Overcoming Performance Pitfalls in Rate-Diverse High Speed WLANs Mart´ın Zubeld´ıa * , Andr´ es Ferragut, Fernando Paganini Universidad ORT Uruguay, Montevideo, Uruguay Abstract Recent developments on the IEEE 802.11 family of standards promise significant increases in speed by incorporating multiple enhancements at the physical layer. These high modulation speeds apply to the data portion of the transmitted frames, while headers must remain at lower speeds; this has motivated the use of frame aggregation to increase data payloads in the newer standards. However, this simple method may still utterly fail to deliver the promised speeds, due to a series of cross-layer effects involving the transport and multiple access layers: the downward equalization of throughputs imposed by TCP under physical rate diversity, the excessive impact of the TCP ACK stream, or the unreasonable fraction of access opportunities taken by uplink flows when competing with the more numerous downlink connections. A first contribution of this paper is to demonstrate these impediments and isolate their causes through a series of experiments with the ns3 packet simulator, on the 802.11n and 802.11ac protocol versions. Our analysis leads us to propose a desirable resource allocation for situations of rate-diverse competition, and an ar- chitecture for control at the access-point to achieve it. Our implementation is compatible with the standard, involving a combination of known techniques: packet aggregation, multiple queues with TCP-ACK isolation, and control of the MAC contention window. The main contribution here is to provide a practical, comprehensive solution that imposes the desired efficiency and fairness model addressing all the previously indicated limitations. We demonstrate analyt- ically and through extensive simulation that our method is able to provide significant enhancements in performance under a variety of traffic conditions. Keywords: IEEE 802.11, Rate-diversity, Cross-layer analysis, Performance evaluation 1. Introduction Wireless local area networks (WLANs) based on IEEE 802.11 [1] are present in nearly every network- ing deployment around the world. WLAN hotspots are shared by multiple users at a time through Medium Ac- cess Control (MAC) protocols, and newer versions of the standard have progressively upgraded the available physical channel speeds. For instance, in the IEEE 802.11n version [2], many new enhancements in modulation and transmission techniques (OFDM, MIMO) have been incorporated to allow stations to transmit at rates reaching 600 Mbps. * Corresponding Author Email address: zubeldia@ort.edu.uy Postal address: Cuareim 1451, 11.100 Tel./Fax: (+598) 29021505 / 29081370 In future releases such as IEEE 802.11ac [3], modula- tion speeds are expected to grow up to 7 Gbps, a 10- fold increase. It is clear, however, that these higher data rates are only achievable in the best channel condi- tions, and thus stations are allowed to transmit at lower data rates if necessary to reduce frame transmission er- rors. The net effect of this adaptation is that multiple users with diverse data rates coexist in the same cell. This fact is not taken into account in the medium access control layer, where typically the Distributed Coordi- nation Function (DCF) mechanism is used for channel access. This mechanism provides roughly equal access opportunities to all stations, regardless of their physi- cal rate; as we demonstrate below, this leads to severe inefficiency. Channel access differentiation is allowed in the Enhanced Distributed Channel Access (EDCA) function of the standard, but its intended use is to differ- Preprint submitted to Computer Networks April 25, 2013