Open systems interconnection Open Systems Interconnection (OSI)
Open Systems Interconnection is the basic osnovopologashchy model describing structure of data transmission from one application to another. It is used as the abstract scheme of the description of-level approach of the description of work the OSI Model consists of seven conceptual levels. Each of these levels corresponds to a konkuretny task, corresponds to accomplishment of a certain part of a certain algorithm. Thanks to the OSI model the paradigm of interaction of network equipment and the software becomes more clear. It is also necessary to note that establishing today is considered the family of protocols TCP/IP which concept goes to a section with OSI, however the reference circuit of open system interconnection cannot sweep aside practical and educational value in any way. The OSI model is developed by the International Organization for Standardization of ISO. The provision describing the considered structure passes at number ISO 7498. You should not transfer OSI as a projection to the existing network system owing to some idealizirovannost of model.
Levels
- Level 0. Wednesday. This level represents the intermediaries connecting final components of network structure: cables, radio lines, etc. As this level de facto is not circuit element, he specifies only on Wednesday.
- Level 1. Physical. Includes physical aspects of transfer of binary information on the communication line. In details describes, for example, tension, frequencies, the nature of the transferring environment. Maintenance of communication and reception-transmission of a bit flow is imputed to this level a duty. The faultlessness is desirable, but it is not required.
- Level 2. Channel. The first provides faultless transfer of data units through the level which by transfer can distort data. This level should define the beginning and the end of a frame in a bit flow, to create of the data transferred by the physical layer, the personnel or the sequences of the personnel, to include the procedure of verification of presence of errors and their correction. This level (and only it) operates with such elements as the bit sequences, encoding techniques, markers. It bears responsibility for the correct data transmission (packets) on sections between directly coupled elements of network. Provides transfer media access control.
- Level 3. Network. This level uses the opportunities given by the second level for ensuring communication of any two points in network. This level performs wiring of messages on network which can have many communication lines, or on a set jointly of the working networks that requires routing, i.e. determination of a way by which it is necessary to send data. Routing is made at the same level. Executes processing of the addresses and also multiplexing and demultiplexing. Basic function of the software at this level is selection of information of a source, its conversion to packets and the correct transfer to an appointment point.
- Level 4. Transport. Regulates transfer of data between the processes executed on computers of network. Completes the organization of data transmission: controls the data stream passing on route on an end-to-end basis, determined by the third level: correctness of transfer of data units, correctness of delivery in the necessary destination point, their completeness, safety, a sequence. Collects information from blocks in its former type. Or operates with datagrams, i.e. expects a response - receipt acknowledgment from a destination point, checks correctness of delivery and addressing, repeats sending of the datagram if the response did not come.
- Level 5. Session. Coordinates interaction of the communicating processes: establishes connection, interacts, recovers under abnormal condition ended sessions. It coordinates not computers and devices, and processes in network, supports their interaction. So manages communication sessions between processes of the application layer. The same level is responsible for cartography of network. Actually it transforms the addresses convenient for people, to the real network addresses, for example, in Internet it corresponds to conversion of regional (domain) computer names to the numerical addresses global and vice versa.
- Level 6. Data views. This level deals with syntax and semantics of the transmitted data. Here mutual understanding of two reported computers how they provide is established and understand on receipt the transmitted data. At this stage such problems as code conversion of text information and images, compression and unpacking, support of network file systems (NFS), abstract data structures are solved.
- Level 7. Applied. Provides the interface between the user and network, makes available to the person various services. At this level, at least, five application services are implemented: file transfer, remote terminal access, electronic transfer of messages, help desk and network management. In specific implementation is defined by the user according to his need and requirements.
See Also
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