Understanding the OSI Model.


In this world of interconnected computers! If we send a message from one computer to another, it passes through various parts of a network. So to understand it better, the network system is divided into various layers [Known as OSI-Layers] and the full set of layers is called an OSI MODEL. And this whole structure is divided into 7 layers and is known as  The OSI model (The Open System Interconnection).


The 7 Layers of the OSI Model

LAYER 7                                   APPLICATION
LAYER 6                                   PRESENTATION
LAYER 5                                   SESSION
LAYER 4                                   TRANSPORT
LAYER 3                                   NETWORK
LAYER 2                                   DATA LINK
LAYER 1                                     PHYSICAL


💎Application layer (Layer 7) 💎

This is the uppermost layer, or the closest layer to the user, with which user interacts. Everything in this layer is application based. Applications like your browsers (chrome, firefox, etc). This includes everything that comes under an application like WWW browsers, NFS, SNMP, Telnet, HTTP, FTP 

Eg: if there is any misconfiguration in my browser and I am unable to access the internet, it is an application layer issue!


💎 Presentation Layer (Layer 6) 💎

The presentation layer is the layer in which our application is running. In the above case, it will be the operating system as the applications like chrome, firefox, etc are running on an operating system like Windows, Linux etc. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.


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💎 Session Layer (Layer 5) 💎

This layer builds up a session or a connection between the server and the computer in which we are using the internet! A session is always formed to communicate between two connected network devices.

Like: If we are downloading something from the internet and we PAUSE our download, we are able to resume it, which is all because a session is formed and that session has not ended!

Eg:  NFS, NetBIOS names, RPC, SQL.


💎 Transport Layer (Layer 4) 💎

OSI Model, Layer 4, provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer. And also handles the amount of data incoming and outgoing in a session.

 g: SPX, TCP, UDP.


💎 Network Layer (Layer 3) 💎

This is the most important layer of the OSI model. Here all the networking part takes place like routing the connection from one point to another. As you know, any internet request is carried from one point to another by passing through various hubs and switches! So this layer is responsible for the proper routing of data. 

Eg:  AppleTalk DDP, IP ADDRESS, IPX.


💎 Data Link Layer (Layer 2) 💎

Here data packets are encoded and decoded into bits. The layer is concerned with packaging data into frames and transmitting those frames on the network, performing errors and uniquely identifying network devices using MAC address and flow control.

Eg:  PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay. 


💎 Physical Layer (Layer 1) 💎

This layer is the lowermost layer which covers all the physical device and wires! which includes the wires connecting the servers! It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards, and physical aspects.

Eg: Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.


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