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Daniel Eran

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A Brief History of Remote Display
Part III - Selling even more licenses: Terminal Server
Meanwhile, Microsoft realized that a considerable amount of money was being spent on new client hardware; to aim all that cash into licensing revenue instead, Microsoft teamed up with Citrix in 1997 to deliver a new wrinkle on an old idea: Windows Terminal Services.

Companies would now centralize their processing power on server hardware. This time around, it'd would be running a specialized version of Windows NT designed to provide multiple client sessions to client PCs. Thin client PCs would act like terminals in simply displaying the results of the computing work done on the server side.

Windows NT was not originally built to handle multiple Windows user sessions. Microsoft's deal with Citrix gave the company access to the Windows NT kernel source code, and Citrix hacked the NT kernel to service multiple concurrent users. The resulting multiuser Windows product was very different than Unix's support for concurrent users.

Unix had efficiently served many concurrent user sessions on a single machine for decades. To create multiuser sessions in Windows however, Citrix had to launch an entire Win32 subsystem environment for each user, creating a protected virtual machine environment on the server that consumed significant resources, typically 20-40 MB of RAM per user.

Scaling Terminal Services up to serve lots of users therefore requires tremendous hardware resources, often referred to as a "server farm," and of course, buying lots of licenses: the Windows Terminal Server licenses, the client machine licenses, and most importantly, the Client Access Licenses, for the right for a client to access the server.

Since the client is now simply displaying the results of a remote Windows session, it can be anything: a Unix workstation, a stripped Windows PC, a Mac or specialized thin client terminal appliance. The alluring idea was that companies no longer had to constantly pay for to upgrade hardware and maintain client desktops.

With thin client terminals, companies were supposed to be able to achieve a graphic version of the old text console terminals: easy to roll out, flexible, manageable units that required little or no configuration. They also provided some protection against theft, since unlike a PC, a stolen terminal is mostly worthless without its terminal server.

Microsoft later built a limited version of terminal server technology into the desktop version of Windows XP. It is artificially limited to support one remote display session, so if a user connects remotely, they have to log out the local user first. Terminal server is also used to deliver Fast User Switching in Windows XP, but FUS stops working once you join a domain. This is partly due to how Microsoft designed the Registry and how users log in; they have plans to fix this in Vista.

VNC: the other thin client


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