One of the most familiar SCADA (supervisory control and data acquisition) application protocols now is OPC protocol. This interface is supported by almost all SCADA, visualization, and process control systems. There are many research efforts tried to design and implement an approach to access an OPC DA server through the Internet. To achieve this goal they used diverse of modern IT technologies like XML, Web services, Java and AJAX. In this paper, we present a complete classification of the different approaches introduced in the literature. A comparative study is also introduced. Finally we study the feasibility of the realization of these approaches based on the real time constraints imposed by the nature of the problem.