Layer 2: The Data Link Layer

The second layer of the OSI Reference Model is called the Data Link Layer. As all the layers do, it has two sets of responsibilities: transmit and receive. It is responsible for providing end-to-end validity of the data being transmitted, typically across a physical link.

On the transmit side, the Data Link Layer is responsible for packing instructions, data, and so forth into frames. A frame is a structure indigenous to the Data Link Layer that contains enough information to ensure that the data can be successfully sent across a physical link (such as a Local Area Network) to its destination.

Successful delivery means that the frame reaches its intended destination intact. Thus, the frame must also contain a mechanism to verify the integrity of its contents upon delivery.
Two things must happen for guaranteed delivery to occur:

1. The originating node must receive an acknowledgment of each frame received intact by the destination node.
2. The destination node, prior to acknowledging receipt of a frame, must verify the integrity of that frame’s contents.

Numerous situations can result in transmitted frames either not reaching the destination or becoming damaged and unusable during transit. The Data Link Layer is responsible for detecting and correcting any and all such errors. The Data Link Layer is also responsible for reassembling any binary streams that are received from the Physical Layer back into frames. Given that both the structure and content of a frame are transmitted, however, the Data Link Layer isn’t really rebuilding a frame. Rather, it is buffering the incoming bits until it has a complete frame.

Layers 1 and 2 are required for each and every type of communication, regardless of whether the network is a LAN or WAN. Typically, network cards inside computers implement the Layer 1 and Layer 2 functionality.