It defines seven network layers.
All media are functionally equivalent.
The main difference is in convenience and cost of installation and maintenance. Converters from one media to another operate at this level. Layer 2 - Data Link Data Link layer defines the format of data on the network. A network data frame, aka packet, includes checksum, source and destination address, and data. The data link layer handles the physical and logical connections to the packet's destination, using a network interface.