Open Systems Interconnection model (OSI model)

Open System Connection or OSI one of the many reference models that are used to define how data is transferred or sent from one host to another host through a physical medium. It was created in 1984 and now it stands as an architectural model for computer communications and data transfer. OSI is divided into 7 different layers all of which are responsible for smaller tasks that combined form the main functions of the OSI model. Each layer performs its functions independently of each other.

The 7 layers of the OSI model are categorized into two parts:

  1. Upper Layer – consists of layers that are responsible for managing the tasks of the application responsible for sending and receiving data. It is implemented in the software. It consists of the application, presentation, session, and transport layers with the application layer being closest to the end-user host.
  2. Lower Layer – consists of layers responsible for sending data across the network to the receiver. It consists of network, data-link, and the physical layers with the physical layer being closest to the physical medium.



The seven layers of the OSI model are:

Physical Layer – It is responsible for transmitting data in binary format (0 or 1). This layer receives data and then converts it into 0s and 1s and then transmits the binary data to the data-link layer. It is also responsible for synchronization of bits by the means of a clock and also defines the transmission mode among Simplex, Half-Duplex or Full-Duplex. The topology or the arrangement of the network devices is also specified by the physical layer. It also maintains the physical connection between the network and the host.

Data-Link Layer – This is the layer responsible for error handling and thus making sure an efficient and error-free transmission of frames. This layer is responsible for putting the data into frames and adds a header that contains the destination address (MAC Address). It specifies the format of data transmitted through the network and also the flow of control of data on both sides of the network. The data rate is constant so that it does not get corrupted during or after transfer. The protocols of the data-link layer determine which network or device has control at any given time.

Network Layer – The Network Layer is the layer responsible for data transfer from one host system to another host system. The IP addresses of the sender system and the receiver system are stored in the data frame as a header by this layer. This layer is mainly responsible for choosing and determining which route the data packets have to take to get from the source to the destination. It mostly chooses a path that is both the shortest and most suitable for data transfer. Logical addressing is also done in this layer which is required for the unique identification of data in a web of data. The IP addresses stored in the header is used to distinguish each device globally.

Transport Layer – It is responsible for the process of Segmentation and end-to-end transmission of data. It breaks the data into smaller units called segments. It utilizes TCP/ UDP protocols for transmission of data across the network in an efficient way. It also checks if the transmission of data is successful and returns an error if the transfer failed. TCP protocols are used for this verification using a TCP handshake. UDP on the other hand does not verify the connectivity and errors before sending the data across the network. Transport Layer is known as the Heart of OSI model.

Session Layer – The Session Layer marks the beginning of the Upper Layer. The Session Layer is solely used for the maintenance of sessions and the establishment of simultaneous connections between host computers. It can be used for establishing and terminating a session or connection between two or more hosts. It is also responsible for organizing the flow of data stream and communication between different network applications. It is also responsible for adding synchronization points which are required for the handling of errors in the endpoints of the network.

Presentation Layer – This layer is known as the Translation Layer. It is responsible for converting ASCII characters to EBCDIC and sets the data in the required format for transmission over the network. One of its main functions is the encryption at the sender end and decryption at the receiver end. It carries out data compression and reduces the amount of data which is transmitted over the network. The major role of data compression is to reduce the number of bits from the side of the end-user.

Application Layer – This layer lies at the top of the OSI Model. This layer deals with the activities of the applications and is handled by the software and its corresponding engineers. There are various types of protocols that work on the application layer such as FTP for the transfer for files, HTTP for the transfer of text, TELNET to handle the IP graphics. The application layer is responsible for producing and generating the data which is to be transferred over the network.


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