What Is OSI (Open Systems Interconnection) Model?


Open Systems Interconnection Model or OSI Model is an architecture to specify computer networks functions in a clear and compatible way. OSI provides interoperability of diverse communication systems with standard communication and transmission architecture, layers, and protocols.

OSI History

In the early- and mid- 1970s the computer networks largely owned and operated by the governments or related institutions. These computer networks were using proprietary standards, devices, and protocols related to IBM, DEC, etc. UK National Computing Center started working on “Why Distributed Computing” which resulted in ISO meeting about standardized networks. First meeting held in France in 1977 to create a standard that will lead to the OSI Model. After years of work and published drafts the OSI Reference Model is published in 1984 with the ISO 7498 standard.

OSI Layers

OSI Model simply refers to a model that proposes multiple logical layers that define functions, features, interoperability. OSI model consist of 7 layers each of them provide unique network service and function to create a complete model for network communications. The OSI Referance model works as top-down approach where a transmission starts from top to down.

  • Layer 7 – Application Layer
  • Layer 6 – Presentation Layer
  • Layer 5 – Sessions Layer
  • Layer 4 – Transport Layer
  • Layer 3 – Network Layer
  • Layer 2 – Data Link Layer
  • Layer 1 – Physical Layer

Host Layers

There are some grouping about these layers where from Layer 7 to Layer 4 are called as Host Layers. Application Layer (Layer 7), Presentation Layer (Layer 6), Sessions Layer (Layer 5), Transport Layer (Layer 4) are called Host Layers because the host or operating system and network stack is responsible with these layers.

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Media Layers

Layers from Layer 3 to Layer 1 are called as Media Layers .Network Layer (Layer 3), Data Link Layer (Layer 2), Physical Layer (Layer 1) are called Media Layers because these layers are implemented on the Network Interface Card and Media and mainly run on inter-network devices like Switch, Router etc.

Layer 7: Application Layer

Application Layer is the closest layer to the end-user. Applications layer mainly operated by applications the user is using. Browsers, Twitter Client, Zoom, Games, etc. run on the applications layer of the OSI Reference Model. Applications layer uses popular protocols like HTTP, HTTPS, NTP, SMB, VNC, RDP which is mainly run by the user side application.

Layer 6: Presentation Layer

Presentation Layer is located as Layer 6. Presentation layer is mainly designed to provide different presentations of the data between different applications for the same applications layer protocols. This layer is mainly used for encoding like ASCII, EBCDIC, Serialization, Compression etc.

Layer 5: Session Layer

Session Layer controls the connections/sessions between hosts/computers. This layer manage the communication to be full-duplex, half-duplex, simplex and operations like session suspension, restart, termination, start etc for the Layer 4 protocol like TCP and UDP.

Layer 4: Transport Layer

Transport Layer is responsible for transmitting data to the remote computer/host by using protocols TCP and UDP. Transport layer provides operations like flow control, segmentation, transmission connection management, re-transmission, transmission error detection, transmission error recover, timeout management, reliable transport etc. TCP and UDP are two popular transport layer protocols used today computer networks.

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Layer 3: Network Layer

Network Layer mainly responsible to transmit the data by the transport layer to the destinated remote computer/host. The network layer uses addresses to define different networks and hosts and routes between them. A network contains a single or multiple hosts or nodes which can communicate with other hosts/nodes located in different networks. The network layer provides:

  • Host Addressing
  • Network Addressing
  • Data Transfer Between Hosts/Nodes
  • Routing
  • Hop by Hop Transmission
  • Network Segmentation

IP is the most popular network layer protocol used for 35 years. IP is used with the TCP and UDP protocols in a very integrated way and dominated the computer network in today’s world.

Layer 2: Data Link Layer

Data Link Layer is used to transfer data from Node to Node in the same network which are connected direct link. Data Link Layer works in the same network. Data Link layer provides:

  • Node to Node datagram transmission
  • Data Flow Control

Ethernet or Medium Access Control (MAC) protocols are the most known and used protocols in computer networks. Ethernet is defined with IEEE 802.3 standard. Wi-Fi is another popular protocol that is mostly used for laptops, tablets, smartphones, TVs to connect to LAN devices wireless. Wi-Fi is defined with IEEE 802.11 standard. Bluetooth and USB are another popular data link layer protocols used in computer networks.

Layer 1: Physical Layer

Physical Layer provides transmission medium for the data link layer. Physical layer transmit the datagrams over it with different type of signals. Physical Layer provides:

  • Encoding
  • Signalling
  • Error Detection
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The physical layer can operate over a copper wire, air, USB cable, fiber cable. The physical layer also uses simplex, half-duplex and full-duplex signal transmission modes.

Example Packet For OSI Layers and Protocols

Below you will find a network packet OSI layers. Frame is related with the physical layer andEthernet II is the data link layer. Internet Protocol Version 4 is the network layer and Transmission Control Protocol is transmission layer.

Example Packet For OSI Layers and Protocols

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