features can be added to the basic Rel99 simulation model, and the required changes on the protocol structure and the transport network need to be implemented by modifying the corresponding layers in the Rel99 simulation model. 5.3 Simulation Environment In this thesis, the simulation models were developed with the OPNET Modeler (version 10.0) [Opnet03]. OPNET is one of the most widely used network simulators and has been designed to support the modeling and simulation of communication networks and distributed systems. OPNET provides a comprehensive development environment to analyze both behavior and performance of modeled systems by performing discrete event simulations. OPNET has a three-layer modeling hierarchy. The highest layer referred to as the network model allows definition of system topologies. The second layer referred to as the node model allows the definition of node architectures (data flow within a node). The third layer is the process model that specifies the logic or control flow among components in the form of a Finite State Machine (FSM). OPNET provides a set of built-in model libraries providing a variety of communication networks and technologies, e.g. UMTS, ATM or WLAN. In this thesis, the author did not choose the OPNET UMTS module and its library. There are three major reasons. Firstly the OPNET built-in UMTS model is a complex UMTS simulation model including an exhaustive modeling of the radio interface and the core network, which is however not the main focus for the objective of the simulation models in this thesis. Secondly, the OPNET UMTS model only supports packet switched traffic, and does not support circuit switched traffic. But traditional voice traffic is a main traffic type to be considered in the UMTS network. At last, there is no modeling for HSDPA and HSUPA in the built-in OPNET UMTS model family. Therefore, in this thesis the UMTS simulation models were developed separately with own implemented UMTS functions and protocols based on a number of OPNET built-in models, e.g. A