d at the Node B as an important transport network QoS metric. It is defined that whenever the measured FP PDU delays exceed a predefined delay threshold, the corresponding FP PDUs are discarded. In addition, the protocol overheads for all these layers are implemented according to the 3GPP specifications. Modeling of the Iub Transport Network The modeling of the lub transport network is based on the OPNET provided AAL and ATM modules. They include the detailed modeling of the AAL and ATM protocols according to the standardizations. The ATM module models establishing the ATM VP or VCs, providing various ATM QoS categories and defining the QoS parameters, ATM switching 5.4 Simulation Model of UMTS Rel99 and routing, cell encapsulation, etc. For the lub user plane, the AAL2 protocol is used. The AAL2 module contains the modeling of segmentation and reassembly, AAL2 multiplexing, etc. The protocol details are introduced in section 3.3.3.2 and 3.3.3.3. In this work, the AAL2 module was extended to map the UMTS transport channels onto the AAL2 connections. For each transport channel, one AAL2 connection was modeled with a unique connection identifier. Moreover, an AAL2 shaping function was implemented which limits the rate of sending the AAL2 packets to the ATM layer according to the given PCR rate on the VP/VC. At the AAL2 layer, a QoS scheme was implemented to prioritize different QoS classes. In the current simulation model, only two QoS classes were considered: real time (RT) and non real time (NRT). For each QoS category, an individual AAL2 buffer was created. The traffic belonging to the same QoS class is sent to the corresponding buffer, depending on the applications and services of the user connections. And between the two AAL2 buffers, a strict priority packet scheduler was implemented: the RT traffic is given higher priority over the NRT traffic. The implemented QoS structure at the AAL2 layer is illustrated in Figure 5.4. Real Time Non Real Time (RT) services (NRT) services Buffer Buffer Layer Priority Schedu