OSI (Open System Intercommunication) Model

In this topic, you will learn about, OSI (Open System Intercommunication) Model.

The OSI model was developed to standardize the procedure for exchanging information between processing systems. The OSI is the communication System reference model that has been defined by the international standard organization (ISO). It has seven layers of communication protocol intended as a standard for the development of a communications system worldwide. When a message is sent from one machine to another, it travels down the layers on one machine and then up the layers on the other machines. As the message travels down the first stack, each layer passes through except the physical layer and application layer, which adds a header. These headers contain pieces of control information that are read and a process by the corresponding layer on the receiving stack.

The Layers of the OSI Models are:

  1. Physical Layers: Physical Layer defines the actual sets of wires, plugs, and electrical signals that connect the sending and the receiving devices to the network.
  2. Data-Link Layer: the data link layer is responsible for gaining access to the network and transmitting the physical blocks and data from one device to another. It includes the error checking necessary to ensure accurate transmission. This layer uses communication protocols that is most commonly referenced and often imply signification for the layers as well.
  3. Network Layer: The network layer makes a routing decision as forwarding packets for devices that are farther away than the single link. A link connects the two network devices and is implemented by the data-link layer. Two devices connected by a link communicate directly with each other and not prove the third device. in a larger network there may be intermediate systems between two ends systems and the network layer makes it possible for the transport layer and the layers above it to send packets without being concerned about whether the end system is immediately adjacent.
  4. Transport Layer: The transport layer breaks large messages from the session layer into packets to be sent to the destination computer and reassembles packets into a message to be presented to the session layer. The transport layer typically sends an acknowledgment to the originator for the message received.
  5. Session Layer: The session layer establishes and terminates the session, queues of the incoming message, and is responsible for recovery from an abnormally transmitted session. This layer allows applications on separate computers to share a connection called a session. This layer provides services such as name lookup and security to allow two programs to find each other and establish the communication link. The session layer also provides for data synchronization and checkpoints so that in the event of a network failure only the data sent after the point of failure need be resent.
  6. Presentation Layer: The presentation layer translates data between the formats of the layer network required and the formats the computer expects. The presentation layer does the protocol conversion, data translation, compression and encryption, and the intercession of graphical commands. A network redirector makes the file on a file server visual to the client computer.
  7. Application Layer: The application layer is the topmost layer of the OSI model. It provides services that directly support user applications, such as database access, emails, and file transfers. It also allows application on another computer s. when a programmer writes an application program that uses network services, this is the layer the application program will access.
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