The classic OPC specifications define component interfaces based on the Microsoft (D)COM technology. (D)COM still is an important part of all Windows operating system versions. Native Windows applications need COM for inter-process communication. Because of DCOM’s roots in older binary and component-based protocols, it fails to deliver the flexibility needed in today's networked environments, is problematic with Firewalls and is cumbersome to deploy and maintain.
In 2002 Microsoft introduced .NET with Web Services and .NET Remoting as the base for distributed applications. The Windows Communication Foundation (WCF) was introduced with .NET3 with extended features for inter-process communication and Web services based interoperability, loosely coupled programming models, and communication based on standards-based protocols such as XML, WSDL, SOAP.
The OPC Foundation created the XML DA specification for OPC Data Access applications based on the Web Service technology. This was followed by the OPC UA and OPC .NET 3 (Xi) specifications. Despite these developments still most OPC applications are built on the DCOM based classic OPC DA specification. The transition to the newer web services based OPC specifications is slow but designers of new applications should be aware of the available options.