OSI MODEL


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  • Open System Interconnection (OSI)  network  architecture, developed by International  Organization for  Standardization, is  an  open standard for  communication in the network across different equipment and appliactions by different vendors.
  • Though not  widely deployed, the  OSI 7 layer model is considered the primary  network architectural model  for inter-computing  and inter-networking  commouncations.
  • In addition to the  OSI network architecture  model, there exist  other  network  architecture nodels  by many vendors, such as IBM SNA  (Systems Network Architecture), Digital  Equipment  Corporation (DEC;  now part  of HP) DNA  (Digital Netwrok Architecture), Apple  computers Apple  Talk,and  Novells  NetWare.
  • Network architecture  provides only  a  conceptual  framework for communcations  between   computers.
  • The model itself does not providew specific methods of  communcation.
  • Actual  communication is defined by various communcaiton protocols.

                                            

                            OSI MODEL IMAGE     

Physical Layar: The physical layer, the lowest laryer of the OSI model, is concerned with the transmission and receptiton of the unstructured raw bet stream over a physical medium. It describes the electrical/optical, mechanical, and functional  interfaces to the physiclal medium, and  carrues the signals  for  all of  the  higher  layers. It provides:

  • Data  encoding: moidifes the simple digital signla pattern (1s and 0s) used by the PC to tbetter accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. I determines:

(i) What signal state represent a binary 1.

(ii) How the receiving staon known  when “bit-time” starts 

(iii) How the receiving station delimits a frame

  • Physical medoum attachement, accommodating various possibilities in the medium:

  (i) Will ab external transceiver  [MAU] be used to connect to the  medium?

(ii) How many pins do the connectors  have and  waht is each pin used for?

  • Transmission technique: determines whether the encoded bits will be transmitted by baseband [digital] or broadband analog signaling.
  • Physical medium transmission: tranmits bits as electrical or optical sigmals appropriate for the physical medium, and determines:

 (i) What physical medium options can be used

(ii) HOw many volts/db should be used to represent a given signal state, using a given physical medium

2) DATA LINK LAYER

The data link layer provides error-free transfer of data frame from one node to another over the physicsal layer, allowing  layers above it to assume virtually error-free transmission over the link. to do this, the data link layer provides:

  • Link establishment and termination: estabilshes and terminates the logical link between two nodes.
  • Frame sequencing: transmits/ receives frames sequientially.
  • Frame delimiting: creates and recognizes frames boundaries
  • Media access management: determines when the node “has the right”to use tyhe physical medium.
  • Fraem error checking: checks received frames for integrity.

3) NETWORK LAYER

The network layer controls the operation of ther subnet, deciding which physical path the data  should  tale based on network conditions, priority of service, and other factors. It provides:

  • Routing: routes frames among  networks.
  • Subnet traffic control: routers can instruct   sending station to “throttle back” Its frame  transmission when router’s buffer fills up.
  • Frame fragmentation: if it determines that a downstream router’s maximum transmission unit (MTU) sizew is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
  • Logical-physical address mapping: translates logical addresses, or names into physical addresses.
  • Subnet usage accounting: has accounting functions to keep track of frames farwarded by subnet intermedate systems, to produce billing information.

4  TRANSORT LAYER

The transport layer ensures that messages are delivered error-free, in sequence, and with no loesses or duplications. It relieves the higher  layer protocols from any concern with the transfer of data between them and their peers.

The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the tramsport layer protocol should include extensive error detection and recovery.

The transport layer provides:

  • Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller unit (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
  • Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
  • Message traffic control: tells the transmitting station to “back off” when no message buffers.
  • Session multiplexing: multiplexes several message streams, oir sessions into one lolgical link  and keeps track of which message belong to which sessions.

   Typicallly, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame.

The transport layer header information must then include control information, such as message start and message end flags, to  enable the transport layer on the other end to recognize message boundaries. In  addition, if the lower layer s do not maintain sequence, the transport header must  contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.

5 ) SESSION LAYER

The session layer allows session establishment between processes running on different stations. It provides:

6) PRESENTATION LAYER

The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate the common format to a format known to the apploication layer into a common  format at the sending station, then translate the common format to a format known to the application  layer  at the receiving station.

The presentation layer provides:

  • Character code translation: for example, ASCII to EBCDIC.
  • Data conversion: bit order, CR-CR /LF, integer-floating point, and so on.
  • Data compression: reduces the number of bits that need to be transmitted on the network.
  • Data encryption: encrypt data for  security purposes. For example, password encrytion.

  7)      APPLICATION LAYER

The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:

  • Resource sharing and device redirection
  • Remote file access 
  • Remote printer access
  • Inter- process communication 
  • Network management
  • Diriectory services
  • Electronic messaging  (such as mail)
  • Network virtual terminals