Authors would like to thank Noritoshi Demizu (OMRON), Nobuhiko Tada (Panasonic/Matsushita), Youki Kadabayashi (Osaka University) and the members of the Information Network laboratory at the Graduate School of Information Science, Nara Institute of Science and Technology for their active discussion and valuable suggestions. The Authors also thank Yoshiki Sameshima and Hideki Fujioka (Hitachi Software Engineering) for their great cooperation as proofreaders.