Layer 2 protocols specify the encapsulation of a packet into a frame and the techniques for getting the encapsulated packet on and off each medium. The technique used for getting the frame on and off media is called the media access control method.
As packets travel from source host to destination host, they typically traverse over different physical networks. These physical networks can consist of different types of physical media such as copper wires, optical fibers, and wireless consisting of electromagnetic signals, radio and microwave frequencies, and satellite links.
The packets do not have a way to directly access these different media. It is the role of the OSI data link layer to prepare network layer packets for transmission and to control access to the physical media. The media access control methods described by the data link layer protocols define the processes by which network devices can access the network media and transmit frames in diverse network environments.
Without the data link layer, network layer protocols such as IP, would have to make provisions for connecting to every type of media that could exist along a delivery path. Moreover, IP would have to adapt every time a new network technology or medium was developed. This process would hamper protocol and network media innovation and development. This is a key reason for using a layered approach to networking.
The animation in the figure provides an example of a PC in Paris connecting to a laptop in Japan. Although the two hosts are communicating using IP exclusively, it is likely that numerous data link layer protocols are being used to transport the IP packets over various types of LANs and WANs. Each transition at a router may require a different data link layer protocol for transport on a new medium.