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03: OSI (Open systems Interconnection) reference model


Almost every discipline has a theoretical aspect to study, and computer networks is no exception. OSI (Open Systems interconnection) model  is a conceptual model that describes how two or more end points communicate, which means that it is a theoretical part of computer networks; in other words, OSI model is not tangible, that is, It performs no function in the networking process or in the real data transmission. OSI reference model was developed in 1984 by ISO (International Standard Organization).

OSI reference model development reasons and goals
one of the key goals of its development is to allow different vendors or manufacturers to interoperate. When networks first emerged, networking software or devices could merely connect to or communicate with software or devices which were fabricated by the same manufacturer and consequently devices of different vendors like computers, servers, routers etc would not be able to exchange data. Thus, OSI model was primarily meant to help vendors create interoperable devices and software in the form of protocols so that vendors' networks could work with each other.

Another important aim of OSI model is the depiction of how data and network information are transmitted from one application on a given networking device through the network media to an application on another networking device. to make it more clear, let us consider the following example:

Let's suppose that these two computers' users are using facebook to communicate. When PC-1 sends a message to PC-2, it passes through different stages. OSI model breaks those stages into seven layers. In this case The message is transferred from PC-1 to PC-2 passing through OSI model layers starting from the highest layer which is the Application Layer of the sender, then getting down to the Physical Layer and transmitting through physical link, thereafter getting up to the Application Layer of the receiver. All of these layers are connected and linked together.  

OSI reference model example of analogy

the following example may make it plain for you:

Let's assume that the seven OSI model layers are a building with seven floors. A person living in the seventh floor of building A, for example, wants to give something (a laptop) to the person dewilling in the seventh floor of building B. Is it reasonable that he throws a laptop through a window? the laptop may fall in the street and breaks down ! so, he must go down until the first floor (from the Application Layer until the Physical Layer), getting out of the building and entering building B (transmission of data through physical cables), then going up until the seventh floor(from the Physical Layer until the Application Layer) and finally giving the laptop to the recepient (data arrived at the receiver's machine).

OSI reference model advantages

- It breaks down the network communication process into smaller and simpler components, which facilitates the finding of network problems (trouble shooting), if there any.

- It allows different types of network hardware and software to communicate.
- It prevents changes in one layer from influencing other layers.
- It encourages industry standardization by defining what functions occur at each layer.
- It allows multiple-vendors development through the standardization of network components.

OSI reference model functions

- OSI model seven layer are divided into two groups, the upper layers (Application, Presentation, and Session) and the lower layers (Transport, Network, Data-link, Physical). The upper layers define how the applications within the end station will communicate with each other. The lower layers define how data is transmitted end to end.
-It provides a framework for creating and implementing networking devices and internetworking schemes.
-It is served as a guideline for application developers, helping them in creating and implementing applications that run on the network.
-It aids in data transmission between disparate hosts.

OSI model layers' functions in brief

Layer 7: Application Layer> the interface between user and network.
Layer 6: Presentation Layer> encryption, compression, translation.
Layer 5: Session Layer> opens a channel between sender and receiver, dialog control.
Layer 4: Transport Layer> end to end connection.
Layer 3: Network Layer> routing.
Layer 2: Data-link Layer> framing.
Layer 1: Physical Layer> physical connection.

Note !! this is just a brief statement of OSI model layers' functions. Each layer is explained in more detail in the next lessons.

03: OSI (Open systems Interconnection) reference model Reviewed by BOUFTIRA on 9:22:00 PM Rating: 5

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