Data Link Layer of the OSI Model: Protocol, Functions & Design

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  • 0:00 What Is the OSI Model?
  • 0:35 Data Link Layer
  • 1:15 Regulating Data Flow
  • 2:44 Protocols
  • 3:40 Data Flow to the Network Layer
  • 4:09 Lesson Summary
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Lesson Transcript
Instructor: Alisa Perry

Alisa has taught college Computer Technology and has a master's degree in Computer Science.

We all want our information to get where it is supposed to go quickly and without any problems. In network communication, this starts in the second layer, the data link layer.

What Is the OSI Model?

Networks are quite complex. The OSI, or Open Systems Interconnection, model is used to understand how networks or systems operate. In trying to understand this model, it is best to look at it based on the seven layers. The seven layers are the physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer. After breaking the OSI model down into the seven layers, focusing on one specific layer makes it easier to understand how systems communicate.

Data Link Layer

The data link layer ensures that all packets of information are passed on free of errors. It makes sure the appropriate physical protocol is assigned to the data. The data link layer is the second layer in the OSI Model. The three main functions of the data link layer are to deal with transmission errors, regulate the flow of data, and provide a well-defined interface to the network layer. When it transfers the data to the network layer, it uses timers and sequence numbers to check for errors to ensure that all data is successfully received. The data link layer has different services and protocols to complete its tasks. The protocols are the rules required to pass data successfully to the next layer.

Regulating Data Flow

The data link layer is much like the postal service. Every piece of mail or package must meet certain requirements to ensure it gets to its specified location. So, let's look at the data link layer as a postal service. First, a sender brings in a package to send to a specific destination. We will consider these packages frames of data. The packages are input into the system by a postal worker (the first phase) to establish a connection and allow input of the necessary variables to keep track as it travels to the receiver. Second, the package is put onto a conveyor belt to move it to the correct truck for shipping. Third, after it is put on the truck, it makes its way to the zip code specified on the package. The second and third phases are the transmitting of frames until they have been sent, and the connection is terminated.

Many times, like regular mail packages, frames will successfully make it to the receiver, but there are times where portions of data may be lost, or high traffic has significantly bogged down the network causing congestion. To accommodate for this, timers are put in place. In the case of mail, a date is set for the package's arrival. For the data link layer, when frames are sent, the sender will start a timer. Once the receiver gets the package, the timer is stopped. Since it is possible that all of the frames or only a portion of frames were received, a sequence number is given to identify original frames from retransmitted frames.

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