To support our communication, the OSI model divides the functions of a data network into layers. Each layer works with the layers above and below in order to transmit data. Two layers within the OSI model are so closely tied, that according to the TCP/IP model, they are in essence one layer. Those two layers are the data link layer and the physical layer.

On the sending device, it is the role of the data link layer to prepare data for transmission and control how that data accesses the physical media. However, the physical layer controls how the data is transmitted onto the physical media by encoding the binary digits that represent data into signals.

On the receiving end, the physical layer receives signals across the connecting media. After decoding the signal back into data, the physical layer passes the data to the data link layer for acceptance and processing.

This chapter begins with the general functions of the physical layer and the standards and protocols that manage the transmission of data across local media. It also introduces the functions of the data link layer and the protocols associated with it.