each specific transport channel. The Transport Format is defined as a format offered by L1 to the MAC (and vice versa) for the delivery of a Transport Block Set during a Transmission Time Interval (TTI) on a Transport Channel. The Transport Format defines the parameters for a specific transport channel, such as Transport Block Size and Transport Block Set Size, Transmission Time Interval which determines the time period between two consecutive TBS that arrives at the air interface. The feasible TTIs are 10, 20, 40, and 80 ms. Dependent on the transport format selection by the MAC layer, a set of MAC PDUs make a Transport Block Set which is sent at every TTI from the MAC layer down to the physical layer. In other words, the MAC layer groups a number of MAC PDUs from the same Transport Channel into one Transport Block Set and forwards it per TTI to the WCDMA air interface. In the UTRAN lub interface, the Transport Block Sets are handed over to the Frame Protocol layer. Thus, the information contained in the TBS is conveyed by the Frame Protocol to