Whether you call it server-based, server-centric or thin-client computing, interest is growing in the technology that enables applications and desktops to be hosted on remote servers instead of local PCs.

Reduced cost of ownership is, typically, cited as the main benefit, most obviously as a result of being able to replace conventional desktop PCs with cheaper thin-client devices. But others include being able to manage applications and user desktops centrally, reducing the amount of support required and enabling product rollouts and updates to be accomplished in hours instead of days.

Enhanced security is another key benefit. And the technology is increasingly used to extend application access to remote and mobile workers, customers and partners. It's these, and other, benefits which appear to be driving renewed interest in server-based computing, causing hardware and software vendors alike to rethink their offerings.

The principles behind server-based computing are best described by comparison with the way more conventional networks operate. In those, employees get their own desktop PC able to run general office applications and custom line of business (LOB) programs locally.

The documents these programs work on and the databases they connect to are, typically, held on remote network servers, but almost all of the processing work is performed by the desktop PC.

With the server-based model, users run the same applications, but on remote network servers rather than locally. The servers can run several user sessions concurrently and all the local desktop hardware does is provide the necessary user interface, in the form of a screen, keyboard and mouse, much like old-style mainframe terminals, albeit with a more modern graphical interface.

Thin clients only have to handle the local user interface, so can be smaller and cheaper than a conventional desktop PC. Special protocols are employed to allow the graphical user interface (GUI) information to be exchanged between the server and its desktop clients.

The client hardware is referred to as thin because it doesn't need a powerful processor, lots of memory or even a hard disk to do this job. There's just a modest processor and enough Ram to handle the GUI, plus a small amount of Flash memory to store settings and, on some devices, hold drivers for local peripherals such as printers and scanners.

The operating system is also loaded from Flash memory and is typically an implementation of either Windows or Linux. Either way, it will be a slimmed-down version focused on providing the GUI support, not running applications although, as we'll discuss shortly, there are exceptions.

Along with applications it's possible to host the whole user environment on the server including the desktop, icons, individual user settings and so on. Nothing at all then needs to be stored on the local device, making it possible to support a range of different thin-client platforms, handheld devices, kiosk access and so on, yet still provide users with the same desktop they're used to.

As far as the benefits are concerned, we've summarised the chief advantages, leaving it up to you to draw your own conclusions. Similarly, when it comes to the disadvantages, there are a number of reasons the server-based approach might not be suitable and, again, we've summarised some of them.

The type of industry you work in is also relevant, with education and manufacturing often cited as particularly suited to the technology.

It's not for everyone though, and bear in mind that moving from a conventional network to server-based computing involves a lot of upheaval and can also be something of a culture shock - especially for those used to having full control of what's installed and run on their desktop PCs.

Server-based computing and Windows
When it comes to real products, Unix was the first platform to offer server-based computing. Already a multi-user operating system, all that was required was the development of an extra layer of technology, called X-Windows, to allow a GUI to be hosted remotely.

However, it took until the mid-1990s before Windows got similar facilities and then it wasn't Microsoft that led the way. Instead it was down to independent developer Citrix Systems which used expertise gained implementing server-based computing under OS/2 to produce a product called Winframe for use with the Windows NT platform.

Launched in 1995, Citrix Winframe was based on a custom NT kernel with multi-user extensions added, together with a thin-client communication protocol known as Independent Computing Architecture (ICA) which we'll talk more about shortly.

Since then Microsoft has licensed back the Citrix technology, first selling a separate Terminal Server version of NT then bundling the technology into the core server package. Renamed Windows Terminal Services, the built-in multi-user support first appeared in Windows 2000 Server and was further updated recently (and confusingly renamed back to Terminal Server) as part of the Windows Server 2003 release.

Microsoft has also dropped the Citrix ICA protocol in favour of its own Remote Desktop Protocol (RDP). ICA can still be used if you add the Citrix Metaframe product, but Microsoft has developed its own clients to use RDP, including an ActiveX client for browser use, and built support for RDP into Windows XP.

As well as a built-in thin client, the RDP facilities in Windows XP are used to support the remote assistance feature whereby users can allow another (expert) access to their desktop over the Internet. On XP Pro, RDP supports the remote desktop option, which enables users to access their office PCs when travelling or at home.

In both cases the XP system acts as the Terminal Server but is limited to hosting a single remote client session with no extra licensing required.

The Terminal Server software can be employed in a similar manner on a Windows server, specifically to support remote administration. Used this way, up to two sessions can be supported without the need for any additional licences.

However, to get full multi-user computing, additional Windows Terminal Server licences need to be purchased, with a custom licence server included to enforce compliance when configured this way.

For companies looking to try out Terminal Server, the software can be installed and used for 90 days without licensing and, although straightforward, the way the product is licensed takes a little explaining.

The server-side software itself, for example, is covered by the main Windows server licence but you need a Client Access Licence (Cal) for every user, just as when accessing a Windows server from an ordinary network PC.

In addition, however, every session hosted by the Terminal Server also requires a separate TS-Cal, which in the Windows Server 2003 version can be purchased on a per user as well as a per device basis. Prices depend on individual licensing agreements, so consult your Windows supplier for details. Also check with thin-client vendors, as licences for Terminal Server may be included.

Terminal Server and 2003
Because it's built into the Windows Server software, Terminal Server is very easy to evaluate. The fact that you can use 'soft' thin clients running on a PC or in a browser also makes it easy to trial.

However, anyone considering implementation in anger would be best advised to go for the latest Windows Server 2003 version, as it has been significantly enhanced compared to the implementation in Windows 2000.

To start with, there's the XP-like desktop, added to which the 2003 version takes full advantage of the extra performance provided by the new operating system to support more users on the same hardware. Unfortunately there are no hard and fast limits here and a degree of experimentation is required to get the hardware right.

However, tools for sizing Terminal Server hardware are available from Microsoft and there's support in Terminal Server for the latest network load balancing technology in Windows Server 2003 to further help scale the solution.

The RDP protocol has been upgraded for the latest server release too. For instance, with RDP 5 in Windows 2000 clients were limited to 256 colour displays, but with the 5.1 protocol implemented in Windows Server 2003 up to 24bit colour can be selected at a maximum resolution of 1,600 x 1,200.