Interconnected heterogeneous networks are presented and their performance analyzed. The interconnected networks consist of token ring, fiber distributed data interface (FDDI), and carrier sense multiple access with collision detection (CSMA/CD) networks, each of which connects a large population of users. In the interconnected networks, the local users on the token ring send intra- or internetwork packets. The internetwork packets are transmitted to the CSMA/CD network via gateways and the FDDI network. In the meantime, the local users on the FDDI and CSMA/CD networks generate intranetwork packets. Since the maximum packet sizes of these networks are different, packet fragmentation and reassembly are sometimes necessary. These functions are performed by gateways. The model focuses on the end-to-end delay between two end users on the token ring and CSMA/CD network. Analytic models for calculating the delays for the FDDI network and the interconnected networks are presented, and the measurements from the models are compared with that from the simulation models, validating the analytic models.