title pagepage 2QoS Enhancement in IEEE802.11 Wireless Local Area Networkspage 2page 3page 4page 5page 6page 7MERL – A MITSUBISHI ELECTRIC RESEARCH LABORATORYhttp://www.merl.comQoS Enhancement in IEEE802.11 WirelessLocal Area NetworksDaqing Gu and Jinyun ZhangTR-2003-67 July 2003AbstractIn this article, a distributed medium access scheme called EDCF, which is adopted in an up-coming standard IEEE802.11e to allow prioritized medium access for applications with QoSrequirements, is described and discussed. Its performance is also evaluated via simulations.This work may not be copied or reproduced in whole or in part for any commercial purpose. 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First printing, TR-2003-67, July 2003QoS Enhancement in IEEE802.11 Wireless Local Area Networks Daqing Gu, Jinyun Zhang Mitsubishi Electric Research Laboratories Cambridge, MA 02139 E-mail: {dgu, jzhang}@ merl.com Abstract In this article, a distributed medium access scheme called EDCF, which is adopted in an upcoming standard IEEE802.11e to allow prioritized medium access for applications with QoS requirements, is described and discussed. Its performance is also evaluated via simulations. Introduction IEEE802.11 wireless local area network (WLAN) is a shared-medium communication network that transmits information over wireless links for all IEEE802.11 stations in its transmission range to receive. It is one of the most deployed wireless networks in the world and is high likely to play a major role in multimedia home networks and next-generation wireless communications. The main characteristic of the IEEE 802.11 WLANs is its simplicity, scalability and robustness against failures due to its distributed nature. IEEE802.11 wireless networks can be configured into two different modes: ad hoc and infrastructure modes. In ad hoc mode, all wireless stations within the communication range can communicate directly with each other, whereas in infrastructure mode, an Access Point (AP) is needed to connect all stations to a Distribution System (DS), and each station can communicate with others through the AP. IEEE802.11 standards actually include a family of standards. Among them, the original standard called IEEE802.11 provides the data rates up to 2 Mbps at 2.4 GHz ISM band [1]. Later, IEEE802.11 working group published its enhanced version named IEEE802.11b that extends the data rate up to 11 Mbps at this ISM band [2]. Its high-speed version at 5 GHz UNII band, i.e. IEEE802.11a, was also defined later [3]. IEEE802.11a standard can achieve data rate of up to 54Mbps by using OFDM (Orthogonal Frequency Division Multiplexing) modulation technique at physical layer. Today, IEEE802.11 wireless networks are widely installed at homes, corporate buildings and hot spots. With the applications over 802.11 WLAN increasing, the customers demand more and more new features and functions of IEEE802.11 WLAN. One very important feature is the support of applications with Quality of Service (QoS) in 802.11 wireless networks. So, the support of video, audio, real-time voice over IP and other multimedia applications over 802.11 WLAN with QoS requirements is the key for 802.11 WLAN to be successful in multimedia home networking and future wireless communications. Many researchers have shown much interest in developing new medium access schemes to support QoS [4, 5]. Accordingly, IEEE 802.11 working group is currently working on a new standard called 802.11e to enhance the original 802.11 MAC (Medium Access Control) sublayer to support QoS [6]. The original 802.11 WLAN MAC sublayer employs a DCF (Distributed Coordination Function), which is based on CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance), for medium access, and is best known for its asynchronous best-effort data transfer. In order to support QoS in 802.11 WLAN, the upcoming IEEE802.11e standard adds a new function called HCF (Hybrid Coordination Function) which includes both controlled contention-free and contention-based channel access methods in a single channel access protocol. The HCF uses a contention-based channel access method called the enhanced DCF (EDCF) that operates concurrently with a controlled channel access mechanism that is based on a central pollingmechanism. HCF supports both prioritized and parameterized medium access. This article will briefly review the main features and functions of the upcoming 802.11e standard. The detailed discussion will be focusing on EDCF functions. The comparison between DCF and EDCF are also given. Original 802.11 Medium Access Mechanisms The architecture of IEEE802.11 standard includes the definitions of MAC sublayer and Physical (PHY) Layer. The original 802.11 MAC sublayer has two access mechanisms: DCF (Distributed Coordination Function) and PCF (Point Coordination Function). DCF uses CSMA/CA protocol, and it is best known for asynchronous data transmission (or best-effort service). PCF uses a central-controlled polling method to support synchronous data transmission. Unlike DCF, the implementation of PCF is optional as stated in the standard [1]. DCF is the basic medium access mechanism for both ad hoc and infrastructure modes. In DCF mode, each station checks whether the medium is idle before attempting to transmit. If the medium has been sensed idle for a DIFS (Distributed InterFrame Space) period, which is 50 µs for 802.11b, the transmission can begin immediately. If the medium is determined to be busy, the station shall defer until the end of the current transmission. After deferral, the station will select a random backoff interval and shall decrement the backoff interval counter while the medium is idle. Once the backoff interval has expired, the station begins the transmission. More specifically, the station