We present an empirical, i.e, measurementbased, characterization of the instantaneous throughput of a station in an 802.11b WLAN as a function of the number of competing stations sharing the access point. Our methodology is applicable to practically any wireless MAC protocol. Our findings show that as the number of stations increases, the overall throughput decreases and its variance increases. Furthermore, the per-station performance depends significantly on the wireless card implementation and does not depend as much on the station's processing capacity.

Rether[1] was originally developed to support guaranteed Quality of Service (QoS) for shared Ethernet LANs. With the growing popularity of wireless LANs, we modified the Rether protocol to provide QoS guarantee on wireless networks. In this paper, we present the design and implementation of the Wireless Rether protocol for 802.11 networks. We also describe our experiences with wireless LAN hardware. Wireless Rether supports QoS for TCP and UDP traffic in both upstream and downstream directions. The protocol can seamlessly inter-operate with any priority-based QoS mechanisms (such as Diserv [6]) on the wired networks that connect the wireless access network to the rest of the Internet. QoS requirements of real-time applications are specified as a simple configurable policy table. Legacy networking applications can benefit from QoS guarantees provided by Wireless Rether without requiring any modifications.

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The need for "information anywhere anytime" has been a driving force for the increasing growth in Web and Internet technology, wireless communication, and portable computing devices. The field of mobile computing is the merger of these advances in computing and communication with the aim of providing seamless and ubiquitous computing environment for mobile users. Mobile computing environments are characterized by severe resource constraints and frequent changes in operating conditions. This has motivated research in many challenging problems which span several areas of computer science, computer engineering and electrical engineering, such as network protocols to support mobility, efficient and adaptive resource management techniques for wireless bandwidth and battery power, predicting mobility patterns, performance modeling and simulation of mobile applications, and supporting mobile real-time multimedia applications.

Reliable single-hop wireless multicast is needed for various applications such as multicast file transfer in a smart classroom. Mobility and wireless channel characteristics such as high bit error rate (BER) and the hidden terminal problem make providing reliability a challenging task. Further, due to the broadcast nature of the wireless medium, feedback packets such as acknowledgements and control packets from several multicast receivers can potentially collide at the multicast source. In this paper

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apport de recherche N 0249-6399Unité de recherche INRIA Rhône-Alpes