THE DATA LINK LAYER

This is the layer that defines the format of data, it organizes bits into frames. This is the second layer of the OSI model, the data link layer provides node-to-node data transfer, a link between two directly connected nodes on a network segment (same level on the network) and does not pass its boundary. It detects and possibly corrects errors that may occur in the physical layer. It defines the protocol to establish and terminate a connection between two physically connected devices. It also defines the protocol for flow control between them.

The Data Link Layer is often divided into two sub-layers;

• The Media Access Control(MAC)

• The Logical Link Control(LLC)

The Media Access Control: According to Wikipedia, this is the sub-layers that determine who is allowed to access a media one at a time. This is also referred to as the frame structure that delivers based on the MAC Address. The Media Access Control is then subdivided into two; divided and centralized. The MAC also performs the function of determining the start and end of each frame of data in the transmission bit stream. In short, the MAC handles interfacing with the physical layer.

The Logical Link Control: This layer provides for the addressing and control of the data link layer. The main function of the LLC is to act as an interface between the lower MAC sublayer and the network layer in the OSI model while the transfer of data is ongoing. We will discuss the network layer very soon. One of the ways the LLC performs its function is via multiplexing.

Multiplexing: This is a method by which multiple analog or digital signals are combined into one signal over a shared medium.

This method allows multiple network protocols to exist within a single multiple access point network. Additionally, while the LLC layer acts as an interface between the MAC and network layers, the MAC acts as an interface between the LLC and physical layers. This includes encapsulating frames so they are ready for transmission via the physical medium.