Fayez Gebali

Performance Analysis of the IEEE 802.11 DCF

This paper presents a Markov chain analysis for studying the performance of the IEEE 802.11 Distributed Coordination Function. The model supports an arbitrary back-off strategy. We found that the most important parameter affecting the performance of binary exponential back-off is the initial back-off window size. Our experimental results show that the probability of collision can be reduced when the initial back-off window size equals the number of stations. Thus, the throughput of the system increases and, at the same time, the delay to transmit the frame is reduced.

DSDMAC: Dual Sensing Directional MAC Protocol for Ad Hoc Networks with Directional Antennas

Applying directional antennas in wireless ad hoc networks can theoretically achieve higher spatial multiplexing gain and, thus, higher network throughput. However, in practice, deafness, hidden-terminal, and exposed terminal problems are exaggerated with directional antennas, and they cause the degradation of the overall network performance. Although there are several random-access-based medium-access control (MAC) protocols being proposed in the literature for networks with directional antennas, the deafness, hidden-terminal, and exposed terminal problems have yet to be fully solved. In this paper, we present a new MAC protocol called the dual-sensing directional MAC (DSDMAC) protocol for wireless ad hoc networks with directional antennas. Different from existing protocols, the DSDMAC protocol relies on the dual-sensing strategy to identify deafness, resolve the hidden-terminal problem, and avoid unnecessary blocking. The integrity of the DSDMAC protocol is verified and validated using , which is a formal protocol verification and validation tool. We further develop an analytical framework to quantify the performance of the DSDMAC protocol and conduct extensive simulations, which verify the accuracy of the analysis. The protocol verification, analysis, and simulation results show the robustness and superior performance of the DSDMAC protocol, which can achieve a much higher network throughput and lower delay utilizing the spatial multiplexing gain of the directional antennas. The results presented in this paper show that the proposed DSDMAC protocol can substantially outperform the state-of-the-art protocols.

Power optimization for application-specific networks-on-chips: A topology-based approach

This paper analyzes the main sources of power consumption in Networks-on-Chip (NoC)-based systems. Analytical power models of global interconnection links are studied at different levels of abstraction. Additionally, power measurement experiments are performed for different types of routers. Based on this study, we propose a new topology-based methodology to optimize the power consumption of complex NoC-based systems at early design phases. The efficiency of the proposed methodology is verified through a case study of an MPEG4 video application. Experimental results show a promising improvement in power consumption (8.55%), average number of hops (10.80%), and number of global links (56.25%) compared to the best known related work.

A fast string search algorithm for deep packet classification

In this paper, we propose a string search algorithm that requires reduced time complexity. It also requires a small amount of memory, and shows better performance than any other algorithm for deep packet classification based on their payload data. The proposed algorithm is based on Boyer-Moore algorithm but requires a much reduced number of operations. In addition, our algorithms memory requirement is lower than Boyer-Moore algorithm without sacrificing its speed. We have done time complexity analysis and verified its time complexities through extensive numerical simulations. These simulations show that our algorithms performance is better for long text, long pattern, and large alphabet set and even its worst case time complexity linearly depends on the length of the text.

Networks-on-Chips: Theory and Practice

This book addresses many challenging topics related to the NoC research area. It starts by studying 3D NoC architectures and progresses to a discussion of NoC resource allocation, processor traffic modeling, and formal verification. NoC protocols are examined at different layers of abstraction. Also, several emerging research issues in NoC are highlighted in this book, such as NoC Quality of Service (QoS), testing and verification methodologies, NoC security requirements, and real-time monitoring. The book also tackles power and energy issues in NoC-based designs, as power constraints are currently considered among the bottlenecks that limit embedding more processing elements on a single chip.

Analytical evaluation of blocking probability in optical burst switching networks

In this paper we present a new analytical model for evaluating the blocking probability in Just-Enough-Time-based optical burst switching networks. The proposed analytical model takes into consideration the effects of the burst offset time and the burst length on the blocking probability. We use a (M+1)-state nonhomogenous Markov chain to describe the state of an output link carrying M wavelength channels. In addition, we model each wavelength channel by a 2-state Markov chain. The offset time is drawn from a specified distribution so that wavelength reservation requests, made before a given time, build up to be a workload whose mean value declines with the reservation starting time. Furthermore, we express the blocking probability in terms of first passage time distributions to account for the burst length. To verify its accuracy, the model results are compared with the results of a sophisticated discrete-event simulation model. The model results were found to be in satisfactory agreement with simulation results.

Modeling the Throughput and Delay in Wireless Multihop Ad Hoc Networks

In wireless multihop ad hoc networks, the use of the RTS/CTS mechanism does not completely eliminate the hidden-terminal problem. Considering the hidden-terminal problem adds complexity to the existing analysis for single-hop networks. In this paper, we provide precise and accurate analytical models for quantifying the throughput and delay in wireless multihop ad hoc networks. The proposed analysis is applicable to many wireless MAC protocols and applications. The accuracy of our analytical models are verified by extensive NS-2 simulations. Our analysis reveals how the throughput and delay in wireless multihop ad hoc networks are affected by the hidden-terminals and by the transmission and interference ranges of wireless devices. These results are important for network planning and protocol optimization in wireless multihop ad hoc networks.

Power-aware topology optimization for networks-on-chips

The choice of a network topology for a networks- on-chip based application significantly impacts its power consumption. In this paper, we propose a new methodology to reduce the total power consumption of the global router-to-router links by selecting the optimal network topology. The proposed methodology merges three mapping approaches: network partitioning, standard topology mapping, and long-range insertion. Analytical power models for global links are studied at different levels of abstraction. The proposed methodology is validated through a case study. Experimental results show the power consumption improvement compared to related work.

A Topology-based Design Methodology for Networks-on-Chip Applications

The topological structure of interconnection networks plays an important role in the design of Networks-on-Chip based Systems. With an enormous number of regular and irregular topologies, acquiring the optimum network topology for a specific application is one of the most complex design problems. In this paper, we propose a new design methodology to automatically acquire the most adapted topology for a given application. A Matlab-based tool called OptNoC is developed to generate the interconnection network using the proposed methodology. The granularity of the OptNoC library facilitates a tool to explore ten different topologies for each design. The current version of the tool mainly aims at minimizing the interconnection network power consumption by using an exhaustive search mapping technique. As a proof of concept, a case study is discussed to show how OptNoC can improve the interconnection network power consumption compared to other tools.

Outage Analysis of Practical FSO/RF Hybrid System With Adaptive Combining

Hybrid free-space optical (FSO)/radio-frequency (RF) systems have emerged as a promising solution for high-data-rate wireless transmission. We present and analyze a transmission scheme for the hybrid FSO/RF communication system based on adaptive combining. Specifically, only FSO link is active as long as the instantaneous signal-to-noise ratio (SNR) at the FSO receiver is above a certain threshold level. When it falls below this threshold level, the RF link is activated along with the FSO link and the signals from the two links are combined at the receiver using a dual-branch maximal ratio combiner. Novel analytical expression for the cumulative distribution function (CDF) of the received SNR for the proposed hybrid system is obtained. This CDF expression is used to study the system outage performance. Numerical examples are presented to compare the outage performance of the proposed hybrid FSO/RF system with that of the FSO-only and RF-only systems.