Data Link Layer

Data Link Layer

The data link layer is the 2nd layer in an OSI model.  It has four primary functions.  They are communication with the network layer, segmentation and reassembly, bit ordering, and communication with the physical layer.  It does not go outside a local area network. 

Its main focus is to provide the functional and procedural means to transfer data between networks.  Some examples of these protocals are Ethernet, point to point protocol, and ADCCP connections.

There are two sub layers within the data link layer.  They are the logical link control sub layer and the media access control sub layer(MAC).

The logical link control sub layer is the uppermost sub layer.  They are the LLC multiplexes protocols that run on top of the data link layer.  They optionally provides flow control, acknowledgement, and error notifications. 

The media access control sub layer determines who is allowed to access the media at any one time.  There are two forms of media access control, they are distributed and centralized.  Both may be compared to communication between people.  In these “conversations”, the MAC looks for points where there are two connections are “talking” at the same time and forces one to pause for a random amount of time then attempt to proceed again.

This sub layer puts a specified amount of time between frames, counts remaining characters in the frame’s header, proceeds frames with a special byte sequence by the means of bit stuffing.  This bit stuffing replaces the start and end marks with flags consisting of a special bit pattern.

In summary the Data link layer allows for proper and controlled communication from a WAN to a LAN and allows the communication to continue unimpeded, provides the formatting of data, provides end-to-end delivery of data between hosts, provides delivery of data between two applications, and provides for the exchange of data over a common local media.

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