To understand the functions offered by the data link layer, it is necessary to look at the layer which precedes it; there physical layer enables, disables and monitors the physical connections between two systems. The data can thus be transferred from the sender to the recipient. However, this layer does not carry out any evaluation: it transfers the data without checking their content and without structuring them.
This is where the data link layer comes into play. She permits encode, decode and organize the different bits, but also to prepare them in the form of data frames. Thus, it creates packets of data or breaks the largest of them into smaller sets.
The data is then transferred from the data link layer to the layer which succeeds it. They are therefore addressed and transferred to the network layer. The « data link layer » ensures the correct transfer of frames and makes it possible to turn insecure system connections into secure system connections.
Communication at the data link layer can be established with or without connection. In the first case, all data to be transferred has a source address and a target address. In the second case, it is necessary to establish logical connections between the sender and the receiver beforehand.
