OPC Xi (ExpressInterface) converters can be used to build highly flexible Tunneling solutions.

OPC Tunnelers are used to replace DCOM communication.
DCOM is difficult to configure in many network configurations. Users look for a simpler solution and multiple vendors offer OPC Tunneler products that address the issue.
These products typically replace DCOM with a proprietary TCP/IP based communication. There are tunneler products that only provide such a communication channel and there products that add other features such as aggregation of data from multiple OPC servers or .NET interfaces. The latter type of products are not called 'Tunnelers' but they are also designed to replace the DCOM communication.

With Tunneler products available, why use Xi as a Tunneler solution?
OPC Xi uses WCF communication instead of DCOM and provides a standardized communication solution for simple and complex network configurations.
This is exactly what a Tunneler has to provide.
A pair of OPC Xi converters build a Tunnel that replaces DCOM communication in a much more flexible way than Tunneler products that use proprietary communication protocols.

Xi Tunneler Advantages
With Xi converters no interface is vendor specific, components from different vendors can be mixed.
Xi can be configured to use any WCF supported communication protocols for:
- high performance
- high security with data encryption, and client / server authentication
- HTTP can be used for even simpler configuration of connections through firewalls

Thanks to the standardized interfaces, Tunneler modules can be eliminated when Xi servers become available and replace DA servers.