Naoki Wakamiya

Challenging issues in visual sensor networks

Wireless sensor networks are an important research area that has attracted considerable attention. Most of this attention, however, has been concentrated on WSNs that collect scalar data such as temperature and vibration. Scalar data can be insufficient for many applications such as automated surveillance and traffic monitoring. In contrast, camera sensors collect visual data, which are rich in information and hence offer tremendous potential when used in WSNs. However, they raise new challenges such as the transmission of visual data with high computational and bandwidth requirements in mainly low-power visual sensor networks. In this article we highlight the challenges and opportunities of VSNs. We discuss major research issues of VSNs, specifically camera coverage optimization, network architecture, and low-power visual data processing and communication, and identify enabling approaches in this area.

Biologically inspired self-adaptive multi-path routing in overlay networks

Using randomness to find optimal solutions in selecting network paths.

QoS Mapping between User’s Preference and Bandwidth Control for Video Transport

In this paper, we present a method of QoS mapping between user’s preference on video quality and a required bandwidth to transport the video across the network. We first investigate the mapping method from QoS parameters to the required bandwidth on the network. For this purpose, we assume that the underlying network supports some bandwidth allocation mechanism, such as DBR service class in ATM, RSVP, IPv6 and so on. Then, for given QoS parameters in terms of spatial, SNR, and timely resolutions, the required bandwidth to support the MPEG-2 video transmission is determined by analyzing the traced MPEG-2 streams. We next consider the mapping method between QoS parameters and the user’s perceived video quality, which is quantified by MOS (Mean Opinion Score) evaluation. Based on the above results, we discuss a systematic method to estimate the required bandwidth to guarantee user’s preference on video quality.

High-Speed Distributed Video Transcoding for Multiple Rates and Formats

This paper describes a distributed video transcoding system that can simultaneously transcode an MPEG-2 video file into various video coding formats with different rates. The transcoder divides the MPEG-2 file into small segments along the time axis and transcodes them in parallel. Efficient video segment handling methods are proposed that minimize the inter-processor communication overhead and eliminate temporal discontinuities from the re-encoded video. We investigate how segment transcoding should be distributed to obtain the shortest total transcoding time. Experimental results show that implementing distributed transcoding on 10 PCs can decrease the total transcoding time by a factor of about 7 for single transcoding and by a factor of 9.5 for simultaneous three kinds of transcoding rates.

Synchronization-Based Data Gathering Scheme for Sensor Networks

By deploying hundreds or thousands of microsensors and organizing a network of them, one can monitor and obtain information of environments or objects for use by users, applications, or systems. Since sensor nodes are usually powered by batteries, an energy-efficient data gathering scheme is needed to prolong the lifetime of the sensor network. In this paper, we propose a novel scheme for data gathering where sensor information periodically propagates from the edge of a sensor network to a base station as the propagation forms a concentric circle. Since it is unrealistic to assume any type of centralized control in a sensor network whose nodes are deployed in an uncontrolled way, a sensor node independently determines the cycle and the timing at which it emits sensor information in synchrony by observing the radio signals emitted by sensor nodes in its vicinity. For this purpose, we adopt a pulse-coupled oscillator model based on biological mutual synchronization such as that used by flashing fireflies, chirping crickets, and pacemaker cells. We conducted simulation experiments, and verified that our scheme could gather sensor information in a fully-distributed, self-organizing, robust, adaptive, scalable, and energy-efficient manner.

A distributed clustering method for energy-efficient data gathering in sensor networks

Since sensor nodes operate on batteries, energy-efficient mechanisms for gathering sensor data are indispensable in prolonging the lifetime of a sensor network as long as possible. In this paper, we propose a novel clustering method where energy-efficient clusters are organised in a distributed and self-organising way through local communication among sensor nodes. Our method is based on the idea of ANTCLUST, a clustering algorithm which applies the colonial closure model of ants. Through simulation experiments, we showed that our method could gather data from more than 80% of the sensor nodes, longer than other clustering methods by over 30%.

MPEG-4 Video Transfer with TCP-Friendly Rate Control

It is widely known that network bandwidth is easily monopolized by distributed multimedia applications due to their greedy UDP traffic. In this paper, we propose TCP-friendly MPEG-4 video transfer methods which enable realtime video applications to fairly shares the bandwidth with conventional TCP data applications.We consider how video applications should regulate video quality to adjust video rate to the desired sending rate which is determined by TCP-friendly rate control algorithm. Carelessly applying TCP-friendly rate variation to the video application would seriously degrade the application-level QoS. For example, the control interval should be long enough to avoid the fluctuation of video quality caused by too frequent rate control. However, popular TCP-friendly rate control algorithms recommend that a non-TCP session regulates its sending rate more than once a RTT. Through simulation experiments, it is shown that high-quality and stable video transfer can be accomplished by our proposed methods.

Resilient multi-path routing based on a biological attractor selection scheme

In this paper we propose a resilient scheme for multi-path routing using a biologically-inspired attractor selection method. The main advantage of this approach is that it is highly noise-tolerant and capable of operating in a very robust manner under changing environment conditions. We will apply an enhanced attractor selection model to multi-path routing in overlay networks and discuss some general properties of this approach based on numerical simulations. Furthermore, our proposal considers randomization in the path selection which reduces the selfishness and improves the overall network-wide performance.

Reaction-diffusion based autonomous control of wireless sensor networks

Taking into account requirements of sensor networks, we need fully-distributed and self-organising control mechanisms which are scalable to the size of a network, robust to failures of sensor nodes, and adaptive to different and dynamically changing topology and changes in wireless communication environment. To accomplish this goal, our research group focuses on behaviour of biological systems, which inherently are scalable, adaptive and robust. In this paper, we first verify the practicality of control mechanisms adopting a reaction-diffusion equation, which explains emergence of patterns on the surface of body of fishes and mammals, and then propose two methods for faster pattern generation to save energy consumption. Prom simulation and practical experiments on a prototype, it was shown that a stable pattern could be generated in a wireless sensor network in several minutes, even when packets were lost for collisions in wireless communication.

Scalable and Efficient Ant-Based Routing Algorithm for Ad-Hoc Networks

SUMMARY Ants-based routing algorithms have attracted the attention of researchers because they are more robust, reliable, and scalable than other conventional routing algorithms. Since they do not involve extra message exchanges to maintain paths when network topology changes, they are suitable for mobile ad-hoc networks where nodes move dynamically and topology changes frequently. As the number of nodes increases, however, the number of ants (i.e., mobile agents or control messages) also increases, which means that existing algorithms have poor scalability. In this paper, we propose a scalable ant-based routing algorithm that keeps the overhead low while keeping paths short. Our algorithm uses a multistep TTL (Time To Live) scheme, an effective message migration scheme, and an efficient scheme for updating the probability of packet forwarding. Simulation experiments have confirmed that our proposed algorithm can establish shorter paths than the conventional ant-based algorithm with the same signaling overhead.