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osi models or network layers

Posted: 2014-02-19

OSI (Open Systems Interconnection) model was formed by the International Organization for Standardization (ISO), an internationally standardizing organization. It was intended to be a reference model for describing the functions of a communication system. It has seven layers, with each layer describing a different function of data traveling through the network. The OSI model consists 7 stages or layers. Each layer defines an state of the data in transmission. The layers are usually numbered from the bottom up, meaning that the Physical layer to the Application Layer. The Physical layer is the first or 1 in the OSI model. The 7 defined layers are 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Datalink 1. Physical The vendors of network equipment commonly use this reference model to implement some functionality into a networking device, which then enables easier interconnect with devices from other vendors. Brief description of individual layers Physical – defines the data bits from one device to another. It specifies the wiring and functions of cables, connectors, network interface cards, and other network devices as well as data in bits. Data Link – The data link layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the physical layer. A frame contains a header and a trailer that enable devices to communicate. A header, most commonly, contains a source and a destination MAC address. A trailer contains the Frame Check Sequence field, which is used to detect transmission errors. The data link layer has two sub layers: A. Logical Link Control – used for flow control and error detection B. Media Access Control – used for hardware addressing and controlling the access method Network – Defines the switching and routing methods, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing,internetworking, error handling, congestion control and packet sequencing. It assigns a logical address or IP on each of a network. Transport – Segments chunks of data received from the higher layer protocols. Establishes and provides transparent transfer of data between end hosts. Used for flow control and data recovery. Defines methods of transporting data between hosts such as connection-less (UDP) or connection oriented (TCP). Session – Defines the methods of establishing, managing and terminating connections between applications. It coordinates, and terminates associating, interactions, and dialogues between the applications connected ends. It deals with session and connection synchronization. Presentation – defines data formats, compression and encryption. It presents the data in a form readable by applications or resamples content. Application – This layer is the closest to the user’s interactions on the network. It enables network applications to communicate with other network applications via HTTP, FTP, TFTP, TELNET, SMTP, SSH, etc.

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