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Posted on August 17th, 2012, by

This paper is devoted to the OSI seven layers model for networks. We are going to study the historical information related to creation of this model, the structure of the model, detailed description of each of the layers and its functions, in relation to the whole network.

The system of seven layers, as a model for networking framework, was worked out by Open System Interconnection or the OSI. One of the key features of the model is the passing of the control from one layer to the next one, starting from the application layer, then going to the bottom layer, then to the next station and back up.

The OSI model actually presents an abstract description for layered communications and computer network protocol design (Ben, 1993). The whole network architecture is thus divided into seven layers, which are: the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. Due to the number of layers, this model received the second name- the OSI Seven Layer Model. Under the layer here we understand “a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it”¯ (Ben, 1993). The connection between layers is done in archaic form, i.e. a layer provides service to the layer above him and demands service from the below layer. The two instances of one layer are untied by horizontal protocol connection. Compatible internal connection between network devices is vitally important for reliable network communications.

Nowadays the OSI protocol is not so widely used, but the model itself is important for other protocols operating, as for example ATM or TCP/IP and so on.

As it was already mentioned above there are total of seven layers in the OSI model, each having its own function. We will start from the 7th layer, which supports application and end ”“ user processes (Ben, 1993). Here identification of communication partners as well as quality of service is done, privacy is considered and data syntax constrains are defined. At this layer everything is related to application, including application for file transfers, emails or any other network software services. Such applications as FTP and Telnet can be found only in this application layer.

The 6th layer is the layer of presentation, data is transformed from application to the format of the network. The task of the presentation layer is transformation of data into the format, acceptable in the application layer. This layer is responsible for sending data in the network without compatibility problems. The 6th layer has also another name- the syntax layer.

The session layer or the 5th one is responsible for starting, controlling and finishing of conversations. Actually thanks to the session layer transaction sequencing is managed. As there might be a lot of messages, there is the need in sorting out those, which fail. It is able also to define, whether the message is at the middle of conversation or at the end of it. As soon as the conversation is completed, the data would be passed to the next level ”“ the 6th layer.

The 4th layer is needed for ordering and reassembly of the packets, which could have been broken up (Ben, 1993).

There are special protocols in this layer, which are responsible for errors’ improving. After reordering of data it is passed to the 5th layer.

The network layer 3 ensures the passing of packets end to end, providing a logical addressing scheme for this transmitting. It can be both connection ”“ oriented as well as connectionless and depends upon the path, which the data goes through. The routing packets are defined in this layer, as well as breaking the bigger packets into the smaller ones for various media. This process is also called ”“ packet switching. When the data from the previous level ”“ layer 2 comes, this layer studies the destination address, in case it corresponds to its own address, the data will be passed further to the 4th layer.

The second layer is also called data link layer. It gets the data with the help of specific medium and individual links, providing one or more data link connections between two networks (Ben, 1993).It is necessary to separately identify the end points for the network layer sequencing. The correct sequence is important for keeping the frames, this processes is assisted by Flow control and Quality of Service parameters, which include Service Availability, Throughput, Transit Delay (Ben, 1993)..

The Frame Check Sequence helps to make the error checking at this level. In case there is a problem in some frame – it is discarded. The information between the header and the trailer is sent to the 3d layer. “The MAC layer concerns itself with the access control method and determines how use of the physical transmission is controlled and provides the token ring protocols that define how a token ring operates”¯ (Ben, 1993).

At last the first layer got the other name ”“ physical layer, because it is related to the physical aspects of the media, which is used for transmitting of the data. Such means as the electrical, mechanical, procedural and functional ones are related to this layer. “This defines things like pinouts, electrical characteristics, modulation and encoding of data bits on carrier signals. It ensures bit synchronisation and places the binary pattern that it receives into a receive buffer”¯ (Ben, 1993).

Overall, we have studied briefly the history of the creation of the seven layers of the OSI Model and their functioning and roles.

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