What is the OSI model?
Before knowing the OSI model, it is important to know why the OSI model was developed?
Why was the OSI model developed?
In the beginning, when we had to communicate between networks or transport data, it was possible only between devices manufactured by one company/vendors, we cannot communicate or transfer data between devices manufactured by different companies. Because each company used to implement its network model for network communication the same company was entitled to that network model.
That's why we needed a network model that could communicate between different companies' devices. Therefore a standard OSI model was developed for networking and telecommunication systems.
Why is it called OSI?
This model is called Open System Interconnection (OSI) because this model allows any two different systems to communicate with each other regardless of their internal architecture. It groups the model, logical functions, and set of rules, which are called Protocols. To establish and conduct communication between two or more systems, it is necessary to group logical functions and a set of rules. The OSI reference model is now considered a primary standard for Internetworking and Inter computing.
- The full name of OSI is Open Systems Interconnection.
- OSI model को ISO(International Organization for Standardization) ने develop किया।
- There are seven layers in the OSI model, and each layer has its own specific function.
- Each layer is self-contained so that each layer can perform the assigned task/task independently.
- The OSI model is very flexible because any protocol can be implemented in it.
The OSI reference model
Application Layer
The Application Layer is the topmost layer of the OSI Model. This layer is for those applications which are involved in the communication system.
The Application Layer is where the actual communication is initiated and reflected. The application layer takes the help of the transport and all the layers below it to communicate or transfer its data to the remote host.
Some of the services provided by the Application Layer include: E-mail, transferring files, distributing results to users, directory services, network resources, etc.
Application Layer Protocol: Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol (SMTP), Domain Name System (DNS)
Presentation Layer
The presentation layer is also known as the translation layer, as this layer acts as a data translator for the network
The main responsibility of the Presentation Layer is to provide or define data format and encryption. The presentation layer is also called the Syntax layer as it is responsible for maintaining the proper syntax of data that either receive it or transmits it to another layer.
It performs data compression to reduce the bandwidth of the data to be transmitted. The primary role of data compression is to reduce the number of bits to transmit. It is important in transmitting multimedia like audio, video, text, etc.
Presentation Layer Protocol: TLS, SSL, Telnet
Session Layer
The Session Layer allows users on different machines to establish active communication sessions between them.
Its main aim is to establish, maintain and synchronize the interaction between communicating systems.
The Session Layer manages and synchronizes conversations between two different applications.
The Session layer is responsible for establishing, managing, synchronizing and terminating sessions between end-user application processes.
Session Layer Protocol: NetBIOS, SAP
Transport Layer
The basic function of the transport layer is to accept data from the upper layer, split it into smaller units, pass these data units to the network layer, and ensure that all units reach the other end correctly.
The transport layer provides services to the application layer and takes services from the network layer. In the transport layer, the data is referenced in the form of segments. It is responsible for the end-to-end delivery of the complete message.
The transport layer also provides acknowledgment of successful data transmission and re-transmits the data if any error is found.
Transport Layer Protocol: TCP, UDP
Network Layer
The Network Layer works for the transmission of data from one host to another, located in a different network. The Network Layer controls the operation of the subnet.
The main job of this layer is to deliver packets from source to destination in multiple links. It routes signals through different channels to the other end and acts as a network controller.
It also takes care of packet routing i.e. choosing the shortest route out of the number of routes available to transmit the packet. The IP addresses of the sender and receiver are placed in the header by the network layer.
It divides outgoing messages into packets and assembles incoming packets into messages for higher levels.
The network layer works for the transmission of data from one host to another, located in a different network. It also takes care of packet routing i.e. choosing the shortest route out of the number of routes available to transmit the packet.
The network layer is implemented by networking devices such as routers.
Network Layer Protocol: IPv4, IPv6, ARP, RARP, ICMP
Data Link Layer
The Data Link Layer is responsible for delivering the message node to node. The main function of this layer is to ensure that the data transfer at the physical layer is error-free from one node to another. When a packet arrives in the network, it is the responsibility of the data link layer to deliver it to the host using its MAC address.
The Data link layer is divided into two sublayers:
1. Logical Link Control (LLC).
2. Media Access Control (MAC).
Data Link Protocol: RAPA, PPP, Frame Relay, ATM
Physical Layer
1. Physical layer is the first and lowest layer of the OSI model. This layer is also called the bit unit layer.
2. This layer establishes the physical and electrical connection between two or more devices.
3. The transmission mode (simplex, half-duplex, full-duplex mode) is also defined in this layer.
4. The network topology (bus, tree, ring, etc.) is also defined in this layer.
5. This layer also tells whether the connection will be wired or wireless.
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