Brief functioning of upper layers


Transport layer is operated by the Operating System. It is a part of the OS and communicates with the Application Layer by making system calls.
Transport Layer is called as Heart of OSI


Software layers are also called as Upper layers

  1. Session Layer (Layer 5) :

    This layer is responsible for establishment of connection, maintenance of sessions, authentication and also ensures security.
    The functions of the session layer are :
    1. Session establishment, maintenance and termination: The layer allows the two processes to establish, use and terminate a connection.
    2. Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into the data. These synchronization point help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.
    3. Dialog Controller : The session layer allows two systems to start communication with each other in half-duplex or full-duplex.

  2. Presentation Layer (Layer 6) :

    Presentation layer is also called the Translation layer.The data from the application layer is extracted here and manipulated as per the required format to transmit over the network.
    The functions of the presentation layer are :

    1. Translation : For example, ASCII to EBCDIC.
    2. Encryption/ Decryption : Data encryption translates the data into another form or code. The encrypted data is known as the cipher text and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.
    3. Compression: Reduces the number of bits that need to be transmitted on the network.

  3. Application Layer (Layer 7) :

    At the very top of the OSI Reference Model stack of layers, we find Application layer which is implemented by the network applications. These applications produce the data, which has to be transferred over the network. This layer also serves as a window for the application services to access the network and for displaying the received information to the user.
    Ex: Application – Browsers, Skype Messenger etc.
    **Application Layer is also called as Desktop Layer.

     The functions of the Application layer are :

    1. Network Virtual Terminal
    2. FTAM-File transfer access and management
    3. Mail Services
    4. Directory Services