Data Link Layer

Data link layer has a number of specific functions. These functions include providing a well defined service interface to the network layer and dealing with transmission error and regulating the flow of data so that slow receivers are not swamped by fast senders.

Data link functions provide bits that separate and identify each field in a frame.. Each frame has a frame header, a payload field (to hold  packet), and a frame trailer.

The data link layer’s function is to provide services to the network layer .The Principal Service is transferring data from the network layer on the source machine to the network layer on destination machine. On the source machine is an entity, call it a process, in the network layer that hands some bits to the data link layer for transmission to the destination. The job of the data link layer is to transmit the bits to the destination machine so they can be handed over to the network layer there.

The data link layer offers various services. The actual services offered can vary from system to system. Three possibilities that are commonly provided are

• Unacknowledged connectionless service.
• Acknowledged connectionless service.
• Acknowledged connection-oriented service.

In Unacknowledged connectionless service, the source machine sends independent frames to the destination machine without having the destination machine acknowledge them. No logical connection is established beforehand or released afterward.

Acknowledged connectionless service is reliable .When the service is offered there are still no logical connections but each frame sent is individually acknowledged. In this way, the sender knows whether a frame has arrived correctly.If it has not arrived within a specified time interval it can be sent again. This service is useful over unreliable channels such as wireless systems.

When connection-oriented service is used transfers go through three distinct phases. In the first phase the connection is established by having both sides initialize variables and counters needed to keep track of which frames have been received and which ones have not. One or more frames are actually transmitted in the second phase. In the third and final phase the connection is released freeing up the variables buffers and other resources used to maintain connection.