modeled. When coming to the ATM-based transport network, each DCH is mapped onto a separate AAL2 connection, while there is one individual AAL2 connection for RACH and FACH respectively. UE Modeling As explained in the overall simulation model framework in section 5.2, the UEs are not modeled as individual nodes in the simulation model but instead each UE connection is represented by an application session and identified by its session identifier. The UE model includes the application layer, the transport protocol, and the involved UMTS radio access protocols. The UE supports a variety of applications and services in this model, where for each application a certain transport protocol can be specified, e.g. TCP or UDP. The OPNET ATM workstation model used only allows applications running over IP using either TCP or UDP protocol, without any circuit-switched path. Therefore, the OPNET ATM workstation model was extended within this work to add another transport path to enable circuit-switched type of applications like voice telephony. With this extension, the simulation model can perform simulations for various traffic mix scenarios: either pure packet-switched traffic which is mainly based on IP, or pure circuit-switched traffic, or both mixed together. Another important feature about the UE modeling is that the UEs can be generated statistically in the simulations. Each UE can be modeled to be active for a limited duration. Depending on different applications, different statistical distributions can be used to define the arrival process of the UEs and their durations. Moreover, the application traffic for a certain user is generated according to the traffic models (a detailed introduction is given in section 5.8). At 5 Introduction of Simulation Models last, the UMTS radio protocols of a UE, i.e. RLC and MAC as well as the corresponding channels were modeled. Their implementations are explained below. Air Interface Modeling In the Rel99 simulation model, the modeling of the air interface is simplified. Two effects