The OSI Reference Model

The OSI Reference Model:
The early development of networks was chaotic in many ways. The early 1980s saw tremendous increases in the number and sizes of networks. As companies realized that they could save money and gain productivity by using networking technology, they added networks and expanded existing networks as rapidly as new network technologies and products were introduced.

By the mid 1980s, companies began to experience difficulties from all of the expansions they had made. It became more difficult for networks using different specifications and implementations to communicate with one another. The companies realized that they needed to move away from proprietary networking systems—those systems that are privately developed, owned and controlled. In a computer industry, “proprietary” is the opposite of “open“.

Proprietary means that one company or a small group of companies controls all use of the technology. Open means that use of the technology is available free to the public.

To address the problem of networks being incompatible and unable to communicate with one another, the International Organization for Standardization (ISO) researched different network schemes. As a result of this research, the ISO created a model that would help vendors create networks that would be compatible with, and operate with, other networks.

In other simple words,

أنظمة وبرمجيات مختلفة بين شبكتين يعني غير قدرة على التخاطب بين الشبكتين أو التوصيل بينهم لأن نظام عمل الشبكة في كلا الطرفين مختلف

The OSI reference model, released in 1984, was the descriptive scheme that the ISO created. It provided vendors with a set of standards that ensured greater compatibility and interoperability between the various types of network technologies produced by companies around the world.

Although other models exist, most networks vendors today relate their products to the OSI reference model, especially when they want to educate customers on the use of their products.

Now pay attention, the OSI model separates network functions into seven categories. This separation of networking functions is called layering. The OSI model has seven numbered layers, each illustrating a particular network function. The OSI model defines the network functions that occur at each layer. More importantly, the OSI model facilitates an understanding of how information travels throughout a network. In addition, the OSI model describes how data travels from application programs through a network medium, to an application program located in another computer, even if the sender and receiver are connected using different network media.

Benefits of using the OSI model:

  1. The OSI model breaks network communication into smaller, simpler parts.
  2. The OSI model standardizes network components to allow multiple-vendor development and support.
  3. The OSI model allows different types of network hardware and software to communicate with one another.
  4. The OSI model prevents changes in one layer from affecting the other layers, allowing for quicker development.
  5. The OSI model provides for effective updates and improvements to individual components without affecting other components or having to rewrite the entire protocol.
  6. The OSI model breaks network communications into smaller components to make learning easier.

The 7 layers of the OSI model:

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

Yes, the layers are in descending order, from now on when you read layer3 it means Network layer, and you better memorize their names and number by heart.

Unlike what usually happens, I’m going to try to shed some light on the 7 layers starting with layer7, the reason is that next topic should be about encapsulation and that requires a closer connection with the first 4 layers, layer1 to 4, because all the fun happens their behind the scenes.

Application, layer7:

The application layer is the OSI layer that is closest to the user. This layer provides network services to the application of the user, such as e-mail, files transfer and terminal emulation.
The application layer differs from the other layers in that it does not provide services to any other OSI layer, but only to applications outside the OSI model. The application layer establishes the availability of intended communication partners and sync and establishes agreement on procedures for error recovery and control of data integrity.

Presentation, layer6:

The presentation layer ensures that the information sent at the application layer of one systerm is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the same characters. If necessary, the presentation layer translates between multiple data formats by using a common format.

Session, layer5:

The session layer establishes, manages and terminates sessions between two communicating hosts. The session layer also sync dialog between the presentation layers of the two hosts and manages their data exchange. For example, web servers have many users, so there are many communications processes open at a given time. It is important, then, to keep track of which user communicates on which path, a user opens two different web pages but both referes to the same domain, the website respondes to the user clicks and send back data, session layer knows to which window it will send each information provided by layer4 based on some information that I will provide later. In addition to session regulation, the session layer offers provision for efficient data transfer, Class of Service (CoS), and exception reporting of session layer, presentation layer and application layer problems.

* time of death, 2:10PM — I will continue this topic as soon as possible 🙂 call House now 😛

I have written the same tutorial (and more) on Sec-Geeks forum, in Arabic, I thought it would be nice if I get you a copy of it since all what I write here is English.. with no further talk, here is it


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