The continued appeal of Frame Relay (FR) and ATM coupled with the growing attractiveness of Ethernet as a network service has resulted in end user demand for heterogeneous WANs that support a mix of these technologies. Network service providers can capitalize on this demand by supporting service interworking across FR, ATM and Ethernet. Ciena's DN 7000 Series is an extremely cost-effective, feature-rich, and reliable solution to address carrier-grade service interworking requirements. This paper substantiates the business case for service interworking functionality, explains switching equipment requirements for service interworking and provides a "Service Interworking Technical Primer."