Ahmed Khattab

Elastic Rate Limiting for Spatially Biased Wireless Mesh Networks

IEEE 802.11-based mesh networks can yield a throughput distribution among nodes that is spatially biased, with traffic originating from nodes that directly communicate with the gateway obtaining higher throughput than all other upstream traffic. In particular, if single-hop nodes fully utilize the gateways resources, all other nodes communicating with the same gateway will attain very little (if any) throughput. In this paper, we show that it is sufficient to rate limit the single-hop nodes in order to give transmission opportunities to all other nodes. Based on this observation, we develop a new rate limiting scheme for 802.11 mesh networks, which counters the spatial bias effect and does not require, in principle, any control overhead. Our rate control mechanism is based on three key techniques. First, we exploit the systems inherent priority nature and control the throughput of the spatially disadvantaged nodes by only controlling the transmission rate of the spatially advantaged nodes. Namely, the single-hop nodes collectively behave as a proxy controller for multi-hop nodes in order to achieve the desired bandwidth distribution. Second, we devise a rate limiting scheme that enforces a utilization threshold for advantaged single-hop traffic and guarantees a small portion of the gateway resources for the disadvantaged multi-hop traffic. We infer demand for multi-hop flow bandwidth whenever gateway resource usage exceeds this threshold, and subsequently reduce the rates of the spatially advantaged single-hop nodes. Third, since the more bandwidth the spatially disadvantaged nodes attain, the easier they can \emph{signal} their demands, we allow the bandwidth unavailable for the advantaged nodes to be elastic, i.e., the more the disadvantaged flows use the gateway resources, the higher the utilization threshold is. We develop an analytical model to study a system characterized by such priority, dynamic utilization thresholds, and control by proxy. Moreover, we use simulations to evaluate the proposed elastic rate limiting technique.

Mechanistic Characteristics of Asphalt Binder and Asphalt Matrix Modified with Nano-Fibers

The Hot Mix Asphalt (HMA) mainly consists of air voids, coarse aggregate and asphalt matrix (AM), which includes asphalt cement (AC) and fine aggregates. The coarse aggregate is stiffer than the AM and is elastic in nature, whereas, the AM makes the HMA a visco-elastic material. The AM is considerably weaker than the coarse aggregate and highly susceptible to damage due to external loads and environment. This study focuses on the preparation and mechanistic characterization of AC and AM mixtures modified with Carbon Nano-fibers (CNF). The AM mixtures were made using lime-stone aggregates and three AC types, neat, processed and CNF-modified. The AC was modified with varying percentage of CNF by weight of AC. To achieve the highest degree of CNF dispersion in AC, two different dispersion techniques were utilized. First, the CNF were sonicated for a specified time to initially disperse them into a solvent. Then the mixture was mixed with AC using a mechanical mixer at medium to high temperature. The dynamic shear rheometer was utilized to determine complex shear modulus (G*) and creep compliance (J[t]) of AC and AM mixtures for a range of temperatures and loading frequencies. The G*-master curve and J[t] analysis revealed that the AC modified with CNF significantly improves the visco-elastic response of the AC and AM. It is recommended to expand the research to different types of aggregate, aggregate gradation, and types of asphalt in order to identify key parameters that can facilitate the understanding of improvements in HMA performance using nano-fibers.

Mechanical and thermal characterization of carbon nanofiber reinforced low-density polyethylene composites:

The mechanical, thermal, and fracture morphological characterization of low-density polyethylene (LDPE) composites reinforced with vapor-grown carbon nanofibers (VGCNFs) is presented in this article. VGCNF/LDPE composites with different VGCNF weight percentages were prepared by single-screw extrusion followed by injection molding. Scanning electron microscopy showed that VGCNFs were predominantly dispersed uniformly within the LDPE matrix. Differential scanning calorimetry has shown that the VGCNF/LDPE composites crystallized at higher temperatures than pure LDPE polymer. Tensile testing has shown that strength and Young’s modulus of VGCNF/LDPE composites were improved by 15% and 44%, respectively, when the VGCNF loading reached 3 wt%.

Opportunistic Spectrum Access: From Theory to Practice

In this article, an experimental study of the less studied topic of distributed OSA implementation have been presented. Hardware technologies do not provide the cognitive transceiver requirements needed to exploit OSA to its full potential, suboptimal OSA approaches developed to target that low-complexity transceivers can achieve significant performance gains compared to approaches theoretically optimized disregarding the practical system constraints, is demonstrated. Theoretical OSA approaches can exploit the gains available to the individual practical components. A radio transceiver with cognition capabilities and real-time reconfigurability can be used to realize cognitive radio networks.

A Low-Power Parallel Architecture for Finite Galois Field GF(2 m ) Arithmetic Operations for Elliptic Curve Cryptography

In this paper, a parallel, power-efficient and scalable word-based crypto architecture is proposed that performs the operations required for scalar point multiplication including add, multiplication and inversion operations on GF(2) operands. The proposed architecture distinguishes itself from exiting architectures, including our prior architecture, by the fact that its resource usage and power-consumption is based on the input data. Hence, such architecture might be used for various operand sizes without modifying or reconfiguring the underlying hardware. The architecture has also the ability to perform several different operations in parallel when each operation requires a small key size which significantly increases the overall performance and throughput of the system. In the absence of parallel requests, the remaining unused modules will be turned off in order to save power. The experimental results show significant improvement in the timing, throughput and energy performances with a slight overhead in the circuit area.

Redundant Bit Security in RFIDs: Architecture Design and Security Performance Evaluation

This paper presents an analysis of the security performance and evaluation of the hardware architecture of the redundant bit security (RBS) cryptosystem. RBS is a lightweight symmetric encryption algorithm that targets resource-constrained RFID devices. Unlike the existing cryptosystems, RBS simultaneously provides confidentiality, authentication, and integrity of the plaintext by inserting hash-generated redundant bits among the already modified plaintext data. A flexible-length hash algorithm in our optimized hardware architecture allows RBS to support different key sizes which allows flexibility in the security level. Our analysis shows the resilience of RBS against powerful and well-known attacks such as differential attacks and known-plaintext attacks. We compare the performance of the RBS cryptosystem against other distinguished ciphers developed for RFID systems. Simulation results show that RBS results in approximately 100%, 239%, and 153% higher hardware efficiencies while requiring 48%, 56%, and 59% less energy-per-bit compared to H-PRESENT, HB-2, and Grain, respectively. Such results present confirmatory evidence that RBS is a superior solution for providing security in resource-constrained systems such as RFID systems especially when authentication is a priority.

Cure Cycle Effect on High-Temperature Polymer Composite Structures Molded by VARTM

This paper presents an analytical and experimental investigation of cure cycle effect on carbon-fiber reinforced high-temperature polymer composite structures molded by vacuum assisted resin transfer molding (VARTM). The molded composite structure consists of AS4-8 harness carbon-fiber fabrics and a high-temperature polymer (Cycom 5250-4-RTM). Thermal and resin cure analysis is performed to model the cure cycle of the VARTM process. The temperature and cure variations with time are determined by solving the three-dimensional transient energy and species equations within the composite part. Several case studies were investigated by the developed analytical model. The same cases were also experimentally investigated to determine the ultimate tensile strength for each case. This study helps in developing a science based technology for the VARTM process for the understanding of the process behavior and the effect of the cure cycle on the properties of the molded high-temperature polymer composites.

Probabilistic framework for opportunistic spectrum management in cognitive ad hoc networks

Existing distributed opportunistic spectrum management schemes do not consider the inability of todays cognitive transceivers to measure interference at the primary receivers. Consequently, optimizing the constrained cognitive radio network performance based only on the local interference measurements at the cognitive senders does not lead to truly optimal performance due to the existence of hidden (or exposed) primary senders. In this paper, we present a probabilistic framework for opportunistic spectrum management in cognitive ad hoc networks that optimizes the constrained cognitive user goodput while taking the unavoidable inaccuracy of spectrum sensing into account. The proposed framework (i) randomly explores individual spectrum bands as local interference measurements lead to inaccurate spectrum access decisions and (ii) adopts a non-greedy probabilistic spectrum access policy that prevents a single cognitive transmission from monopolizing an available spectral opportunity. In contrast to existing techniques, our approach allows multiple cognitive flows to fairly share the available opportunities without explicit inter-flow coordination. We analytically formulate the cognitive user performance optimization problem as a mixed-integer non-linear programming to derive the optimal parameter values. We use packet-level simulations to show that our approach achieves up to 138% higher goodput with significantly better fairness characteristics compared to greedy approaches.

Development of Virtual Flow Model for Process Design in Vacuum Assisted Resin Infusion Molding Operations

A virtual flow model for process design in vacuum assisted resin infusion operations is developed. It uses a control volume technique based on finite difference method to characterize flow behavior during resin infusion in molding woven fiber composite structures. In order to enhance the visual capability of the developed virtual model, a geometric reconstruction scheme is used to present the resin flow front at fixed time increment. The Graphic Interchange Format (GIF) is used to combine images into a single file to create animation. This model provides capabilities for prediction of flow pattern, pressure distribution inside the mold, and evolved defects. Several case studies were conducted to evaluate the effectiveness of the developed model.Copyright