To support our communication, the OSI model divides the functions of a data network into layers.
To recap:
- The application layer provides the interface to the user.
- The transport layer is responsible for dividing and managing communications between the processes running in the two end systems.
- The network layer protocols organize our communication data so that it can travel across internetworks from the originating host to a destination host.
For network layer packets to be transported from source host to destination host, they must traverse different physical networks. These physical networks can consist of different types of physical media, such as copper wires, microwaves, optical fibers, and satellite links. Network layer 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.
Upper OSI layer protocols prepare data from the human network for transmission to its destination. The physical layer controls how data is transmitted on the communication media by encoding the binary digits that represent data link layer frames into signals. It then transmits and receives these signals across the physical media (i.e., copper wires, optical fiber, and wireless) that connect network devices.
This chapter introduces the general functions of the data link layer and the protocols associated with it. It also covers the general functions of the physical layer and the standards and protocols that manage the transmission of data across local media.