Celesti, A., F. Tusa, M. Villari, and A. Puliafito,  "An Approach To Enable Cloud Service Providers To Arrange Iaas, Paas, And Saas Using External Virtualization Infrastructures",  Services (SERVICES), 2011 IEEE World Congress on, Los Alamitos, CA, IEEE Computer Society, pp. 607?611, 9, 2011.