[1] Electrical and Computer Eng. Dept., The Polytechnic Faculty, Aristotle University of Thessaloniki, Gr-54124 Thessaloniki, Greece
[2] Info. and Telematics Institute (ITI), 1st Km Thermi-Panorama Road, Gr-57001 Thermi-Thessaloniki, Greece
[3] Computer Science Department, University of Crete, P.O. Box 2208, Gr-71409 Heraklion, Greece
[4] School of Electrical and Comp. Eng., National Technical Univ. of Athens, Gr-15773 Athens, Greece