An Information Network Overlay Architecture for the NSDL
  We describe the underlying data model and implementation of a new architecture for the National Science Digital Library (NSDL) by the Core Integration Team (CI). The architecture is based on the notion of an information network overlay. This network, implemented as a graph of digital objects in a Fedora repository, allows the representation of multiple information entities and their relationships. The architecture provides the framework for contextualization and reuse of resources, which we argue is essential for the utility of the NSDL as a tool for teaching and learning. 
Carl Lagoze, Dean B. Krafft, Susan Jesuroga, Tim Cornwell, Ellen J.   Cramer, Eddie Shin