Power Point 2007 presentation that explains the architecture and the features of OPC .NET 3.0 (Xi)

Video Presentation
Emerson's Lee Neitzel, who was the technical lead for the Xi project, in this 15 minutes podcast provides a layman's overview of what Xi is, why it was developed, and how it is a common interface for the OPC Data Access (DA), OPC Alarms & Events (A&E), and OPC Historical Data Access (HDA). He describes the security and performance models.
After this introduction, the presentation gets more specific and provides more detail for software developers.