Citrix Systems is changing its emphasis on virtualization. From now on, it's less about launching virtual machines in the data center and more about delivering virtual services to user desktops, it will tell attendees at its three-day user group meeting, Synergy 2008, which opens today in Houston.

Citrix Delivery Center will consist of a group of products, both existing and new software. Among the existing pieces are XenApp, for virtualizing on central servers a set of applications to be accessed by end users, and NetScaler, its software for delivering application services over the Internet. Among the new pieces will be: Citrix App Receiver and Citrix Desktop Receiver, both lightweight clients for running end user machines by receiving the virtualized applications from a central server.

It may sound simple but, so far, there are few examples of enterprises virtualizing their desktops on a large scale and keeping a variety of end users happy -- with costs savings involved. If Delivery Center works the way Citrix plans, it will accomplish just that.

Citrix is starting from a well-defined base of virtualizing applications. XenApp is the former Presentation Server, Citrix's high-end application virtualization server. With it and related software, Citrix can authenticate hundreds or thousands of end users and supply them application services over the local area network. Now, with a new hardware appliance thrown in, the Citrix Branch Repeater, it wants to start deploying virtual applications and virtual desktops out beyond the local area network to branch offices and remote workers.

Citrix Desktop Receiver will sit on a thin client at the user's desktop and offer instant-on capability that starts up in a fraction of the time it takes to warm up a PC desktop. It's the equivalent of "a set-top box, getting a signal and amplifying it for the receiving TV set," said Sanjay Uppal, VP of the Citrix Application Networking Group.

"Citrix is focused on getting a signal from the data center to the [Branch] repeater to the end user," he added, but instead of an analog TV signal, the signal will be application services from a data center server that may be far away at corporate headquarters. Citrix can deliver services from running applications, in response to user inputs, or stream an application to end users to run on their machines.

Desktop Receiver sits on an end user thin client or PC and allows the client to talk to a virtual machine assigned to the end user in a data center. The set up relies on Citrix's high-speed ICA protocol rather than Windows Terminal Services so that the user experience is closer to that of a running desktop at the user's site.

App Receiver is also lightweight client software "tuned to receive an application from anywhere," said Uppal. It is a framework for plug-ins that can connect to the core components of Delivery Center. Both existing Citrix client software and third-party client software will be redesigned as plug-ins for App Receiver, allowing it to be the intermediary for multiple user interfaces.

App Receiver is designed to "dramatically simplify the application delivery process for IT administrators" by requiring a single client footprint at the end user's location, he said. That way, an end user who wants to work with Unix applications won't need different software installed from a Windows XP user.

Bala Kasiviswanathan, Microsoft's director of branch and storage solution, Windows Server Division, said the Citrix Branch Repeater appliance is a key part of the Delivery Center's architecture. Without it, desktop virtualization implementers run into performance problems as they try to serve remote sites. By sitting between the data center and the end user, it can take one application and stream it to multiple users inside the branch office, without requiring it to be sent multiple times over the network.