Experimental Evaluation of Asymmetric QoS in IEEE 802.11g Wireless Networks Qi Gang (William) Hu School of ICT Southern Alberta Institute of Technology & University of Calgary Calgary, Canada william.hu@sait.ca Carey Williamson Department of Computer Science University of Calgary Calgary, Canada carey@cpsc.ucalgary.ca Abraham O. Fapojuwo Department of Electrical and Computer Engineering University of Calgary Calgary, Canada fapojuwo@ucalgary.ca ABSTRACT Quality of Service (QoS) can be provided in a Wireless Local Area Network (WLAN) using the Enhanced Distributed Channel Access (EDCA) mechanism specified in IEEE 802.11e. However, 802.11e WLANs are not widely deployed and not all WLAN vendors implement the 802.11e mechanisms. In this paper, we propose and evaluate an asymmetric QoS solution, in which QoS support is provided only at the wireless Access Point (AP). We believe that this approach provides a practical solution for many cases where wireless clients may not support EDCA QoS. The feasibility of this solution is studied, using an experimental approach. A QoS testbed is designed and implemented using a centralized wireless controller and lightweight AP. The measurement results show that VLAN-based asymmetric QoS provides effective prioritization and excellent performance for high-priority traffic classes, including Voice over IP (VoIP) and TCP traffic, even during severe congestion conditions. Furthermore, this approach can be easily implemented using minimal equipment. Categories and Subject Descriptors C.2.1 [Computer-Communication Networks]: Network Architecture and Design – Wireless Communication. General Terms Performance Keywords WLAN QoS; 802.11e EDCA; Asymmetric QoS; VoIP over WLAN 1. INTRODUCTION IEEE 802.11 Wireless Local Area Networks (WLANs) have been widely deployed in enterprise and campus networks over the past decade, and have become a fundamental component of the converged network infrastructure in enterprises. Today’s WLAN environments need to support many different types of traffic, such as delay-sensitive voice, video streaming, and mission-critical traffic with specific Quality of Service (QoS) requirements. The provisioning of QoS in WLAN environments is challenging, and becoming increasingly important in many cases. Most of today’s WLANs use conventional IEEE 802.11 protocols, such as the Distributed Coordination Function (DCF). DCF is a contention-based Medium Access Control (MAC) protocol that uses Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA) to arbitrate channel access among the contending wireless stations. With 802.11 DCF, all stations associated with the same Access Point (AP) operate independently and share the channel bandwidth. Therefore, it is difficult to satisfy the low delay and jitter requirements of VoIP traffic over WLANs [1]. In order to support VoIP over WLANs, IEEE proposed Enhanced Distributed Channel Access (EDCA) in 802.11e, which is a QoS extension to legacy 802.11 DCF. With EDCA, high priority traffic has a higher probability of being sent than low priority traffic. In other words, a station with high priority traffic waits less, on average, than a station with low priority traffic. This is accomplished by using a smaller Contention Window (CW), and shorter Arbitration Inter-Frame Space (AIFS) values for higher priority packets. In addition, EDCA provides sustained access to the channel in a contention-free fashion for a period called a Transmit Opportunity (TXOP). A TXOP is a bounded time interval during which a station can send as many frames as it wishes, provided that the duration of the TXOP is not exceeded. If a frame is too large to fit within a single TXOP, it is fragmented into smaller frames. The use of TXOPs ameliorates the problem of low-rate stations monopolizing use of the channel in the legacy 802.11 DCF MAC [8]. 1.1 Problem Statement Many research efforts have studied the problem of supporting Voice over IP (VoIP) on 802.11 WLANs, but few works focus on practical implementations [3]. In a converged network environment, WLANs process many types of traffic. It is important to investigate and evaluate the performance of the actual wireless QoS implementations based on the widely deployed wireless equipment and wireless adapter cards. While the 802.11e EDCA standard has been adopted by major wireless vendors, the EDCA QoS configuration is still not widely supported on wireless adapters, PCs, and laptops. The challenges of provisioning QoS over real wireless LAN include: • Is it possible to implement WLAN QoS on the AP only? (i.e., Asymmetric QoS) • How would the network perform if QoS is only configured on the AP? 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