The Web offers abundant amounts of information anytime, anyplace. Multimedia provision companies bloom and die, seeking to attract a viable share of the data dissemination market. More often than not, failure is attributed to unsuitable content orientation. The present work contributes a novel content specialization scheme for periodic, push-based data streaming services. Its goal is to maximize the ratio of user requests served within their respective deadlines. In this sense, optimality is proven to be achievable only by discarding all requests for certain data items, focusing solely on the streaming of the remaining ones. Thus, the service achieves the content specialization that yields the most clients. The methodology workflow is as follows. By means of mathematical analysis we quantify data compatibility in terms of boosting the system’s service ratio. A data selection algorithm is then proposed, which follows a sort-and-segment approach. Through simulations, the algorithm is shown to surpass related solutions, while coinciding with bruteforce results and analytical expectations.

In environments with locality of client demands, the use of multiple directional antennas at the Broadcast Server has been shown to increase performance. In many cases however, such broadcasting systems fail to exploit the full potential of the multiple antennas as they do not take into account the geographical distribution of clients within the coverage area of the system. This letter proposes an adaptive smart antenna-based wireless push system where the beamwidth of each smart antenna is altered based on the current placement of clients within the system area. Coupled with a modification of the broadcast schedule, the proposed approach significantly increases the performance observed by the system clients.

Despite being a fundamental networking primitive, data broadcasting has so far received little attention in underwater networks. This paper proposes an adaptive push system for data broadcasting in underwater acoustic wireless networks with locality of client demands. It achieves adaptation of its broadcast schedule according to the a-priori unknown needs of the clients, it efficiently combats the problem of high latency of the underwater acoustic wireless environment and it exploits the bandwidth distance relationship of this environment in order to improve performance in topologies with locality of client demands. Simulation results show superior performance of the proposed approach compared to existing systems.

IEEE 802.16 (WiMAX) constitutes one of the most promising broadband access technologies for high-capacity and high-distance wireless access networks, supporting user mobility. The contribution of this paper is twofold. Firstly, a novel mapping scheme for IEEE 802.16 Mobile standard is introduced, applying horizon mapping. Secondly, an efficient adaptive prediction-based scheme is devised, which is able to adjust the downlink sub-frame capacity, accordingly to the traffic load, since the standard allows the downlink-to-uplink subframe ratio to be changeable from 3:1 to 1:1. The novel adaptive horizon burst mapping (AHBM) scheme is evaluated by simulation experiments, which indicate that the proposed scheme operates effectively and efficiently, by reducing the number of unserved users, the number of unserved traffic requests, and the portion of wasted bandwidth.

This paper proposes an adaptive push system for dissemination of data in underwater acoustic wireless networks. Apart from achieving adaptation of its broadcast schedule according to the a-priori unknown needs of the clients, the proposed system also efficiently combats the problem of high latency of the underwater acoustic wireless environment. Simulation results show superior performance of the proposed system in the underwater environment compared to adaptations of existing terrestrial push systems. Moreover, its performance is not affected by increasing the coverage area of the broadcast server.

This paper proposes an adaptive push system that operates efficiently in the high-latency environment of underwater acoustic wireless networks. Using a simple technique, the proposed system efficiently utilizes the scarce bandwidth resource of the underwater acoustic environment. Simulation results show superior performance of the proposed system compared to adaptations of existing terrestrial push systems. Moreover, they show that the efficient utilization of bandwidth, its performance can be improved up to 17%.

Quality of Service (QoS) support has become a key factor in designing Media Access Control (MAC) protocols. This paper introduces a novel scheduling scheme which supports priority based QoS in Wavelength Division Multiplexing (WDM) star networks. The proposed Interval-based Prioritized Orderly Scheduling Strategy (IPOSS) employs a collision-free scheduling approach and handles variable-length data packets. In practice, it is designed to handle real-time traffic, on the basis that each node may generate high- and low-priority packets with high-priority packets being scheduled prior to low-priority ones. Moreover, the proposed scheme differentiates the packets' schedule order, by prioritizing the long-length over the short-length packets. The performance of IPOSS is evaluated under Bernoulli traffic and simulation results indicate that the novel scheme achieves a significantly high throughput-delay performance for real-time traffic, without sacrificing the performance for non-real-time traffic.

The Broadcast Disks method is commonly used to schedule the data transmission in wireless push based networks. The performance of a system that utilizes this method depends upon several parameters. This paper presents the Optimization- Based Procedure (OBP), an analytical approach for estimating the optimal values of these parameters. The validity of the analysis is verified through simulation and comparison with other popular and traditional approaches. The new approach was found to be accurate and dominant in the vast majority of a wide set of test cases. It also provided a thorough insight of the system's stability and behavior in general.

Optical burst switching (OBS) has emerged as a viable switching alternative in backbone optical networks since it can support high data rates with an intermediate granularity compared to wavelength routing and optical packet switching. At the edges of an OBS cloud, packets are assembled to form bursts which enter the network core and are switched on the fly using bandwidth previously reserved by their control packets at each node. A key problem in OBS networks is the assignment of wavelengths to incoming bursts, i.e. the scheduling of bursts. This paper proposes two new techniques which are shown to improve burst scheduling algorithms by lowering their complexity. The first proposed technique is based on a triangular estimator that defines a ldquodrop zonerdquo; bursts that fall into this area are considered to have a very low probability of finding a suitable wavelength and as such, no effort is made to schedule them. According to the second approach, the drop zone is defined dynamically based on the burst drop history. Simulation results show that both approaches yield burst drop rates marginally higher or identical to the LAUC-VF scheduling algorithm while reducing the number of channel or void checks and thus the algorithm complexity and execution time.

Several well-known scheduling schemes like EATS and MSL seem to suffer notable performance degradation under the realistic conditions of non-uniform destinations and non-negligible transceiver tuning latencies that apply in practical WDM star networks. Our EMPS protocol has been shown to noticeably increase the performance of practical networks of this kind. Nonetheless, EMPS tries to balance the offered load by only considering the heads of the message queues maintained by the source nodes; therefore, its performance is degraded, especially for medium to high offered loads and for rather slowly tunable transceivers. In this article we propose the Minimization of Probability and Size of scheduling Latency (MinPSL) protocol, which examines each source node's message queue at a certain depth and reorders messages intelligently on the basis of receiver availability. MinPSL perfectly minimizes the probability of having to schedule two or more messages intended for the same destination simultaneously or at close times. The proposed scheme also minimizes the actual performance penalty incurred, when scheduling of consecutive messages with the same destination cannot be avoided. Extensive simulations are carried out in order to study the performance of MinPSL and compare it to EMPS under various conditions. The simulation results ascertain the fact that MinPSL always brings about a considerable performance improvement.

With the increasing popularity of wireless networks and mobile computing, data broadcasting has emerged as an efficient way of delivering data to mobile clients having a high degree of commonality in their demand patterns. This paper proposes an adaptive push system that operates efficiently in environments characterized by high broadcasting speeds and a-priori unknown client demands for data items. The proposed system adapts to the demand pattern of the client population in order to reflect the overall popularity of each data item. We propose a method for feedback collection by the server so that the client population can enjoy a performance increase in proportion to the broadcasting speed used by the server. Simulation results are presented which reveal satisfactory performance in environments with a-priori unknown client demands and under various high broadcasting speeds.

In this work we present the early design steps of a system for supporting person-centered learning (PCL), focusing on the process of transforming PCL principles into system specifications. Although person centered learning has gained significant attention in higher education, it is not clear how the instructor can be efficiently supported to manage the additional workload emerging from the effort to promote PCL and address individual studentspsila learning traits and preferences. To cope with this issue we argue that specific technology systems can be built for supporting students and instructors in implementing various PCL-related tasks. In our approach we develop a pedagogical model based on common PCL principles, which will be supported by a Web-based system and will be used to help students and teachers in tasks like: identifying learning and teaching preferences, increasing their meta cognitive awareness, defining homogeneous or heterogeneous workgroups and negotiating their preferences.

With the increasing popularity of wireless networks and mobile computing, data broadcasting has emerged as an efficient way of delivering data to mobile clients having a high degree of commonality in their demand patterns. This paper proposes a push system that continuously adapts to the demand pattern of the client population in order to reflect the overall popularity of each data item. The adaptation is accomplished using a simple feedback from the clients. We propose that the simple feedback is sent only from clients whose distance from the server does not incur a significant timing overhead for the acknowledgment of an item. Simulation results are presented which reveal satisfactory performance in highspeed environments with a-priori unknown and dynamic client demands.

Today’s wireless LANs require efficient integration of multimedia and traditional data traffic. Multimedia network applications are time-bounded and have stricter QoS demands. The IEEE 802.11e workgroup is standardizing a new QoS enhanced access scheme for wireless networks. It is based on a mechanism called Enhanced Distributed Channel Access (EDCA). EDCA seems capable of differentiating the traffic, however, it exhibits great overhead that limits the actually available bandwidth and degrades the overall performance. This work proposes an alternative protocol which could be used in place of EDCA. The Priority Oriented Adaptive Polling (POAP) is collision free, it prioritizes the different kinds of traffic, and it is able to provide QoS for all types of multimedia network applications, while efficiently supporting background data traffic. POAP compared to EDCA, provides higher channel utilization, distributes network resources to the mobile stations adapting to their real needs, and generally exhibits superior performance.

We present a new scheduling algorithm, which is a synthesis of all the remarkable features of pretransmission coordination-based protocols for broadcast and select star networks. The new algorithm follows a novel policy for switching, in order to improve the efficiency of the network. It is based on all notable elements of a specific family of protocols and tries to accommodate the different ways of scheduling. The protocol manages to bring some improvement, in terms of channel utilization and network throughput, which is proven by simulation results.

Summary form only given. The communication systems of commercial banks support four classes of traffic: Alarm, BSC, SNA and IP traffic, with each class of traffic having different priority requirements. In this paper, a new approach to the design of multiple access protocols which are capable of handling the above priority classes is introduced. Furthermore, a hybrid multiple access protocol that has been designed according to the proposed approach is presented and evaluated by means of extensive simulation results.

In many data broadcasting applications clients are grouped into several groups, each one located at a different region, with the members of each group having similar demands. This paper proposes a mechanism that exploits locality of demand in order to increase the performance of wireless data dissemination systems. It trades the received energy per bit redundancy at distances smaller than the radius of the service area for an increased bit rate and thus transmission speed for items demanded by clients at such distances. The bit rate for an item transmission is dynamically determined from the distance between the server's antenna to the group of clients that demand this item via a simple feedback from the clients. Additionally, a simple mechanism is introduced that protects performance around the geographical area of interest from degradation caused by clients that are located elsewhere and demand the same information items with clients inside that area

QoS/HyTDMA-HTS is a new collision-free MAC sublayer protocol which provides the well known HyTDMA-HTS protocol with QoS extensions which enable it to offer quality services in demanding environments with critical applications for which time sensitivity is a key issue. QoS/HyTDMA-HTS can operate in bursty traffic networks, successfully regulating QoS parameters of hosts running real-time services even under high traffic loads, while at the same time, the traffic shaping techniques which it employs, prevent other hosts from sending large amounts of traffic and monopolizing the available bandwidth.

We report an innovative solely optical architecture to implement the centralized wavelength conversion module of the CWC (controlled wavelength conversion) protocol [Papadimitriou, G.I. et al., 1999]. The proposed module was designed to distribute the conversion wavelengths to the corresponding converters in an unbiased manner taking into consideration the information carried by the control wavelength. The design of the optically controlled module is based on the use of optical logic circuits. These optical circuits distribute the available wavelengths by eliminating the need of optical to electronic translation of the network's feedback information and thus reducing drastically processing time. Furthermore, due to the all-optical nature of the network hub the reliability of the system is improved.

With the increasing popularity of wireless networks and mobile computing, data broadcasting has emerged as an efficient way of delivering data to mobile clients having a high degree of commonality in their demand patterns. This paper proposes a push system that continuously adapts to the demand pattern of the client population in order to reflect the overall popularity of each data item. The adaptation is accomplished using a simple feedback from the clients. We propose that the simple feedback from the clients can be sent at regular time intervals in order to avoid the overhead that would be an incurred for acknowledging all item broadcasts. Simulation results are presented which reveal satisfactory performance in environments where client demands change over time with the nature of these changes being unknown to the broadcast server

An adaptive MAC protocol for distributed wireless LANs, capable of operating efficiently under bursty traffic conditions, is introduced. According to the proposed protocol, the mobile station that is granted permission to transmit is selected by means of a neural-based algorithm. The neural-based algorithm takes into account the network feedback information in order to update the choice probability of each mobile station. The proposed protocol is compared via simulation to TDMA and IEEE 802.11 and is shown to exhibit superior performance under bursty traffic conditions even when the network feedback is noisy.

A carrier-sense-assisted learning automata-based MAC protocol for wireless LANs, capable of operating efficiently under bursty traffic and unreliable channel feedback, is introduced. According to the proposed protocol, the mobile station that is granted permission to transmit is selected by means of learning automata. At each station, the learning automaton takes into account the network feedback information in order to update the choice probability of each mobile station. The proposed protocol utilizes carrier sensing in order to reduce the collisions that are caused by different decisions at the various mobile stations due to the unreliable channel feedback.

A new class of P-model absorbing learning automata is introduced. The proposed automata are based on the use of a stochastic estimator in order to achieve a rapid and accurate convergence when operating in stationary random environments. According to the proposed stochastic estimator scheme, the estimates of the reward probabilities of actions are not strictly dependent on the environmental responses. The dependence between the stochastic estimates and the deterministic ones is more relaxed for actions that have been selected only a few times. In this way, actions that have been selected only a few times, have the opportunity to be estimated as "optimal," to increase their choice probability and consequently, to be selected. In this way, the estimates become more reliable and consequently, the automaton rapidly and accurately converges to the optimal action. The asymptotic behavior of the proposed scheme is analyzed and it is proved to be epsilon-optimal in every stationary random environment. Furthermore, extensive simulation results are presented that indicate that the proposed stochastic estimator scheme converges faster than the deterministic-estimator-based DP(RI) and DGPA schemes when operating in stationary P-model random environments.

We describe a controlled experiment concerning the use of a learning aid during the instructional procedure. The core issue of investigation is whether this instructional aid can augment the cognitive transfer of the learners by personalizing the offered knowledge. A controlled experiment was performed with the participation of 79 students. The results have shown that for the transfer of simple information this "lesson sheet" does not provide any statistically significant advantage, yet for complex information a statistically significant better performance is observed for the student group that used the tool.

In this letter, a new medium-access sublayer self-adaptive protocol for broadcast networks is introduced. This new protocol is capable of operating efficiently under heavily bursty traffic conditions and can handle stations, which transmit packet of arbitrary length. TDMA, RTDMA and other fixed-assignment protocols have various drawbacks, that learning protocols such as LTDMA are known to resolve. But as a result of LTDMS's fixed-sized timeslots, reduced channel usage, wasted timeslots, as well as network equipment overloading can occur. The VPL/LTDMA protocol, which is here introduced, is tested using real LAN traffic traces and is proved to be successfully resolving the above issues. In addition it achieves a high throughput-delay performance when operating under realistic traffic conditions.

Multimedia offer a variety of coding formats for presenting information and this variability is generally valued as a positive characteristic of any multimedia learning environment. This study investigates the idea that students' attitudes concerning the use of multiple media for learning exhibit a considerable variability (different feelings, modes of use, preferences and dislikes) and information about these various stances may help designers and educators to better integrate and use multiple media for educational purposes. Employing a phenomenographic research methodology students' attitudes were recorded, categorised and related to parameters such as their learning styles. Results of the study indicate that a) visual dynamic media (animation and digital movies) capture students' interest and generate subjective feelings of better learning, b) digital movies may enhance the perceived authenticity of the educational setting and this also increases students' interest, c) students transfer to the digital medium the need for learning techniques that they use in the printed medium and experience feelings of restriction if the environment does not adequately support the same techniques or satisfactory alternatives, d) acceptance of audio varies significantly depending on perceptual preferences of students and e) students displaying a high abstract conceptualisation learning style express preference for studying more abstract content using printed matter.

A self-adaptive neural-based MAC protocol for distributed wireless LANs, capable of operating efficiently under bursty traffic conditions, is introduced. According to the proposed protocol, the mobile station that is granted permission to transmit is selected by means of a neural-based algorithm. The neural-based algorithm takes into account the network feedback information in order to update the choice probability of each mobile station. The proposed protocol is compared via simulation to TDMA and is shown to exhibit superior performance under bursty traffic conditions even when the network feedback is noisy.

A learning automata-based polling (LEAP) protocol for wireless LANs, capable of operating efficiently under bursty traffic conditions, is introduced. We consider an infrastructure wireless LAN, where the access point (AP) is located at the center of a cell which comprises a number of mobile stations. According to the proposed protocol, the mobile station that is granted permission to transmit is selected by the AP by means of a learning automaton. The learning automaton takes into account the network feedback information in order to update the choice probability of each mobile station. It is proved that the learning algorithm asymptotically tends to assign to each station a portion of the bandwidth proportional to the station's needs. LEAP is compared to the randomly addressed polling and group randomly addressed polling protocols and is shown to exhibit superior performance under bursty traffic.

An adaptive medium access control protocol for broadcast networks, which is capable of operating efficiently under bursty traffic conditions, is introduced. According to the proposed protocol, the station which grants permission to transmit at each time slot is selected by taking into account the network feedback information. In this way, the number of idle slots is minimized and the network performance is significantly improved. Furthermore, the portion of the bandwidth assigned to each station is dynamically adapted to the station's needs.

A TDMA-based randomly addressed potting protocol (TRAP) is proposed. TRAP employs a variable-length TDMA-based contention stage with the length based on the number of active stations. Simulation results are presented that reveal the superiority of TRAP against the RAP protocol in cases of medium and high offered loads. Furthermore the implementation of TRAP is much simpler than that of CDMA-based versions of RAP, since no extra hardware is needed for the orthogonal reception of the random addresses.

L EARNING automata [1] have attracted a considerable interest in the last three decades. They are adaptive decision making devices that operate in unknown stochastic environments and progressively improve their performance via a learning process. They have been initially used by psychologists and biologists to describe the human behavior from both psychological and biological viewpoints. Learning automata have made a significant impact on all areas of engineering. They can be applied to a broad range of modeling and control problems, which are characterized by nonlinearity and a high degree of uncertainty. Learning automata have some key features, which make them applicable to a broad range of applications: they combine rapid and accurate convergence with a low computational complexity. Learning is defined as any permanent change in behavior as a result of past experience, and a learning system should therefore have the ability to improve its behavior with time, toward a final goal. In a purely mathematical context, the goal of a learning system is the optimization of a function not known explicitly [2]. Thirty years ago, Tsypkin [3] introduced a method to reduce the problem to the determination of an optimal set of parameters and then applied stochastic hill-climbing techniques. Tsetlin [4] started the work on learning automata during the same period. An alternative approach to applying stochastic hillclimbing techniques, introduced by Narendra and Viswanathan [5], is to regard the problem as one of finding an optimal action out of a set of allowable actions and to achieve this using stochastic automata. The difference between the two approaches is that the former updates the parameter space at each iteration while the latter updates the probability space. The stochastic automaton attempts a solution of the problem without any information on the optimal action. One action is selected at random, the response from the environment is observed, action probabilities are updated based on that response,

Although new high-bandwidth network technologies are being introduced and widely deployed, asynchronous transfer mode (ATM) is still considered one of the most important network technologies currently in use. A number of ATM switch architectures have been proposed in the literature. However, industry has shown that is better to use the well-known shared-medium technique in the design of these ATM switches. In this paper, four variations of a new distributed scheme are proposed for the arbitration of a shared bus of an ATM switch. These schemes are based on learning automata. By taking advantage of the bursty nature of ATM traffic, the new arbitration scheme demonstrates superb performance compared to the time division multiple access (TDMA) scheme.

Due to its fixed assignment nature, the well-known time division multiple access (TDMA) protocol suffers from poor performance when the offered traffic is bursty. In this paper, an adaptive TDMA protocol, which is capable of operating efficiently under bursty traffic conditions, is introduced. According to the proposed protocol, the station which is granted permission to transmit at each time slot is selected by means of learning automata (LA). The choice probability of the selected station is updated by taking into account the network feedback information. The system which consists of the LA and the network is analyzed and it is proven that the choice probability of each station asymptotically tends to be proportional to the probability that this station is not idle. Although there is no centralized control of the stations and the traffic characteristics are unknown and time-variable, each station tends to take a fraction of the bandwidth proportional to its needs. Furthermore, extensive simulation results are presented, which indicate that the proposed protocol achieves a significantly higher performance than other well-known TDMA protocols when operating under bursty traffic conditions.

An absorbing learning automaton which is based on the use of a stochastic estimator is introduced. According to the proposed stochastic estimator scheme, the estimates of the reward probabilities are computed stochastically. Actions that have not been selected many times have the opportunity to be estimated as optimal, to increase their choice probabilities, and consequently, to be selected. In this way, the automaton's accuracy is significantly improved. This proposed automaton is proven to be absolutely expedient in all stationary environments, while the simulation results demonstrate that the proposed scheme achieves a significantly higher performance compared with deterministic estimator based schemes.

An adaptive protocol for WDM passive star networks, which is capable of operating efficiently under bursty, and correlated traffic, is introduced. According to the proposed protocol, the stations which are granted permission to transmit at each time slot, are selected by taking into account the network feedback information. Although the traffic parameters are unknown and time-variable, the bandwidth of each wavelength is allocated to the stations according to their needs. In this way, the number of idle slots is reduced, resulting in a significant increase of the network throughput.

Due to its fixed assignment nature, the well-known TDMA protocol suffers from poor performance when the offered traffic is bursty. In this paper, a new time division multiple access protocol which is capable of operating efficiently under bursty traffic conditions is introduced. According to the proposed protocol, the station which grants permission to transmit at each time slot is selected by means of stochastic estimator learning automata. The system which consists of the automata and the network is analyzed and it is proved that the probability of selecting an idle station asymptotically tends to be minimized. Therefore, the number of idle slots is drastically reduced and consequently, the network throughput is improved. Furthermore, due the use of a stochastic estimator, the automata are capable of being rapidly adapted to the sharp changes of the dynamic bursty traffic environment. Extensive simulation results are presented which indicate that the proposed protocol achieves a significantly higher performance than other well-known time division multiple access protocols when operating under bursty traffic conditions.

A client/server model which employs a polling policy as its access strategy is considered. We propose a learning-automata-based approach for polling in order to improve the throughput-delay performance of the system. Each client has an associated queue and the server performs selective polling such that the next client to be served is identified by a learning automaton. The learning automaton updates each client's choice probability according to the feedback information. Simulation results have shown that the proposed polling policy is beneficial in comparison to the conventional round-robin polling when operating under bursty traffic conditions.

The centralized filtering of the transmitted packets has been the base of a family of high-performance protocols for WDM star networks. Protocols of this family have been proved to achieve a significantly higher performance than the well-known contention-oriented or round-robin protocols. However, the need for electronic control of the filtering mechanism as well as the use of slowly tunable acoustooptic filters have been limiting factors in their performance. Furthermore, since the network hub of WDM star networks represents a single point of failure, the extensive use of electronic circuits at this point reduces the reliability of the system. In this paper, an all-optical centralized protocol is introduced. According to this protocol, the passing of the transmitted packets to the star coupler is controlled by means of optical logic circuits, without the need of optical to electronic translation or electronic processing of the network feedback information. In this way, the processing time is drastically reduced, while the need for acoustooptic filters is eliminated. Therefore, a significant performance improvement is achieved. Furthermore, due to the all-optical nature of the network hub, the reliability of the system is improved. The performance of the proposed protocol is studied via extensive analytical and simulation results which indicate that a WDM star network operating under this protocol achieves a high throughput-delay performance under any load conditions.

MERMAID is a European Union (EU)-financed maritime telemedicine project with global reach and 24-h multilingual capability, so as to serve multinational crews working in the isolation of the world's oceans. It provides a model for the provision of healthcare services based on the electronic transmission of medical information via ISDN-based video conferencing. This model is not limited to medical diagnostics, but it encompasses all cases in which the actual delivery of healthcare services involves a patient who is not located where the provider is. Its implementation requires the commissioning of an extensive telecommunications infrastructure that includes both satellite transmission for ship to shore communication and an extensive ground-based network for summoning expert medical help from around the world so as to meet the project's multilinguality requirements and, therefore, the exploration of a number of solutions. In fact, all categories of telemedical applications (audio and video conferencing, multimedia communications, flat file and image transfer with low-, medium-, and high-bandwidth data requirements) are considered, while the full range of network choices (digital land lines, cellular/wireless, satellite, and broadband) are being tested in terms of cost/performance tradeoffs that are inherent to them and the developmental stage each of these options occupies in their lifecycle. Finally, out of that, MERMAID utilizes advanced land-based line transmission technologies to aid the remote patient by making available the specialist care that is best suited in the particular case.