Long description

At the top, one of the backbones, is normally operated by a backbone operator. It consists of several routers connected by high-bandwidth fiber optics, with connections to backbones operated by other (competing) telephone companies. Usually, no hosts connect directly to the backbone, other than maintenance and test machines run by the telephone company. At the medium layer, various routers from the backbone operator are connected to the regional network via medium bandwidth fiber, and some routers are connected with the routers at I S P. From routers at I S P, various A D S L lines to home P C are connected. At the lower layer, various routers from the regional network are connected to Local routers via fiber or copper wires. The local routers are connected to the Ethernet and all hosts at lower levels are connected to Ethernet.