介绍poc-ua很好的外国文档，比较难得的，简单易懂。
1.Overview
The idea of setting up a remote OPC connection can even bring fear to those who are vastly familiar with DCOM and classic remote OPC. Out of desperation, it may even be common practice to disable firewalls and also expose the computer to unauthorized or anonymous users. Even if a firewall is in place, DCOM requires that port 135 be added to the exception list by default. This port is used for Microsoft’s Remote Procedure Call and has been the target of many malicious attacks. So how can a safe and secure remote OPC connection be established?
The solution comes in the form of a new specification known as OPC Unified Architecture (OPC UA). OPC UA does not require callbacks and the use of DCOM for remote connections. This greatly simplifies firewall configuration and completely eliminates the headache known as DCOM.
The OPC Tunnel is not a product in itself, but rather a feature that is created out of
