vices, video sharing sites, etc. Telco cloud services. Telecom operators (i.e. telcos) can provide enterprise cloud sys- tems via managed IP networks because they have their own networks and can guaran- tee QoS end-to-end when they act as CSPs [36]. So, in this case QoS can be guaranteed end-to-end, between CSPs and CSCs. Also, telecom operators may provide QoS net- working for the federators between different cloud entities, including public, private, community, and hybrid clouds [2]. 6.8.1 QoS Metrics for Cloud Services There are certain QoS metrics that can be specified for cloud services, as given in Table 6.10. Some can be further broken down into submetrics. For example, elasticity as a metric can be split into submetrics which include provisioning interval (that is, time needed to access or release the requested cloud resources), agility, and scaling (flexibility in service provisioning with increasing or decreasing number of CSCs) [37]. For higher agility, the CSP should have enough agile cloud system that can scale with the increased workload upon the needs of the CSCs. After elasticity as first basic metric for cloud QoS, the response time can be considered as the second basic cloud metric. Further, throughput can be considered as the third basic cloud metric, which is the most important one for the cloud services and can be measured also by cloud customers (on their side, i.e. client side, considering that the cloud services are based on the Internet/IP client-server principle). Table 6.10 QoS metrics for cloud services. QoS metric Description Elasticity The ability of the CSP to accommodate the CSC's requirements Agility The ability of the cloud service to scale with the increased or decreased workload Response time It is defined as the time it takes for a request made by a client or workload generator to be received by the client Throughput Throughput should, be measured in the context of the CSC's workload, such as bit/s, packet/s, or tasks/second Availability The ability of a CSU to access the clou