Search

Jamming attacks have become prevalent during the last few years facilitated by the open access to the shared wireless medium as well as the increased motivation and easiness to create damage as a result of sophistication of wireless devices, both legitimate and jamming ones. Among the challenges that a wireless network faces while trying to confront the jammer, jammer localization is of utmost importance. This entails estimating the physical location of the jammer. Successful jammer localization can trigger a series of corrective measures to ensure sustainable network operation. However, locating the jammer is a difficult problem. Our primary goal in this paper is to design a simple, lightweight and generic approach for localizing a jamming device through a set of measurable parameters. The key observation guiding our design, is that the Packet Delivery Ratio (PDR) that can be readily measured locally by a device decreases as a receiver moves closer to the jammer. Further, we draw on the gradient-descent principle from optimization theory, and we adapt it to operate on the discrete plane of the network topology so that the jamming device location can be estimated. The very nature of the gradient-descent algorithm allows the distributed execution of our localization scheme. In this paper, we compute and experimentally validate the impact of jammer on the PDR of a link and we show that this impact decreases as the link moves away from the jammer. We further design a distributed, lightweight jammer localization system, which does not require any modifications to the driver/firmware of commercial NICs, while we implement a prototype system to evaluate our scheme on our 802.11 indoor testbed. Finally, we evaluate the performance of our system via extensive simulations in larger scale settings. Its performance in terms of average location estimation error in combination with its simplicity and distributed operations hold great promise.

The unlicensed 60 GHz band brings the promise of multi-gigabit data rates to support new applications such as high definition video over wireless links. Signal propagation in the 60 GHz band significantly differs from that in the traditionally used 2.4 and 5 GHz bands. The propagation and penetration losses in the 60 GHz band are much higher. Furthermore, the signals are often reflected in indoor settings. Previous physical layer studies show that the use of directional antennas can significantly help in coping with these effects. In this paper, we address the problem of neighbor discovery in the 60 GHz band. We account for not only discovery via direct line-of-sight paths, but also via reflected beams. To the best of our knowledge, none of the previous efforts on higher layer protocols for use with directional antennas account for reflections. We consider two approaches for neighbor discovery (a) direct discovery where each node explicitly discovers its neighbors and, (b) gossip-based discovery where nodes exchange information with regards to their already discovered neighbors. We develop analytical models to capture the performance of the two approaches and validate the models via simulations in indoor settings with obstacles that reflect the transmitted signals. As one might expect, the gossip based discovery incurs a lower neighbor-discovery latency than direct discovery. We examine the impact of system parameters such as varying beamwidth and node density. Our study provides insights on the right choice of system parameters for efficient neighbor discovery in the 60 GHz regime.

This paper highlights some of the results obtained while studying Ni20Co18Cr12.5Al0.6Y (NiCoCrAlY) type metallic bond-coat properties of a thermal-barrier coated (TBC), AE-437A Ni base superalloy m ...

In wireless cellular networks, a roaming mobile station (MS) is expected to move from one cell to another. In order to ensure that ongoing calls are not dropped while the owner mobile stations roam among cells, handoff calls may be admitted with a higher priority than newly generated calls. Predictive channel reservation (or pre-reservation) schemes allow the reservation of a channel for an ongoing call in an adjacent cell before its owner MS moves into that cell, so that the call is sustained when the MS moves into the adjacent cell. Pre-reservations are made when the MS is within some distance of the new cell boundary. This distance determines the area in which the MS can make channel reservations. In this paper, we study the effect of the pre-reservation area size on handoff performance in wireless cellular networks. Our studies show that if the reserved channels are strictly mapped to the MSs that made the corresponding reservations, as we increase the pre-reservation area size, the system performance (in terms of the probability that the handoff calls are dropped) improves initially. However, beyond a certain point, the performance begins to degrade due to a large number of false reservations. The optimal pre-reservation area size is closely related to the traffic load of the network and the MSs' mobility pattern (moving speed). We provide an analytical formulation that furthers understanding with regard to the perceived behavior in one-dimensional networks (in which all cells are along a line). With the objective of improving handoff performance and alleviating this sensitivity to the area size, we propose an adaptive channel pre-reservation scheme. Unlike prior pre-reservation methods, the key idea in our scheme is to send a channel pre-reservation request for a possible handoff call to a neighboring cell not only based on the position and orientation of the owner MS, but also as per its speed towards the target cell. The newly proposed scheme uses GPS measurements to determine when channel pre-reservation requests are to be made. Simulation results show that the adaptive channel pre-reservation scheme performs better in all typical scenarios than a previously proposed popular pre-reservation method that does not take mobility into account.

In group communications, we find that current multicast protocols are far from ''one size fits all'': they are typically geared towards and optimized for particular scenarios. As a result, when deployed in different scenarios, their performance and overhead often degrades significantly. A common problem is that most of these protocols incur high overheads with a high density of group members and in high mobility. Our objective is to design a protocol that adapts in response to the dynamics of the network. In particular, our objective is to provide efficient and lightweight multicast data dissemination irrespective of the density of group members and node density. Our work is motivated by two observations. First, broadcasting in some cases is more efficient than multicasting. Second, member and node layout distributions are not necessarily homogeneous. For example, many MANET applications result in a topological clustering of group members that move together. Thus, we develop Fireworks, an adaptive approach for group communications in mobile ad hoc networks. Fireworks is a hybrid 2-tier multicast/broadcast protocol that adapts to maintain performance given the dynamics of the network topology and group density. In a nutshell, our protocol creates pockets of broadcast distribution in areas with many members, while it develops a multicast backbone to interconnect these dense pockets. Fireworks offers packet delivery statistics comparable to that of a pure multicast scheme but with significantly lower overheads.

Application layer multicasting has emerged as an appealing alternative to network layer multicasting in wireline networks. Here, we examine the suitability of application layer multicast in ad hoc networks. To this effect, we propose a flexible receiver-driven overlay multicast protocol which we call Application Layer Multicast Algorithm (ALMA). ALMA constructs an overlay multicast tree in a dynamic, decentralized and incremental way. First, ALMA is receiver-driven: the member nodes find their connections according to their needs. Second, it is flexible, and thus, it can satisfy the performance goals and the needs of a wide range of applications. Third, it is highly adaptive: it reconfigures the tree in response to mobility or congestion. In addition, our protocol has the advantages of an application layer protocol: (a) simplicity of deployment, (b) independence from lower layer protocols, and (c) capability of exploiting features such as reliability and security that may be provided by the lower layers. Through extensive simulations, we show that ALMA performs favorably against the currently best application layer and network layer protocols. In more detail, we find that ALMA performs significantly better than ODMRP, a network layer, for small group sizes. We conclude that the application layer approach and ALMA seem very promising for ad hoc multicasting.

Nodes in a mobile ad hoc network are often vulnerable to failures. The failures could be either due to fading effects, battery drainage, or as a result of compromised nodes that do not participate in network operations. Intermittent node failures can disrupt routing functionalities. As such, it is important to provide redundancy in terms of providing multiple node-disjoint paths from a source to a destination. In line with this objective, we first propose a modified version of the widely studied ad hoc on-demand distance vector routing protocol to facilitate the discovery of multiple node-disjoint paths from a source to a destination. We find that very few of such paths can be found. Furthermore, as distances between sources and destinations increase, bottlenecks inevitably occur and thus, the possibility of finding multiple paths is considerably reduced. We conclude that it is necessary to place what we call reliable nodes (in terms of both being robust to failure and being secure) in the network to support efficient routing operations. We propose a deployment strategy that determines the positions and the trajectories of these reliable nodes such that we can achieve a framework for reliably routing information. We define a notion of a reliable path which is made up of multiple segments, each of which either entirely consists of reliable nodes, or contains a preset number of multiple paths between the end points of the segment. We show that the probability of establishing a reliable path between a random source and destination pair increases tremendously even with a small number of reliable nodes when we use our algorithm to appropriately position these reliable nodes.

In order to facilitate scalability in a mobile ad hoc network (MANET) and to provide connectivity between partitions that might occur in wireless networks as a consequence of mobility, we can envision a 'range extension' network that consists of airborne communication platforms, and satellites. These airborne or satellite nodes will maintain communication links with specific 'gateway' nodes among the mobile ground nodes. To communicate with a node that is geographically distant or belongs to a different network partition, an ad hoc node can relay its data packets through an appropriate mobile gateway and via the range extension network. If we envision that the MANET is divided into different groups and a mobile gateway is deployed for each such group, an objective then, is to determine the trajectory of the mobile gateway to best serve the ad hoc group to which it belongs, in terms of network performance metrics such as throughput and latency. In this paper, this problem of computing the optimal position for a gateway is reduced to a linear optimization problem by means of some simplifying but realistic assumptions. We suggest methods that may be deployed to enable the gateway to follow this optimal trajectory as closely as possible (within the practical constraints imposed by its velocity and maneuverability). Simulation results for various scenarios show a 10-15% improvement in the throughput and in latency (per group containing a gateway) if a gateway has a dynamic trajectory whose locus follows the computed optimal position, as compared to a gateway that is statically placed at a regular position, or to a gateway that has a random trajectory.

The provision of guaranteed services in low earth orbit (LEO) satellite networks is rather complicated due to the mobility of satellites. Users may be handed-off from one spot beam to another and/or from one satellite to another frequently during the lifetime of their calls. Traffic on inter-satellite links (ISLs) incident on a particular satellite change as the user traffic served by the satellite changes due to satellite mobility. The variation of user traffic over the ISLs may cause violation of quality of service (QoS) requirements of ongoing connections. We propose a novel protocol which we call the predictive routing protocol (PRP), which exploits the deterministic nature of the LEO satellite topology to maximize the total number of users served by the system, while maintaining each user's QoS requirements. PRP predicts the traffic load on the ISLs up to a short time in the future by using the deterministic knowledge of the LEO satellite topology and user location information. PRP determines multiple paths for a particular connection that effectively help avoid possible future bottlenecks as predicted by estimated future traffic on the ISLs. The protocol is compared with other non-predictive routing protocols such as IP routing by simulations and it is shown that PRP can deliver deterministic QoS guarantees (such as delay or delay jitter), without over-reserving channel bandwidth. An admission control curve has also been obtained which may be used to limit the number of users in order to ensure that the desired QoS metrics may be guaranteed.

The present study aims at characterizing the rate-dependent non-linear behavior of two polymeric composite material systems, the unidirectional S2-glass/8553-40 and the woven 7781/Fl55 E-glass fabric in the range of 0.0001 to 1/s under monotonic and proportional loading situations. The objective is to develop a viscoplasticity and a rate-dependent failure model. Uniaxial tension tests were conducted on various off-axis coupon specimens to obtain stress/strain curves for various strain rates. A three-parameter constitutive model is proposed to fit the experimental data. A single rate-dependent parameter is shown to be adequate for capturing the effect of strain rate on strength in both the unidirectional and the woven material systems.

The present study was aimed at modeling the high strain rate response of the unidirectional S2Glass/8553-40 polymeric composite material system. High strain rate experiments were conducted on a split Hopkinson pressure bar (SHPB), on balanced symmetric laminates from which the stress, strain and strain rate histories within the laminates were recovered. Strain rates of the order of 1000/s were achieved in the specimen. A 3-D viscoplasticity model was used to describe the high strain rate response of the composite. A computer code was written to incorporate the constitutive equations into the finite element code ABAQUS, which was subsequently used to analyze the dynamic response of the composite specimen in the high strain rate SHPB experiment. Comparison of the analysis result with experimental data indicates that the viscoplasticity model determined using low strain rate testing can still be valid for modeling the high strain rate responses.