2.2 The OSI Model (Contd.)

In previous tutorial, we learned about Layered Structure in communications, the OSI Model and also the 3 network support layers of the model. Now, we will proceed to next 4 layers i.e. the Transport layer and the User Support layers.

4. Transport Layer
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The transport layer is responsible for process to process delivery of the data. A process is an application running on a host. It is responsible for the service point addressing. It helps in segmentation and reassembly of the data packets. It also controls the connection type i.e. connection less or connection orientation. It is also responsible for flow and error controls.

5. Sessions Layer
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The Session Layer establishes, maintains and synchronizes the interaction among the systems that are communicating. The credential validation i.e. the login username and password verification is handled in this layer. The mechanism for controlling the conversation among the machines. This is done by defining how to start, manage and end dialogues with the help of Sessions (Dialogue Control). A check pointing mechanism is also implemented in this layer which check the discrepancy in data between the checkpoints. If there is in error in transmission of data between the checkpoints, the data id sent back to previous checkpoint (Synchronization).

6. Presentation Layer
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The Presentation Layer enables translation, compression and encryption of data. It defines the format in which the data is transferred during communication. It’s function is also to change the data from sender dependent to a common format on sender’s end. It also converts from common format to receiver dependent format on receiver’s end. It is also indulged in security enhancement by encryption/decryption of data. It also compresses data so that the number of bits transmitted increases.

7. Application Layer
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The Application layer provides services for the user. It provides a User interface that helps user to interact with the communication systems. It’s examples are the file manager, login screens etc. This layer takes the commands for the data receiving/sending from the user via the application.

These layers are the building blocks of the communicating systems. Understanding them would help you learn about the technical aspects of Networking. In coming tutorials, we will learn more about networking, protocols and addressing.

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