Note: Asynchronous means "without respect to time". In terms of data transmission, asynchronous means that no clock or timing source is needed to keep both the sender and the receiver synchronized. Without the benefit of a clock, the sender must signal the start and stop of each character so that the receiver knows when to expect data.

A mainframe environment depends on a single computer or group of computers that can easily be centrally managed and maintained. This configuration has the additional advantage of having more security.

At its peak in the late 70s and early 80s, the mainframe (and later the minicomputer, which was a smaller, less expensive line of mainframes) market was dominated by IBM and Digital Equipment Corporation. These high-powered computers, however, came with high price tags. The cost of entry into the mainframe market was typically several hundred thousand to several million U.S. dollars. The minicomputer began to bring similar capabilities at a lower price, but the minicomputer configurations were often over ten thousand U.S. dollars as well.

Mainframes continue to play a large role in corporate computing. It is estimated that there are still 24 million dumb terminals in use worldwide. In addition, 15 million PCs are currently deployed to function primarily as mainframe terminal emulators. These dumb terminals are American Standard Code for Information Interchange (ASCII) character based devices and are often referred to as "green screens" because many display green characters.

There are several advantages of mainframes:

• Scalability, the ability to add more users as the need arises
• Centralized management
• Centralized backup
• Low cost desktop devices (dumb terminals)
• High level of security

There are several disadvantages of mainframes:

• Character based applications
• Lack of vendor operating system standards and interoperability in multivendor environments
• Expensive, with a high cost for set up, maintenance, and initial equipment
• Potential single point of failure (non-fault tolerant configurations)
• Timesharing systems, which means that there is a potential for a bottleneck

Personal Computers
With the advent of the PC, the Graphical User Interface (GUI) gained wide introduction to users. Figure  shows an example of a GUI found in Windows. The GUIs first appeared in Xerox and Apple computers. Along with the GUI, thousands of Windows-based applications were also introduced. As PC technology has improved, the power of the PC has risen to the point that it can perform enterprise level functions.

There are several advantages of PC computing:

• Standardized hardware
• Standardized, highly interoperable operating systems
• GUI interface
• Low cost devices (when compared to mainframes), low cost of entry
• Distributed computing
• User flexibility
• High productivity applications

There are several disadvantages of PC computing:

• Desktop computers cost, on average, five times as much as dumb terminals, according to some industry estimates
• No centralized backup
• No centralized management
• Security risks can be greater (physical, data access, and virus security)
• High management and maintenance costs, although they are generally cheaper to maintain than mainframes