TM, IP, and Ethernet models. The following sections will give a detailed introduction of the developed UMTS simulation models in OPNET. 5.4 Simulation Model of UMTS Rel99 This section introduces the modeling of UMTS Rel99 with ATM-based UTRAN. The implementation of the Rel99 simulation model is according to the simulation model framework presented in section 5.2, taking considerations of the overall requirements on the simulation model which are pointed out in section 5.1. The structure of Rel99 simulation model is shown in Figure 5.2, with the same structure as the simulation model framework presented in Figure 5.1. The required protocol stacks for the 5.4 Simulation Model of UMTS Rel99 UEs, corresponding nodes, and the UTRAN lub interface (user plane) as shown in Figure 5.2 are fully implemented in the simulation model. The protocol stack of UTRAN lub user plane is according to the 3GPP Rel99 specifications, which is introduced in section 3.3.1. For the UEs and the corresponding nodes, the application layer and the corresponding transport layer protocols like TCP or UDP are modeled. In addition, the UEs include the RLC and MAC entities to communicate with the UTRAN. The air interface is modeled between the UE and the Node B, and the core network was between the corresponding node entity and the RNC. In UMTS Rel99, ATM is defined as the transport technology for the UTRAN transport network. Due to this, the UMTS Rel99 simulation model is implemented based on the OPNET ATM workstation and ATM server model (see Appendix A.8). Their node models are shown in Figure 5.3. Both models have the same node structure, except that the ATM workstation model is working as the client and the ATM server model is used as the server. Both the OPNET ATM workstation and server model consist of the application module, the transport protocol modules (TCP, UDP and IP), the AAL and the ATM module, and the ATM physical layer. These modules represent the essential elements for constructing an ATM- based UMTS network. Based on them, the UM