The basic concept behind WinFrame is simple. Citrix licensed NT Server from Microsoft and modified the kernel components to let multiple users concurrently sign on to an NT Server system. Then Citrix invented a protocol called Independent Computing Architecture (ICA) to transport screen, video, and mouse information between the server and a client system. The client system can be another computer running client-side ICA software, or it can be a Windows terminal, which is a terminal-like device that supports the ICA protocol.
Originally there was WinFrame, and that was that. But WinFrame did not address all of the needs in the market. Specifically, WinFrame did not support connections from X terminals. To address that market segment, Citrix licensed WinFrame to Tektronix, NCD, and Insignia. These three companies then added support for the X11 protocol (as well as other bells and whistles) and marketed their WinFrame variants under different names (WinDD, WinCenter, and Ntrigue, respectively).
Separately from Citrix, another company, Prologue, also licensed NT Server and produced a multiuser version that supports only X terminal clients. This product is called WiNTimes. As in the case of WinFrame, Prologue licensed its technology to several companies, so WiNTimes is available from Exodus as Nterprise and is available from several other companies. See Mark Smith, "Thin Is In," September 1997 for more details on multiuser NT variants.
The one element all multiuser NT products have in common is that they are based on the 3.51 version of NT Server. Microsoft refused to license the NT 4.0 code to Citrix and Prologue and instead launched a full-scale effort to deliver a multiuser version of NT Server. To accomplish this goal, Microsoft licensed technology from both Citrix and Prologue. Thus, when the Microsoft multiuser product, Windows-based Terminal Server (formerly code-named Hydra), comes to market, it will be based on NT Server 4.0 and will feature core technology adapted from Citrix and Prologue. Citrix and Prologue will then be in a position to sell add-on products to Hydra to extend its functionality.