The frame header contains the control information specified by the data link layer protocol for the specific logical topology and media used.
Frame control information is unique to each type of protocol. It is used by the Layer 2 protocol to provide features demanded by the communication environment.
The figure displays the Ethernet frame header fields:
- Start Frame field: Indicates the beginning of the frame.
- Source and Destination Address fields: Indicates the source and destination nodes on the media.
- Type field: Indicates the upper layer service contained in the frame.
Different data link layer protocols may use different fields from those mentioned. For example other Layer 2 protocol header frame fields could include:
- Priority/Quality of Service field: Indicates a particular type of communication service for processing.
- Logical connection control field: Used to establish a logical connection between nodes.
- Physical link control field: Used to establish the media link.
- Flow control field: Used to start and stop traffic over the media.
- Congestion control field: Indicates congestion in the media.
Because the purposes and functions of data link layer protocols are related to the specific topologies and media, each protocol has to be examined to gain a detailed understanding of its frame structure. As protocols are discussed in this course, more information about the frame structure will be explained.