OSI & TCP/IP Model

 Introduction of OSI Model:-
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1. The OSI model was developed by the ISO (International Organization for Standardization) in 1984.
2. This is a reference model, i.e. there is no use in its real life. In real life, you use the built-in TCP / IP (Transmission control protocol / Internet Protocol) model.
3. The OSI model is designed to understand the journey of data. Through OSI model you can understand how the data goes from one network to another. And what is the processing with the data during this time.
4. The OSI model consists of 7 layers. All these layers process anything or something with the data. And when the data reaches the same layer on the other side, it is removed from processing data. The data on every layer is known by different names.

• These layers are on both sides of the sender side and also on the receiver side. And these layers are in the descending order i.e. the last layer comes in the top and the first layer comes in the bottom.

Application Layer_(PDU = DATA):-

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1. The application layer provides an interface between the user's application and the network. Such as a web browser (Internet Explorer, Mozilla Firefox, chrome) or an email client (Outlook, Thunderbird) etc. 
2. All these applications provide you the interface to work on the network. The application of the user is not in the application layer but rather the protocol that controls user operations. 
3. The user interacts with the application and interacts with the application network. Such as opening a web address. Many protocols are used on the application layer, some of which are being given below. 

@ HTTP(Hyper text transfer protocol) @ FTP (File Transfer Protocol) @ POP 3 (Post Office Protocol) @ SMTP (Simple mail transfer protocol) @ Telnet All of these protocols are used to interact with the network. The application layer performs some tasks that are given below.

The communicating partner recognizes the application layer. # Keep track of the availability of data. # Synchronize communication. # Providing Basic Email Service # Starting File Transfer

Presentation Layer_(PDU = DATA):-

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1. Presentation layer is the 6th layer of the OSI model.  
2. This layer is responsible for the presentation of the data. This layer verifies that the data sender is sending the receiver side. For this both the receiver and the sender follow some data standards. 

Data standards of Presentation Layer is:- 

1) Text – RTF(Rich Text Format), ASCII(American Standard Code

for Information Interchange) 

2) Images – JPG(Joint Photographic Experts Group),

GIF(Graphics Interchange Format) 

3) Audio – MP3, WAV (Waveform Audio File) 

4) Movies – AVI(Audio Video Interleave), MPEG (Moving Picture

Experts Group) 

1. These are some common data standards on which both sides agree. For example, if the sender is sending an image then it should be in JPG format so that the receiver can see it. 
2. This formatting layer data. From this layer the data goes directly to the application layer, where it shows the user. Therefore, all this responsibility is of the presentation layer how the data user will be present. 
3. If sender and receiver do not support the same format then presentation layer translation and conversion services are also provided.

Functions of Presentation Layer is:-

1. It encrypts it. 
2. And compress that data.

Session Layer_(PDU = DATA):-

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1. Session layer is the 5th layer of the OSI model.
2. This layer establishes a session between the sender and the receiver, while maintaining that session until the data transfer is done and after the data transfer, it terminates the session.
3. If any session breaks in, this layer has the ability to recover it. This layer is also responsible for data synchronization.
   

Functions of Presentation Layer is:-

1. Session Establish
2. Maintain the session
3. Terminate the session

Transport Layer_(PDU = Segment):-

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1. The Transport layer is the 4th layer of the OSI model.  
2. This layer is responsible for the reliable transfer of data. It is the responsibility of this layer to get more error free in the data order.  
3. Transport layer 2 communicates like connectionless and connection oriented. 
4. For connection-less communication, UDP and connection oriented are used for TCP / IP protocols. Connectionless communication is fast but does not guarantee the data free of error and access to it properly. 
5. Connection guarantees the communication of data communication to be free and accessible.

This communication provides some services:- 

1) Segmentation - Data is converted into small sections before sending it. 

2) Sequencing - Every segment is given a sequence number. 

3) Connection establishment - Before sending data, connection is established between sender and receiver. 

4) Acknowledgment - When the segment reaches, it is acknowledged that the segment of such number has come, it does not need to be re-sent. 

5) Flow control - Data transfer rate is confirmed. 

Network Layer_(PDU = Packet):-

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1. This layer is the 3rd layer of the OSI model. This layer is responsible for network communication.
2. The network layer is converted into data packets. Network layer has two main functions, which are given below. Logical addressing - The network layer provides the IP address to travel data to the network, which is responsible for reaching the destination of IP address data. Routing - sending data from one network to another is also the responsibility of the network layer. On the network layer, an IP (Internet Protocol) is used.

Data Link Layer_(PDU = Frame):-

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1. Data link layer is the 2nd layer of the OSI model. This layer is responsible for transmitting data to the network. The Data link layer has 2 sub layers. 
2. LLC sub-layer physical layer and the rest establish a link between the above layers. 
3. Media access control - MAC sub layer controls access to a physical medium. 
4. Data link layer packs network layer data into frames. The data link layer is converted into data frames. So that data can be sent through a physical medium. This process is called framing.
5. Frames contain the hardware address of source and destination devices. Hardware address is used to uniquely identify hosts in a network. The most common hardware address is the MAC address of Ethernet.

Physical Layer_(PDU = Bit):-

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1. The Physical layer is the 1st layer of the OSI model. This layer gets converted into data bits.
2. Through this layer the data is transferred through physical mediums such as Cables etc. This is the only layer of the OSI model that establishes communication between physically .
3. NIC cards and different cables come in the devices of the Physical Layer.

The functions of the physical layer are given below. 1) Data rate - This layer defines data rate, such as how many bits will be transferred in a second. 2) Synchronization - This synchronize layer sender and receiver to bit level. 3) Signals - This sends the layer bits into converters.

Thanks & Regards

Abhishek Pathak

+91-9621134014

abhishek9621134014@outlook.com