The OSI (Open Systems Interconnect) reference model was developed in the early 1970s by the International Standards Organization (ISO). Provides a set of general design guidelines for data-communications systems and also gives a standard way to describe how various portions (layers) of data-communication systems interact.

The hierarchical layering of protocols on a computer that forms the OSI model is known as a stack. A given layer in a stack sends commands to layers below it and services commands from layers above it.

The Purpose of the OSI Model -
- breaks network communication into smaller, simpler parts that are easier to develop.
- facilitates standardization of network components to allow multiple-vendor development and support.
- allows different types of network hardware and software to communicate with each other.
- prevents changes in one layer from affecting the other layers so that they can develop more quickly.
- breaks network communication into smaller parts to make learning it easier to understand.

The seven layers in order from highest to lowest are Application, Presentation, Session, Transport, Network, Data Link, and Physical can be remembered by using the following memory aide: All People Seem To Need Data Processing.

The Application layer - includes network software that directly serves the user, providing such things as the user interface and application features. The Application layer is usually made available by using an Application Programmer Interface (API), or hooks, which are made available by the networking vendor.

The Presentation layer - translates data to ensure that it is presented properly for the end user, also handles related issues such as data encryption and compression, and how data is structured, as in a database.

The Session layer - comes into play primarily at the beginning and end of a transmission. At the beginning of the transmission, it makes known its intent to transmit. At the end of the transmission, the Session layer determines if the transmission was successful. This layer also manages errors that occur in the upper layers, such as a shortage of memory or disk space necessary to complete an operation, or printer errors.

The Transport layer - provides the upper layers with a communication channel to the network. The Transport layer collects and reassembles any packets, organizing the segments for delivery and ensuring the reliability of data delivery by detecting and attempting to correct problems that occurred.

The Network layer's main purpose is to decide which physical path the information should follow from its source to its destination.

The Data Link layer - provides a system through which network devices can share the communication channel. This function is called media-access control (MAC).

The Physical layer - provides the electro-mechanical interface through which data moves among devices on the network.

For more information on Networking Basics, the OSI Model and Server Administration:
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