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:
- Which nodes are in communication with each other?
- When does communication between individual nodes begin and when does it end?
- Which errors occurred while the nodes communicated?
- Which nodes will communicate next?
Unlike the other PDUs that have been discussed in this course, the data link layer frame includes:
- Header: Contains control information, such as addressing, and is located at the beginning of the PDU.
- Data: Contains the IP header, transport layer header, and application data.
- Trailer: Contains control information for error detection added to the end of the PDU.
These frame elements are shown in the figure, and will be discussed in greater detail.