Web Services are a high-level application-to-application communication technology embraced by virtually all platform providers including Microsoft, IBM, Sun, and Linux.
XML (eXtensible Markup Language) is one of the key enabling technologies for achieving global inter-connectivity and it has industry-wide support as a standardized way of handling data among systems using a variety of operating systems. On a higher layer the SOAP specification defines remote procedure call messages in a platform independent fashion, using XML. Web services such as XML DA or UA base on SOAP and are defined in WSDL documents.

XML DA
In October 1999 the OPC Foundation announced that it will publish XML schema based on OPC specifications that define application interoperability and communications between disparate industrial automation devices, systems and applications running across the manufacturing enterprise.
In July 2003 the first version of the XML DA specification was released.
XML DA specifies a rather simple web service with eight methods that handle all of the features typically used in the OPC Data Access applications.

The XML DA specification didn't get the attention it deserves, mainly because the OPC Foundation announced the United Architecture specification shortly after the release of XML DA and allocated its resources on this new specification. Another reason is performance concerns. Due to the XML based communication XML DA is much slower than the COM based OPC DA. This caused users to decide for proprietary tunnel solutions instead of XML DA. However, it needs to be noted that XML DA is actually fast enough for many OPC data access applications.

XML DA Characteristics

• Simple to implement
• Limited functionality, Data Access only
• Covers the requirements of most OPC Data Access applications
• Can be used with web service extensions for secure communication
• Much slower than COM based OPC
• Products available

WCF (Windows Communication Foundation) introduced in .NET3 eliminates many of the web services short comings. The communication can now be configured for either high performance and/or security.

The XML DA specification defines SOAP messages for requests and responses. With .NET2 the communication is restricted the HTTP transport. With .NET3 (WCF) the communication can be configured in a wide range to include TCP and Pipe transports and many security options. The WCF basicHttp binding is compatible with .NET2.

Advosol OPC XML DA Products
Advosol offers software development tools for XML-DA clients, XML-DA Servers and Gateways (converters). The products are provided in two version, for IIS web services and for WCF.

• XML-DA server-side Gateway
  The XDAGW-SS gateway enables OPC-DA servers to be accessed from XML-DA clients. Using the gateway is the quickest way to make an existing OPC DA server accessible from a remote location.
  XDAGW-SS can be configured to restrict access to a item subset or allow only read access.
  
  
• XML-DA client-side Gateway
  The XDAGW-CS gateway allows OPC DA clients to access XML-DA servers. Only minimal configuration is required. XDAGW-CS is OPC DA V2.05 and V3 compliance tested.
  
  
• XML-DA Client Component
  The XMLDA.NET client component allows XML-DA clients to access local OPC DA V2 server directly, without going through XML serialization, offering highly improved performance.
  Clients can be developed as state of the art web services clients even if OPC DA COM server need to be accessed.