The ever-increasing status of the cloud computing hypothesis and the budding concept of federated cloud computing have enthused research efforts towards intellectual cloud service selection aimed at developing techniques for enabling the cloud users to gain maximum benefit from cloud computing by selecting services which provide optimal performance at lowest possible cost. Cloud computing is a novel paradigm for the provision of computing infrastructure, which aims to shift the location of the computing infrastructure to the network in order to reduce the maintenance costs of hardware and software resources. Cloud computing systems vitally provide access to large pools of resources. Resources provided by cloud computing systems hide a great deal of services from the user through virtualization. In this paper, the cloud data center is modelled as queuing system with a single task arrivals and a task request buffer of infinite capacity.