Data Link Layer

The network layer encapsulates data from the transport layer and sends it down to the data
link layer (OSI Layer 2)
Data Link Layer: Accessing the Media
The data link layer provides a means for exchanging data over a common local media. The
data link layer links the many upper-layer services responsible for packaging the data for
communication between hosts with the services to transfer that data across the media.
To transfer this data across the local media, the data link layer repackages it into frames and
controls the frames’ access to the media. Because an indescribable number of physical
media exist, a wide variety of data link layer protocols define different types of frames and
different methods of controlling access to the media.
Some types of frames and data link layer services support communications for LANs; others
support communications across WANs. Some frame types are used on a specific type of
media; others can be used on multiple types of media.
So generalizing the functions of the data link layer is not an easy task. To learn about the
data link layer, this chapter examines generalizations of the data link layer as described by
the OSI model and then provides insight into different types of data link and physical technologies,
including some of the logical topologies. Finally, Ethernet is presented as an
example for the physical and data link layers.