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Open Systems Interconnection Reference Model


The Open Systems Interconnection Reference Model  (OSI model) is an open layered model for the communication of information processing systems. These are standardized procedures and rules for the exchange of data.

It has been developed since 1979 and was adopted by the International Organization for Standardization (ISO) in 1984. The OSI model serves as the basis for a number of vendor-independent network protocols that are used almost exclusively in public transport in the transport network.

In a computer network, various services are provided to the various hosts, and indeed by the other users in the network. The communication required for this is not as trivial as it seems at first glance, because a multitude of tasks must be mastered and requirements with regard to reliability, safety, efficiency, etc. must be met. The problems to be solved range from issues of electronic transmission of signals through a regulated order in communication to more abstract tasks that arise within the communicating applications.

An example of communication between an NIS server and a client is to illustrate the meaning of the layers of the 7-layer OSI model. The server runs on a 64-bit machine and the client runs on a 32-bit machine, that means the data is available on the participating computers in different formats.

Process of communication

The NIS client sends a request. Since he can not know the data format understood by the server, he brings this into a general format (in connection with RPC, External Data Representation XDR is mostly used). The remote procedure call organizes the "remote procedure call" by packaging the data along with information about the desired service. At the transport level u.a. the data is broken down into blocks of a certain size, the switching takes over the addressing of the target computer at the IP level. The backup layer is quite hardware specific and takes care of the error-free transmission on the actual medium. Finally, the transmission is via a physical medium whose characteristics are specified by the transmission layer protocol. On the server side, the layer 2 packet is handed over. If this detects the absence of errors, it reaches the network layer. At the transport level, the packets generated by the sender are combined to form the complete message. The RPC server process evaluates the requirements. If they can be fulfilled, the data is transferred to the required format and the NIS server is consulted. This generates an answer and the game repeats itself with distributed roles ...