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:
- Frame start and stop indicator flags: Used by the MAC sublayer to identify the beginning and end limits of the frame.
- Addressing: Used by the MAC sublayer to identify the source and destination nodes.
- Type: Used by the LLC to identify the Layer 3 protocol.
- Control: Identifies special flow control services.
- Data: Contains the frame payload (i.e., packet header, segment header, and the data).
- Error Detection: Included after the data to form the trailer, these frame fields are used for error detection.
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.