Dr. Ejaz Ahmed - Academia.edu (original) (raw)

Papers by Dr. Ejaz Ahmed

The explosive growth in the number of devices connected to the Internet of Things (IoT) and the e... more The explosive growth in the number of devices connected to the Internet of Things (IoT) and the exponential increase in data consumption only reflect how the growth of big data perfectly overlaps with that of IoT. The management of big data in a continuously expanding network gives rise to non-trivial concerns regarding data collection efficiency, data processing, analytics, and security. To address these concerns, researchers have examined the challenges associated with the successful deployment of IoT. Despite the large number of studies on big data, analytics, and IoT, the convergence of these areas creates several opportunities for flourishing big data and analytics for IoT systems. In this paper, we explore the recent advances in big data analytics for IoT systems as well as the key requirements for managing big data and for enabling analytics in an IoT environment. We taxonomized the literature based on important parameters. We identify the opportunities resulting from the convergence of big data, analytics, and IoT as well as discuss the role of big data analytics in IoT applications. Finally, several open challenges are presented as future research directions.

In past few years, advancement in mobile applications and their integration with Cloud computing ... more In past few years, advancement in mobile applications and their integration with Cloud computing services has introduced a new computing paradigm known as Mobile Cloud Computing. Although Cloud services support a wide range of mobile applications, access to these services suffers from several
performance issues such as WAN latency, jitter, and packet
losses. Cloudlet frameworks are proposed to overcome these
performance issues. More specifically, Cloudlets aim to bring the Cloud or a specific part of the Cloud closer to the mobile device by utilizing proximate computing resources to perform compute intensive tasks. This paper presents a comprehensive survey on the state-of-the-art mobile Cloudlet architectures. We also classify the state-of-the-art Cloudlet solutions by presenting a hierarchical taxonomy. Moreover, the areas of Cloudlet applications are also identified and presented. Cloudlet discovery, resource management, data security, mobility, application offloading, and most importantly incentives to deploy a Cloudlet are areas that still needs to be investigated by the research community. The critical aspects of the current Cloudlet frameworks in Mobile Cloud Computing are analyzed to determine the strengths and weaknesses of the frameworks. The similarities and differences of the frameworks based on the important parameters, such as scalability, mobility support, Internet dependency, dynamic configuration, energy savings, and execution cost are also investigated. The requirements for deploying the Cloudlet in a Local Wireless Network are also highlighted and presented. We also discuss open research challenges that Cloudlet deployments face in Local Wireless Networks.

Mobile Edge Computing is an emerging technology that provides cloud and IT services within the cl... more Mobile Edge Computing is an emerging technology that provides cloud and IT services within the close proximity
of mobile subscribers. Traditional telecom network operators
perform traffic control flow (forwarding and filtering of packets),
but in Mobile Edge Computing, cloud servers are also deployed in each base station. Therefore, network operator has a great responsibility in serving mobile subscribers. Mobile Edge Computing platform reduces network latency by enabling
computation and storage capacity at the edge network. It also
enables application developers and content providers to serve
context-aware services (such as collaborative computing) by
using real time radio access network information. Mobile and
Internet of Things devices perform computation offloading for
compute intensive applications, such as image processing, mobile gaming, to leverage the Mobile Edge Computing services. In this paper, some of the promising real time Mobile Edge Computing application scenarios are discussed. Later on, a state-of-the-art research efforts on Mobile Edge Computing domain is presented. The paper also presents taxonomy of Mobile Edge Computing, describing key attributes. Finally, open research challenges in successful deployment of Mobile Edge Computing are identified and discussed.

Recently, Cloud-based Mobile Augmentation (CMA) approaches have gained remarkable ground from aca... more Recently, Cloud-based Mobile Augmentation (CMA)
approaches have gained remarkable ground from academia and industry. CMA is the state-of-the-art mobile augmentation model that employs resource-rich clouds to increase, enhance, and optimize computing capabilities of mobile devices aiming at execution of resource-intensive mobile applications. Augmented mobile devices envision to perform extensive computations and to store big data beyond their intrinsic capabilities with least footprint and vulnerability. Researchers utilize varied cloud-based computing resources (e.g., distant clouds and nearby mobile nodes) to meet various computing requirements of mobile users. However, employing cloud-based computing resources is not a straightforward panacea. Comprehending critical factors (e.g., current state of mobile client and remote resources) that impact on augmentation process and optimum selection of cloud-based
resource types are some challenges that hinder CMA
adaptability. This paper comprehensively surveys the mobile augmentation domain and presents taxonomy of CMA approaches. The objectives of this study is to highlight the effects of remote resources on the quality and reliability of augmentation processes and discuss the challenges and opportunities of employing varied cloud-based resources in augmenting mobile devices. We present augmentation definition, motivation, and taxonomy of augmentation
types, including traditional and cloud-based. We critically
analyze the state-of-the-art CMA approaches and classify them
into four groups of distant fixed, proximate fixed, proximate
mobile, and hybrid to present a taxonomy. Vital decision making and performance limitation factors that influence on the adoption of CMA approaches are introduced and an exemplary decision making flowchart for future CMA approaches are presented. Impacts of CMA approaches on mobile computing is discussed and open challenges are presented as the future research directions.

Accepted In Journal of Network and Computer Applications

Seamless application execution is vital for the usability of various delay-sensitive mobile cloud... more Seamless application execution is vital for the usability of various delay-sensitive mobile cloud applications. However, the resource-intensive migration process and intrinsic limitations of the wireless medium impede the realization of seamless execution in mobile cloud computing (MCC) environment. This work is the first comprehensive survey that studies the state-of-the-art cloud-based mobile application execution frameworks (CMAEFs) in perspective of seamless application execution in MCC and investigation of the frameworks suitability for the seamless execution. The seamless execution enabling approaches for the CMAEFs are identified and classified based on the implementation locations. We also investigate the seamless application execution enabling approaches to identify advantages and disadvantages of employing such approaches for attaining the seamless application execution in MCC. The existing frameworks are compared based on the significant parameters derived from the taxonomy of the seamless application execution enabling approaches. The principles for enabling the seamless application execution within the MCC are also highlighted. Finally, open research challenges in realizing the seamless application execution are discussed.

Cognitive radio is an enabling technology of dynamic spectrum access (DSA) networking. In DSA, un... more Cognitive radio is an enabling technology of dynamic spectrum access (DSA) networking. In DSA, unlicensed secondary users can coexist with primary licensed users and can share the radio spectrum opportunistically. Broadcasting is an important networking primitive that is useful for many CRN applications such as control information dissemination, warning notification, etc. Unfortunately, the sporadic channels availability degrades the performance of broadcast routing. The quality of a broadcast transmission on a particular channel depends on the channel quality of all the receivers for the same transmitter. Current broadcast routing protocols lack transmission quality awareness. In this paper, we develop a transmission quality-aware broadcasting framework, comprising algorithm for transmission quality-aware broadcast routing in multi-radio dynamic-spectrum-access CRNs, and formulate a transmission quality metric wherein we consider a receiver-centric view rather than a transmission-centric view. We perform a detailed simulation performance evaluation of our proposed framework using OMNeT++. The proposed broadcast routing algorithm is validated by comparing results with state-of-the-art routing algorithms. Analysis of the results shows average performance gains of approximately 40 percent in throughput and packet delivery ratio.

Mobile cloud computing (MCC) enables the development of computational intensive mobile applicatio... more Mobile cloud computing (MCC) enables the development of computational intensive mobile applications by leveraging the application processing services of computational clouds. Contemporary distributed application processing frameworks use runtime partitioning of elastic applications in which additional computing resources are occurred in runtime application profiling and partitioning. A number of recent
studies have highlighted the different aspects of MCC. Current studies, however, have overlooked into the mechanism of application partitioning for MCC. We consider application partitioning to be an independent aspect of dynamic computational offloading and therefore we review the current status of application partitioning algorithms (APAs) to identify the issues and challenges. To the best of our knowledge, this paper is the first to propose a thematic taxonomy for APAs in MCC. The APAs are reviewed comprehensively to qualitatively analyze the implications and critical aspects. Furthermore,
the APAs are analyzed based on partitioning granularity, partitioning objective, partitioning model, programming language support, presence of a profiler, allocation decision, analysis technique, and annotation. This paper also highlights the issues and challenges in partitioning of elastic application to
assist in selecting appropriate research domains and exploring lightweight techniques of distributed application processing in MCC.

IEEE Communication Magazine

The emergence of Software Defined Networks (SDNs) promises to dramatically simplify network manag... more The emergence of Software Defined Networks (SDNs) promises to dramatically simplify network management and enable innovation through network programmability. Despite all the hype surrounded by the SDNs, exploiting its full potential is demanding. Security is still being the key concern and is an equally striking challenge that reduces the growth of SDNs. Moreover, the deployment of novel entities and the introduction of several architectural components of SDNs pose new security threats and vulnerabilities. Besides, the landscape of digital threats and cyber-attacks is evolving tremendously while considering SDNs as a potential target to have even more devastating effects than using simple networks. Security is not considered as part of the initial SDN design; therefore, it must be raised on the agenda. The paper discusses the state-of-the-art security solutions proposed to secure SDNs. We classify the security solutions in the literature by presenting a thematic taxonomy based on SDN layers/interfaces, security measures, simulation environments, and security objectives. Moreover, the paper points out the possible attacks and threat vectors targeting different layers/interfaces of the SDNs. The potential requirements and their key enablers for securing SDNs are also identified and presented. Besides, the paper gives great guidance for secure and dependable SDNs. Finally, we discuss open issues and challenges of SDN security that may deem appropriate to be tackled by researchers and professionals in the future.

(Accepted in IEEE Communication Surveys and Tutorials IF 6.4)

The cognitive radio is an emerging technology that enables dynamic spectrum access in wireless ne... more The cognitive radio is an emerging technology that enables dynamic spectrum access in wireless networks. The cognitive radio is capable of opportunistically using the available portions of a licensed spectrum to improve the application performance for unlicensed users. The opportunistic use of the available channels in the wireless environment requires dynamic channel assignment to efficiently utilize the available resources while minimizing the interference in the network. A challenging aspect of such algorithms is the incorporation of the channels’ diverse characteristics, highly dynamic network conditions with
respect to primary users’ activity, and different fragmented
sizes of the available channels. This paper presents
a comprehensive survey on the state-of-the-art channel
assignment algorithms in cognitive radio networks. We
also classify the algorithms by presenting a thematic
taxonomy of the current channel assignment algorithms
in cognitive radio networks. Moreover, the critical aspects
of the current channel assignment algorithms in cognitive
radio networks are analyzed to determine the strengths
and weaknesses of such algorithms. The similarities and
differences of the algorithms based on the important
parameters, such as routing dependencies, channel models,
assignment methods, execution model, and optimization
objectives, are also investigated. We also discuss open
research issues and challenges of channel assignment in
the cognitive radio networks. (Accepted in IEEE Communication Surveys and Tutorials IF 6.4)

Mobile Cloud Computing alleviates the limitations of resource-constrained mobile devices by lever... more Mobile Cloud Computing alleviates the limitations of resource-constrained mobile devices by leveraging the cloud resources. Currently, software-level solutions, also known as computational offloading, migrate the cloud-based mobile applications at runtime to the cloud datacenter to optimize the application execution time. However, the application execution frameworks mainly focus on migrating the application without considering the various critical network-centric parameters, such as traffic load and mobility speed, in application migration decision. In this paper, we analyze the effect of network-centric parameters on the application migration process. The performance of the migration process is analyzed by simulating the migration process in OMNeT++. The effects of various parameters, such as number of users in a WLAN, size of a file containing the application and its running states, traffic load on the wireless access point, message length, number of hops to the cloud, and mobility speed, are studied on the application performance metrics such as application migration time and packet drop ratio. Our analysis shows that the application and its running states migration time is affected by the changes in the network conditions. Based on our research findings, we recommend application execution framework designers to incorporate the network-centric parameters along with other parameters in the decision process of the application migration. Network-centric performance analysis of runtime application migration in mobile cloud computing, Simulation Modeling Practice and Theory, in press, 2014.

Mobile Cloud Computing enables execution of compute-intensive mobile applications by leveraging t... more Mobile Cloud Computing enables execution of compute-intensive mobile applications by leveraging the services of the cloud. In spite of enabling compute-intensive mobile application execution on resources constrained Smart Mobile Devices, the execution in MCC is not able to carry out without disruption; which degrades the end user experience, hence, deteriorates the usability of emerging applications. The state-of-the-art solutions focus only on the mobile application execution in MCC but do not incorporates all diverse features of the heterogeneous environment to make the execution seamless for the end user. Seamless refers to uninterrupted application execution in MCC with minimal user involvement and interaction to deliver enhanced performance, compared to the local application execution. In this paper, we propose Multi-Objective Optimization model for seamless execution of distributed application in mobile cloud computing that optimizes the execution time subject to execution cost and energy consumption. The problem is formulated as nonlinear integer programming.

In Cognitive Radio Networks, the application throughput is not only affected by primary user acti... more In Cognitive Radio Networks, the application throughput is not only affected by primary user activity but also by numerous environment factors such as interference. Therefore, channel assignment for cognitive radio networks should not only consider channel idle time but also an error rate perceived on the channel. The spectrum-aware channel assignment is vital to efficiently utilize the network resources. In this paper, we propose Spectrum-aware Channel Assignment (SaCA) algorithm for multi-radio, multi-channel cognitive radio networks. We have simulated our proposed algorithm in OMNeT++, an open source discrete event simulator, and compare its performance with the spectrum-unaware channel assignment (SuCA) algorithm. The performance of channel assignment is evaluated for packet delivery ratio and number of channel switches by varying the number of primary users, number of channels and primary user activity ratio. The performance of SaCA is better for large number of channels, primary users and higher primary user activity ratio in the network. In comparison with SaCA, average packet delivery ratio more sharply decreases with increase in number of primary users for SuCA. . His academic qualifications were obtained from UK's universities bachelor and master degrees from the University of Hull, and Ph.D from the University of Sheffield. He has vast teaching experience due to having worked in various educational institutions locally and abroad -schools, teaching college, ministry of education, and universities. His interest in research started in 1983 when he was chosen to attend Scientific Research course in RECSAM by the Ministry of Education, Malaysia. More than 100 academic papers have been published in conferences and respectable journals. He actively supervises many students at all level of study -Bachelor, Master and PhD. His interest of research includes self-organized system, reinforcement learning and wireless-related networks. He is now working on mobile cloud computing with High Impact Research Grant of 1.5 M for the period of 2011-2016.

The latest developments in mobile computing technology have increased the computing capabilities ... more The latest developments in mobile computing technology have increased the computing capabilities of smartphones in terms of storage capacity, features support such as multimodal connectivity, and support for customized user applications. Mobile devices are, however, still intrinsically limited by low bandwidth, computing power, and battery lifetime. Therefore, the computing power of computational clouds is tapped on demand basis for mitigating resources limitations in mobile devices. Mobile cloud computing (MCC) is believed to be able to leverage cloud application processing services for alleviating the computing limitations of smartphones. In MCC, application offloading is implemented as a significant software level solution for sharing the application processing load of smartphones. The challenging aspect of application offloading frameworks is the resources intensive mechanism of runtime profiling and partitioning of elastic mobile applications, which involves additional computing resources utilization on Smart Mobile Devices (SMDs). This paper investigates the overhead of runtime application partitioning on SMD by analyzing additional resources utilization on SMD in the mechanism of runtime application profiling and partitioning. We evaluate the mechanism of runtime application partitioning on SMDs in the SmartSim simulation environment and validate the overhead of runtime application profiling by running prototype application in the real mobile computing environment. Empirical results indicate that additional computing resources are utilized in runtime application profiling and partitioning. Hence, lightweight alternatives with optimal distributed deployment and management mechanism are mandatory for accessing application processing services of computational clouds.

The recent efforts in Mobile Cloud Computing (MCC) focus on defining novel methods, policies and ... more The recent efforts in Mobile Cloud Computing (MCC) focus on defining novel methods, policies and mechanisms for efficiently leveraging the services of cloud datacenters for mitigating resources constraints in mobile devices. To test these newly developed methods and policies, researchers need tools for evaluating the hypothesis prior to deployment on the real time system. Simulation based approaches in evaluating the deployment mechanism for accessing distributed services and application behaviors offer significant benefits, as they allow mobile application developers to test performance of the operational logic of distributed deployment in a repeatable and controllable environment free of cost and to tune the performance bottlenecks before real world deployment on commercial MCC environment. This paper presents SmartSim; which is a simulation toolkit developed for modeling the application processing capabilities of Smart Mobile Devices (SMDs). SmartSim simulates the system and behavior modeling of mobile devices. The toolkit provides an easy to configure simulation environment for modeling application processing attributes of SMD and arbitrary resources intensive mobile application. SmartSim models the mechanism of runtime partitioning of elastic mobile application and determines resources utilization on SMDs during the execution of the elastic application.

Recently, Cloud-based Mobile Augmentation (CMA) approaches have gained remarkable ground from aca... more Recently, Cloud-based Mobile Augmentation (CMA) approaches have gained remarkable ground from academia and industry. CMA is the state-of-the-art mobile augmentation model that employs resource-rich clouds to increase, enhance, and optimize computing capabilities of mobile devices aiming at execution of resource-intensive mobile applications. Augmented mobile devices envision to perform extensive computations and to store big data beyond their intrinsic capabilities with least footprint and vulnerability. Researchers utilize varied cloudbased computing resources (e.g., distant clouds and nearby mobile nodes) to meet various computing requirements of mobile users. However, employing cloud-based computing resources is not a straightforward panacea. Comprehending critical factors (e.g., current state of mobile client and remote resources) that impact on augmentation process and optimum selection of cloudbased resource types are some challenges that hinder CMA adaptability. This paper comprehensively surveys the mobile augmentation domain and presents taxonomy of CMA approaches. The objectives of this study is to highlight the effects of remote resources on the quality and reliability of augmentation processes and discuss the challenges and opportunities of employing varied cloud-based resources in augmenting mobile devices. We present augmentation definition, motivation, and taxonomy of augmentation types, including traditional and cloud-based. We critically analyze the state-of-the-art CMA approaches and classify them into four groups of distant fixed, proximate fixed, proximate mobile, and hybrid to present a taxonomy. Vital decision making and performance limitation factors that influence on the adoption of CMA approaches are introduced and an exemplary decision making flowchart for future CMA approaches are presented. Impacts of CMA approaches on mobile computing is discussed and open challenges are presented as the future research directions.

During the past few years, cognitive radio networks (CRNs) have emerged as a solution for the pro... more During the past few years, cognitive radio networks (CRNs) have emerged as a solution for the problems created due to fixed spectrum allocation such as inefficient usage of licensed spectrum. CRNs aim at solving this problem by exploiting the spectrum holes (the spectrum not being used by primary radio nodes at a particular time) and allocating the spectrum dynamically. In this paper, we address the problem of dynamic channel assignment for cognitive radio users in multi-radio multichannel cognitive radio networks (MRMC-CRNs). We propose an efficient spectrum-aware dynamic channel assignment (SA-DCA) strategy for such networks. SA-DCA utilizes available channels and assigns them to multiple radio interfaces of cognitive radio nodes based on primary radio unoccupancy, minimum interference to primary radio nodes, maximum connectivity and minimum interference between cognitive radio nodes. We perform simulations in NS-2 and compare the performance of SA-DCA with two related strategies. Simulation results show that SA-DCA assigns channels efficiently and results in significantly reduced interference to primary radio nodes and increased packet delivery ratio in MRMC-CRNs.

The rising density of wireless devices, combined with the availability of a plethora of wireless ... more The rising density of wireless devices, combined with the availability of a plethora of wireless web applications, has overcrowded the radio frequency spectrum. Majority of the present wireless radio spectrum is already licensed; however, studies have shown that the licensed spectrum is significantly underutilized. Cognitive Radio Networks (CRNs) are envisioned to utilize the radio spectrum more efficiently. CRNs may be required to handle both unicast and broadcast traffic, which makes the task of channel assignment in CRNs more challenging, as communication mode required for each type of traffic is different. Unicast traffic may suffer due to interference if the same channel is assigned to and used by neighboring nodes; on the other hand for broadcast traffic if a larger set of neighbors share a common channel, any particular node may exploit wireless broadcast advantage to communicate with a maximum number of neighbors in a single transmission. Existing channel assignment schemes either favor unicast or broadcast traffic only. In this paper a Unified Channel Assignment (UCA) algorithm is proposed which assigns channels according to their respective interference and connectivity parameters depending on the proportions of unicast and broadcast traffic in the network.

In recent times, wireless communications has established itself as a popular access technology du... more In recent times, wireless communications has established itself as a popular access technology due to the user preference for the flexibility of untethered communication. The single biggest problem that still impedes broader uptake of wireless technology is scarceness of wireless capacity. The lack of wireless capacity scaling is primarily due to two factors: firstly, wireless interference that limits a wireless channel to only possible transmission at any given time, and secondly, the current radio spectrum management scheme based on licensing frequency spectrum which is known to be very inefficient. Two well-known techniques that address parts of our considered problem space in wireless networks include: 1) cognitive radio networks (CR) or dynamic-spectrum-access (DSA) networks that utilize programmable software defined radios to address the wireless standards interoperability problem, and 2) multi-radiomulti-channel (MRMC) technology, which addresses the wireless scalability problem, in which each node is equipped with multiple radio interfaces (that can tune to any one of the available orthogonal channel) to allow multiple overlapping transmissions. In this work, we aim to investigate the benefits of a hybrid of these approaches: an approach that call C-MRMC technology. In C-MRMC wireless networks, each node is equipped with multiple cognitive radio interfaces. We investigate in our work the potential improvement in performance (which we gauge in metrics such as throughput, packet delivery ratio) gained by such an approach through extensive simulations. Our results demonstrate that having such an approach is viable and can lead to significant performance gains.

Bluetooth devices face a significant delay for Pico-net formation due to relatively slow device d... more Bluetooth devices face a significant delay for Pico-net formation due to relatively slow device discovery (DD) process. Device discovery is a crucial stage before starting useful services in a highly dynamic ultra-wideband (UWB) personal area networks. The channel utilization methods induce significant latencies due to the collision and high signaling load during network hierarchy formulation. The proposed scheme not only provides the Bluetooth (BT) devices fast connection establishment but also provides power optimized communication. The MICA Mote services provide an alternate radio channel free of contention to
achieve channel allocation and device discovery with very low latencies. Our approach is robust, scalable and adaptable. The results show large latency gain as compare to bare-born device discovery process used in Bluetooth.

Device Discovery process in Bluetooth-enabled devices produces significant delay before the start... more Device Discovery process in Bluetooth-enabled devices produces significant delay before the start of useful data communication among the devices. Inefficient Scatter-net formation mechanism adds further delays due to delegation of responsibilities to one of the slave device. This decision needs to be carefully taken because of the residual resource constraints of slave devices. In this paper, Mica Mote has been introduced for exchanging the control information during the Device Discovery and Scatter-net formation process. Further decision making has been modified for selecting suitable candidate for master role allocation from existing set of un-parked and parked devices so as the residual resources; particularly energy level is significantly high for selected master device. Our proposed algorithm provides the fast and reliable Scatter-net formation mechanism with less power consumption.

Accepted In Journal of Network and Computer Applications

IEEE Communication Magazine

(Accepted in IEEE Communication Surveys and Tutorials IF 6.4)

Recently, Cloud-based Mobile Augmentation (CMA) approaches have gained remarkable ground from aca... more Recently, Cloud-based Mobile Augmentation (CMA) approaches have gained remarkable ground from academia and industry. CMA is the state-of-the-art mobile augmentation model that employs resource-rich clouds to increase, enhance, and optimize computing capabilities of mobile devices aiming at execution of resource-intensive mobile applications. Augmented mobile devices envision to perform extensive computations and to store big data beyond their intrinsic capabilities with least footprint and vulnerability. Researchers utilize varied cloudbased computing resources (e.g., distant clouds and nearby mobile nodes) to meet various computing requirements of mobile users. However, employing cloud-based computing resources is not a straightforward panacea. Comprehending critical factors (e.g., current state of mobile client and remote resources) that impact on augmentation process and optimum selection of cloudbased resource types are some challenges that hinder CMA adaptability. This paper comprehensively surveys the mobile augmentation domain and presents taxonomy of CMA approaches. The objectives of this study is to highlight the effects of remote resources on the quality and reliability of augmentation processes and discuss the challenges and opportunities of employing varied cloud-based resources in augmenting mobile devices. We present augmentation definition, motivation, and taxonomy of augmentation types, including traditional and cloud-based. We critically analyze the state-of-the-art CMA approaches and classify them into four groups of distant fixed, proximate fixed, proximate mobile, and hybrid to present a taxonomy. Vital decision making and performance limitation factors that influence on the adoption of CMA approaches are introduced and an exemplary decision making flowchart for future CMA approaches are presented. Impacts of CMA approaches on mobile computing is discussed and open challenges are presented as the future research directions.