The open system Interconnection (OSI) Reference Model

Models are useful because they help to understand our difficult terms and wrapped systems. If it concerns networks, there be several models which are used for the role of different technologies, and how them interact. Of these, most popular and most used the open Systems Interconnection (OSI) are Reference model.

Definition: An abstract description of the digital communication between an application processes in separate systems.

Layers: The model has a hierarchical structure of the seven layers. Each low service with added value performs at the request of adjacent higher low and, for its part, request more basic provisions of adjacent lower layer:

1. Physical Layer (Layer 1)
   Physical layer services perform services at the request of the data link layer. Performs the most important functions and services by physical layer is:
   
   • Determining and the suspension a connection with a communication medium.
     
     
   • Participation to the process where the communication resources is effectively shared with several users.
     
     
   • Conversion between the representation of the digital data of sent the users and the corresponding indicators concerning a communication channel.
     


2. Data Link Layer (Layer 2) These low reacts to requests of the service network layer and questions service requests to physical low. The data risky Layer possibly stipulates the functional and procedural resources to transfer of data between network and entities to trace and corrects of errors which can appear in physical low.


3. Network Layer (Layer 3) These low reacts to requests of the service transport Layer and questions service requests to the data risky Layer. The Network Layer stipulate the functional and procedural resources of the transfer of variable length data sequences of a source to a destination by means of one or more networks, with conservation of the quality of the service by the transport Layer. The Network Layer conduct network routing, flow control, segmentation/de-segmentation and wrongly control functions.
   
   
4. Transport Layer (Layer 4) These low reacts to requests of the service Session Layer and questions service requests to the network layer. The aim of the transport Layer is supplying transparent transfer of data between end-users, so that the upper layers of only concern concerning supplying reliable and cost effective data transfer.
   
   
5. Session Layer (Layer 5) these low approach to the requests of the service presentation Layer and questions service requests to the transport Layer. The Session Layer offer application processes to the mechanism for the management of the dialogue between end-user. It provides for a duplex or half-duplex-working and puts check pointing, discontinuance, suspension, and restart procedures.
   
   
6. Presentation Layer (Layer 6) These low reacts to requests of the service Application Layer and questions service requests to the Session Layer. The presentation Layer lighted the Application Layer of care on the syntactic differences in data representation in end-user the systems.
   
   
7. Application Layer (Layer 7) These low interfaces directly and run into common application for the application processes, but also questions request Layer to the presentation. The common application services offer, semantic conversion linked between application processes.