What is the OSI Data Link Layer?

Description: This is an article attempting to develop knowledge and understanding about the OSI model, specifically the Data Link Layer.

To understand the Data Link layer, you must first understand frames as they are what the data link layer uses. The easiest way to understand frames is to think of the train on the tracks we mentioned before. The data link layer is the cars - a frame is one car, which holds so much "cargo" or information inside.

One thing you may see on the side of a train car is where the car is headed. If your train is headed from Los Angeles to Denver, you may put "Denver" on the side of the car. The Data Link layer does this in the form of a hardware address, or MAC address. This helps to identify the specific device on the other end.

The next thing the Data Link Layer does once creating these frames and addressing them is to provide redundancy and error-checking in the form of a 4-byte frame check sequence (FCS). Using this frame check sequence, the network devices perform a cyclic redundancy check (CRC). This may sound complicated, but what it breaks down to is that when a device is sending a frame of data, it runs some math on that frame that generates a small code and sends that too. The computer that receives the data frame runs the same math on it, and makes sure that the codes match. That process is critical for making sure that there is no data corruption when transferring information through a network.

The last function of the data link layer is to verify that the frame has been received. Once the device sends the information, it waits for a message from the other end saying that it received and successfully read the frame, and that there were no errors.

Be sure to check back soon for the next layer of the OSI model - the Network layer!

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