Application Layer - defines overall protocols, user interface, and actions.
    HTTP (IE, Firefox), telnet & ssh (Putty), FTP (Filezilla), etc, 
    i.e display of windows that allow you to drag and drop a file name to 
      trigger the FTP transfer.

  Presentation Layer - guarantees common syntax of data transmitted.
    When submitting a program to MVS, convert ASCII text to EBCDIC. 

    Also, tasks such as data encryption or compression.

  Session Layer - provides a specific transaction between machines.
    Establishes task definition and control.
      Example :
        FTP - connect, login, transfer of multiple files without having
        to log in each time, close.

        Putty (ssh) - connect, login, continued interaction between screen 
          and keyboard, log-off and close.

      And guarantees that the file transmitted was correctly reassembled,
       i.e; if connection lost and re-established, 
         transfer continues at interruption.

    A session is maintained using a 4 number identifier.
      Source IP (network layer), source port (transport layer)
      Destination IP (network layer), Destination port (transport layer)

  Transport Layer - parses data into packets of workable size for network
    layer.

    Connects software client to server.
      Establishes the 'virtual' connection between two system.

      Service access point between individual system/software and network
      technology. i.e Support of specific protocol, FTP, ssh, HTTP, etc.

    Responsible for sequencing created packets and rebuilding them on 
      delivery with minimal errors for a particular transaction.

      Transfer of a particular file with FTP.

    Level at which communicating machines interact. 

      TCP, UDP, etc.