We have proposed a fish-farm monitoring system. In our system, the transmission range of acoustic waves from sensors attached to the undersides of the fish is not omnidirectional because of obstruction from the bodies of the fish. In addition, energy-efficient control is highly important in our system to avoid the need to replace the batteries. In this letter, we propose a data-gathering method for fish-farm monitoring without the use of control packets so that energy-efficient control is possible. Instead, our method uses the transmission-range volume as calculated from the location of the sensor node to determine the timing of packet transmission. Through simulation evaluations, we show that the data-gathering performance of our proposed method is better than that of comparative methods.

In this paper, we develop a system to support learning of OpenFlow by visually associating controller configuration information and logical topology. In our system, the constructed topology is visually shown on a touchscreen display and a learner can easily confirm the topology. In addition, a learner can configure the controller by visual and simple operation using the touchscreen display. Therefore, a learner can learn the behavior of the OpenFlow even if the learner is a beginner. Through experimental evaluation, we confirmed that our system can be applied to an OpenFlow network consists of dozens of OpenFlow switches.

We have proposed a fish farm monitoring system for the efficient farming of tuna. In our system, energy efficient and adaptive control of sensor node is highly important. In addition, since a sensor node is attached to the fish, the transmission range of sensor node is not omnidirectional. In this paper, we propose a data gathering mechanism for fish farm monitoring by extending a traditional desyncronization mechanism. In our proposed mechanism, by utilizing acknowledgment packets from the sink node, distributed and adaptive timing control of packet transmission is accomplished. In addition, we apply a reassignment mechanism and a sleep mechanism for improving the performance of our proposed mechanism. Through simulation experiments, we show that the performance of our proposed mechanism is higher than that of comparative mechanisms.

In this paper, we propose a novel energy-efficient architecture for software-defined data center infrastructures. In our proposed data center architecture, we include an exhaust heat reuse system that utilizes high-temperature exhaust heat from servers in conditioning humidity and air temperature of office space near the data center. To obtain high-temperature exhaust heat, equipment such as server racks and air conditioners are deployed in tandem so that the aisles are divided into three types: cold, hot, and super-hot. In this paper, to investigate the fundamental characteristics of our proposed data center architecture, we consider various types of data center models and conduct numerical simulations that use results obtained by experiments at an actual data center. Through simulation, we show that the total power consumption by a data center with our proposed architecture is 27 percent lower than that by data center with a conventional architecture. In addition, it is also shown that the proposed tandem equipment arrangement is suitable for obtaining high-temperature exhaust heat and decreasing the total power consumption significantly under a wider range of conditions than in the conventional equipment arrangement.

As a method for supporting fish farming, this paper presents a video scene detection when farmed fry start instantaneously in a tank due to environmental stimuli. Although some environmental stimuli such as sound noises or lighting startle the fry and the stimuli bring about the instantaneous response, actual situations around the tanks in which the stimuli occur are unclear in detail. From the fact the fry often die due to crashes to the tank's wall and between the fry by the response, a monitoring system for the fry and situation around the pool could find causes of the stimuli, and it could result in decrease of the death number of the fry. In this research, the fry which swim in a tank are monitored by a video cam- era and the video scenes at the response are detected by a SVM with a feature value which represents fry's acceleration using sequential frames of the moving image. Preparing the moving images which include scenes of the response by fish in a tank, performances of the proposed method were examined. From experimental results, accuracy ratios of the recall and the precision for the scene detection have shown more than 80% on average and 100% under normal illuminances (108.5 lux on average), respectively.

In this paper, we propose a workload assignment policy for reducing power consumption by air conditioners in data centers. In the proposed policy, to reduce the air conditioner power consumption by raising the temperature set points of the air conditioners, the temperatures of all server back-planes are equalized by moving workload from the servers with the highest temperatures to the servers with the lowest temperatures. To evaluate the proposed policy, we use a computational fluid dynamics simulator for obtaining airflow and air temperature in data centers, and an air conditioner model based on experimental results from actual data center. Through evaluation, we show that the air conditioners' power consumption is reduced by 10.4% in a conventional data center. In addition, in a tandem data center proposed in our research group, the air conditioners' power consumption is reduced by 53%, and the total power consumption of the whole data center is exhibited to be reduced by 23% by reusing the exhaust heat from the servers.

In recent years, sensor network technologies have attracted attentions in the field of the primary industry such as the aquaculture industry. We have proposed a fish farm monitoring system for developing efficient farming of tuna. In our monitoring system, a sensor node is attached at fish and its sensing data are gathered at a sink node. In the monitoring environment, the size of fish is varied between 5 cm and 100 cm depending on the age of fish. Therefore, the communication environment is also varied depending on the age. In this paper, we investigate the effect of fish body size on the performance of communication in our monitoring system. Through simulation evaluations, we show that the size of fish significantly affects to the data gathering performance.

One of the promising sensor network application is for the primary industry and we have proposed a fish farm monitoring system. In our system, sensor data monitored at a fish-mounted sensor node is transmitted to a sink node by using acoustic communication. Since we assume that a sensor node is attached at an abdominal cavity region of fish, the transmission range of acoustic wave is not omni-directional due to obstruction of fish body. In addition, energy efficient control is highly important in our system since it is difficult to replace battery of sensor node. In this paper, we propose a data gathering method taking into account energy efficiency and directional transmission range. In our method, we use the volume of transmission range to determine the timing of packet transmission. Our method does not require control packets for determining its transmission timing. Through simulation evaluations, we show that the data gathering performance of our proposed method is higher than that of comparative methods.

In tuna farming, turbidity measurement is highly important since one of the cause of death of tuna is a choking death due to muddy water. In this paper, we consider to use visible light communication modules for estimation of turbidity in underwater environments. If these modules can be used as turbidity meters, additional introduction of dedicated turbidity meters is not necessary. In this paper, through experimental evaluations using off-the-shelf visible light communication modules, we investigate the effect of turbidity on the performance of visible light communication in an underwater environment.

In network education, there are several benefits for conducting cooperative practice among a variety of learning levels of learners. We have proposed a hands-on IP network practice system by constructing a virtual network on a cloud server. In our past study, we introduced a pseudo cooperative practice function to our system so that cooperative practice with a companion agent can be accomplished even if there is no other learner for constructing a group. In this paper, we extend the pseudo cooperative practice function to deal with a variety of learning levels of learners. In our system, the companion agent changes its role automatically depending on the learner's learning level. Through questionnaire investigation, we show the effectiveness of pseudo cooperative practice.

OpenFlow network technologies has attracted attention along with popularization of cloud environment and server virtualization since it enables flexible network configuration. It is expected that existing networks of many organizations will migrate to OpenFlow networks in the future. Since it takes a huge cost to replace all existing network devices to OpenFlow switches, many organizations consider to employ overlay OpenFlow networks where virtual OpenFlow devices are introduced at endpoint servers. In overlay OpenFlow networks, the cost for migration increases according to the number of servers. In this paper, we propose a system for supporting migration to an overlay OpenFlow network. Our proposed system acquires settings of servers in the existing network endpoints and reflects them to the overlay OpenFlow network. Through experimental evaluations, we confirm the behavior of the configuration acquisition function of our proposed system.

Software-Defined Networking (SDN) has attracted attention along with the popularization of cloud environment and server virtualization. In SDN, the control plane and the data plane are decoupled so that the logical topology and routing control can be configured dynamically depending on network conditions. To obtain network conditions precisely, a network monitoring mechanism is necessary. In this paper, we focus on OpenFlow which is a core technology to realize SDN. We propose, design, implement, and evaluate a network monitoring system for OpenFlow networks. Our proposed system acts as a proxy between an OpenFlow controller and OpenFlow switches. Through experimental evaluations, we confirm that our proposed system can capture packets and monitor traffic information depending on administrator's configuration. In addition, we show that our proposed system does not influence significant performance degradation to overall network performance.

In the case of cycling at night, it is helpful for cyclist if information on bad road surface condition is obtained autonomously. In this paper, we propose a system for autonomously monitoring road surface condition by using laser light and camera modules. In our system, the laser light module emits grid laser light, and the camera module captures the image of front road area. When our system detects bad road surface condition by processing obtained images, it alerts to the cyclist. In this paper, we apply template matching for detecting obstacles in the front of bicycle. In addition, we implement the system using off-the-shelf node, Raspberry PI, a laser module and a typical bicycle. Through experimental evaluations, we confirm that the monitoring system can detect obstacles in the front of bicycle.

Monitoring bad road surface condition, such as break, potholes, obstacles, is helpful to prevent accidents in the case of cycling. In this paper, we consider to use low-cost ultrasonic distance sensor for monitoring road surface condition. We design and implement a monitoring system using an off-the-shelf node with a low-cost ultrasonic distance sensor. In the system, ultrasonic signal is periodically emitted to measure the distance from the system to the road so that the road surface condition can be estimated. To limit the sensing region, the ultrasonic distance sensor is covered by a plastic shield plate. Through experimental evaluations, we show that the monitoring system can detect the 360 cm away obstacle on road in the front area of bicycle. In addition, we show that the system also can detect a hole in the front area of bicycle.

In undersea environment, development of low-cost communication module is highly important for dissemination of undersea wireless sensor networks. Some undersea wireless sensor network applications do not require high performance communication capability such as transmission distance. If the terrestrial low-cost communication module can be used in the undersea environment, it is possible to use the module in some undersea wireless sensor networks. In this paper, we evaluate the communication capability of a commercially available WiFi device in an undersea environment. In the experiment, we use a mobile access point as a sender node and an Android tablet as a receiver node. Through evaluations, we show that the transmission distance of WiFi device is around 0.1 m in an undersea environment. In addition, we evaluate a fish farm monitoring system based on WiFi modules through simulations.

Education of network security has attracted a lot of attentions due to increasing unauthorized access, lack of engineers who have network security skills, and so on. Although practice using actual computer networks is highly important for network security education, it requires costs to prepare a computer network for practice. We have proposed a network security learning system by constructing a virtual network on one PC so that low-cost, easy and safe practice can be accomplished. In this paper, we introduce an attacker agent that automatically attacks servers in a fixed pattern for supporting practical learning of defense techniques safely on a virtual network.

In the case of cycling at a blind road, at night or cycling by elderly people, it is helpful to prevent accidents if information on bad road surface condition, such as break, potholes, obstacles, bumps, is preliminary obtained. In this paper, we consider to use a low-cost ultrasonic distance sensor attached on a bicycle for monitoring the road surface conditions in the front area. We design and implement a road surface condition monitoring system using an off-the-shelf node Arduino, an ultrasonic distance sensor, and a typical bicycle. In the implemented system, to limit the sensing region, the ultrasonic distance sensor is covered by a plastic shield plate. Through experimental evaluations, we show that the monitoring system can detect the 223 cm away obstacle on road in the front area of moving bicycle.

As interdomain routing protocol, BGP is fairly simple, and allows plenty of policies based on ISPs' preferences. However, recent studies show that BGP routes are often non-optimal in end-to-end performance, due to technological and economic reasons. To obtain improved end-to-end performance, overlay routing, which can change traffic routing in application layer, has gained attention. However, overlay routing often violates BGP routing policies and harms ISPs' interest. In order to take the advantage of overlay to improve the end-to-end performance, while overcoming the disadvantages, we propose a novel interdomain overlay structure, in which overlay nodes are operated by ISPs within an ISP alliance. The traffic between ISPs within the alliance could be routed by overlay routing, and the other traffic would still be routed by BGP. As economic structure plays very important role in interdomain routing, so we propose an effective and fair charging and pricing scheme within the ISP alliance in correspondence with the overlay routing structure. Finally, we give a simple pricing algorithm, with which ISPs can find the optimal prices in the practice. By mathematical analysis and numerical experiments, we show the correctness and convergence of the pricing algorithm.

In this paper, we propose a method for estimating the number of pedestrians walking in opposite directions, as in cases of a shopping street or a sidewalk in a downtown area. The proposed method utilizes a compound-eye sensor that is constructed by placing two binary sensors for the pedestrians' movement direction and multiple binary sensors for the vertical direction of the pedestrians' movement direction. A number of Monte Carlo simulations about the movement of pedestrians are conducted, and the output history of the compound-eye sensor is obtained in each simulation. The simulation scenario with a small difference of the output history of the compound-eye sensor is selected to estimate the number of pedestrians. Evaluation results show that in the field whose width is 8 [m] the relative error in the proposed method is the smallest by using 2 x 8 binary sensors.

In IEEE 802.16j relay networks, time division multiple access (TDMA) is adopted for sharing wireless network resources while avoiding radio wave interference. Time slot assignment determines the degree of spatial reuse of wireless network resources and radio interference strength, which affects network performance. The relay network performance is also influenced by various factors, such as background noise level, path loss exponent, density of relay nodes, and transmission signal strength. In this paper, we present the results of multiple regression analysis of IEEE 802.16j relay network throughput. We first summarize the network environment parameters that affect the throughput of relay networks and conduct extensive simulation experiments with various sets of parameter values. From the simulation results, we investigate the relative effect of parameters on throughput performance by multiple regression analysis. The analysis results show that background noise level and path loss exponent are the key parameters for determining relay network throughput. We also obtain a regression equation to estimate the throughput with enough accuracy.

In this paper, we propose two heuristic algorithms for time slot assignment in IEEE 802.16j relay networks. In the proposed methods, we calculate an appropriate group of time slots for each link in the network or calculate an appropriate group of links for each time slot. We show that the latter algorithm is better in terms of the balance between the calculation time and network performance.

In time division multiple access (TDMA) based wireless networks, graph coloring based link scheduling mechanisms are often considered to assign time slots to links. A graph coloring based link scheduling mechanism requires a radio interference estimation method to make a conflict graph from a wireless network topology, and the protocol method is usually used among radio interference estimation methods. In this paper, we propose a radio interference estimation method for wireless relay networks, where the network topology is a tree. Through simulation experiments, we show that the accuracy of radio interference estimation with our proposed method is improved compared to that with the protocol method.

We proposed a workload assignment policy for reducing power consumption by air conditioners in data centers. Power consumption was estimated by computational fluid dynamics for both the conventional equipment arrangement and the tandem arrangement newly developed to reuse the exhaust heat from servers. To reduce the air conditioner power consumption by raising the temperature set points of the air conditioners, the temperatures of all server back-planes were equalized by moving workload from the servers with the highest temperatures to the servers with the lowest temperatures. Consequently, the air conditioners' power consumption was reduced by 10.4% in the conventional arrangement. In the tandem arrangement, the air conditioners' power consumption was reduced by 53%, and also the total power consumption of the whole data center was exhibited to be reduced by 23% by reusing the exhaust heat from the servers.

Under regular or heavy traffic conditions, contention-free mechanisms such as time division multiple access (TDMA) are considered as under layer mechanisms in wireless networks. In TDMA mechanisms, links are scheduled by assigning different time slots for links that interfere each other. In this paper, we propose a self-organizing link scheduling mechanism for single-hop wireless networks considering traffic load of each wireless link. The proposed mechanism is based on a conventional desynchronization-based TDMA mechanism. Through simulation evaluations, it is shown that our proposed mechanism can schedule links in a self-organizing manner in accordance with traffic load of each link.

We propose a method for tracking multiple pedestrians using a binary sensor network. In our proposed method, sensor nodes are composed of pairs of binary sensors and placed at specific points, referred to as gates, where pedestrians temporarily change their movement characteristics, such as doors, stairs, and elevators, to detect pedestrian arrival and departure events. Tracking pedestrians in each subregion divided by gates, referred to as microcells, is conducted by matching the pedestrian gate arrival and gate departure events using a Bayesian estimation-based method. To improve accuracy of pedestrian tracking, estimated pedestrian velocity and its reliability in a microcell are used for trajectory estimation in the succeeding microcell. Through simulation experiments, we show that the accuracy of pedestrian tracking using our proposed method is improved by up to 35% compared to the conventional method.

In this paper, we propose self-organizing trans mission and sleep scheduling mechanisms for periodically gathering sensor data at a base station in wireless sensor networks. In our proposed mechanisms, we use a conventional self-organizing communication mechanism based on phase-locking in a pulse-coupled oscillator model to propagate sensor data from the edge of the network to the base station in order of hop counts from the base station. In addition, we propose two mechanisms to avoid collisions among sensor nodes with the same hop count, a randomization-based mechanism and a desynchronizationbased mechanism based on anti-phase synchronization in the pulse-coupled oscillator model. Through simulation experiments, we show that our proposed mechanism significantly improves the data gathering ratio and energy-efficiency in comparison with the conventional mechanism. ©2013 Engineering and Technology Publishing.

Multihoming is widely used by Internet service providers (ISPs) to obtain improved performance and reliability when connecting to the Internet. Recently, the use of overlay routing for network application traffic is rapidly increasing. As a source of both routing oscillation and cost increases, overlay routing is known to bring challenges to ISPs. In this paper, we study the interaction between overlay routing and a multihomed ISP's routing strategy with a Nash game model, and propose a routing strategy for the multihomed ISP to alleviate the negative impact of overlay traffic. We prove that with the proposed routing strategy, the network routing game can always converge to a stable state, and the ISP can reduce costs to a relatively low level. From numerical simulations, we show the efficiency and convergence resulting from the proposed routing strategy. We also discuss the conditions under which the multihomed ISP can realize minimum cost by the proposed strategy.

We consider a pedestrian tracking system where sensor nodes are placed only at specific points so that the monitoring region is divided into multiple smaller regions referred to as microcells. In the proposed pedestrian tracking system, sensor nodes composed of pairs of binary sensors can detect pedestrian arrival and departure events. In this paper, we focus on pedestrian tracking in microcells. First, we investigate actual pedestrian trajectories in a microcell on the basis of observations using video sequences, after which we prepare a pedestrian mobility model. Next, we propose a method for pedestrian tracking in microcells based on the developed pedestrian mobility model. In the proposed method, we extend the Bayesian estimation to account for time-series information to estimate the correspondence between pedestrian arrival and departure events. Through simulations, we show that the tracking success ratio of the proposed method is increased by 35.8% compared to a combinatorial optimization-based tracking method.

The Internet is composed of many distinct networks, operated by independent Internet Service Providers (ISPs). The traffic and economic relationships of ISPs are mainly decided by their routing policies. However, in today's Internet, overlay routing, which changes traffic routing at the application layer, is rapidly increasing and this challenges the validity of ISPs' existing agreements. We study here the economic implications of overlay routing for ISPs, using an ISP interconnection business model based on a simple network. We then study the overlay traffic patterns in the network under various conditions. Combining the business model and traffic patterns, we study the ISPs' cost reductions with Bill-and-Keep peering and paid peering. We also discuss the ISPs' incentive to upgrade the network under each peering strategy. Copyright © 2013 The Institute of Electronics, Information and Communication Engineers.

IEEE 802.16j-based wireless relay networks adopt time division multiple access, where time slots are assigned to communication links as transmission opportunities. When we employ adaptive modulation and coding for data transmission on wireless links, the transmission bitrate is affected by the signal to interference and noise ratio. Therefore, algorithms that assign time slots to links are important for enhancing the performance of relay networks. When we employ relay networks to access backhaul networks, the bottleneck link is likely to be located near the gateway node, and dominates the performance of the relay network. In this paper, we propose a time slot-adding algorithm to improve bottleneck link throughput in IEEE 802.16j relay networks. Although we employ an existing method for the initial assignment of time slots to links in the network, we introduce novel algorithm to add more time slots to the bottleneck link. Performance evaluations confirm that the proposed method improves data transfer throuzhput by up to around 30%. © 2013 IEEE.

Time slot assignment algorithms in IEEE 802.16j multi-hop relay networks often assume simple radio interference models based on the distance or the number of hops between nodes for simplicity. However, by using more detailed models that consider the signal to interference plus noise ratio, we expect to obtain more effective time slot assignment which gives higher network performance. In this paper, we evaluate the performance of time slot assignment algorithms based on various radio interference models by considering differences in the radio propagation environment and the use of adaptive modulation and coding. Through performance evaluations, we clarify the effect of the parameter settings in the radio interference models on the network performance. We also demonstrate that by carefully choosing the parameters in the simple models, we can obtain almost the same network performance as from the detailed models.

In this paper, the authors focus on upstream transmission in TDMA-based IEEE 802.16j and propose two time slot assignment algorithms to decrease end-to-end transmission latency. One of the proposed algorithms assigns time slots considering the hop count from a gateway node, and the other takes the path from the relay node to the gateway node into account. In addition, a restriction in assigning time slots is introduced to reduce the delay at each relay node. The algorithms with the restriction assign later time slots considering the time slot order of links connecting a relay node. The performance of the proposed algorithms is evaluated through simulation experiments from the viewpoints of frame size and end-to-end transmission latency, and it is confirmed that the proposed algorithms achieve small transmission latency regardless of packet generation rate in the network, and decrease the transmission latency by up to 70% compared with the existing algorithm.

In this paper, we propose a method for estimating the number of pedestrians walking in opposite directions, as in the case of a corridor. The proposed method utilizes a ceiling-mounted compound-eye sensor composed of two binary sensors with partially overlapping sensing regions. The output of the compound-eye sensor is sent to a monitoring server, where a number of Monte Carlo simulations are conducted. The simulation scenario with the smallest difference from the output history of the compound-eye sensor is selected as an estimate of the number of pedestrians. Simulations show that in crowded situations the relative error in the proposed method decreases by up to 45% compared to an existing method.

Wireless sensor network technologies have attracted a lot of attention in recent years. In this paper, we propose an energy-efficient data gathering mechanism using traveling wave and spatial interpolation for wireless sensor networks. In our proposed mechanism, sensor nodes schedule their message transmission timing in a fully-distributed manner such that they can gather sensor data over a whole wireless sensor network and transmit that data to a sink node while switching between a sleep state and an active state. In addition, each sensor node determines the redundancy of its sensor data according to received messages so that only necessary sensor data are gathered and transmitted to the sink node. Our proposed mechanism does not require additional control messages and enables both data traffic and control traffic to be drastically reduced. Through simulation experiments, we confirmed that with our proposed mechanism, the number of message transmissions can be reduced by up to 77% and the amount of transmitted data can be reduced by up to 13% compared to a conventional mechanism.

Multihoming is widely used by Internet service providers (ISPs) to obtain improved performance and reliability when connecting to the Internet. Recently, the use of overlay routing for network application traffic is rapidly increasing. As a source of both routing oscillation and cost increases, overlay routing is known to bring challenges to ISPs. In this paper, we study the interaction between overlay routing and a multihomed ISP's routing strategy with a Nash game model, and propose a routing strategy for the multihomed ISP to alleviate the negative impact of overlay traffic.We prove that with the proposed routing strategy, the network routing game can always converge to a stable state, and the ISP can reduce costs to a relatively low level. From numerical simulations, we show the efficiency and convergence resulting from the proposed routing strategy. We also discuss the conditions under which the multihomed ISP can realize minimum cost by the proposed strategy. © 2012 IEEE.

We propose self-organizing transmission scheduling mechanisms for periodically gathering sensor data at a base station in wireless sensor networks. In our proposed mechanisms, sensor nodes self-organize message transmission and sleep timings. To propagate sensor data from the edge of the network to the base station in order of hop counts from the base station, we use a conventional self-organizing communication mechanism based on phase-locking in a pulse-coupled oscillator model. We propose two mechanisms to avoid collisions among sensor nodes with the same hop count, a randomization-based mechanism and a desynchronization-based mechanism based on anti-phase synchronization in the pulse-coupled oscillator model. Simulation experiments confirm that our mechanism significantly improves the data gathering ratio in comparison with the conventional mechanism. © 2012 IEEE.

In general, wireless mesh networks provides wireless Internet access service to client terminals that exist in the coverage area of the network. In such networks, the coverage area size and its overlap degree have a significant effect on network performance. In order to determine the coverage area size for each node, it is important to use the location information of neighboring nodes. However, such information cannot be always obtained. In the present paper, we introduce the method to determine the coverage area size only by utilizing the distance information estimated from received signal strength. Specifically, in proposed method, each node estimates the relative locations of its neighboring nodes based on the estimated distance information. It then calculates its Voronoi area, and sets its coverage area size based on the shape of Voronoi area. We conduct simulation experiments to evaluate the performance of the proposed method in terms of total covered ratio, coverage overlap, and energy consumption. We confirm that our method can keep more than 99% of total covered ratio regardless of the node density and reduce the energy consumption more than 50% than that of existing method. © 2012 IEEE.

Wireless sensor network technologies have attracted a lot of attention in recent years. In this paper, we propose an energy-efficient data gathering mechanism using traveling wave and spatial interpolation for wireless sensor networks. In our proposed mechanism, sensor nodes schedule their message transmission timing in a fully-distributed manner such that they can gather sensor data over a whole wireless sensor network and transmit that data to a sink node while switching between a sleep state and an active state. In addition, each sensor node determines the redundancy of its sensor data according to received messages so that only necessary sensor data are gathered and transmitted to the sink node. Our proposed mechanism does not require additional control messages and enables both data traffic and control traffic to be drastically reduced. Through simulation experiments, we confirmed that with our proposed mechanism, the number of message transmissions can be reduced by up to 77% and the amount of transmitted data can be reduced by up to 13% compared to a conventional mechanism. © 2012 Information Processing Society of Japan.

In time division multiple access (TDMA)-based wireless mesh networks, interference relationships should be considered when time slots are assigned to links. In graph theory-based time slot assignment algorithms, the protocol interference model is widely used to determine radio interference information, although it is an inaccurate model of actual radio interference. On the other hand, the signal-to-interference-plus-noise-ratio model (SINR model) gives more accurate interference relationships but is difficult to apply to time slot assignment algorithms since the radio interference information cannot be determined before time slot assignment. In this paper, we investigate the effect of the parameters of the protocol interference model on the accuracy of the interference relationships determined using this model. Specifically, after assigning time slots to links based on the protocol interference model with various interference ratios, which is the major parameter of the protocol interference model, we compare the interference relationship among links in the protocol interference and SINR models. Through simulation experiments, we show that accuracy of the protocol interference model is improved by up to 15% by adjusting the interference ratios of the protocol interference model.

In the present paper, we propose an object tracking method called scenario-type hypothesis object tracking. In the proposed method, an indoor monitoring region is divided into multiple closed micro-cells using sensor nodes that can detect objects and their moving directions. Sensor information is accumulated in a tracking server through wireless multihop networks, and object tracking is performed at the tracking server. In order to estimate the trajectory of objects from sensor information, we introduce a novel concept of the virtual world, which consists of virtual micro-cells and virtual objects. Virtual objects are generated, transferred, and deleted in virtual micro-cells according to sensor information. In order to handle specific movements of objects in micro-cells, such as slowdown of passing objects in a narrow passageway, we also consider the generation of virtual objects according to interactions among virtual objects. In addition, virtual objects are generated when the tracking server estimates loss of sensor information in order to decrease the number of object tracking failures. Through simulations, we confirm that the ratio of successful tracking is improved by up to 29% by considering interactions among virtual objects. Furthermore, the tracking performance is improved up to 6% by considering loss of sensor information.

The performance of wireless sensor networks in terms of coverage, connectivity, lifetime and robustness is greatly influenced by the initial physical placement of the sensor nodes. In this paper we propose a novel deployment method for sensor nodes, when a large number of nodes is distributed from the air in a monitoring region that is inaccessible from the ground. In our deployment method, each sensor node is equipped with a parachute and a device to switch between its two falling behaviours (gliding and falling) in the air. After being dropped from midair, sensor nodes stochastically coordinate their falling behaviour in order to distribute themselves uniformly in the monitoring region. In addition, each sensor node exchanges messages with neighbouring nodes while being airborne and it changes its falling behaviour according to the density of the neighbouring nodes in order to reduce non-uniform areas. Through simulation evaluations, we confirm that our proposal can achieve a high uniformity in the placement of the nodes compared to a traditional airdrop method.

Radio interference should be taken into account to assign time slots to links in time division multiple access (TDMA)-based wireless mesh networks. In many graph theory-based time slot assignment algorithms, the protocol interference model is widely used to obtain radio interference information, although this model is considered to be inaccurate when compared with actual radio interference. On the other hand, the signal-to-interference-plus-noise-ratio model (SINR model) is a well-known and accurate model of radio interference. However, the SINR model requires time slot information to obtain radio interference relationship, and thus it is difficult to apply the SINR model to graph theory-based time slot assignment algorithms. In this paper, we extend the protocol interference model to represent radio interference more accurately for wireless mesh networks. To do this, we adjust the interference ratio parameter of the protocol interference model by considering SINR. We propose three methods for adjusting this parameter. Through simulation, it is shown that higher accuracy of the protocol interference model can be achieved by adjusting the interference ratio parameter for each node.

In IEEE 802.16j wireless multihop networks, transmission latency from relay nodes to a gateway node is one of the important performance metrics. The transmission latency is mainly affected by a scheduling delay at every relay node, which is determined by algorithms for assigning time slots to wireless links between relay nodes. In this paper, we propose 2 kinds of time slot assignment algorithms for upstream wireless links in IEEE 802.16j multihop networks. One of the proposed algorithms assigns time slots considering the hop count from a gateway node, and the other takes the path from the relay node to the gateway node into account. We evaluated the performance of the proposed algorithms through simulation experiments and confirmed that our algorithms can decrease the upstream latency by up to 15% compared with the existing method, without increasing the average transmission latency of the entire network.

In this paper, we propose an autonomous and energy-efficient data gathering mechanism with transmission reduction for wireless sensor networks (WSNs). In our proposed mechanism, sensor nodes schedule their message transmission timing in a self-organizing manner such that they can gather sensor data over a whole WSN and transmit that data to a sink node while switching between a sleep state and an active state. In addition, each sensor node determines the redundancy of its sensor data according to overheard messages so that only necessary sensor data are gathered and transmitted to the sink node. Our proposed mechanism does not require additional control messages and enables both data traffic and control traffic to be drastically reduced. Through simulation experiments, we confirmed that the number of message transmissions can be reduced by up to 77% with our proposed mechanism compared to a conventional mechanism. © 2011 IEEE.

In this paper, we propose an assignment algorithm of time slots for wireless links in IEEE 802.16 multi-hop relay networks. The main purpose is to reduce transmission latency between user stations and base stations as well as maintaining total network throughput. The proposed algorithm is based on the existing method, which aims better network throughput and guarantees certain frame length, but changes the order of time slot assignment without network throughput changed to decrease the transmission latency. It also accommodates the weighted time slot assignment, which is consistent with IEEE 802.16j specifications. Performance evaluation is conducted through both of simulation experiments and numerical calculations and confirm that the proposed algorithm can decrease the transmission latency by up to 15% on average compared with the existing random assignment algorithm, and can provide an time slot assignment with smaller transmission latency in shorter calculation time. © 2010 IEEE.

In recent years, wireless communication technologies such as ZigBee have been developed. By constructing a wireless sensor network from infrared sensors with those wireless technologies, it is possible to track moving objects in a wide monitoring region. In this paper, we propose an object tracking method by dividing a wide monitoring region into small multiple regions called "micro cells". Pair of infrared sensors is placed at borders among micro cells to detect objects and their movement direction, and sensor information are gathered to a tracking server through wireless or wired networks. In our proposed method, the tracking server maintains virtual micro cells and virtual objects which indicate a variety of hypotheses of object trajectories. A virtual object is generated, moved, and removed in virtual micro cells appropriately according to sensor information and interactions among virtual objects, so that the tracking server keeps a number of object trajectory hypotheses for each object. When an object departs from the monitoring region, the virtual object with highest likelihood is selected as the estimated object trajectory. Through preliminary simulation evaluations, we confirm that tracking accuracy is improved up to 1.94 times higher by making virtual objects appropriately. © 2010 IEEE.

In wireless mesh networks based on IEEE 802.16 WiMAX, each mesh node has its own service area that provides wireless Internet access service to client terminals. The network performance degrades due to radio interference when multiple service areas overlap with each other. To avoid this problem, information of the exact location and size of the service area of neighboring mesh nodes is useful for each mesh node to determine its own service area size. However, the availability of the information depends on the topology construction procedure. In this paper, we discuss a method to determine the size of service areas using the information obtained in topology construction based on IEEE 802.16 WiMAX. Specifically, each mesh node estimates the distance to the nearest neighboring node, and sets the service area size based on this distance. Through performance evaluation, we confirm that our method can increase the size of a single-covered area by an average of 32% compared with the existing method, which is the constant service radius, and can reduce the maximum number of overlapped service areas by approximately 50%.© 2009 IEEE.

By applying a proxy mechanism widely used in WWW systems to video streaming systems, low-delay and high-quality video distribution can be accomplished without imposing extra load on the system. The video streaming system proposed in this paper consists of a video server and multiple proxy servers. In our mechanism, proxies communicate with each other and retrieve missing video data from an appropriate server by taking into account transfer delay and offerable quality. In addition, the quality of cached video data is adapted appropriately at a proxy to cope with the client-to-client heterogeneity in terms of the available bandwidth, end-system performance, and user preferences on the perceived video quality. Through simulation experiments, it is shown that our proposed mechanism can provide users with low-delay and high-quality video streaming services. Furthermore, to verify our mechanism, we implement a real system for MPEG-4 video streaming services and show that our proxy caching system can provide users with a continuous video distribution under dynamically changing network conditions. © 2008 Academy Publisher.

In wireless sensor networks (WSNs), the frequency of sensing and data gathering depends on application requirements and surrounding conditions. In this paper, we propose a data gathering scheme adaptive to sensing requirements for WSNs composed of sensor nodes with multiple sensing capabilities. To accomplish self-organizing control, we adopt the response threshold model for adaptive sensing task engagement and the pulse-coupled oscillator model for energy-efficient transmission and sleep scheduling. Through preliminary simulation experiments, we confirm that autonomous and energy-efficient data gathering can be accomplished satisfying dynamically changing sensing requirements. © 2008 ESRGroups.

In this paper, we propose, design, implement, and evaluate a proxy caching system for MPEG-4 video streaming services for heterogeneous users. In our system, a video stream is divided into blocks for efficient use of the cache buffer and the bandwidth. A proxy retrieves a block from a server, deposits it in its local cache buffer, and provides a requesting client with the block in time. It maintains the cache with limited capacity by replacing unnecessary, cached blocks with a newly retrieved one. It also prefetches video blocks that are expected to be required in the near future. In addition, the proxy server adjusts the quality of block appropriately, because clients are heterogeneous, in terms of the available bandwidth, end-system performance, and so on. Through evaluations, we proved that our proxy caching system could provide users with a continuous and high-quality video streaming service in a heterogeneous environment.

We have proposed a simple and energy-efficient communication mechanism which can organize a variety of communication depending on dynamically changing application requirements. In this demonstration, we show that our mechanism can gather or diffuse information in accordance with application requirements in a dynamic wireless sensor network.

In this paper, we propose and evaluate a self-organizing communication mechanism for wireless sensor networks where a large number of sensor nodes are deployed. To accomplish application-oriented periodic communication without any centralized controls, we adopt traveling wave phenomena of a pulse-coupled oscillator model by regarding sensor nodes as oscillators and emission of radio signals as firing. Through simulation experiments, we confirm that our mechanism delivers sensor information to / from a designated node in a more energy-efficient manner than other method, although it takes time to generate a traveling wave. In addition, we implement our mechanism using MOTE MICAz and verify its practicality. © 2007 IEEE.

In this paper, we propose a self-organizing communication mechanism for a wireless sensor network where a large number of sensor nodes are deployed. To accomplish application-oriented periodic communication without any centralized controls, we adopt traveling wave phenomena of a pulse-coupled oscillator model by regarding sensor nodes as oscillators and the emission of radio signals as firing. We first investigat conditions of a phase-response curve to attain wave-formed firing patterns regardless of the initial phase of oscillators. We adopte the derived phase-response curve to accomplish the desired form of message propagation through local and mutual interactions among neighboring sensor nodes. Through simulation experiments, we confirm that our mechanism can gather or diffuse information effectively in accordance with the application's requirements.

The development of low-cost microsensor equipment having the capability of wireless communications has caused sensor network technology to attract the attention of many researchers and developers. In this paper, we propose a fully-distributed and self-organizing data gathering scheme for a network where a large number of sensor nodes are deployed. To accomplish the periodic data gathering without centralized controls, we adopt the pulse-coupled oscillator model based on biological mutual synchronization such as that observed in flashing fireflies. By adjusting parameters and functions in the model according to application's requirements and conditions surrounding a sensor network, we can control the frequency, the form, and the direction of information propagation which is observed as a wave. In this paper, we show how to organize traveling waves in the model and how to adapt the model to data gathering in sensor networks.

The proxy mechanism widely used in WWW systems offers low-delay and scalable delivery of data by means of a "proxy server". By applying proxy mechanism to video streaming systems, high-quality and low-delay video distribution can be accomplished without imposing extra load on the system. We have proposed proxy caching mechanisms to accomplish the high-quality and highly-interactive video streaming services. In our proposed mechanisms, proxies communicate with each other, retrieve a missing video data from an appropriate server by taking into account transfer delay and offerable quality. In addition, the quality of cached video data can be adapted appropriately in the proxy to cope with the client-to-client heterogeneity, in terms of the available bandwidth, end-system performance, and user preferences on the perceived video quality. In this paper, to verify the practicality of our mechanisms, we implemented them on a real system for MPEG-4 video streaming services, and conducted experiments. Through evaluations, it was shown that our proxy caching system can provide users with a continuous and high-quality video distribution in accordance with network condition.

The proxy mechanism widely used in WWW systems offers low-delay data delivery by means of "proxy server." By applying proxy mechanisms to video streaming system, we expect that high-quality and low-delay video distribution can be accomplished without introducing extra load on the system, in addition, it is effective to adapt the quality of cached video data appropriately in the proxy if user requests are diverse due to heterogeneity in terms of the available bandwidth, end-system performance, and user's preferences on the perceived video quality. In this paper, we propose proxy caching mechanisms to accomplish high-quality and low-delay video streaming services. In our proposed system, a video stream is divided into blocks for efficient use of cache buffer. A proxy cache server is assumed to be able to adjust the quality of cached or retrieved video blocks to requests through video filters. We evaluate our proposed mechanisms in terms of the required buffer size, the play-out delay and the video quality through simulation experiments. Furthermore, to verify the practicality of our mechanisms, we implement our proposed mechanisms on a real system and conducted experiments. Through evaluations from several performance aspects, it is shown that our proposed mechanisms can provide users with a low-latency and high-quality video streaming service in a heterogeneous environment.

We proposed self-organizing transmission scheduling mechanisms for periodically gathering sensor data at a base station in wireless sensor networks. In the proposed mechanisms, sensor nodes self-organize message transmissionand sleep timings. To achieve self-organizing control of sensor networks, we apply phase-locking and anti-phase synchronization in a pulse-coupled oscillator model. Simulation experiments confirm that our mechanism significantly improves the data gathering ratio and energy-efficiency in comparison with the conventional mechanism.

Wireless sensor networks have attracted a great deal of attention. The performance of wireless sensor networks is greatly influenced by the initial physical placement of the sensor nodes. In this research, we proposed a novel deployment method for sensor nodes, when a large number of nodes is distributed from the air in a monitoring region that is inaccessible from the ground. In our deployment method, each sensor node is equipped with a parachute and a device to switch between its two falling behaviors in the air. After being dropped from midair, sensor nodes stochastically coordinate their falling behavior in order to distribute themselves uniformly in the monitoring region. Through simulation evaluations, we confirmed that our proposal can achieve a high uniformity in the placement of the nodes.

Recently, sensor devices are being ubiquitously deployed. These devices send and receive a large amount of sensor data over the network. Due to these advances of sensor and network technologies, various services that make use of those 'ubiquitous network contents' are expected to be widely developed. We studied on techniques to collect, manage, discover, and analyze those massive sensor data.