What layer of transmission service do Leased Lines define?

Leased lines define a service primarily at the Physical Layer. This layer is responsible for the physical connection between devices and the transmission of raw bit streams over a medium. Leased lines provide a dedicated and continuous point-to-point connection between two end points, offering reliable transmission for data without being affected by other users.

In a leased line scenario, the service includes aspects related to the physical properties of the transmission medium, such as signaling, line encoding, and synchronization. These details are crucial for ensuring reliable data transmission, which is why the Physical Layer is the most appropriate descriptor of the service provided by leased lines.

While other layers, such as the Data Link Layer and higher layers, do play roles in a complete networking model, they do not encompass the fundamental aspects of a leased line service, which is rooted in the physical aspect of the data transmission.