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Osi Model

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OSI model

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OSI Model

7 Application layer

6 Presentation layer

5 Session layer

4 Transport layer

3 Network layer

2 Data link layer

LLC sublayer

MAC sublayer

1 Physical layer

The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnection initiative. It is also called the OSI seven layer model.

Contents [hide]

1 History

2 Description of OSI layers

2.1 Layer 7: Application layer

2.2 Layer 6: Presentation layer

2.3 Layer 5: Session layer

2.4 Layer 4: Transport layer

2.5 Layer 3: Network layer

2.6 Layer 2: Data Link layer

2.7 Layer 1: Physical layer

3 Interfaces

4 Examples

5 Humour

6 See also

7 External links

[edit] History

In 1977, the International Standards Organization (ISO), began to develop its Open Systems Interconnection (OSI) networking suite. OSI has two major components: an abstract model of networking (the Basic Reference Model, or seven-layer model), and a set of concrete protocols. The standard documents that describe OSI are for sale and not currently available online.

Parts of OSI have influenced Internet protocol development, but none more than the abstract model itself, documented in OSI 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it. Protocols enable an entity in one host to interact with a corresponding entity at the same layer in a remote host.

[edit] Description of OSI layers

OSI Model

Data unit Layer Function


layers Data Application Network process to application

Presentation Data representation and encryption

Session Interhost communication

Segments Transport End-to-end connections and reliability (TCP)


layers Packets Network Path determination and logical addressing (IP)

Frames Data link Physical addressing (MAC & LLC)

Bits Physical Media, signal and binary transmission

[edit] Layer 7: Application layer

The application layer is the seventh level of the seven-layer OSI model. It interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer.

The common application layer services provide semantic conversion between associated application processes. Note: Examples of common application services of general interest include the virtual file, virtual terminal, and job transfer and manipulation protocols.

The application layer of the four layer and five layer TCP/IP models corresponds to the application layer, the presentation layer and session layer in the seven layer OSI model.

[edit] Layer 6: Presentation layer

The Presentation layer transforms data to provide a standard interface for the Application layer. MIME encoding, data encryption and similar manipulation of the presentation is done at this layer to present the data as a service or protocol developer sees fit. Examples of this layer are converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of XML.

[edit] Layer 5: Session layer

The Session layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for either full-duplex or half-duplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet protocols suite.

[edit] Layer 4: Transport layer

The Transport layer provides transparent transfer of data between end users, thus relieving the upper layers from any concern while providing reliable data transfer. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the transport layer can keep track of the packets and retransmit those that fail. The best known example of a layer 4 protocol is the Transmission Control Protocol (TCP). The transport layer is the layer that converts messages into TCP segments or User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc. packets. Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatching and classification of mail and parcels sent.

[edit] Layer 3: Network layer

The Network layer provides the functional and procedural



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