Thin client computing has been around for a long time. In fact, you might say that the mainframe/terminal computing model of the early days of computing was, in essence, a thin client solution. Today’s thin clients have come a long way from those character-based “dumb terminal” systems, though. We’re now able to combine the advantages of the centralized mainframe model with the advantages of the decentralized PC network model, in which applications run and processing is done on individual desktops that access files across the network.

In this Daily Drill Down, I’ll provide a brief look at how thin client computing works, with a focus on the most recent trends and newest software and hardware in the thin client arena.

Overview of thin client computing
Thin clients are systems that can run on a very small operating system or use a very low level of system resources, while delivering high-powered computing by connecting to a server on which the actual processing is done. The client provides an interface to the user, but the local machine itself doesn’t need a fast processor or lots of memory because it’s not running the applications.

Modern thin clients provide a full graphical interface and use fast protocols for client-server communication so that, to the user, it’s as if he or she is running the programs locally.


Thin client computing is also sometimes referred to as server-centric or server-based computing.

Thin client computing was made more popular by the inclusion of Windows Terminal Services in Microsoft’s Windows 2000 Server products. Before that, you had to buy a special (expensive) edition of Windows NT that included terminal services to use Microsoft’s thin client implementation, which is based on Citrix technology.

How thin client computing works
A thin client can be a low-powered PC (for example, a 486-based computer running Windows 3.x to access a Windows 2000 desktop through a Win2K terminal server), a terminal that boots from and downloads its operating system from a boot server, or a device running Windows CE or another embedded operating system. Client (or terminal services client) software can also be run on DOS, Linux, and Macintosh systems.

Individual “sessions” or virtual machines are created on the server, so that multiple users can access their individual desktops simultaneously. Printing, audio output, etc. can be redirected to the local client machines.

Ultra-thin or zero client technologies
Ultra-thin clients such as the BUDDY systems don’t have a CPU, memory, or any storage devices. Clients that rely completely on the host server and cannot do anything on their own are sometimes called zero client technologies. The server has multiple input and output ports for monitors, keyboards, and pointing devices, and runs software that creates a virtual machine for each set of I/O devices. The zero client consists of this set of I/O devices, which send their input and receive output directly from the host machine.


For more information on zero client technology (and how it compares to traditional thin client technology), check out Cyclopstech.

Advantages of the thin client
Thin client computing provides the following advantages over running programs locally on networked PCs:

  • Cost: Hardware for thin client machines is much less expensive than a full-powered PC.
  • Management: Because all processing is done on the server, management is centralized. For example, if an application needs to be upgraded, the administrator only has to upgrade it once on the server, rather than on a number of desktop computers.
  • Reliability: Because administrators have control over the centralized system, thin clients are not subject to the application errors that often occur when users accidentally delete files or change settings on individual computers.
  • Physical size/footprint: Many of the thin client systems are small and easy to transport. A low-powered laptop that doesn’t have enough memory to run Windows 2000 can still access the Windows 2000 interface through Terminal Services.


For an example of a client that is not only thin but extremely small, check out the XtremePC EL series. It’s billed as the smallest and lightest thin client product, weighing only six ounces.

Trends in thin client computing
A few years ago, thin client computing concentrated primarily on providing access to applications on a server from low-powered desktop clients within a local network. For example, if a number of employees had computer needs that included typing documents with a word processing program, using a spreadsheet application for accounting/record keeping, sending e-mail, and perhaps using the Web for research, these users could be provided with thin clients at their desks rather than traditional PCs. The users could access applications that were running on a server, saving money in hardware costs, and making application management easier for the administrator.

As thin client implementations grew more common and more cost-effective, other uses were found:

  • Users could access programs provided by an external application service provider (ASP), eliminating the need for the company to license or maintain the application software.
  • Walk-up browsing stations could be set up in common areas of offices, hospitals, malls, etc. to provide access to designated applications.
  • Thin clients could be used as stations at Internet cafés or in libraries where a number of people need access to an application such as a Web browser or database.
  • Thin clients could be used in the educational environment, providing Internet and application access to students in the classroom at a low cost.
  • Thin clients could be used as remote access solutions, allowing users to dial or VPN into a company network from a low-powered home computer to connect to a terminal server and access network applications.
  • Thin client kiosks could be set up to provide specific information or limited access at airports, on sales floors, etc.
  • Home users who only need e-mail and/or Web capabilities and don’t need to run a “fat” operating system, or high-powered applications could use a thin client.

Components of thin client computing
Thin client computing requires a basic set of components: thin client hardware running thin client software, a network connection to a host server, and a common thin client protocol for communication between the client and server.

Thin client hardware
As we’ve seen, the thin client can take a number of hardware forms. The most popular include:

  • The Windows-based Terminal (WBT): Microsoft has released a standard for hardware vendors who make thin client products based on the Windows CE operating system.
  • Network PC (NetPC): This is a low-cost PC that can be used as a thin client (with the proper software installed) and doesn’t have a floppy or CD-ROM drive. It is capable of booting from a network server.
  • Network Computer: This is similar to the NetPC but is based on non-Intel processors and runs a Java-based operating system.
  • Wireless Tablet: This runs an embedded operating system such as Windows CE, is powered by a battery, and uses wireless technology such as 802.11 to connect to the network.
  • Computing Appliance: This is a device running preinstalled software; it usually doesn’t have a hard disk and often is designed primarily for Internet connectivity.
  • “Fat” client: This is a full-fledged PC running thin client software and protocols and connecting to a server that runs applications.


Wyse, a leading vendor of WBTs, has released a terminal that runs on the Linux operating system, for those who need more power than a CE-based terminal provides.

Thin client software
The client software required to access the host server may be preinstalled, or it may be installed as an add-on program (for example, you can install the Windows Terminal Services client or Remote Desktop Connection (RDC) client on a computer running Windows 3.x, 9x/Me, or 2000. Note that the RDC client is already installed with the operating system on Windows XP Professional. You can install the Citrix ICA client on Windows, Macintosh, Linux, and DOS computers).


Windows Terminal Services can also use the Terminal Services Advanced Client, which allows you to access the terminal server and run a terminal session through the Internet Explorer browser.

Network connectivity
There must be a network connection between the client and server; thus, both machines must be running a common networking protocol (typically, TCP/IP) and each must have a physical cabled connection or a wireless (such as radio or infrared) connection to the same network.

The network connection can also be remote, over phone or dedicated leased lines, or through a VPN, with both machines connected to the Internet.


For more information on networking, try these articles:

The host server
The server that will run the applications must be configured to allow thin client or terminal connections. For example, a Windows server must be running Terminal Services and configured through Terminal Services Manager as an application server to allow multiple connections from nonadministrative users.

Terminal Services

For more information about Windows Terminal Services see “Introducing Windows 2000 Terminal Services.”

Thin client protocols
The thin client protocol provides for transmission of the user interface from server to client and transmission of user input from client to server. The most popular thin client protocols are:

  • The Remote Desktop Protocol (RDP): This runs on TCP/IP as the underlying network protocol and is used by Microsoft for Windows Terminal Services. The content of the user interface is transmitted as bitmaps. RDP is based on the ITU T.120 protocol as defined by the International Telecommunications Union. RDP allows for cutting and pasting between the local applications and terminal applications and printing to a local printer from terminal applications.
  • The Independent Computing Architecture (ICA) protocol: The ICA protocol is much more robust and includes more functionality than RDP and is used by Citrix for their Winframe and Metaframe products. Unlike RDP, ICA will run on IPX and NetBIOS, as well as IP. It also allows for clients running on non-Microsoft platforms such as Linux, DOS, Macintosh, and JAVA-based operating systems, and performance is faster due to the use of data compression. Citrix also offers a security package (SecureICA) for encryption and authentication.
  • The X Protocol: X can be used as a thin client protocol between UNIX-based systems (X server and X terminal) but is more limited in functionality since this is not the use for which it was designed.

In addition to the networking transport protocols and thin client protocols, other protocols may come into play. For example, network security protocols, such as SSH, IPSec, and SSL/TSL can be used for added security. Mobile devices will require special protocols (802.11, IrDA, Bluetooth) for wireless communication. Bootstrap protocols are necessary for booting from the network.


For a comprehensive discussion of protocols involved in thin client networking, see this article.

Thin client security
Security is a top priority in all aspects of computer networking today; thin client computing is no exception. Some administrators are implementing thin clients as a security solution; because the clients require no local disk drives and because administrators can better control sessions that run on a centralized server, thin clients are by their very nature easier to secure than full-fledged PCs.

Even when PCs are used as the clients, features such as Windows 2000 Terminal Service’s ability to limit or restrict local device redirection and set up other security restrictions for individual users make it easier for administrators to maintain control. However, the terminal server represents a single point of failure; if its security is compromised, the desktops/computing sessions of multiple users will be affected.

Security issues to be aware of include:

  • Logon to Terminal Services. Microsoft has improved security for its .NET terminal server by including support for smart card logon authentication.
  • Secure transmission of data across the network. Standard security protocols such as IPSec can be used to encrypt data as it travels across the network. Windows 2000 Terminal Services includes RDP encryption for protecting transmissions between the terminal server and client.
  • The Citrix ICA protocol: This provides more security than RDP; because of its layered architecture, you can use any third-party encryption you prefer.


For information on securing a Windows 2000 terminal server, see “Analyze your server’s security with the Security Configuration and Analysis Snap-in.”

Thin and on the move: Wireless/mobile solutions
Mobility is the rage and wireless is the solution in today’s on-the-go world. Thin clients are perfect for mobile connectivity because their low resource requirements allow them to be compact and low-cost.

Many manufacturers are designing thin client solutions incorporating wireless technology. An example is the EXILIS wireless pen tablet that runs on Windows CE. The tablet is “ruggedized” to withstand rough handling and rough environments.

Another wireless tablet, which supports both the RDP protocol used by Microsoft’s Terminal Services and the ICA protocol used by Citrix, is the NCD ThinSTAR Voyager. The Voyager provides a high-resolution 12.1-inch Active Matrix LCD screen and runs the Microsoft Internet Explorer web browser locally.

Compact, wireless thin clients open up a new world of computer access. They can easily be carried to meetings to look up information or access e-mail, carried on rounds by doctors to connect to digital patient records or medical databases, or put to hundreds of other uses.

Thin client resources

A number of Web sites and e-zines are devoted to thin client computing. For continuously updated non-vendor-specific information, see:

For a comprehensive overview and list of links pertaining to Microsoft’s Windows Terminal Services, see the Microsoft Terminal Services Web site.

For more information about Citrix and Microsoft products and links to vendors of numerous add-on utilities for both, see the Citrix Hardcore User site.

Thin client solutions are becoming more popular than ever. Terminal Services is included in Windows 2000 server products and is further improved in .NET server. Citrix, New Moon Systems, and other vendors provide add-on products that lend even more functionality to Windows Terminal Services. Linux-based terminals, Java-based terminals, and other alternatives abound and make it easier than ever for administrators to take advantage of the cost-effectiveness and administrative control that comes with implementing thin clients. As wireless thin clients become more common and more affordable, the popularity of server-based computing is sure to increase even more.