OSI (Open Systems Interconnection) model was created by the International Organization for Standardization (ISO), an international standard-setting body. It was designed to be a reference model for describing the functions of a communication system. The OSI model provides a framework for creating and implementing networking standards and devices and describes how network applications on different computers can communicate through the network media. Show
The OSI model has seven layers, with each layer describing a different function of data traveling through a network. Here is the graphical representation of these layers: The layers are usually numbered from the last one, meaning that the Physical layer is considered to be the first layer. It is useful to remember these layers, since there will certainly be a couple of questions on the CCNA exam regarding them. Most people learn the mnemonic „Please Do Not Throw Sausage Pizza Away“: So, what is the purpose of these layers? They are most commonly used by vendors. They enable them to implement some functionality into a networking device, which then enables easier interoperability with devices from other vendors.
The OSI Model is a logical and conceptual model that defines network communication used by systems open to interconnection and communication with other systems. The Open System Interconnection (OSI Model) also defines a logical network and effectively describes computer packet transfer by using various layers of protocols. In this tutorial, you will learn: Characteristics of OSI ModelHere are some important characteristics of the OSI model:
Why of OSI Model?
History of OSI ModelHere are essential landmarks from the history of OSI model:
7 Layers of the OSI ModelOSI model is a layered server architecture system in which each layer is defined according to a specific function to perform. All these seven layers work collaboratively to transmit the data from one layer to another.
Upper and Lower layers further divide network architecture into seven different layers as below
Let’s Study each layer in detail: Physical LayerThe physical layer helps you to define the electrical and physical specifications of the data connection. This level establishes the relationship between a device and a physical transmission medium. The physical layer is not concerned with protocols or other such higher-layer items. Examples of hardware in the physical layer are network adapters, ethernet, repeaters, networking hubs, etc. Data Link Layer:Data link layer corrects errors which can occur at the physical layer. The layer allows you to define the protocol to establish and terminates a connection between two connected network devices. It is IP address understandable layer, which helps you to define logical addressing so that any endpoint should be identified. The layer also helps you implement routing of packets through a network. It helps you to define the best path, which allows you to take data from the source to the destination. The data link layer is subdivided into two types of sublayers:
Important Functions of Datalink Layer:
Transport Layer:The transport layer builds on the network layer to provide data transport from a process on a source machine to a process on a destination machine. It is hosted using single or multiple networks, and also maintains the quality of service functions. It determines how much data should be sent where and at what rate. This layer builds on the message which are received from the application layer. It helps ensure that data units are delivered error-free and in sequence. Transport layer helps you to control the reliability of a link through flow control, error control, and segmentation or desegmentation. The transport layer also offers an acknowledgment of the successful data transmission and sends the next data in case no errors occurred. TCP is the best-known example of the transport layer. Important functions of Transport Layers:
Network Layer:The network layer provides the functional and procedural means of transferring variable length data sequences from one node to another connected in “different networks”. Message delivery at the network layer does not give any guaranteed to be reliable network layer protocol. Layer-management protocols that belong to the network layer are:
Session LayerSession Layer controls the dialogues between computers. It helps you to establish starting and terminating the connections between the local and remote application. This layer request for a logical connection which should be established on end user’s requirement. This layer handles all the important log-on or password validation. Session layer offers services like dialog discipline, which can be duplex or half-duplex. It is mostly implemented in application environments that use remote procedure calls. Important function of Session Layer:
Presentation LayerPresentation layer allows you to define the form in which the data is to exchange between the two communicating entities. It also helps you to handles data compression and data encryption. This layer transforms data into the form which is accepted by the application. It also formats and encrypts data which should be sent across all the networks. This layer is also known as a syntax layer. The function of Presentation Layers:
Application LayerApplication layer interacts with an application program, which is the highest level of OSI model. The application layer is the OSI layer, which is closest to the end-user. It means OSI application layer allows users to interact with other software application. Application layer interacts with software applications to implement a communicating component. The interpretation of data by the application program is always outside the scope of the OSI model. Example of the application layer is an application such as file transfer, email, remote login, etc. The function of the Application Layers are:
Interaction Between OSI Model LayersInformation sent from a one computer application to another needs to pass through each of the OSI layers. This is explained in the below-given example:
Protocols supported at various levels
Differences between OSI & TCP/IPHere, are some important differences between the OSI & TCP/IP model:
Advantages of the OSI ModelHere, are major benefits/pros of using the OSI model :
Disadvantages of the OSI ModelHere are some cons/ drawbacks of using OSI Model:
Summary
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