Ali Shahrabi

Congestion Avoidance Routing Protocol for QoS-Aware MANETs

With the increase of multimedia traffic over the past few years and traffic differentiation introduced by IEEE 802.11e, nodes with delay-sensitive multimedia traffic tend to be busy for long periods, thus exacerbating the congestion problem in mobile ad hoc networks (MANETs). Although most of the existing routing protocols are based on the shortest path algorithm, some other metrics like load and delay have also been considered in some other research. In this paper, we first expose that the performance of MANETs routing protocols is highly dependent on the type of traffic generated or routed by intermediate nodes. This paper proposes a Type of service aware routing protocol (TSA), an enhancement to AODV, which uses both the ToS and traditional hop count as route selection metrics. TSA avoids congestion by distributing the load over a potentially greater area and therefore improving spatial reuse. Our simulation study reveals that TSA considerably improves the throughput and packet delay of both low and high priority traffic under different network operational conditions.

Performance Evaluation of Pilot-Assisted PAPR Reduction Technique in Optical OFDM Systems

The pilot-assisted peak-to-average power ratio (PAPR) reduction technique proposed for optical orthogonal frequency division multiplexing (OFDM) communication systems is evaluated empirically and theoretically in this letter. The PAPR reduction is achieved by rotating the phase of data symbols with P iterations of randomly generated pilot symbol sequence. The results of our hardware implementation and analysis show a close agreement to that of computer simulations. In comparison with basic OFDM, where no PAPR reduction technique is implemented, experimental PAPR reduction gain of pilot-assisted OFDM at a complementary cumulative distribution function of 10-3 with P = 5 is ~2 dB. This gain is ~0.2 dB less than that of analytical results. The experimental results also show that the pilot-assisted technique does not cause any significant deterioration of the bit error performance.

ANCH: A New Clustering Algorithm for Wireless Sensor Networks

The adaptable and distributed nature of wireless sensor networks has made them popular in a broad range of applications. Clustering is a widely accepted approach for organising nodes in sensor networks to address the network congestion and energy efficiency concerns. In clustering, the number and uniform distribution of the cluster heads are crucial for the effectiveness of an algorithm. In this paper, we propose a new clustering algorithm for wireless sensor networks that reduces the networks energy consumption and significantly prolongs its lifetime. This is achieved by optimising the distribution of cluster heads across the network. The results of our extensive simulation study show considerable reduction in network energy consumption and therefore prolonging network lifetime.

Fuzzy-based probabilistic broadcasting in Mobile Ad Hoc Networks

Broadcasting in Mobile Ad Hoc Networks is a fundamental process for nodes to maintain network connectivity and stability, providing support to management procedures in order to exchange control information and aiding routing protocols to keep up to date records of routes to all possible destinations. The most commonly used broadcast scheme in MANETs is Simple Flooding (SF) due to its simplicity. SF is found to be neither resource efficient nor reliable, especially in dense and highly populated networks. Various adaptive schemes have been proposed to alleviate the effects caused by SF that take into account network conditions and other limitations. Adaptivity is most commonly implemented with the support of hello protocols. It is critical to decide upon the best frequency value to be used by the hello protocol in order to avoid introducing unnecessary control overhead. In this paper, we demonstrate the need for a hello protocol with frequency adaptivity compared to either fixed or plain dynamic frequency implementations as part of an adaptive probabilistic broadcast scheme. Furthermore, we introduce the use of Fuzzy Logic to control the frequency of a hello protocol and thus to achieve optimum results for a novel broadcast algorithm that bases its rebroadcast decision on a probability model. Our simulation results clearly demonstrate the superiority of our proposed scheme when Fuzzy Logic is incorporated by the hello protocol used to implement adaptivity against fixed and dynamic frequency alteration approaches.

A probability-based adaptive scheme for broadcasting in MANETs

The broadcast process is a vital mechanism in Mobile Ad Hoc Networks (MANETs), as it is the key element for maintaining network connectivity and stability by aiding management tools to distribute control packets and routing protocols to keep their routing tables up to date. Simple Flooding (SF) is the simplest broadcast scheme to use in MANETs. It is neither resource efficient nor reliable, especially in high populated and dense networks. The Probability-Based (PB) scheme is one of the schemes proposed to alleviate the SF effects, also known as Broadcast Storm Problem. The algorithm for PB dictates the use of a fixed and pre-determined probability threshold value regardless of network dynamics. In this paper, we propose an Adaptive Probability-Based (ProbA) scheme, which locally makes a decision upon the density volume of the network and adjusts the probability threshold accordingly. ProbA does not rely on exchanging of HELLO packets or using GPS systems. Our extensive simulation study shows that our proposed broadcast scheme considerably outperforms the PB scheme while achieving minimal volumes of energy consumption.

A cross-layer TCP enhancement in QoS-aware mobile ad hoc networks

With the increase of multimedia traffic over the Internet, current network protocols are largely concerned with the QoS requirements of delay-sensitive applications. In Mobile Ad-hoc Networks (MANETs), the majority of protocols developed to date provide QoS mechanisms by assigning high priority to delay-sensitive applications. While todays Internet traffic is still dominated by TCP-based applications, the negative effects of the IEEE 802.11e service differentiation scheme on TCP performance in the presence of high priority traffic have not been adequately addressed in the literature. In this paper we first evaluate the performance of TCP in 802.11e MANETs when competing with high priority VoIP traffic. We then propose a novel TCP-friendly scheme, called IEDCA, to improve IEEE 802.11e EDCA mechanism. Our simulation-based performance study demonstrates that our proposed scheme IEDCA not only improves the performance of TCP significantly, it also facilitates the voice traffic transmission.

Quarc: A Novel Network-On-Chip Architecture

This paper introduces the Quarc NoC, a novel NoC architecture inspired by the Spidergon NoC. The Quarc scheme significantly outperforms the Spidergon NoC through balancing the traffic which is the result of the modifications applied to the topology and the routing elements.The proposed architecture is highly efficient in performing collective communication operations including broadcast and multicast. We present the topology, routing discipline and switch architecture for the Quarc NoC and demonstrate the performance with the results obtained from discrete event simulations.

TCP Enhancement in IEEE 802.11e Wireless Networks

Wireless networks and multimedia applications are two rapidly emerging technological trends. The IEEE 802.11e protocol has been developed to support QoS for such delay-sensitive applications in wireless networks. This protocol has also been extensively studied in the literature over the past few years, and several mechanisms to improve its performance have been proposed concentrating on ways to reinforce QoS guarantees for delay-sensitive applications. While the Internet traffic is still dominated by TCP-based applications, the negative effects of IEEE 802.11e service differentiation scheme on the performance of TCP have not received enough attention. In this paper, we first, as an attempt to highlight these effects, evaluate the performance of TCP in 802.11e WLANs when competing with high priority VoIP traffic. We then evaluate the enhancement achievable by our proposed cross-layer schemes, IEDCA and RE-TCP, which exploit the TCP bidirectional nature to alleviate TCP starvation and consequently improving its performance. Our simulation results show significant improvements in terms of TCP goodput, retransmissions and segment delay (without any negative effect on the performance of delay-sensitive applications).

A Performance Model of Communication in the Quarc NoC

Networks on-chip (NoC) emerged as a promising communication medium for future MPSoC development. To serve this purpose, the NoCs have to be able to efficiently exchange all types of traffic including the collective communications at a reasonable cost. The Quarc NoC is introduced as a NOC which is highly efficient in performing collective communication operations such as broadcast and multicast. This paper presents an introduction to the Quarc scheme and an analytical model to compute the average message latency in the architecture. To validate the model we compare the model latency prediction against the results obtained from discrete-event simulations.

Constant-Width Zones Broadcast Algorithm in Mobile Ad-Hoc Networks

Broadcast operation is perhaps the most fundamental services utilized frequently by other communication mechanisms in Mobile Ad-hoc Networks (MANETs).Supporting efficient broadcast operation is therefore very crucial for such networks. A novel distance-based broadcast algorithm, called Constant-Width Zones (CWZ), is proposed in this paper. CWZ can effectively alleviate the redundant rebroadcast (overlaying) problem by defining a constant upper limit for the width of all rebroadcast zones and, consequently, reducing the number of forwarding hosts. The results of our simulation-based performance study show that the proposed CWZ algorithm is able to rebroadcast packets more effectively in order to achieve higher reachability while reducing the latency especially for heavy host density networks.