The OSI Model

OSI - Standardization within Computer Networks

A network is a complex structure within computers. Here, all important data transmissions are gathered, and it represents all connections for internal device communication and coupling to the control unit. As this connection describes complex processes, protocols must be used. Protocols make it possible to control the communication in a system in order to assure that the data of any formatting is directed to the addressee at the time he needs to receive them. Protocols also control the accuracy of the transmitted messages and information. This means that the contents which are supposed to be in a data packet actually are in the packet. As there are several technologies in this area, standards and norms are being specified continuously in order to obtain an internationally recognized standard. The OSI model, also known as the OSI layer model or the OSI reference model, is one of them.

The Structure of the OSI Model

The development of this model began in 1979, and four years later it was specified as a standard. For regulation of tasks which occur with communication between the participants of a network, they are divided into layers, and the OSI model has a total of seven layers. Each layer is occupied by an instance, where the instances must operate according to prescribed rules on the side of the sender as well as the receiver, in order to come to an agreement about the method of data processing. This regulation also claims to be a logical, horizontal connection. However, this only applies to the instances within a layer, as the data flow between a higher and a lower layer runs vertically. The seven layers of the OSI model build upon each other, but they differ in the way that they realize their requirements in the form of a communication protocol. Over time, this had the consequence that each individual layer by now has several protocol types available.

Layer 1
This layer in the OSI model serves for bit transfer. It makes mechanical as well as electrical resources available, which serve for activation and deactivation of physical connections, as well as maintenance of these lines and the transfer of bits. Within this layer, bits are sent on a line-bound transfer path or on one without a line. Finally, the requirements also include the regulation of the transfer method, namely which information carrier is selected.

Layer 2
This is the security layer, which can also be called the section security layer, the connection security layer, or the data security layer. After all, this layer in the OSI model serves for exactly this, assuring a secure and as far as possible error-free transfer by securing connections, data, and sections. Furthermore, this instance also performs regulation of the access to the transmission medium. Additionally, checksums are added here, a process taking place within the frame of a channel coding. The purpose of this measure is to find faulty blocks and enabling them to be dropped by the receiver. These blocks can be corrected when required, but once dropped blocks cannot be requested again in this layer.

Layer 3
This is the network layer, and it takes care of the switching of connections within performance-oriented services and transferring of data packets with packet-oriented services. as the transmission of the information always covers the entire communication network, finding routes between the nodes of a network is also one of the tasks of this layer. More precisely the main tasks of the third layer of the OSI model provide addresses via networks, update routing tables and create them, negotiate and assure a special service quality, and fragment the data packets to be transferred.

Layer 4
This is the transport layer of the OSI model, which enables uniform access for the superior layers and takes care of segmenting of the data flow and avoiding of congestion. Five different service qualities are defined within the layer, which can be used by the layers 5 to 7 and which contain services with multiplex mechanisms and error correction methods.

Layer 5
This layer is the session layer, serving for communication of the processes of two systems and making available services of the organized and synchronized data exchange. These avoid a crash of the session. To fulfill these tasks, check points, i.e. fixed points. are specified, so that new synchronization can be performed from this position in case of a failure of the transports.

Layer 6
The presentation layer enables syntactically correct flow of the data exchange by selecting a representation of the data that is independent of system and form. Data compression, encryption, and assuring the readability of data by a different system are also part of this instance. When required, the presentation layer of the OSI model also assumes the position of a translator between two systems.

Layer 7
This is the application layer and also the top layer in the OSI model. Access to network services takes place here.


Conclusion for the OSI Model

Without this model, realization of various processes within the communication technology would not be possible, as it is the base for the development of numerous protocol types and still serves as a design model for uniform protocols and thus is indispensable for the order of a network.