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As users become more mobile, IT departments have had to find different ways of lowering the support cost while maintaining corporate security and software standards. One way to do this, for users that have access to a network or internet connection is through the use of Thin Client technologies.

Thin Client is a way of delivering applications to users without installing them on local devices. Instead, they are installed and run from a server, which the user connects to and then launches their application. Their keyboard and mouse inputs are captured and sent to the server which returns screen information back to them.

There are several benefits. For the user, it means that they can access applications from any machine as long as they know their username and password. They can leave applications running, log out of a session and when they return later, their desktop will be just as they left it. For the IT department, it means that applications and data are kept securely within the server farm. As the amount of bandwidth is low, it will also run over a network connection where there is very limited bandwidth such as mobile phone networks. 

While the benefits look compelling, there is a lot of careful planning and thought that needs to go into deploying any Thin Client technology. As everything is run from the server, it is critical that users are categorised and their usage patterns understood. The servers need to be correctly sized in order to support the user workloads and load balancing, while spreading the load over several machines should maintain application performance.

The Citrix Access Suite consists of several server packages. Not every package needs to be on its own server but there are three distinct server groupings. The first is the Secure Gateway. This validates the user when they log in and runs their user profile. The traffic through here tends to be very concentrated at certain times of day and, depending on the number of users, should be load balanced across multiple blades.

The second is the Access Servers where components such as the Password Manager, License Server and the Web Interface sit.

The third grouping is the Presentation Server Farm. This is a collection of servers that will execute the installed applications for the users. Applications should be spread across the farm and it must be configured for load balancing to prevent problems with server overload.

Alongside these servers will be your standard application servers such as mail, database, file and print.

To determine how many computers you need, it is important to build a profile of the workload of the users. Citrix typically breaks users into three categories, light, medium and heavy. User loads are very application dependent. In the example below we use an environment where Microsoft Office is being deployed over Thin Client.

Light users are likely to use just one application at a time and little in the way of computational resources. They are most likely to be word processing users with the occasional use of spreadsheets.

Medium users might have multiple applications simultaneously open but not too often. They would be doing word processing and spreadsheets with some graphic applications either based on the spreadsheet or the occasional presentation.

Heavy users are likely to have multiple applications open all the time, do a lot of calculations and use graphical applications such as PowerPoint.

Each of these user groups places a different set of stresses on the servers. For example, when a user logs in they will need their user profile which details what applications and resources they need. This means that the logon server requires fast disk access.

Processor and memory are then the key restrictions. Spreadsheets and graphic applications tend to be processor intensive while opening large files and running big data sets will require memory. If a user logs in, loads their desktop and then disconnects leaving applications open rather than logging out, memory will be needed to maintain the state of their desktop.

Sizing for Thin Client is not difficult, but it does require the creation of representative workloads based on users. These workloads are then run on test servers with the number of connected users slowly being increased until the server begins to exhibit a reduction in performance. Citrix and HP both provide tools to help with this.

Thin Client is an ideal solution where IT departments want control over applications and data. It also works well when users are likely to logon from a variety of machines from desktop to public access through hotel business centres and internet café's, to Internet enabled mobile devices such as phones and PDAs.