OSI Model

OSI Model

In the beginning the development of networks was chaotic. Each vendor had its own proprietary solution.The bad part was that one vendor’s solution was not compatible with another vendor’s solution.. This is where the idea for the OSI-model was born, having a layered approach to networks our hardware vendors would design hardware for the network, and others could develop software for the application layer. Using an open model which everyone agrees on means we can build networks that are compatible with each other.

To fix this problem the International Organization for Standardization (ISO) esearched different network models and the result is the OSI-model which was released in 1984.Nowadays most vendors build networks based on the OSI model and hardware from different vendors is compatible.

The OSI-Model isn’t just a model to make networks compatible; it’s also one of the BEST ways to teach people about networks. Keep this in mind since I’ll be referring a lot to the OSI-model, it’s very useful.


Layer 7 Application
Layer 6 Presentation
Layer 5 Session
Layer 4 Transport
Layer 3 Network
Layer 2 Data Link
Layer 1 Physical

All People Seem To Need Data Processing”

This is the OSI-model which has seven layers; we are working our way from the bottom to the top.

Let’s Start with physical Layer:

Physical Layer: This layer describes stuff like voltage levels, timing, physical data rates, physical connectors and so on. Everything you can “touch” since it’s physical.
Data Link: This layer makes sure data is formatted the correct way, takes care of error detection and makes sure data is delivered reliably. This might sound a bit vague now, for now try to remember this is where “Ethernet” lives. MAC Addresses and Ethernet frames are on the Data Link layer.
Network: This layer takes care of connectivity and path selection (routing). This is where IPv4 and IPv6 live. Every network device needs a unique address on the network.
Transport: The transport layer takes care of transport, when you downloaded this book from the Internet the file was sent in segments and transported to your computer.

  • TCP lives here; it’s a protocol which send data in a reliable way.
  • UDP lives here; it’s a protocol which sends data in an unreliable way.

these four layers tare just described more important for networking, and the upper three layers are about applications.

Session: The session layer takes care of establishing, managing and termination of sessions between two hosts. When you are browsing a website on the internet you are probably not the only user of the webserver hosting that website. This webserver needs to keep track of all the different “sessions”.
Presentation: This one will make sure that information is readable for the application layer by formatting and structuring the data. Most computers use the ASCII table for characters. If another computer would use another character like EBCDIC than the presentation layer needs to £reformat” the data so both computers agree on the same characters.
Application: Here are you applications. E-mail, browsing the web (HTTP), FTP and many more.

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