Measurement-Based Approaches for Accurate Simulation of 802.11-based Wireless Networks Anand Kashyap Computer Science Dept. Stony Brook University Stony Brook, NY, USA anand@cs.sunysb.edu Samrat Ganguly NEC Corporation of America New York,NY, USA samrat.ganguly@necam.com Samir R. Das Computer Science Dept. Stony Brook University Stony Brook, NY, USA samir@cs.sunysb.edu ABSTRACT In this work, we address the issue of unrealistic simula- tions of wireless networks using a measurement-based ap- proach. The idea is to use empirical modeling using mea- surement data as a mechanism to model physical layer be- havior. We demonstrate the power of this approach for 802.11-based networks using ns2, a packet-level network sim- ulator. Specifically, we develop two versions of the ns2 sim- ulator that model the wireless physical layer with different levels of fidelity. In both versions, the deferral and reception model are built using measurements. For propagation mod- eling, one version uses direct measurements and the other uses an empirically derived model. In validation experiments with a 12-node mesh testbed, both these versions were found to be reasonably accurate (85 percentile errors within about 10% of the capacity) relative to regular simulations (85 per- centile errors within 50% of capacity). Categories and Subject Descriptors I.6.5 [Simulation and Modeling]: Model Development— Modeling methodologies .; C.2.1 [Computer-Communication Networks]: Network Architecture and Design—Wireless communication General Terms Experimentation, Measurement. Keywords Network simulator, wireless network capacity, wireless inter- ference, 802.11. 1. INTRODUCTION Simulation-based modeling is a useful tool for evaluating performance of network protocols. Simulations served the networking community well for wired networking regime. However, simulations for wireless networks have often been questioned [13, 6], primarily due to the lack of realistic lower layer models. However, the research community has not yet Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. MSWiM’08, October 27–31, 2008, Vancouver, BC, Canada. Copyright 2008 ACM 978-1-60558-235-1/08/10 ...$5.00. practiced serious validation exercises for wireless network simulators barring minor exceptions [14]. Our goal in this work is to revisit the issue of unrealistic simulation mod- els of wireless networks for the lower layers, and address the problem using a new approach that uses measurement-based modeling. Network simulators widely used in wireless networking lit- erature such as ns-2 [4], qualnet [3], opnet [2] etc. implement the network protocol layers in the same fashion as in a real system. The upper layer implementations (such as trans- port and network) are fairly accurate. This is because they are implemented in software in a real system. This makes it easier to model them in the simulation software. This is also true for MAC-layer models as detailed specs and firmware implementations are available to a serious simulation mod- eler. However, the wireless physical layer has been hard to model. While theoretical models do exist, they make as- sumptions on the propagation environment and the interface characteristics and use various model parameters (e.g., path loss exponent) that are hard to instantiate. Also, often such models work at a much finer timescale (at the bit or sym- bol level, e.g.) while popular network simulators operate at a packet-level time scale. Making the timescale finer may cause a serious slowdown of the simulator eliminating the scalability benefit – one possible reason why such attempts have not been seriously pursued yet. On the other hand, research has shown that physical layer modeling can make serious impact on the upper layer protocol performance [19] thereby making realistic modeling all the more important. Our goal in this paper is to propose measurement-based approaches to model the physical layer of protocol stack so that not only popular packet level simulators can still be used, but also the simulation accuracy is vastly improved. The approach is not simulator specific, but we have used ns-2 because of its popularity. Similarly, our work is not MAC/radio specific, but we focus in this paper on 802.11 because of its ubiquity. We identify three components that comprehensively capture the physical layer behavior in an 802.11-based network. They are (i) signal propagation model, (ii) carrier sensing model on the sender side, and (iii) packet reception model on the receiver side. We propose measurement- based approaches to model the above three components. The idea is to use measurements to preserve realism where analytical models are inadequate. We validate the accuracy of the measurement-based ap- proaches vis-a-vis direct experimentation on a 12-node 802.11- based indoor mesh network testbed. Our general conclusion is that the technique is very accurate when measurement