- February 07, 2003
- News
Summary
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The answer to this question can be found by exploring something called Terminal Services, a standard feature added to Microsoft Server software starting with Windows 2000.
Terminal Services History
Late in the Windows NT 3.51 lifecycle, Citrix Systems out of Florida approached Microsoft with an idea for a multi-user version of Windows similar to what Citrix had been doing with Unix. As a result of that meeting, Microsoft agreed to sell them the Windows NT 3.51 source code, which Citrix then turned into Citrix WinFrame; a version of NT 3.51 that allowed multiple users to run on the same server. Microsoft later licensed the technology back from Citrix and incorporated it into a special version of NT 4.0 (known as NT 4.0 TSE, or Terminal Server Edition) and most recently into all version of Windows 2000 Server.
The Terminal Server version of a Windows operating system allows the operation of standard Windows software under the mainframe model. Users log onto the server using limited display hardware (similar to terminals) and the server creates a session in memory dedicated to that user. Any GUI commands that would normally be sent to a local graphics card are instead compressed and sent to the terminal. Likewise, user keyboard and mouse movements are sent back to the user's task running on the server.
Specialized Thin Client Hardware
These user sessions running on the server were originally connected to standard PCs. Each of these PCs ran a program to display the application screens coming from the server. It was soon realized that the processing power and installed software on these 'client' PCs was largely uninvolved in the control and display of the user interface, and, in fact, could easily get in the way by adding complexity to what was otherwise a very simple process. Hardware companies began making what they originally called Windows Terminals and later Thin Clients, so called because the original client PC was 'thinned' down by the removal of much of the memory and any disk drives. What is left is a lean mean user interface displaying machine.
Using Terminal Services and Thin clients, users can have the best of everything - centralized applications with distributed user interfaces. The benefits for the industrial customer are tremendous. Complex HMI and SCADA packages can be installed, configured and maintained once (on a server in a secure area) but can be realized anywhere on the plant floor. If one of these Thin Clients is lost or destroyed it can be replaced in a matter of minutes instead of the hours or even days that it takes to purchase and rebuild a PC. The operator gets the exact same display he is used to, and in fact cannot tell that he is running on a Thin Client instead of on a full PC.
Terminal Server Hardware and Software Requirements
What is required to make a Thin Client system? No matter which type of solution you choose to go with (more on that later) the basic skeleton of the system remains the same.
First, and most importantly, is a PC that will host the Thin Client sessions (the Terminal Server). Because of the tremendous technology advances in the last few years in entry-level machines, many installations will not even require a true server class computer. Two years ago companies like Wonderware were saying that a user could run up to 25 sessions of their HMI software on a PC with 2 Gbytes of RAM and Dual PIII 500 MHz, processors that are one half the speed available today. The computing requirements for 'lighter' applications (Excel, VB Applications, etc.) are even lower. The main requirement is that the servers have enough memory to handle the applications.
Second, the server must have a Microsoft Terminal Server operating system, which today is pretty much any version of Windows 2000 Server. This is configured at installation to provide support for multiple users.
The third thing required is the proper licensing from Microsoft. Once Microsoft decided to support multiple users on a single PC they needed a way to make up for the revenue lost on operating system sales. A Thin Client system with 1 server and 50 clients only has one Operating System, so they began to require two licenses (a CAL and a TSCAL) for every Thin Client sold.
The final necessary component is the Thin Client itself, and this is where decisions have to be made. The only companies providing a Terminal Services communication protocol are Microsoft and Citrix. Microsoft provides RDP (or Remote Desktop Protocol) for client/server communication and Citrix provides tools to use their own Terminal Server protocol, ICA (Independent Computing Architecture).
Microsoft has a software client available for free on their website called TSAC (Terminal Server Advanced Client) that is simply an Active-X plug in for Internet Explorer. Another way to get an RDP based client running is to use the Terminal Server itself to create a client that can be run as an application on most Windows platforms and can connect back to the server.
Slightly better performance can be achieved by installing the server and client side of the other Terminal Server protocol, ICA, from Citrix. This requires an application running on the server (normally Citrix MetaFrame) and an ICA client program running on each client machine.
Either of these solutions requires a full PC to host the 'Thin Client' session, which is not the best idea. While you do gain the benefits of centralized software administration, you have a client (the PC) that is still subject to the same failures as any other PC. A better solution is to move to dedicated Thin Client hardware, most often found in the form of Windows CE based computers. The commercial versions of these machines are now available for as little as $300, and the major player in this market is Wyse Technologies. The Thin Client business was a logical extension of their dominance in the standard computer terminal market.
Focused on the Industrial Market
While at least one company has brought a Thin Client product into the industrial market by simply taking a commercial Thin Client and putting it in an industrial enclosure, several companies have developed CE based versions of their industrial computers. These offerings can run either an RDP or an ICA client and display sessions running on the Terminal Server. However, by far the largest numbers of true industrial Thin Clients sold today are based on technology developed by Automation Control Products (ACP).
ACP has taken all of the inherent benefits of the Thin Client model and enhanced them specifically for the automation industry. Recognizing the importance of continuous access to the user interface, ACP eliminated the single point of failure inherent in the Terminal Server model by providing for automatic failover to backup servers. They are also the only company in the world to provide for high-speed serial communication directly from the Thin Client, eliminating the need for Ethernet to Serial converts when serial communication to industrial equipment is required. Some of the other enhancements made by ACP include:
- Industry specific I/O (Profibus, etc.) directly from the clients
- Auto login, auto configuration of new clients, and auto replacement
- Centralized support for any combination of the almost 40 models of ACP Enabled client hardware from 9 different companies
- No software resident on the clients
- Included support for the ICA protocol without the need for MetaFrame
So the answer to the original question ("How do Thin Clients run Windows software without a PC?") technically is that it doesn't. You still need a PC (the server) somewhere in the mix. A more accurate question might be posed by substituting the word 'display' for the word 'run', but then you are still discounting the server. I guess you could continue to change it until you had something like "How do multiple Thin Clients display Windows software running on a single server in separate user sessions without a full Windows Operating System (or any user applications) running locally on the client?", but then that's not nearly as catchy a title.
About The Author
This article was written and provided by Automation Control Products (ACP). ACP is a leader on the forefront of Thin Client computer technology. ACP carries a full line of industrial and desktop thin client products including their custom Thin Client configuration and management software, ThinManager™.
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