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Showing papers by "Samir R. Das published in 2007"


Proceedings ArticleDOI
18 Jun 2007
TL;DR: A semidefinite program formulation of the optimization problem to obtain a lower bound on overall network interference is developed and empirical evaluations show that the algorithms perform close to the above established lower bound, with the difference diminishing rapidly with increase in number of radios.
Abstract: In this paper, we consider multi-hop wireless mesh networks, where each router node is equipped with multiple radio interfaces and multiple channels are available for communication. We address the problem of assigning channels to communication links in the network with the objective of minimizing overall network interference. Since the number of radios on any node can be less than the number of available channels, the channel assignment must obey the constraint that the number of different channels assigned to the links incident on any node is atmost the number of radio interfaces on that node. The above optimization problem is known to be NP-hard. We design centralized and distributed algorithms for the above channel assignment problem. To evaluate the quality of the solutions obtained by our algorithms, we develop a semidefinite program formulation of our optimization problem to obtain a lower bound on overall network interference. Empirical evaluations on randomly generated network graphs show that our algorithms perform close to the above established lower bound, with the difference diminishing rapidly with increase in number of radios. Also, detailed ns-2 simulation studies demonstrate the performance potential of our channel assignment algorithms in 802.11-based multi-radio mesh networks.

317 citations


Proceedings ArticleDOI
01 May 2007
TL;DR: A novel algorithm for detecting worm-hole attacks in wireless multi-hop networks that uses only connectivity information to look for forbidden substructures in the connectivity graph is proposed, independent of wireless communication models.
Abstract: We propose a novel algorithm for detecting worm-hole attacks in wireless multi-hop networks. The algorithm uses only connectivity information to look for forbidden substructures in the connectivity graph. The proposed approach is completely localized and, unlike many techniques proposed in literature, does not use any special hardware artifact or location information, making the technique universally applicable. The algorithm is independent of wireless communication models. However, knowledge of the model and node distribution helps estimate a parameter used in the algorithm. We present simulation results for three different communication models and two different node distributions, and show that the algorithm is able to detect wormhole attacks with a 100% detection and 0% false alarm probabilities whenever the network is connected with high probability. Even for very low density networks where chances of disconnection is very high, the detection probability remains very high.

259 citations


Proceedings ArticleDOI
13 Jun 2007
TL;DR: The use of directional antennas and beam steering techniques to improve performance of 802.11 links in the context of communication between amoving vehicle and roadside APs are investigated and a framework called MobiSteer is developed that provides practical approaches to perform beam steering.
Abstract: In this work, we investigate the use of directional antennas and beam steering techniques to improve performance of 802.11 links in the context of communication between amoving vehicle and roadside APs. To this end, we develop a framework called MobiSteer that provides practical approaches to perform beam steering. MobiSteer can operate in two modes - cached mode - where it uses prior radiosurvey data collected during "idle" drives, and online mode, where it uses probing. The goal is to select the best AP and beam combination at each point along the drive given the available information, so that the throughput can be maximized. For the cached mode, an optimal algorithm for AP and beam selection is developed that factors in all overheads.We provide extensive experimental results using a commercially available eight element phased-array antenna. In the experiments, we use controlled scenarios with our own APs, in two different multipath environments, as well as in situ scenarios, where we use APs already deployed in an urban region - to demonstrate the performance advantage of using MobiSteer over using an equivalent omni-directional antenna. We show that MobiSteer improves the connectivity duration as well as PHY-layer data rate due to better SNR provisioning. In particular, MobiSteer improves the throughput in the controlled experiments by a factor of 2 - 4. In in situ experiments, it improves the connectivity duration by more than a factor of 2 and average SNR by about 15 dB.

222 citations


Proceedings ArticleDOI
09 Sep 2007
TL;DR: A practical, measurement-based model that captures the effect of interference in 802.11-based wireless LAN or mesh networks, and provides two solution approaches - one based on direct simulation (slow, but accurate) and the other based on analytical methods (faster, but approximate).
Abstract: We present a practical, measurement-based model that captures the effect of interference in 802.11-based wireless LAN or mesh networks. The goal is to model capacity of any given link in the presence of any given number of interferers in a deployed network, carrying any specified amount of offered load. Central to our modeling approach is a MAC-layer model for 802.11 that is fed by PHY-layer models for deferral and packet capture behaviors, which in turn are profiled based on measurements. The target network to be evaluated needs only O(N) measurement steps to gather metrics for individual links that seed the models. We provide two solution approaches - one based on direct simulation (slow, but accurate) and the other based on analytical methods (faster, but approximate). We present elaborate validation results for a 12 node 802.11b mesh network using upto 5 interfering transmissions. We demonstrate, using as comparison points three simpler modeling approaches, that the accuracy of our approach is much better, predicting link capacities with errors within 10% of the base channel datarate for about 90% of the cases.

128 citations


Proceedings ArticleDOI
05 Nov 2007
TL;DR: This paper develops centralized algorithms in a slotted time model to read all the tags using near-optimal number of time slots for slotted scheduled access of RFID tags in a multiple reader environment and significantly outperform colorwave, an existing algorithm for similar problems.
Abstract: Radio frequency identification (RFID) is a technology where a reader device can "sense" the presence of a close-by object by reading a tag device attached to the object. To improve coverage, multiple RFID readers can be deployed in the given region. In this paper, we consider the problem of slotted scheduled access of RFID tags in a multiple reader environment. In particular, we develop centralized algorithms in a slotted time model to read all the tags using near-optimal number of time slots. We consider two scenarios -one wherein the tag distribution in the physical space is unknown, and the other where tag distribution is known or can be estimated a priori. For each of these scenarios, we consider two cases depending on whether a single channel or multiple channels are available. All the above version of the problem are NP-hard. We design approximation algorithms for the single channel and heuristic algorithms for the multiple channel cases. Through extensive simulations, we show that for the single channel case, our heuristics perform close to the approximation algorithms. In general, our simulations show that our algorithms significantly outperform colorwave, an existing algorithm for similar problems.

98 citations


Proceedings ArticleDOI
01 May 2007
TL;DR: A new method for routing using call statistics that uses prior calling patterns to avoid potentially critical links and a modeling approach that considers models of transmission and interference ranges to develop a polynomial-time algorithm to search for feasible routes is developed.
Abstract: We study the problem of supporting VoIP calls in a wireless mesh network. Specifically, we propose solutions for call admission control (CAC) and route selection for VoIP calls. Call admission decisions must evaluate how the capacity of the mesh network is utilized by the existing calls. We address this issue via a measurement-based modeling effort to model mutual interference between wireless links. The modeling approach evaluates whether capacity constraints (or, required QoS metrics) will be satisfied if a new call is admitted with a given route. Evaluations with a 6-node 802.11a testbed demonstrate excellent accuracy of the model and thus also the CAC performance. We address the issue of route selection by also using a modeling approach that considers models of transmission and interference ranges to develop a polynomial-time algorithm to search for feasible routes. This problem takes exponential time for wireless networks without such modeling. In addition to studying feasibility, we study several routing metrics such as shortest feasible path and maximum residual feasible path. Finally, we develop a new method for routing using call statistics that uses prior calling patterns to avoid potentially critical links. We evaluate the performance of these route selection techniques via extensive simulations and demonstrate the superiority of using max residual feasible path over simply shortest feasible path, and routing using call statistics over max residual feasible path.

59 citations


Proceedings ArticleDOI
09 Jul 2007
TL;DR: Simulation results indicate that the proposed MAC protocol can effectively address deafness and directional hidden terminal problem and increase network performance.
Abstract: We address deafness and directional hidden terminal problem that occur when MAC protocols are designed for directional antenna based wireless multi-hop networks. Deafness occurs when the transmitter fails to communicate to its intended receiver, because the receiver's antenna is oriented in a different direction. The directional hidden terminal problem occurs when the transmitter fails to hear a prior RTS/CTS exchange between another pair of nodes and cause collision by initiating a transmission to the receiver of the ongoing communication. Though directional antennas offer better spatial reuse, these problems can have a serious impact on network performance. In this paper, we study various scenarios in which these problems can occur and design a MAC protocol that solves them comprehensively using only a single channel and single radio interface. Current solutions in literature either do not address these issues comprehensively or use more than one radio/channel to solve them. We evaluate our protocol using detailed simulation studies. Simulation results indicate that our protocol can effectively address deafness and directional hidden terminal problem and increase network performance.

15 citations


Proceedings ArticleDOI
22 Apr 2007
TL;DR: Results of experiments with electronical steerable directional antennas mounted on a car communicating with stationary access points show the benefit of using directional antennas in different environments typical for vehicular communications.
Abstract: This paper presents first results of experiments in vehicular-to-roadside communication using directional antennas. With directional antennas on one side, the duration of connection to a fixed access point or a road side communication unit can be extended and on the other side the interference caused to others can be reduced. In this work results of experiments with electronical steerable directional antennas mounted on a car communicating with stationary access points are presented. The measurements show the benefit of using directional antennas in different environments typical for vehicular communications. The duration of potential 802.11b connections have been compared using directional and omnidirectional antenna patterns when driving through suburban environment. This comparison is based on passive scanning for access points in order to validate the approach in realistic scenarios. The results clearly prove a substantial potential improvement when using directional antennas.

7 citations


Proceedings ArticleDOI
18 Jun 2007
TL;DR: A virtual time based MAC protocol is developed to complement the fair rate estimation and scheduling procedures and demonstrate via extensive simulations the benefit of all these approaches for ensuring fairness relative to the base case that uses 802.11 MAC and FIFO scheduling.
Abstract: The goal in this paper is to develop comprehensive protocol support in all layers to provide max-min fairness for multihop flows in a wireless mesh network. Our approach has three parts. First, we estimate the max-min fair rate of all multihop flows in the network using a distributed protocol. This estimation uses the knowledge of the flow contention graph that the network nodes learn by exchanging local information. Second, the nodes enforce this rate by controlling the rate at which a flow is scheduled to the link layer. Third, a back pressure flow control is used to reduce the transmission rate of a flow if it has been exceeding its fair rate. Finally, we argue that the fair rate estimation can at best be approximated in an 802.11 based MAC protocol. Thus, to complement our fair rate estimation and scheduling procedures, we develop a virtual time based MAC protocol. We demonstrate via extensive simulations the benefit of all these approaches for ensuring fairness relative to the base case that uses 802.11 MAC and FIFO scheduling.

5 citations


Journal ArticleDOI
TL;DR: This work states that existing multihop wireless routing protocols are incapable of reacting quickly to transient degradation of link quality rendering them unsuitable for supporting real-time applications such as streaming/interactive voice and video.
Abstract: Existing multihop wireless routing protocols are incapable of reacting quickly to transient degradation of link quality rendering them unsuitable for supporting real-time applications such as streaming/interactive voice and video. This limitation in the sensitivity of the routing protocols to link quality is difficult to overcome due to the following reasons. All link quality metrics used in routing, such as ETX [1, 2] require active probing. Too frequent probing results in high overhead and can interfere with the actual traffic. Further, a time window of 5 to 10 seconds is required to gather enough probe samples to allow reasonable confidence in the statistics and avoid load sensitivity. Additional latency is also incurred in propagating routing updates.

2 citations