萬代 雅希

This paper discusses a buffering strategy for a delay-tolerant multimedia sensor network (DTMSN), whose typical application is video surveillance. In DTMSN, a sensor node observes events around it and stores the data in its own buffer memory. All the data is collected to the sink. Sensor nodes have restrictions on buffer memory as well as battery capacity. The entire data size is much larger than a single node's memory size. Thus, developing a strategy for buffering satisfying these restrictions is a critical issue for DTMSN. In this paper, we propose a novel buffering scheme for DTMSN called cooperative buffering (CB). In the proposed CB, the sensor node which has a large amount of data cooperates with its neighbor nodes to buffer the data in a distributed manner. CB uses mobile sinks. The cooperatively buffered data are transmitted directly to the mobile sink when it arrives. After proposing CB, this paper discusses extension for easy collection of the sink, extension for multi source nodes, and some sink mobility strategies of sink mobility. It evaluates the power consumption performance of CB via theoretical formulation and computer simulation. As a result, we show from the results that the proposed CB can handle multimedia data while operating at low-power.

近年,指向性アンテナの使用を仮定した指向性MAC(Medium Access Control)プロトコルが提案されている.従来の指向性MACプロトコルでは,理想的なアンテナビームパターンが仮定されていた.しかし,実際のアンテナを用いた場合,サイドローブ,バックローブ等のマイナーローブの存在により,理想アンテナでは考慮されていなかった問題が発生することが考えられる.本論文では,マイナーローブが存在するアンテナビームパターンを用いて既存のMACプロトコルを評価し,問題点を定量的に明らかにする.問題点として,NAV(Network Allocation Vector)が適切に設定されないために発生する四つの問題を明らかにし,それらの問題について考察する.続いて,指向性アンテナのマイナーローブを考慮した指向性MACプロトコルを提案する.提案方式では,指向性巡回受信,NAVリクエストフレーム送信により,周辺端末との衝突を回避する.シミュレーション評価により,理想的なアンテナビームパターンを仮定して設計された従来のMACプロトコルと比較し,提案方式のスループット性能が向上することを示す.

センサネットワークにおいて,観測データの発生が地理的に偏る場合には,センサノードの電力消費に差が生じ,シンク付近のノードが他のノードに比べて早期にバッテリ枯渇を起こす.これが原因となり,長いネットワーク寿命を達成できない.この問題に対し,本論文では,複数のシンクを用い,各ノードが複数のシンクを届け先とするバケット分配送信方式(DispersiveCast方式)を提案する.DispersiveCast方式は,各シンクヘのホップ数をもとに各ノードが分散的にバケット送信割合を決定するB-DOP(Basic DispresiveCast Of Packet)送信方式と,複数のシンクに到達するトラヒックを均衡化させるようにシンクが集中的にバケット送信割合を決定するO-DOP(Optima1 DispersiveCast Of Packet)送信方式からなる.シミュレーションによって,両送信方式の性能を評価し,DispersiveCast方式がネットワーク寿命の延長を可能とすることを示す.

ワイヤレスマルチホップネットワークでは,端末のアクセス数の増加やホップ数の増加に伴い,著しくスループットが低下する問題点がある.我々は,この問題を解決するために,IEEE802.11をベースとしたAsymmetric Wireless Multi-hop Access Network(以下AWiMA Net)を提案している.AWiMA Netではアクセスポイント(以下AP)と携帯端末(以下端末)の通信を上りリンク配信区間と下りリンク配信区間に分離するノンコンテンションデータ転送方式によってデータパケットのふくそうを抑止する.更に下りリンク配信区間においてAPの信号出力を高めAPから端末へは1ホップでデータ送信を行いAPのサービス領域の拡大とスループット向上を行っている.本論文ではAWiMA Netに対するシミュレーションによって得られたスループットの特性評価を行った上で,ネットワークのスループット向上のための下りデータのバースト転送による高速化を提案する.シミュレーションにより提案方式を評価した結果,端末からAPへのデータ配信の要求間隔と,上りリンク配信区間と下りリンク配信区間で構成するサイクルタイム長の設定を適切に行うことで,提案方式によるスループットが3倍から4倍向上することを確認した.

本論文では,データ送信における電力効率を高める目的で送信電力と送信レートを適応制御するアドホックネットワークメディアアクセス制御(MAC)プロトコルを提案する.提案プロトコルでは,あらかじめ送信電力と送信レートのすべての組合せに対して,電力消費の効率を算出した送信電力レート制御テーブルを作成する.提案プロトコルでは制御フレームの交換によって得られた受信電力を用いて,送信電力レート制御テーブルを参照して,二つの送信シーケンスを使い分ける.送受信間の直接送信よりも他のノードが中継を行った方が電力効率が高い場合,リレーシーケンスを適用する.計算機シミュレーションによって提案プロトコルを評価し,提案方式の電力効率が向上できることを示す.

This paper describes a protocol testbed system called UNAGI/ESPAR for ad hoc networks, which enables the evaluation of communication protocols with a gain-controlled antenna. It incorporates ESPAR as a practical smart antenna, IEEE802.15.4/ZigBee, GPS and gyro modules to allow easy installation of different protocols. Some experimental results are also shown.

Several directional MAC protocols for ad hoc networks using directional antennas have been proposed recently. Although directional antennas have great potential such as high spatial reuse and range extension, directional MAC protocols inherently introduce new problems arising from directivity. Deafness is one of the major problems and reduces the performance, caused by a lack of state information from neighbor nodes. This paper presents two directional MAC protocols, DMAC/DA (Directional MAC with Deafness Avoidance) and RI-DMAC (Receiver-Initiated Directional MAC), which handle the deafness problem, and mainly evaluates these protocols through extensive simulation study. DMAC/DA is a proactive handling method for deafness. In DMAC/DA, WTS (Wait To Send) frames are transmitted to notify the on-going communication to potential transmitters that may experience deafness. In this paper, DMAC/DA is enhanced by the next packet notification, called DMAC/DA with NPN, to distinguish transmitters from neighbors. On the other hand, RI-DMAC handles deafness reactively using a combination of sender-initiated and receiver-initiated operations. In RI-DMAC, each node polls a potential deafness node using RTR (Ready To Receive) after the completion of every dialog. The experimental results show that our proposed protocols outperform existing directional MAC protocols in terms of throughput, control overhead and packet drop ratio.

Several directional MAC protocols for ad hoc networks using directional antennas have been proposed recently. Although directional antennas have great potential such as high spatial reuse and range extension, directional MAC protocols inherently introduce new problems arising from directivity. Deafness is one of the major problems and reduces the performance, caused by a lack of state information from neighbor nodes. This paper presents two directional MAC protocols, DMAC/DA (Directional MAC with Deafness Avoidance) and RI-DMAC (Receiver-Initiated Directional MAC), which handle the deafness problem, and mainly evaluates these protocols through extensive simulation study. DMAC/DA is a proactive handling method for deafness. In DMAC/DA, WTS (Wait To Send) frames are transmitted to notify the on-going communication to potential transmitters that may experience deafness. In this paper, DMAC/DA is enhanced by the next packet notification, called DMAC/DA with NPN, to distinguish transmitters from neighbors. On the other hand, RI-DMAC handles deafness reactively using a combination of sender-initiated and receiver-initiated operations. In RI-DMAC, each node polls a potential deafness node using RTR (Ready To Receive) after the completion of every dialog. The experimental results show that our proposed protocols outperform existing directional MAC protocols in terms of throughput, control overhead and packet drop ratio.

In recent years, several MAC (Medium Access Control) protocols using directional antennas have been proposed for mobile ad hoc networks (MANETs) including our proposed MAC protocol called SWAMP (Smart antennas based Wider-range Access MAC Protocol). These are typically referred to as directional MAC protocols. This paper first summarizes the proposed directional MAC protocols and outlines these common issues, which reduce the probability of successful transmissions, such as location information staleness, deafness and directional hidden- and exposed-terminal problems arisen due to directional transmissions. This paper formulates and analyzes the issues of directional MAC protocols, and proposes solutions of these issues, especially for location information staleness. The experimental results show that the mobility prediction and the optimization of parameters associated with location information staleness, such as the beamwidth and lifetime of the table information, may mitigate location information staleness and improve the overall network performance.