The second layer of the Open Systems Interconnection (OSI) Model is the Data Link Layer. This layer works with the functions provided by the physical layer below it and provides the functions to be used by the Network layer.
It is basically a protocol layer designed for transferring data between network nodes that are adjacent to each other in both the Wide Area Network (WAN) and Local Area Network (LAN).
The physical layer has been given the responsibility of detecting and correcting errors that might occur in the process of the data transmission but just as a doctor can’t treat his own illness, the physical layer cannot detect and correct the error that may occur in the physical layer. So, the Data Link Layer detects and corrects the error in the Physical Layer.
The protocol data units, which is delivered by this layer is called Data Link Frames. The transmission of these units is done within a local network area (LAN) and the delivery of these frames is possible only between the nodes that are on the same level of the network.
The connection between the nodes can consist of more than one physical line. Some of the options of these physical lines are optical fiber cable, satellite channels, copper and wire cables and microwave link.
The two sub-layers of the Data Link Layer are Logical Link Control and Media Access Control. As suggested by the name itself, Media Access Control deals with the control of media and the previous sub-layer handles protocols, flow control and error control.
The functions and services provided by the Data Link Layer are the following:
- The Data Link Frames are made from the data received from the Network Layer and the forming of the frame is achieved by breaking the data into small manageable units.
- Not only it receives data from Network Layer, but it also permits the transfer of data to it like a two-way road with a barricaded checking point.
- It is a reliable source as the delivery service data because of its’ errorless transmission service over the links.
- The buffer of the receiver can be overflowed with frames and as a result, a lot of frames can be lost. This layer provides the solution for that as well in the form of flow control in which it prevents the sending node from overwhelming the receiver node.
- Half Duplex and Full Duplex services are also provided by the Data Link.
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