**2.1 Cloud computing**

Cloud computing is storing, accessing, and managing huge data and software applications over the Internet. Access to data is protected by firewalls. Users are still using their computers to access the cloud-hosted data/applications. The difference lies in the fact that these data/applications use no or little the storage and compute resources of these computers since they are running in the cloud.

Cloud computing provides the context of offering virtualized computing resources and services in a shared and scalable environment through the network. A big percentage of global IT firms and governmental entities have incorporated cloud services for a multitude of purposes such as those related to mission-oriented applications and thus sensitive data. In order to provide full-support for these applications and their sensitive data, it is vital to include ample provision of environments that incorporate dependable cloud computing.

The National Institute of Standards and Technology (NIST) in SP 800-145, NIST specifies that a cloud infrastructure should have the five essential characteristics listed below:


*Network Function Virtualization over Cloud-Cloud Computing as Business Continuity Solution DOI: http://dx.doi.org/10.5772/intechopen.97369*

NIST also specifies three primary cloud deployment models:


As well as three primary cloud service models:


### **2.2 Essential characteristics**

#### *2.2.1 On-demand self-service*

In cloud computing, users have the ability to provision any IT resource that they require on demand from a cloud, whenever they want. Self-service means that the consumers themselves carry out all the activities required to provision the cloud resource.

To enable on-demand self-service, a cloud provider maintains a self-service portal, which allows consumers to view and order cloud services. The cloud provider publishes a service catalog on the self-service portal. The service catalog lists items, such as service offerings, service prices, service functions, request processes, and so on.

#### *2.2.2 Broad network access*

Consumers access cloud services on any client/endpoint device from anywhere over a network, such as the Internet or an organization's private network.

#### *2.2.3 Resource pooling*

The provider's computing resources are pooled to serve multiple consumers using a multi-tenant model, with different physical and virtual resources dynamically assigned and re-assigned according to consumer demand. Usually, end-users have no knowledge about the exact location of the resources they may want to access, but they may be able to specify location at a higher level of abstraction (e.g., country, state, or datacenter). Examples of such resources include storage, processing, memory, and network bandwidth.

## *2.2.4 Rapid elasticity*

Rapid elasticity refers to the ability for consumers to quickly request, receive, and later release as many resources as needed. The characteristic of rapid elasticity gives consumers the impression that unlimited IT resources can be provisioned at any given time. It enables consumers (in few minutes) to adapt to the variations in workloads by quickly and dynamically expanding (scaling outward) or reducing (scaling inward) IT resources, and to proportionately maintain the required performance level.
