ISO/OSI Model - 7 layers model

Reference model ISO/OSI is defined by standard project of communications system. The reason of standard inception was the need to unify description of communications system. At the beginning PC dotting wasn´t namely fixed rules and nets wasn´t each other compatible.

On this model begined in 70‘ working company ISO (Internacional Standardization Organization). Year 1984 was confirmed and got labelling OSI (Open Systems Interconnection).

This model forms quite 7 layers- physical, datalink, network, transport, session, presentation, application.

Creators of the model were leaded through universal principles serving to as most effective and as simple as possible division of communication to layers. Of course they tried to minimalize count of layers, create a unique section for alternative functions. That all of course must work without fallout to another layers.

Services, which offer sections of various levels, can have character of connection or connectionless services. Connection Oriented Service work on principle like telephone. First you must dial a number and he must answer with turn up the phone. With this rise the connection. Connectionless service is useing like letter post. It doesn´t count witch connection sender with recipient. Transported datas are furnished with address of recipient and transmit abstractedly from another messages even in another sequence, like they was sendet.

Next we can part the sevices to Reliable and Unreliable. Realiable service- datas never get lost. It have also disadvantages too. Recipient is bearing to successful receiving, what means to sending again datas, which were wrong recieved. He accepts it better, then he lose it completely.

Unrelible services are somewhat strange labelling- this services have high reliability rate, but they don´t provide 100% guarantee, because in this way we don´t use confirmation´s mechanisms.

With this have connection even topology of net. This means structure of connection single users of communications net. Like primary structure we can consider tree, star, bus and circular. Each of them have theirs proprieties, advantages and disavantages and its choice have effect on proprieties of net.