Throughput Management for CSMA/CA Networks : IEEE 802.11e Wireless LAN.

Abstract

This thesis investigates the design and development considerations to introduce through- put differentiation and management capabilities into the Medium Access Control (MAC) layer of IEEE 802.11 Wireless Local Area Network (WLAN) systems, while also maximizing overall throughput performance. The final control mechanism highlighted in this thesis requires only an initial user configuration to specify the required throughput differentiation and management rules prior to it operating in a fully autonomous manner. In order to maximize throughput performance the control mechanism is designed to seamlessly adjust the throughput differentiation and management rules to reflect the current network traffic load. Throughout this thesis, we discovered and identified the great difficultly that is inherent in trying to create a control mechanism that is capable of operating autonomously in a broad range of possible traffic load conditions. The difficulties faced stemmed primarily from the design philosophy of a fully self-contained control mechanism within the MAC layer of the AP alone, and employing a completely passive/non-intrusive decision making procedure. Furthermore the idiosyncrasies of TCP-based traffic required special attention which could have otherwise been avoided if cross layer interaction was permitted. The IEEE 802.11e MAC layer standard [1] is chosen to be the foundation for providing throughput management capabilities in a WLAN. In particular, the throughput management capability is provided using the IEEE 802.11e Enhanced Distributed Channel Access (EDCA). The EDCA mechanism supports service differentiation across 4 different Access Categories (AC). Each AC has specific tunable parameters associated to it, which affects the level of probabilistic medium access it obtains against other ACs. The implementation and control objective of tuning parameters within EDCA are left open in the IEEE 802.11e standard [1]. We describe the process of selecting appropriate settings for the tunable parameters associated with each AC such that a specified throughput proportion allocation can be achieved between each AC. The selection process is aided by an analytical model of IEEE 802.11e EDCA under saturation load conditions [2]. The model allows us to identify, when under saturation load conditions, the parameter combinations that achieves a required throughput proportion allocation amongst ACs and at the same time maximizes overall throughput performance. Based on this information, we define Control Scheme-1 that resides within the access point (AP) and, as required, notifies all stations in the WLAN what required parameters should be associated with each AC. Through this process, regardless of dynamically changing active station counts transmitting a particular AC traffic, we demonstrate the control scheme's ability to maintain a required throughput proportion allocation regime between ACs. We then proceed to specify throughput proportion allocations between Downlink (DL) and Uplink (UL) paths in a WLAN under saturation load. EDCA allows for differentiating the AP and wireless client stations through the use of independent tunable parameters specifically associated to the AP. We make use of this feature directly by describing an updated parameter selection process. Therefore based on the new selection of parameters, we describe the new Control Scheme-2 and demonstrate its ability to manage the WLAN. We then investigate a new objective of developing a control that can also operate in non-saturation load conditions. In doing this, we still aim to maintain, wherever possible, the requirements of managing the throughput proportion allocation between the DL and UL and its respective ACs, and all the while focussing on maximizing throughput performance. In order to achieve this goal, we divided the modification of Control Scheme-2 into two parts. The first being modifications required to handle non-saturation load conditions in DL and subsequently modifications required to handle non-saturation load conditions in UL. With both DL and UL modifications in place we verify that the completed control mechanism Control Scheme-3 is able to achieve the required performance results across a wide range of test case scenarios. In addition, we compare these performance results to that obtained when employing the standard reference implementation of IEEE 802.11e EDCA [1].