Data link protocol
A data link protocol manages the interaction of at least two communications devices with a shared transmission medium. Data link protocols concentrate on the most efficient shared use, not the individual physical connections of each device to the medium. Most data link protocols have at least two sublayers, originated in IEEE Project 802 but found to have general utility.
The upper sublayer, called logical link control (LLC) by the IEEE, is basically medium-independent, but identifies the type of information carried in the media access control (MAC) frames below LLC. LLC provides a place for buffering and other features that help make the abstract service user of the data link service isolated from medium-specific technology. Minimally, the LLC header had a delimited protocol type field; the lone byte gave insufficient values and a five-byte subnetwork access protocol trailer to the LLC header gave adequate identification space.
These protocols deal with three broad types of shared media:
- point-to-point, with subsets of dedicated and on-demand (e.g., dialup)
- broadast multiacess, characteristic of local area networks
- nonbroadcast multiacess (NBMA), charateristic of frame relay and asynchronous tranfer mode