When data travels on the media, it is converted into a stream of bits, or 1s and 0s. If a node is receiving long streams of bits, how does it determine where a frame starts and stops or which bits represent the address?

Framing breaks the stream into decipherable groupings, with control information inserted in the header and trailer as values in different fields. This format gives the physical signals a structure that can be received by nodes and decoded into packets at the destination.

As shown in the figure, generic frame field types include:

Not all protocols include all of these fields. The standards for a specific data link protocol define the actual frame format.

Note: Examples of frame formats will be discussed at the end of this chapter.