Open Systems Interconnection (OSI) Reference Model explained

The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to describe the functions of a networking system. The OSI model characterizes computing functions into a universal set of rules and requirements in order to support interoperability between different products and software. In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation and Application.

Created at a time when network computing was in its infancy, the OSI was published in 1984 by the International Organization for Standardization (ISO). Though it does not always map directly to specific systems, the OSI Model is still used today as a means to describe Network Architecture.

The 7 Layers of the OSI Model

Layer 1 : Physical Layer

The lowest layer of the OSI Model is concerned with electrically or optically transmitting raw unstructured data bits across the network from the physical layer of the sending device to the physical layer of the receiving device. It can include specifications such as voltages, pin layout, cabling, and radio frequencies. At the physical layer, one might find “physical” resources such as network hubs, cabling, repeaters, network adapters or modems.

At the data link layer, directly connected nodes are used to perform node-to-node data transfer where data is packaged into frames. The data link layer also corrects errors that may have occurred at the physical layer.

The data link layer encompasses two sub-layers of its own. The first, media access control (MAC), provides flow control and multiplexing for device transmissions over a network. The second, the logical link control (LLC), provides flow and error control over the physical medium as well as identifies line protocols.

Layer 3 : Network Layer

The third layer of OSI model is the Network Layer. The network layer is responsible for receiving frames from the data link layer, and delivering them to their intended destinations among based on the addresses contained inside the frame. The network layer finds the destination by using logical addresses, such as IP (internet protocol). At this layer, routers are a crucial component used to quite literally route information where it needs to go between networks.

Layer 4 : Transport Layer

The fourth layer of the seven layers of Open Systems Interconnection (OSI) network mode is the Transport layer. The transport layer manages the delivery and error checking of data packets. It regulates the size, sequencing, and ultimately the transfer of data between systems and hosts. One of the most common examples of the transport layer is TCP or the Transmission Control Protocol.

This layer provides following functionality:

Segmentation – On sending computer, it breaks data stream into smaller pieces before transmission. Each piece is known as segment and the process of breaking data into smaller pieces is known as segmentation. On receiving computer, it joins all segments back in data stream. So the upper layers receive data in the format in which it was sent.

Data transportation – This layer establishes a logical connection between sending system and receiving system and uses that connection to provide end-to-end data transportation. For data transportation, it mainly uses two protocols; TCP and UDP.

TCP protocol is used for reliable data transportation. TCP is a connection-oriented protocol. UDP protocol is used for unreliable data transportation. UDP is a connection-less protocol.

Main difference between a connection-less and connection-oriented protocol is that a connection-oriented protocol provides reliable data delivery. For reliable data delivery, it uses several mechanisms such as, three way handshake process, acknowledgments, sequencing and flow control.

Multiplexing – Through the use of port numbers, this layer also provides connection multiplexing. Connection multiplexing allows multiple applications to send and receive data simultaneously.

Layer 5 : Session Layer

The position of Session Layer of the Seven Layered Open Systems Interconnection (OSI) model is between Transport Layer and the Presentation Layer. Session layer is the fifth layer of seven layered Open Systems Interconnection (OSI) Model. The session layer controls the conversations between different computers. A session or connection between machines is set up, managed, and terminated at layer 5. Session layer services also include authentication and reconnections.

Layer 6 : Presentation Layer

The sixth layer of OSI model is the Presentation layer. The presentation layer formats or translates data for the Application layer based on the syntax or semantics that the application accepts. Because of this, it at times also called the syntax layer. This layer can also handle the encryption and decryption required by the application layer.

When the presentation layer receives data from the Application layer, to be sent over the network, it makes sure that the data is in the proper format. If it is not, the presentation layer converts the data to the proper format. On the other side of communication, when the presentation layer receives network data from the session layer, it makes sure that the data is in the proper format and once again converts it if it is not.

Formatting functions at the presentation layer may include compression, encryption, and ensuring that the character code set (ASCII, Unicode, EBCDIC (Extended Binary Coded Decimal Interchange Code, which is used in IBM servers) etc.) can be interpreted on the other side. For example, if we select to compress the data from a network application that we are using, the Application Layer will pass that request to the Presentation Layer and it will be the Presentation Layer that does the compression.

Layer 7 : Application Layer

Application Layer is the top-most layer of the seven layered Open Systems Interconnection (OSI) network model. At this layer, both the end user and the application layer interact directly with the software application. This layer sees network services provided to end-user applications such as a web browser or Office 365. The application layer identifies communication partners, resource availability, and synchronizes communication.

In the OSI model, this is the layer that is the “closest to the end user”. It receives information directly from users and displays incoming data it to the user. Real traffic data will be often generated from the Application Layer. This may be a web request generated from HTTP protocol, a command from telnet protocol, a file download request from FTP protocol etc.

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