Although there are many different data link layer protocols that describe data link layer frames, each frame type has three basic parts:

All data link layer protocols encapsulate the Layer 3 PDU within the data field of the frame. However, the structure of the frame and the fields contained in the header and trailer vary according to the protocol.

The data link layer protocol describes the features required for the transport of packets across different media. These features of the protocol are integrated into the encapsulation of the frame. When the frame arrives at its destination and the data link protocol takes the frame off the media, the framing information is read and discarded.

There is no one frame structure that meets the needs of all data transportation across all types of media. Depending on the environment, the amount of control information needed in the frame varies to match the media access control requirements of the media and logical topology.

As shown in Figure 1, a fragile environment requires more control. However, a protected environment, as shown in Figure 2, requires fewer controls.