The data-link layer

The data-link layer falls very clearly into the Media layer's grouping, as entities in this layer provide the actual transfer of data between nodes in a network. It's responsible for error detection from the physical layer, and controls the flow of bits over physical media between nodes. So, for example, in a half-duplex communication setup, an entity in the data-link layer is responsible for restricting the transfer of data in one direction while data is being transferred in the other direction. Entities in this layer almost serve as the traffic lights directing traffic over the roads of a node-to-node connection. The data-link layer is broken down even further into two sub-layers by the Institution of Electrical and Electronics Engineers (IEEE) standard 802. These two sub-layers are as follows:

• The Medium Access-Control (MAC) layer: This sub-layer controls who can transmit data through the data-layer entity, and how that data can be transmitted.
• The Logical Link Control (LLC) layer: This sub-layer encapsulates the logical protocols of network interaction. It is essentially the interface that provides the entities links as a set of abstract protocol operations.

Driving home how narrowly specific the data-link layer is in terms of its responsibilities on a network, its most common protocol is the Point-to-Point Protocol (PPP). This just highlights that entities of the data-link layer really are only concerned with facilitating the connection between two points.