Communication model :
In the early days of computer networking, various manufacturers made communication software in their own way. Due to the different protocols of these hardware-based software, the software made by one company could not exchange data with the software made by another company. This would be a big problem. Later, a committee was formed from ISO (International Standard Organization) to eliminate this problem. This committee created a protocol model called OSI (Open System Interconnection) to create standard standards for harmonizing all communication software. This model is known as communication model.
OSI (Open System Interconnection) Model: The OSI model describes how communication between computers and other networking devices will be developed. The International Organization for Standards (ISO) started working on this model in 1974. The need for the OSI model is felt so that different networking device manufacturers can produce their devices following the same standard. The OSI model was finalized in 1977 after discussions and debates in various circles.
The OSI model is divided into seven layers. Its layers are-
(i) Application Layer
(ii) Presentation Layer
(iii) Session Layer
(iv) Transport Layer (Transport Layer)
(v) Network Layer (Network Layer)
(vi) Data Link Layer (Data Link Layer)
(vii) Physical Layer (Physical Layer)
Each layer has specific devices performing specific tasks. One layer of the OSI model is independent of the other and not dependent on the other. Each layer performs its own task and generates data for the layer above or below it.
Different Layers of OSI Reference Model: In the OSI model each layer is considered as an independent layer. In theory, protocols at one layer can be interchanged with protocols that operate at that layer.
Different levels of OSI reference model work: Application Layer (Application Layer) This is the seventh layer of the OSI model. Its job is to work as a window between the user and the network service, so that if the user wants a network service from an application, he can send it to the next layer of the OSI model. For example, if a mail client wants to compose and send, it will first go to the application layer.
Functions of Application Layer:
(i) Resource Sharing and Device Re-direction
(ii) Remote file, printer access
(iii) Interprocess Communication (IPC)
(iv) Network Management
(v) Directory Services.
Presentation Layer (Presentation Layer): This layer acts as a data translator for network services. At the sending computer, this layer converts the data from the application layer into a form understandable by the layers below it. Again the presentation layer converts the data into a format understandable by the application layer.
Functions of Presentation Layer:
(i) (character) code translation, such as conversion from ASCII to EBCDIC format.
(ii) Data conversion, such as integer to float.
(ii) Downstream compression, so that data can be transferred faster over the network.
(iv) Data encryption (Encryption) and Decryption (Decryption), so that the data transmitted in the network is safe.
Session Layer (Session Layer): This layer handles all types of messages that need to be exchanged from the start of the data transfer process to the end. This message can be one-way or two-way. The function of this layer is to ensure the data exchange to the users at both ends when the data transfer is done. The session layer handles message synchronization. After sending several data packets, the session layer of the source device waits for an acknowledge message from the session layer of the receiving device. After receiving an acknowledge message from the session layer of the receiving device, the source starts sending data again. In this way the message synchronization takes place at a fixed time interval.
In addition, the session layer provides all the support required for various processes to work on the network, such as user name and password verification, resource security verification, etc.
Transport Layer (Transport Layer): The fourth layer of the OSI model is the transport layer, ensuring that the data received from the last layer reaches the other device reliably. To do this, the transport layer divides the data received from the session layer into smaller pieces, so that it can be sent by subsequent layers to other networks. The transport layer uses two types of transmission to ensure that data is sent from one device to another. Namely - connection oriented (Connection-oriented) and connectionless (connectionless). This transport layer also determines what the communication speed will be between the two devices.
Network Layer: This is an important layer of OSI model. The work of this layer is addressing and packet delivery. At this level, the data packet is called a datagram. The datagram is added to the network to which the packet will go. The address used in the network layer is called network address.
The function of network layer is--
(i) By encapsulation by adding the network address to the data packet.
(ii) Maps the network address to the physical address of the device.
(ii) If the network address is different from the current network address then it is the job of this layer to determine the correct path to reach that network.
Data link layer (Data Link Layer): The function of the data link layer is to send the diagram from one device to another device without error through the physical layer. The Data Link Layer is divided into two sublayers. namely--
(i) Logical Link Control : Establishes and maintains the connection of communication devices. This layer is also responsible for error correction and hardware addressing.
(ii) Media Access Control: The process of sharing the data medium (Media) channel by the network devices is done in this sub-layer.
Physical Layer (Physical Layer): The lowest layer of the OSI model is the physical layer. This layer connects one device to another. This layer determines the method by which data will be transmitted from one device to another, how the electric signal or data bit format will be, etc. The physical layer carries the signal to the next layer. It makes physical connections with physical components like twisted pair cable, fiber optic cable, coaxial cable, wireless media etc.
TCP/IP (TCP/IP) model: TCP/IP (TCP/IP) model is actually Set of Communications Protocol. which is used in the Internet and similar networks. The real meaning of TCP/IP model is Transmission Control Protocol/Internet Protocol. The TCP/IP model was developed as part of ARPANET under the American Department of Defense (DOD). A comparison is made with the OSI model to understand this currently widely used protocol. The TCP model is called the DoD model. The OSI model has seven layers, but the TCP/IP model has four layers. These four layers are compared to the seven layers of the OSI model
Layers of TCP/IP Model: The 4 layers of TCP/IP model are--
(i) Application/Process Layer
(ii) Transport layer
(iii) Internet layer
(iv) Network interface layer
(i) Application/Process Layer (Application or Process Layer): The application layer of the TCP/IP model is sometimes called the process layer. Various application level services such as Hypertext Transfer Protocol (HTTP), DHCP, DNS, FTR, IMAP, NTP etc. protocols are used in this layer.
(ii) Transport Layer: The transport layer of TCP/IP model works like the transport layer of OSI model. It uses two protocols – Connection Oriented Transmission Control Protocol (TCP) and Connectionless.
User Datagram Protocol (UDP). Oriented TCP used for connection-oriented services, such as FTP. And connectionless services, such as email, use connectionless UDP.
(iii) Internet Layer (Internet Layer): The network layer of OSI model is called Internet Pair in TCP/IP model. Its function as network layer is IP addressing and packet delivery. This layer uses a logical IP addressing scheme to identify each computer on the Internet. The protocols used in this layer are IP (IPV4, IPV6), ICMP, ICMP6. IGMP etc.
(iv) Network Interface Layer (Network Interface Layer): Network interface layer of TCP/IP model with data link layer of OSI model and physical layer. Its job is to transmit data on the network. Internet layer after properly routing the data is the responsibility of this layer to deliver it to the device.
Essentials of the OSI Reference Model:
In the initial stage there was no international software with specific rules for networking. By doing this, one company started to develop network software and release it in the market. As a result, a problem for users is that one company's computer could not be networked with another company's computer. The problem is that even if the software of all computers is not the same, data cannot be exchanged. The network had to face huge difficulties. In other words, there is a huge problem in data exchange in both international and national fields. To eliminate this difficulty, an organization named International Standard Organization - ISO was formed in 1978. This organization made some rules, by following which software can be created to exchange data with all computers. The organization specified a minimum of seven layers, each of which must be present in communication software.
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