The Open Standards Interconnection model (OSI) is a network architecture reference model developed by the International Organization for Standardization in the 1980s. It is a conceptual model rather than a real-life implementation designed as a reference for developing network architectures and protocols.

Having a standardised, multi-layer model has a number of benefits:

  • Modularisation and abstraction – different components can handle different layers of the stack. If you’re a web developer, you just need to worry about the application layer, not the full stack.

  • Standard interfaces – interaction between layers is well defined so that you can design your product to run on top of a lower layer protocol regardless of who developed that.

  • Interoperability – protocols at each layer are specified such that different vendor equipment should be able to talk to each other at the same layer.

OSI Layers

  1. Physical
  2. Data-link
  3. Network
  4. Transport
  5. Session
  6. Presentation
  7. Application

Application

The application layer provides a communication interface to an application on a device.

Presentation

The presentation layer negotiates to ensure that data is presented to the application layer in the correct format.

Session

The session layer starts, stops and maintains connections, dialogs or sessions between devices.

Transport

The transport layer handles the transfer of data between a source and a destination. This could include flow control, segmentation/desegmentation and error control.

Network

The network layer defines logical addresses and the routing between them.

Data-Link

The data link layer prepares data for transmission across a physical media, including hardware addressing.

Physical

The physical layer determines the physical properties of the media used for data transmission.

Relationship between OSI and TCP/IP

We can loosely equate the two models as follows:

TCP/IP Application Layer

Includes the OSI Application, Presentation and Session layers

TCP/IP Transport Layer

Roughly equates to the OSI Transport Layer

TCP/IP Internet Layer

Roughly Equates to the OSI Network Layer

TCP/IP Link Layer

Includes the OSI Data Link and Physical layers.

Although TCP/IP is the model most people use, engineers will still refer to layers by their OSI layer number. For example IP and routers as layer 3, a network switch as a layer 2 device or TCP as a layer 4 protocol.

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