Dipartimento di Ingegneria Elettrica, Elettronica e Informatica (DIEEI), CNIT (National Inter-University Consortium for Telecommunications) Catania, Italy.|Dipartimento di Ingegneria Elettrica, Elettronica e Informatica (DIEEI), CNIT (National Inter-University Consortium for Telecommunications) Catania, Italy.|Department of Computer Science and Information Systems, Teesside University, Middlesbrough, United Kingdom.|Department of Computer Science and Information Systems, Teesside University, Middlesbrough, United Kingdom.|Dipartimento di Ingegneria Elettrica, Elettronica e Informatica (DIEEI), CNIT (National Inter-University Consortium for Telecommunications) Catania, Italy.|Computer Laboratory, University of Cambridge, Cambridge, United Kingdom.