Leaders Are Born Not Made

Topics: OSI model, OSI protocols, Internet Protocol Suite Pages: 8 (1534 words) Published: August 25, 2013
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The OSI Model's Seven Layers Defined and Functions ExplainedArticle ID: 103884This article was previously published under Q103884Expand all | Collapse allOn This Page * SUMMARY * PHYSICAL LAYER * DATA LINK LAYER * NETWORK LAYER * Communications Subnet * TRANSPORT LAYER * End-to-end layers * SESSION LAYER * PRESENTATION LAYER * APPLICATION LAYER * Properties * Give FeedbackSUMMARYThe Open Systems Interconnect (OSI) model has seven layers. This article describes and explains them, beginning with the 'lowest' in the hierarchy (the physical) and proceeding to the 'highest' (the application). The layers are stacked this way: * Application * Presentation * Session * Transport * Network * Data Link * PhysicalPHYSICAL LAYERThe physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides: * Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:

* What signal state represents a binary 1 * How the receiving station knows when a "bit-time" starts * How the receiving station delimits a frame * Physical medium attachment, accommodating various possibilities in the medium:

* Will an external transceiver (MAU) be used to connect to the medium? * How many pins do the connectors have and what is each pin used for? * Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling. * Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines:

* What physical medium options can be used * How many volts/db should be used to represent a given signal state, using a given physical mediumDATA LINK LAYERThe data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides: 

* Link establishment and termination: establishes and terminates the logical link between two nodes. * Frame traffic control: tells the transmitting node to "back-off" when no frame buffers are available. * Frame sequencing: transmits/receives frames sequentially. * Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt. * Frame delimiting: creates and recognizes frame boundaries. * Frame error checking: checks received frames for integrity. * Media access management: determines when the node "has the right" to use the physical medium.NETWORK LAYERThe network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides: 

* Routing: routes frames among networks. * Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle...
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