OSI Model

    Layer 7 provides an interface between the communications software and any applications that need to communicate outside the computer on which the application resides. It also defines processes for user authentication.
    This layers main purpose is to define and negotiate data formats, such as ASCII text, EBCDIC text, binary, BCD, and JPEG. Encryption also is defined by OSI as a presentation layer service.
    The session layer defines how to start, control, and end conversations (called sessions). This includes the control and management of multiple bidirectional messages so that the application can be notified if only some of a series of messages are completed. This allows the presentation layer to have a seamless view of an incoming stream of data.
    Layer 4 protocols provide a large number of services. Although OSI Layers 5 through 7 focus on issues related to the application, Layer 4 focuses on issues related to data delivery to another computer for instance, error recovery and flow control.
    The network layer defines three main features: logical addressing, routing (forwarding), path determination. The routing concepts define how devices (typically routers) forward packets to their final destination. Logical addressing defines how each device can have an address that can be used by the routing process. Path determination refers to the work done by routing protocols by which all possible routes are learned, but the best route is chosen for use.
    The data link layer defines the rules (protocols) that determine when a device can send data over a particular medium. Data link protocols also define the format of a header and trailer that allows devices attached to the medium to send and receive data successfully. The data link trailer, which follows the encapsulated data, typically defines a Frame Check Sequence (FCS)     field, which allows the receiving device to detect transmission errors.
    This layer typically refers to standards from other organizations. These standards deal with the physical characteristics of the transmission medium, including connectors, pins, use of pins, electrical currents, encoding, light modulation, and the rules for how to activate and deactivate the use of the physical medium.
The International Standards Organisation (ISO) defined a seven layer model to standardise networking processes. The Open System Interconnection (OSI) model facilitates the understanding of the complexities of networking by defining what happens at each stage of the process.

Each of the seven layers in the model communicates with the layers above and below; using standardised coding at the beginning of the message that can be interpreted by another device regardless of who made it.

The seven different layers break up the process of networking, making it easier to understand and to troubleshoot problems. It is possible to test the functionabilty of each layer in sequence, to determine where the problem is, and where to begin repairs.

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