What is the OSI Model?


The Open Systems Interconnect (OSI) reference model is the ISO (International Standards Organization) structure for the “ideal” network architecture. This Model outlines seven areas, or layers, for the network. These layers are (from highest to lowest):

    1. Application : Where the user applications software lies. Such issues as file access and transfer, virtual terminal emulation, interprocess communication and the like are handled here.
    1. Presentation : Differences in data representation are dealt with at this level. For example, UNIX-style line endings (CR only) might be converted to MS-DOS style (CRLF), or EBCIDIC to ASCII character sets.
    1. Session : Communications between applications across a net- work is controlled at the session layer. Testing for out-of-sequence packets and handling two-way communication are handled here.
    1. Transport : Makes sure the lower three layers are doing their job correctly, and provides a transparent, logical data stream between the end user and the network service s/he is using. This is the lower layer that provides local user services.
    1. Network : This layer makes certain that a packet sent from one device to another actually gets there in a reasonable period of time. Routing and flow control are performed here. This is the lowest layer of the OSI model that can remain ignorant of the physical network.
    1. Data Link : This layer deals with getting data packets on and off the wire, error detection and correction and retransmission. This layer is generally broken into two sub-layers: The LLC (Logical Link Control) on the upper half, which does the error checking, and the MAC (Medium Access Control) on the lower half, which deals with getting the data on and off the wire.
    1. Physical : The nuts and bolts layer. Here is where the cable, connector and signaling specifications are defined.