The data link layer prepares a packet for transport across the local media by encapsulating it with a header and a trailer to create a frame. The description of a frame is a key element of each data link layer protocol.

Data link layer protocols require control information to enable the protocols to function. Control information typically answers:

Unlike the other PDUs that have been discussed in this course, the data link layer frame includes:

These frame elements are shown in the figure, and will be discussed in greater detail.