One of the most familiar SCADA (supervisory control and data acquisition)application protocols now is OPC protocol. This interface is supported by almostall SCADA, visualization, and process control systems. There are many researchefforts tried to design and implement an approach to access an OPC DA serverthrough the Internet. To achieve this goal they used diverse of modern ITtechnologies like XML, Webservices, Java and AJAX. In this paper, we present acomplete classification of the different approaches introduced in the litrature. Acomparative study is also introduced. Finally we study the feasibilty of therealization of these approaches based on the real time constraints imposed bythe nature of the problem.