The OSI Model
Applying the OSI Model
 

  • Each layer of the model applies its own header, and sometimes a trailer, containing information about addressing, routing, flow control, and so on.

  • A software application requests data from a server, e.g. email client retreiving mail from a server.
    The Application Layer formats the request for the server, adding an Application header, to formm a PDU (Protocol Data Unit) which is passed on to ...

  • The Presentation Layer then adds a Presentation header with information about encryption, graphics information, as needed, and passes the modified PDU on to ...

  • The Session Layer then adds a Session header, containing information about the connection (e.g. baud rate, authorization, node IDs) and passes the modified PDU on to ...

  • The Transport Layer uses segmentation to optimize the packet size for transmission, adds checksum, flow control and acknowledgement data to the PDU and passes the modified PDU on to ...
Data Transfrmation through the OSI Model 
        {From: Network+ Guide to Networks, 4th Edition, pg 57}
 
  • The Network Layer adds logical addressing information, at which point the PDU becomes a Packet, which is passed on to ...

  • The Data Link Layer adds a header and trailer, containing data for error checking, to the packet to make a frame, and passes the modified PDU on to ...

  • The Physical Layer) does not add anything to the frame but takes care of the actual transmission (and reception) thereof.
  • The process is then reversed at the receiving end.