The OSI model explained in simple terms

OSI Model Illustrated graphicLearning technology isn't sexy, but I am doing my best to keep it interesting. Here I take on the complex subject of the Computer Networking OSI model explained in simple terms. In our previous article, Understanding the mystical OSI Model explained in simple terms we used an analogy to illustrate the OSI model.

Why is the OSI Reference Model important?

Simply put the OSI Reference Model is a THEORETICAL model describing a standard of computer networking. The TCP/IP Reference model is based on the ACTUAL standards of the internet which are defined in the collection of Request for Comments (RFC) documents started by Steve Crocker in 1969 to help record unofficial notes on the development of ARPANET. RFCs have since become official documents of Internet specifications.

The OSI model is important because many certification tests use it to determine your understanding of computer networking concepts. The OSI Reference Model is an attempt to create a set of computer networking standards by the International Standards Organization. A "Reference Model" is a set of text book definitions. You often learn something new by first learning text book definitions. The common protocol suite of computer networking is TCP/IP. The geeks who created TCP/IP were not as anal in creating a pretty "reference model." TCP/IP evolved over many years as it went from a theory to the concept of the internet.


The Internet and the TCP/IP family of protocols evolved separately from the OSI model. Often you find teachers, and websites, making direct comparison of the different models. Don't spend too much time trying to compare one versus the other. The two models were developed independently of each other to describe the standards of computer networking.

The TCP/IP Reference Model is not merely a reduced version of the OSI Reference Model with a straight line comparison of the four layers of the TCP/IP model to seven layers of the OSI model. The TCP/IP Reference Model does NOT always line up neatly against the OSI model. People try to hard to make neat comparisons of one model versus the other when there is not always a neat one to one correlation of each aspect.


The stated purpose of the OSI Model:

  • breaks network communication into smaller, simpler parts that are easier to develop.
  • facilitates standardization of network components to allow multiple-vendor development and support.
  • allows different types of network hardware and software to communicate with each other.
  • prevents changes in one layer from affecting the other layers so that they can develop more quickly.
  • breaks network communication into smaller parts to make learning it easier to understand.


The seven Layers of the OSI Model

The hierarchical layering of protocols on a computer that forms the OSI model is known as a stack. A given layer in a stack sends commands to layers below it and services commands from layers above it.

The seven layers in order from highest to lowest are Application, Presentation, Session, Transport, Network, Data Link, and Physical can be remembered by using the following memory aide: All People Seem To Need Data Processing.

The Application layer includes network software that directly serves the user, providing such things as the user interface and application features. The Application layer is usually made available by using an Application Programmer Interface (API), or hooks, which are made available by the networking vendor.

The Presentation layer translates data to ensure that it is presented properly for the end user, also handles related issues such as data encryption and compression, and how data is structured, as in a database.

The Session layer comes into play primarily at the beginning and end of a transmission. At the beginning of the transmission, it makes known its intent to transmit. At the end of the transmission, the Session layer determines if the transmission was successful. This layer also manages errors that occur in the upper layers, such as a shortage of memory or disk space necessary to complete an operation, or printer errors.

The Transport layer provides the upper layers with a communication channel to the network. The Transport layer collects and reassembles any packets, organizing the segments for delivery and ensuring the reliability of data delivery by detecting and attempting to correct problems that occurred.

The Network layer's main purpose is to decide which physical path the information should follow from its source to its destination.

The Data Link layer provides a system through which network devices can share the communication channel. This function is called media-access control (MAC).

The Physical layer provides the electro-mechanical interface through which data moves among devices on the network.

In the articles that follow we will break down each layer in more detail, covering topics you will need to know as a networking professional.
 

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