OSI MODEL in computer network

OSI MODEL 

What is the OSI model ?

• The full name of the OSI model is Open Systems Interconnection, developed in 1978 by ISO (International Standard Organization) and the model consists of 7 layers.

• The OSI model is a reference model for communication between two users in a network. Each layer of this model does not depend on the other layer but there is a transmission of data from one layer to another.

• The OSI model describes how to send and receive data or information in a network. All layers of the OSI model have their own different functions so that data can be easily accessed from one system to another.

• The OSI model also describes how network hardware and software function as layers with each other.

Layers of OSI MODEL

OSI model has the following 7 layers
                         

layers in OSI model

• PHYSICAL LAYER

• The physical layer is the lowest layer in the OSI model. This layer is responsible for physical and electrical connections such as: - voltage, data rates etc.

• The digital signal in this layer is converted into an electrical signal.

• In this layer, the topology of the network i.e. layout of network (size of network) is also in this layer.

• The physical layer also describes whether the communication will be wireless or wired.

• This layer is also called a bit unit.

Its functions

•  The physical layer defines how two or more devices connect physically together.

•  It defines which transmission mode will be between two devices in the network. Which one will be simplex, half-duplex, and full duplex.

    It determines the signal transmitting   information.

• Data link layer

• In OSI MODEL the data link layer is the second number layer from the bottom. This layer has two sub-layers: -

* MAC (Media Access Control)
* LLC (Logic Link Control)

• In this layer, packets of data sent by the network layer are decode and encode and this layer also ensures that these packets of data are error free.

• This layer is also called a frame unit. In this layer, two protocols are used for data transmission.

1) high-level data link control (HDLC)
2) PPP (Point-to-Point Protocol)

• Its functions

    This layer translates physical raw bit stream into packets. We call these packets frames. And this layer adds header and trailer to these frames.
    Its main function is to control flow. In this, a fixed data rate is maintained from both the receiver and sender. So that no data is corrupted.
    It also controls the error. CRC (cyclic redundancy check) is added with trailer in it so that there is no error in the data.
    When two or more devices are connected to a communication channel, this layer determines which device is given access.

• Network layer

• The network layer is the third layer of the OSI model, in this layer the switching and routing techniques are used. Its function is logical address ie I.P. Address is also to be provided.

• The data in the network layer is in the form of a packet (group of data) and the network layer is responsible for transporting these packets from source to destination.

• This layer is also called a packet unit.

• Its functions

•     Internetworking is the main responsibility of the network layer. It provides logical connection across different devices.

    It adds source and destination address to the header of the frame. The address is used to identify devices in the Internet.

•     The work of this layer is also routing. It determines the best path (s).

• Transport layer

• The transport layer is the fourth layer of the OSI model. This layer is used to transfer data from the middle of the network correctly. The function of this layer is also to provide communication between two computers.

• It is also called segment unit.

• Its functions

•     The main function of the transport layer is to transmit data from one computer to another.

    When this layer recieve the message from the upper layers, it divides the message into very abstract segments. And each segment has a sequence number so that each segment can be easily identified.

•     It provides two types of service: - connection oriented and connection less.

•     It performs both flow control and error control functions.

• Session layer

• The session layer is the fifth layer of the OSI model, which controls connections between many computers. 

• The session layer provides sessions for communication between two devices, that is, whenever a user opens any website, the session is created between the user's computer system and the website's server.

Simply put, the main function of the session layer is to see how the connections are established, maintained and terminated.

• Its function

•     The session layer that is there acts like a dialog controller. This creates a dialog between two processes.

•     It also fulfills the task of synchronization. That is, whenever an error occurs in the transmission, the transmission is repeated again.

• Presentation layer

• The presentation layer is the sixth layer of the OSI model. This layer is used for encryption and decryption of data. It is also used for data compression. This layer is related to the operating system.

• Functions

•     The function of this layer is encryption. It is used for privacy.
  

•     Its main function is also compression. Compression is very important because we can reduce its size by compressing the data.

• Application layer

• The application layer is the seventh (highest) layer of the OSI model. The main function of the application layer is to interface between our actual application and other layers.

• The application layer is closest to the end user. Protocols like HTTP, FTP, SMTP and NFS etc. come under this layer.

• This layer controls how an application accesses the network.

• Functions

•     Through the application layer, the user can access the files from the remote computer and retrieve the files.

•     It also provides the facility to forward and store email.

    Through this, we can access the directory from the database.

A non-technical thing

There are 7 layers in the OSI model, it is a bit difficult to remember them, so below you have been given an easy way to remember it easily: -
P-Pyare
D-Dost
N-Naveen
T-tumhari
S-Shaadi
P-Pe
A-Aaunga

• Advantage of OSI model

• Its benefits are as follows: -

1: - This is a generic model and is considered a standard model.

2: - The layers of the OSI model are very specific for services, interfaces, and protocols.

3: - This is a very flexible model because any protocol can be implemented in it.

4: - It supports both connection oriented and connection less services.

5: - It uses divide and conquer technique whereby all services work in different layers. This makes it easy to administrate and maintain the OSI model.

6: - Even if it is changed in one layer, it does not have any effect in the second layer.

7: - It is very secure and adaptable.

• disadvantage of OSI model

Its disadvantages are as follows: -

1; - It does not define any particular protocol.

2: - It is sometimes difficult to implement new protocols because this model was made before the invention of these protocols.

3: - In this there is duplication of services such as transport and data link layer both have error control method.

4: - All these layers are interdependent on each other.

• Characteristics of OSI model

Now we will know its characteristics. Which are as follows: -

    This model is divided into two layers. One upper layers and the other lower layers.
    Its upper layer mainly handles issues related to the application and these apply only to software. The application layer is closest to the end user.
    The lower layers of the OSI model handle data transport issues. The data link layer and physical layer apply in hardware and software. The physical layer is the lowest layer and is closest to the physical medium. The main function of the physical layer is to keep the data or information in the physical medium.