Cloud computing is frequently equated with grid computing. The latter refers to the breaking down of tasks requiring enormous computing power into smaller units which are then carried out by a network (grid) of computers in parallel. But cloud computing is more than this; it stands for a paradigm shift. An extremely diverse range of services previously handled by a local computer – enterprise software, data, memory, etc. – moves into the Internet “cloud”. The Internet connection serves as the interface between the client and the cloud servers on which the data are stored.   

The idea is not all that new. Similar models appeared a number of years ago, at that time known as “application service providing” (ASP) or “software on demand”. But because technological development and the users were not ready for such a step at that time, the concept was never accepted. Today, thanks to new web technologies such as AJAX and faster Internet service, the same ideas are experiencing a renaissance, albeit under a new name.   

Cloud computing services can step in at various stages along the value chain of ICT service providers. Currently, three models are predominant (see Figure 1, Page 30): “infrastructure as a service” (IAAS), “software as a service” (SAAS), and “platform as a service” (PAAS).  

Services, which provide the flexible use of computing power or storage capacity via the Internet are called “infrastructure as a service”. Examples of this are the web services Simple Storage Service (S3) and Elastic Compute Cloud (EC2) from Amazon.   

“Software as a service” is frequently equated with cloud computing, but it is in reality only one part of it. The concept of software as a service stands for the operation and maintenance of computer applications as a service via the Internet. The ­applications are hosted and updated on the operator’s servers. For example, Google’s Web service “Google Docs” offers a cloud-based alternative to Microsoft Office, and GMail is a web-based e-mail service which can also be used offline.   

The model “platform as a service” is aimed at the developers of Web applications rather than users. In the PAAS model, a provider makes application programming interfaces (APIs) and ­developer tools (software development kits, SDKs) available. Any developer can use them to program, test, and operate their own web applications based on the platform provider’s infrastructure. The SAAS pioneer Salesforce has been offering a platform of this type for some time now with its Web site Force.com.