X can make an excellent thin client protocol when accessing Linux (or other Unix-like OSs). You can run Linux and X on older or stripped-down systems or use dedicated X terminals as thin clients. You can’t easily use X to access Windows servers, though, except in limited ways.

VNC, or the Remote Frame Buffer (RFB), as the protocol is more properly known, is a useful thin client tool. Linux can function as a VNC server or client, enabling you to reuse old computers. You can also run a VNC server on Windows systems to provide remote access, although VNC doesn’t provide multi-user access to Windows. (Using VNC to access Linux isn’t so limited.) Some dedicated thin client appliances support VNC, so it can provide remote access to Linux from such thin clients.

The Remote Desktop Protocol is Microsoft’s favored thin client protocol and is supported by Microsoft’s Terminal Server software package. Linux RDP client packages are available, such as rdesktop (http://www.rdesktop.org) that lets you configure a Linux system as a thin client for a Microsoft server.

The Independent Computing Architecture protocol is favored by Citrix, which provides tools to access Microsoft Windows servers using thin clients that support the ICA protocol. Citrix (http://www.citrix.com) provides a free ICA client for Linux, so Linux can function as a good ICA thin client OS.

By recycling old computers as thin clients or buying new stripped-down computers or dedicated thin clients, you can reduce your expenditures on new desktop systems and upgrades to desktop computers.

Thin clients, being much simpler than conventional desktop systems, are less likely to break down, all other things being equal. This can reduce the need for tedious hardware troubleshooting sessions.

Particularly if you use dedicated thin client hardware or old computers without hard disks, thin clients are likely to be quieter and consume less power than full desktop computers. This also results in less heat, which can help lower air conditioning bills. One partial exception to this rule is if you reuse old CRT monitors rather than buy new LCD monitors. CRT monitors consume a lot of power compared to new LCD monitors, replacing CRTs with LCDs can further reduce your power consumption.

In a thin client environment, any user can use any thin client computer. Users see their own desktop environments and their own files, no matter which thin client is used. This feature can be great in public computing centers or when you want to upgrade hardware or move users between offices.

Administering a single login server computer and dozens of thin clients is typically simpler than administering dozens of desktop computers.

In some sense, this advantage is a corollary of the last one. Simple thin client computers are less likely to become infected by worms or otherwise compromised than are typical desktop systems. This advantage is particularly great if the thin clients are simple dedicated units with their OSs in ROMs or if they can boot from files stored on a network server. If you use Linux systems with their own hard disks as thin clients, an intruder might be more likely to gain access to and modify the clients. Thin clients can also benefit by the fact that it’s seldom necessary to give them public IP addresses; they can reside on entirely private subnets that link only to your local servers and to each other. This can be even more secure than a conventional desktop behind a network address translation (NAT) router, because the thin clients can’t initiate outside connections (say, because of a viral infection).

The cost benefit on the client is at least partially offset by the fact that you’ll need to invest more in the server or servers that users will access. The login server must typically be substantially more powerful than individual workstations, although precisely how much more powerful depends on your site’s computing needs. Light uses, such as word processing, may need a server scaled to the needs of a typical user plus a small increment in RAM and CPU speed for each additional user. CPU- or RAM-intensive tasks such as running scientific simulations may not work well at all with a centralized approach unless you invest in a very powerful server. As a general rule of thumb, a high-end IA-32 system (say, one that sells for about $3,000) can usually support about 30 users.

Although thin clients are likely to be more reliable than individual desktop systems, this approach is essentially one of putting all your eggs in one basket. If the login server computer or your main network infrastructure fails, no user will be able to do any work. For this reason, you should be particularly careful in buying or building a login server for a thin client environment; buy the most reliable hardware possible, and whenever possible, employ reliability-improving tools, such as a Redundant Array of Independent Disks (RAID) array. Keep backups of your data and have backup hardware on hand so that if a critical server hardware component fails, you can quickly replace it. Using server clusters or having redundant servers can also be useful techniques to improve reliability. Also, if you use old PCs as thin clients, their reliability advantage over new desktop systems may evaporate, but the thin clients are cheap and easy to replace should they break.

Using some types of local hardware devices, such as sound cards, scanners, and CD-ROM drives, can become tricky in a thin client environment. Solutions to these problems do exist, but they often require extra configuration to work well. For instance, you might need to configure a thin client computer as a file server to give users access to a local CD-ROM drive.