[1] Univ Crete, Dept Comp Sci, GR-71409 Iraklion, Crete, Greece
[2] Nara Inst Sci & Technol, Grad Sch Informat Sci, Nara 6300192, Japan
[3] Natl Cheng Kung Univ, Dept Comp Sci & Informat Engn, Tainan 701, Taiwan
[4] Oregon Hlth & Sci Univ, Dept Comp Sci & Elect Engn, Beaverton, OR 97006 USA
[5] France Telecom R&D, Orange Labs, F-22307 Lannion, France