Second, the networks are built in layers, from the physical layer, which is concerned with the mechanical, electrical and optical, and functional and procedural means for managing network connections to the data, network, and transport layers, which are concerned with transferring data, routing data across networks between addresses, and ensuring end-to-end
connections and reliability of data transfer to the application layer, which is concerned with providing a particular functionality using the network and with the interface to the user.
More info: entry level network engineer
Both technology (equipment and software) suppliers and service providers tend to specialize in one or two of these layers, each of which seeks to serve all applications and all media. As a consequence, creating a new application may require the participation and cooperation of a set of complementary layered capabilities. This structure results in a horizontal industry structure, quite distinct from the vertically integrated industry structure of the Bell System era.
All these changes suggest a new definition of telecommunications: Telecommunications is the suite of technologies, devices, equipment, facilities, networks, and applications that support communication at a distance.