Modeling and simulation of mixed traffic on a prioritized shared medium

Network access systems (NAS) such as digital loop carriers (DLC) are increasingly utilizing a shared medium, such as Hybrid Fiber Coax (HFC) to provide point to multi-point access from the public switched telephone network (PSTN) to the end user (consumer). New services, such as direct access to the packet switched network (PSN, WWW) have been added to DLC equipment in such a way as to provide for a prioritized set of services over a shared medium in an effort to take advantage of otherwise unused bandwidth. With the introduction of such services comes the added complexity of traffic analysis and modeling these network access systems, particularly when considering the variability in different service type traffic characteristics. This work identifies a traffic engineering problem of prioritized circuit switched and packet switched (PSTN/PSN) traffic over the same shared medium as it may relate to "perceived" quality of service (QoS). A brief review of the evolution of models for communications traffic is presented. Models are then proposed and developed for the various traffic types being considered, which include voice, modem, and data. These models become the basis for running simulations using the OPNET Modeler simulation software package, modeling a prioritized shared medium with finite bandwidth under varying source and offered load conditions. Simulation results suggest that at high (but typical) concentration ratios, small numbers of data users could experience QoS degradation when combined with sufficient numbers of modem and/or voice users. Finally, recommendations for potential future work are offered.