We wish to develop a distributed connection admission control (CAC) scheme where stations independently admit or reject flows. Channel utilization threshold serves as a useful metric as an indirect measure of currently admitted load and delivered performance. However, owing to variable protocol capacity, the advisable threshold also is variable, limiting the efficacy of fixed threshold based CAC. If the admission threshold is tuned to reflect the current protocol capacity, better throughput can be obtained while avoiding WLAN overload.
In this work, we develop a feedback based scheme to tune the utilization threshold for admission. We select the performance metric used for feedback, derive the system model from empirical data, and use control theory to design a feedback controller. Further, we present an analytical model and heuristics using which the controller can be adapted to work under diverse operating scenarios. Simulation results of proportional and proportional-integral controllers in OPNET suggest that the feedback based CAC is able to avoid WLAN overload and achieve high throughput despite large changes in protocol capacity.
Admission control as a mechanism for providing QoS requires an accurate description of the requested flow as well as already admitted flows. Since 802.11 WLAN capacity is shared between flows belonging to all stations, admission control requires knowledge of all flows in the WLAN. Further, estimation of the load-dependent WLAN capacity through analytical model requires inputs about channel data rate, payload size and the number of stations. These factors combined point to a centralized admission control whereas for 802.11 DCF it is ideally performed in a distributed manner. The use of measurements from the channel avoids explicit inputs about the state of the channel described above.
BUFFET, a model based measurement-assisted distributed admission control scheme for DCF developed in this project relies on measurements to derive model inputs and predict WLAN saturation, thereby maintaining average delay within acceptable limits. Being measurement based, it adapts to heterogeneous flows too, making it completely autonomous and distributed. Performance analysis and comparison with two other schemes using OPNET simulations suggests that BUFFET is able to ensure average delay under 7$ms$ at a near-optimal throughput.
IEEE 802.11 has become a common standard for Wireless LAN data communication now. Since this is a multiple-access protocol, the protocol employs complex mechanisms for medium access control. Many simulation studies have been performed to analyze the performance of 802.11 MAC, but due to the complexity involved, analytical models are few. Such models are also limited in their coverage of protocol details. Further, due to the nature of wireless medium, a thorough performance study through an analytical model of the protocol becomes necessary before the protocol can be deployed in applications like real-time communication.
Since the throughput and capacity are also not easily deducible from the protocol details, sizing WLANs is typically based on heuristics. With simple validated analytical models, this activity could well become a scientific activity, suitable for automation, rather than an art. The aim of the project is to evaluate the existing analytical models for their applicability in the application context of WLANs, especially for real-time (soft as well as hard real-time) traffic. Based on the evaluation, if needed, we intend to modify existing models or propose new model. We then propose to apply such a suitable model for performance analysis and sizing of wireless LANs.