  In this paper, we propose a hierarchical semantic overlay network for
searching heterogeneous data over wide-area networks. In this system, data are
represented as RDF triples based on ontologies. Peers that have the same
semantics are organized into a semantic cluster, and the semantic clusters are
self-organized into a one-dimensional ring space to form the toplevel semantic
overlay network. Each semantic cluster has its low-level overlay network which
can be built using an unstructured overlay or a DHT-based overlay. A search is
first forwarded to the appropriate semantic cluster, and then routed to the
specific peers that hold the relevant data using a parallel flooding algorithm
or a DHT-based routing algorithm. By combining the advantages of both
unstructured and structured overlay networks, we are able to achieve a better
tradeoff in terms of search efficiency, search cost and overlay maintenance
cost.
