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How It Works > How It Works - Pg. 74

74 CHAPTER 4 Industrial Network Protocols an implementation of OLE for Process Control Systems. Originally OPC was Distributed Component Object Model (DCOM) based, and many OPC systems in use today use DCOM, although OPC has more recently been updated to use an object-oriented protocol called OPC-Unified Architecture (OPC-UA). 13 OPC provides a common communications interface between diverse industrial control systems and products by leveraging Microsoft's DCOM communications API, reducing the need for device-specific drivers. In place of specific communi- cations drivers for each device, simple device drivers could be written to interface with OPC. The use of OPC therefore minimized driver development and allowed for better optimization of core OPC interfaces. 14 OPC's strengths and weaknesses come from its foundation, which is based upon Microsoft's OLE protocol. OLE is used extensively in Office document generation and is used to embed a common data set in both a Word file and an Excel spread- sheet, for example. This not only allows OPC-connected devices to communicate and interact with minimal operator feedback (as in the case of the Office docu- ments) but also presents significant security challenges. 15 How It Works OPC works in a client/server manner, where a client application calls a local process, but instead of executing the process using local code, the process is executed on a remote server. The remote process is linked to the client application and is responsi- ble for providing the necessary parameters and functions to the server, over an RPC. In other words, the stub process is linked to the client, but when a function is performed, the process is performed remotely, on the server. The server RPC func- tions then transmit the requested data back to the client computer. Finally, the client process receives the data over the network, provides it to the requesting application, and closes the session, as shown in Figure 4.13. FIGURE 4.13 Typical OPC Protocol Operation.