A commonly agreed upon framework for describing cloud computing services goes by the acronym "SPI." This acronym stands for the three major services provided through the cloud: software-as-a-service (SaaS), platform-as-a-service (Paas), and infrastructure-as-a-service (Iaas). Figure 1 illustrates the relationship between services, uses, and types of clouds.

Relevant Technologies in Cloud Computing

Cloud computing isn't so much a technology as it is the combination of many preexisting technologies. These technologies have matured at different rates and in different contexts, and were not designed as a coherent whole; however, they have come together to create a technical ecosystem for cloud computing. New advances in processors, virtualization technology, disk storage, broadband Internet connection, and fast, inexpensive servers have combined to make the cloud a more compelling solution.

Figure 2 illustrates the relevant technologies.

1) Cloud access devices

The range of access devices for the cloud has expanded in recent years. Home PCs, enterprise PCs, network computers, mobile phone devices, custom handheld devices, and custom static devices (including refrigerators) are all online. Interestingly, the growth of the iPhone and the proliferation of applications available from its App Store illustrate an improvement in terms of access to the cloud. This greater access is resulting in greater use and growth of services within the cloud. For example, you can now use skype through the iPhone, thus bringing this peer-to-peer network much closer to users, and Salesforce.com has introduced an application that allows users to access its services from the iPhone, as well as many other vendors.

2) Browsers and thin clients

Users of multiple device types can now access applications and information from wherever they can load a browser. Indeed, browsers are becoming increasingly sophisticated. Enterprise applications, such as SAP and Oracle, can be accessed through a browser interface- -a change from when a client (a so-called "fat") application needed to be loaded onto the desktop. The general population has become more familiar with the browser function and can use a discrete application, where the context is intuitive, without requiring training or user guides.

3) High-speed broadband access

A critical component of the cloud is the broadband network, which offers the means to connect components and provides one of the substantial differences from the utility computing concept of 30 years ago. Broadband access is now widely available, especially in global metropolitan areas. Nearly pervasive wireless access (e.g.. WiFi, cellular, emerging WiMAX) is available, which has established mobile devices as entry points to the IT resources of the enterprise and the cloud.

4) Data centers and server farms

Cloud-based services require large computing capacity and are hosted in data centers and server farms. These distributed data centers and server farms span multiple locations and can be linked via internetworks providing distributed computing and service delivery capabilities.

A number of examples today illustrate the flexibility and scalability of cloud computing power. For instance, Google has linked a very large number of inexpensive servers to provide tremendous flexibility and power. Amazon's Elastic Compute Cloud (EC2) provides virtualization in the data center to create huge numbers of virtual instances for services being requested. Salesforce.com provides Saas to its large customer base by grouping its customers into clusters to enable scalability and flexibility.

5) Storage devices

Decreasing storage costs and the flexibility with which storage can be deployed have changed the storage landscape. The fixed direct access storage device (DASD) has been replaced with storage area networks (SANs), which have reduced costs and allowed a great deal more flexibility in enterprise storage. SAN software manages integration of storage devices and can independently allocate storage space on demand across a number of devices.