Application of Network Layer Mechanisms for Service Differentiation in 802.11 WLANs: Implementation and Experience Vasilios A. Siris and George Stamatakis Institute of Computer Science, Foundation for Research and Technology - Hellas (FORTH) P.O. Box 1385, GR 711 10 Heraklion, Crete, Greece Tel.: +30 2810 391726, fax: +30 2810 391601, email: {vsiris,gstam}@ics.forth.gr Abstract— In this paper we present the design, implementation, and experience in using network layer mechanisms for service differentiation in 802.11 Wireless LANs (WLANs). In particular, we use the Class-Based Weighted Fair Queuing (CBWFQ) mechanism with weights that are dynamically adjusted based on the achieved throughput of the wireless stations and their physical layer transmission rate. The scheme was implemented in a Linux- based testbed, and experiments show that it can effectively support throughput differentiation, and deal with important WLANs problems, such as unfairness due to location-dependent channel errors, uplink-downlink unfairness, and performance degradation for the whole WLAN when a station transmits at a small rate. The experiments involved best-effort traffic, as well as streaming video and voice-over-IP. Keywords: fairness, bursty errors, multirate I. I NTRODUCTION In the last few years, wireless networks based on IEEE 802.11 have been experiencing a tremendous growth. Their success can be attributed to the utilization of unlicensed spectrum, interoperability through standard implementations, low cost, and simple installation and management. The in- creasing use of wireless networks by demanding applications and users with different requirements has created the need for service differentiation, while efficiently utilizing the shared wireless channel. Supporting service differentiation in wireless networks is hindered by several factors, such as location- dependent channel errors, uplink-downlink unfairness, and performance reduction due to the multirate physical layer. Nevertheless, a number of solutions to these problems have been proposed at various network layers. Few of these solu- tions, however, can be implemented in a feasible and effective way without requiring alternation of the 802.11 MAC layer. In this paper we investigate a dynamic Class-Based Weighted Fair Queuing (CBWFQ) scheme, introduced ini- tially in [1], that supports throughput differentiation, while improving fairness and enhancing the efficient utilization of the wireless channel. The current paper contains a discussion of the fairness issues that motivate the proposed scheme, a discussion of its operation and implementation, and present testbed results involving streaming video and voice-over-IP traffic. The proposed mechanism is based on a definition of fairness inherently different from the fairness objective of the IEEE 802.11 MAC protocol. In particular, fairness is expressed in terms of the channel occupation time, which suits best the multirate physical layer of 802.11. The proposed scheme is comprised of two algorithms that utilize cross-layer information to periodically adjust the CBWFQ weights in a way that collectively deals with significant issues of WLANs, such as location-dependent channel errors, uplink downlink unfairness, and unfairness due to the multirate operation of 802.11. The first algorithm monitors the throughput achieved by each node and adjusts weights in order to compensate nodes that suffered losses due to channel errors, or did not have packets to transmit due to traffic burstiness. The second algorithm incorporates transmission rate information in the weight adjustment procedure, so that nodes occupy the channel for a time duration proportional to their weights. The dynamic CBWFQ scheme is a network layer mecha- nism whose implementation does not require changes to the 802.11 MAC layer. Its deployment can be done either in a control station, that interconnects one or more access points with the wired network, or as a service running on an access router. The former approach enables the proposed scheme to work with legacy 802.11 access points. In this paper we focus on the latter deployment approach, and present experiments il- lustrating its effectiveness in supporting service differentiation in the presence of different traffic types, including best-effort traffic, streaming video, and voice-over-IP. The rest of the paper is organized as follows. In Section II we discuss fairness issues in 802.11. In Section III we describe our service differentiation mechanism. In Section IV we describe the testbed implementation and evaluation scenarios, and analyze the experimental results. In Section V we review related work and in Section VI we present our conclusions and identify related ongoing and future work. II. FAIRNESS IN IEEE 802.11 AND RELATED I SSUES The distributed coordination function (DCF) in IEEE 802.11 is responsible for controlling access to the shared wireless medium. DCF is a random access scheme, based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)