Papers by Orhan GEMIKONAKLI
IEEE Conference Proceedings, 2016
KSII Transactions on Internet and Information Systems, 2014
Authentication mechanisms coupled with strong encryption techniques are used for network security... more Authentication mechanisms coupled with strong encryption techniques are used for network security purposes; however, given sufficient time, well-equipped intruders are successful for compromising system security. The authentication protocols often fail when they are analysed critically. Formal approaches have emerged to analyse protocol failures. In this study, Communicating Sequential Processes (CSP) which is an abstract language designed especially for the description of communication patterns is employed. Rank functions are also used for verification and analysis which are helpful to establish that some critical information is not available to the intruder. In order to establish this, by assigning a value or rank to each critical information, it is shown that all the critical information that can be generated within the network have a particular characterizing property. This paper presents an application of rank functions approach to an authentication protocol that combines delaying the decryption process with timed authentication while keys are dynamically renewed under pseudo-secure situations. The analysis and verification of authentication properties and results are presented and discussed.
ESASP, Oct 1, 1991
In this paper, we discuss the possibility of transmitting synchronous digital hierarchy (SDH) sig... more In this paper, we discuss the possibility of transmitting synchronous digital hierarchy (SDH) signals through two-link nonlinear satellite channels. Transmitting such high bit rate signals through standard 54 MHz or 36 MHz transponder bandwidth requires the use of high-level modulation schemes. The techniques and technologies needed to make the use of 16-ary QAM and 64-ary QAM transmissions feasible for future satellite communication systems are examined. We show that it is possible to transmit a synchronous transport modulelevel 1 (STM-1) signal through a standard 54 or 36 MHz transponder bandwidth using 16-ary QAM or 64-ary QAM transmission, respectively, for the 6 / 4 GHz band. However, for higher frequency bands, due to high fade margins needed to achieve the high availability and performance for SDH systems, it is not practical to transmit the STM-1 signal through such standard transponder bandwidths.
The performance of trellis coded 16-ary QAM (quadrature amplitude modulation) signaling transmitt... more The performance of trellis coded 16-ary QAM (quadrature amplitude modulation) signaling transmitted through a two-link, nonlinear satellite channel is studied. A 3/4-rate coding and a 4-state, simple convolutional encoder is considered. The bandlimited signal is distorted by a nonlinear amplifier on board the satellite which introduces AM-AM and AM-PM conversions. Additive white Gaussian noise is assumed to be added to the signal on both up and down links. Bit error rates are computed for uncoded 8-ary coherent phase-shift keying (CPSK) and trellis coded 16-ary QAM transmissions as a function of up and down link carrier-to-noise ratios. A raised cosine filter with a roll-off factor of alpha =0.4 equally split between the transmitter and the receiver, giving a spectral efficiency of 2.14 b/s/Hz, is assumed. It is shown that as output backoff is increased, coding gain is also increased and can reach a value of over 3 dB. However, when the best performances of the two signalings are compared, the coding gain is about 1 dB.<<ETX>>
Telecommunication Systems, Dec 16, 2016
This study presents models for management of voice and data traffic and new algorithms, which use... more This study presents models for management of voice and data traffic and new algorithms, which use call admission control as well as buffer management to optimise the performance of single channel systems such as wireless local area networks in the presence of mobile stations. Unlike existing studies, the new approach queues incoming voice packets as well as data packets, and uses a new preemption algorithm in order to keep the response time of voice requests at certain levels while the blocking of data requests is minimised. A new performance metric is introduced to provide uncorrelated handling of integrated services. Queueing related issues such as overall queue capacity, individual capacities for voice and data requests, the probability of blocking, and effects of waiting time on overall quality of service are considered in detail. Analytical models are presented and the results obtained from the analytical models were validated using discrete event simulations.
Exploring gate-limited analytical models for high-performance network storage servers. Journal of... more Exploring gate-limited analytical models for high-performance network storage servers. Journal of Computer and System Sciences, 77 (5).
Full bibliographic details must be given when referring to, or quoting from full items including ... more Full bibliographic details must be given when referring to, or quoting from full items including the author's name, the title of the work, publication details where relevant (place, publisher, date), pagination, and for theses or dissertations the awarding institution, the degree type awarded, and the date of the award.
Future Generation Computer Systems, 2021
The unprecedented growth in the number of mobile nodes, connected devices, and data traffic lead ... more The unprecedented growth in the number of mobile nodes, connected devices, and data traffic lead to the dense deployment of fifth generation (5G) networks. In such networks, small cells or femtocells are becoming quite popular mainly because of their features such as consistent coverage regardless of the location, transmit power adjustment, energy efficiency, and their ability of providing higher data rates compared to traditional cellular networks. Although the number of femtocells in a heterogeneous cellular network environment can be increased with an aim of satisfying high data traffic in nextgeneration 5G networks, higher numbers of small cell deployments may result in unnecessary, and frequent handovers. Therefore, the need for improvement and optimization of femtocell handover paradigms as well as their coexistence with other technologies should be investigated in the era of 5G cellular networks. This study proposes an analytical model which can be used to analyse integrated heterogeneous wireless cellular networks which consist femtocells. Integration of fourth and fifth generation wireless cellular systems is considered for this purpose. The state diagram of the proposed model can be represented by a two dimensional Markov Chain. A new, novel decomposition approach is employed for solving the system for state probabilities and to obtain performance measures such as mean queue length, throughput and response time. The main advantage of the new solution method is its capability of considering interaction of two systems with large queuing capacities unlike the existing matrix based solution approaches such as spectral expansion and matrix geometric methods. The proposed analytical solution approach has been validated using simulation and a system of simultaneous equations.
Mobility is one of the major issues in the performance characterization of wireless communication... more Mobility is one of the major issues in the performance characterization of wireless communication systems. Wireless communication systems encounter failures. These failures can occur because of software, hardware, human error, or a combination of these factors. The probability of an ongoing call being dropped due to a handoff failure and the probability of a new call being blocked due to
The Journal of Supercomputing, Oct 1, 2015
Various types of wireless networks have been developed and deployed including 3G, WLAN, WiMAX, LT... more Various types of wireless networks have been developed and deployed including 3G, WLAN, WiMAX, LTE and LTE Advanced. User connectivity and network performance can be improved using vertical handover techniques which involve switching between available networks in heterogeneous environments. In this respect, recently there has been an increased interest in the integration of cellular and WLAN systems. In this study, the integrated heterogeneous wireless systems are modelled using two-stage open queuing systems. The proposed analytical model builds a framework for acceptable levels of QoS in heterogeneous environments. This paper gives the detailed analysis of the integrated cellular/WLAN systems based on the deployment of guard channels and buffering at base station for highly mobile users in the B Yonal Kirsal
IEEE Communications Magazine, Dec 1, 2015
Full bibliographic details must be given when referring to, or quoting from full items including ... more Full bibliographic details must be given when referring to, or quoting from full items including the author's name, the title of the work, publication details where relevant (place, publisher, date), pagination, and for theses or dissertations the awarding institution, the degree type awarded, and the date of the award.
Lecture Notes in Computer Science, Nov 24, 2016
Building a prototype VANET testbed to explore communication dynamics in highly mobile environments.
JOURNAL OF EDUCATION AND FUTURE, Jul 26, 2021
It is anticipated that there will be differences in the use of the Internet and mobile technologi... more It is anticipated that there will be differences in the use of the Internet and mobile technologies in accessing information amongst the new generations (digital natives) depending on the development level of the country they live in. Globalization further reinforces these differences. In this study, these differences are investigated for three countries having different levels of development: the United Kingdom, Malta, and Turkey. A total of 443 participants are chosen from university universities studying in different departments in 2017-2018 spring term. Variance analysis (ANOVA) has been used in identifying the differences between the study groups' access to information of the three countries. Findings show differences between the three countries in the use of social networks such as Facebook, Instagram, Twitter and Snapchat. In addition, there are differences in the use of mobile technology devices Notebook, Tablet and Smartphones in accessing information. Finally, differences were identified in the use of information sources in accessing information. These differences are in the use of search engines, wiki, social networks, blogs, and digital libraries. No differences were found in terms of digital newspapers. The findings are discussed comparatively with reference to the scholarly work done in the field and recommendations are given.
Byzantine fault-tolerant (BFT) systems are able to maintain the availability and integrity of IoT... more Byzantine fault-tolerant (BFT) systems are able to maintain the availability and integrity of IoT systems, in the presence of failure of individual components, random data corruption or malicious attacks. Fault-tolerant systems in general are essential in assuring continuity of service for mission-critical applications. However, their implementation may be challenging and expensive. In this study, IoT Systems with Byzantine Fault-Tolerance are considered. Analytical models and solutions are presented as well as a detailed analysis for the evaluation of the availability. Byzantine Fault Tolerance is particularly important for blockchain mechanisms, and in turn for IoT, since it can provide a secure, reliable and decentralized infrastructure for IoT devices to communicate and transact with each other. A continuous-time Markov chain is used to model the IoT systems with Byzantine Fault-Tolerance where the breakdown and repair times follow exponential distributions, and the number of the Byzantine nodes in the network follows various distributions. The presented numerical findings demonstrate the relationship between the number of servers in the system, the proportion of honest users, and the overall availability. Based on the model, it can be inferred that the correlation between the scale of the system (servers) and network availability is non-linear.
In this paper, a simple and flexible approximate technique, capable of overcoming the state space... more In this paper, a simple and flexible approximate technique, capable of overcoming the state space explosion problem in computing the performability of large Markov models is presented. To show the capability of the proposed technique, MQL results are computed for parallel processing systems for large numbers of homogeneous processors and presented. In order to show the accuracy of the method, some of these results are validated using existing techniques whenever possible, and results obtained using different approaches are shown to be in good agreement. It is shown that, this approach can handle large state spaces. A case study is presented for 64, 128, 256, and 512-processor parallel processing systems with break-downs and repairs. This method can be further extended to compute the performability of large and complex networks.
UCL eBooks, 2006
Fault-tolerant systems with repair-upon-failure strategy can become expensive in terms of labour ... more Fault-tolerant systems with repair-upon-failure strategy can become expensive in terms of labour and time. Especially for homogeneous multi-server systems, if no control hierarchy exists, postponing non essential repairs can reduce these costs without affecting the availability of the whole system significantly. While postponing these repairs, it is essential to keep the whole system capable to deal with user requests. For this purpose, usually, a threshold value is defined which represents the minimum number of servers the system administrator should keep operative. Performability evaluation of such systems is very important. In this paper, the performance modelling of a homogeneous, multi-server system, serving a stream of arriving jobs is considered. To analyse the effects of various deferred repair strategies, such systems are modelled and solved for exact performability measures for both bounded and unbounded queuing systems.
2015 IEEE/ACM 8th International Conference on Utility and Cloud Computing (UCC), 2015
In recent years, cloud computing becomes a new computing model emerged from the rapid development... more In recent years, cloud computing becomes a new computing model emerged from the rapid development of the internet. Users can reach their resources with high flexibility using the cloud computing systems all over the world. However, such systems are prone to failures. In order to obtain realistic quality of service (QoS) measurements, failure and recovery behaviours of the system should be considered. System's failures and repairs are associated with availability context in QoS measurements. In this paper, performance issues are considered with the availability of the system. Markov Reward Model (MRM) method is used to get QoS measurements. The mean queue length (MQL) results are calculated using the MRM. The results explicitly show that failures and repairs affect the system performance significantly.
Journal of Education and Future, 2021
It is anticipated that there will be differences in the use of the Internet and mobile technologi... more It is anticipated that there will be differences in the use of the Internet and mobile technologies in accessing information amongst the new generations (digital natives) depending on the development level of the country they live in. Globalization further reinforces these differences. In this study, these differences are investigated for three countries having different levels of development: the United Kingdom, Malta, and Turkey. A total of 443 participants are chosen from university universities studying in different departments in 2017-2018 spring term. Variance analysis (ANOVA) has been used in identifying the differences between the study groups’ access to information of the three countries. Findings show differences between the three countries in the use of social networks such as Facebook, Instagram, Twitter and Snapchat. In addition, there are differences in the use of mobile technology devices Notebook, Tablet and Smartphones in accessing information. Finally, differences ...
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Papers by Orhan GEMIKONAKLI