The Open System Interconnection or OSI defines a networking framework for implementing networking protocols and distributed applications in the International Standard Organization's Open System Interconnect (ISO/OSI) model. OSI Model is a set of protocols that try to identify and homogenize the data communication practices. Developed by representatives of major computer and telecommunication companies beginning in 1983, the OSI Model has the support of most computer, network vendors, and the implementation bodies.

The OSI Model illustrates how data communications should take place in a networking framework. It is a way of sub-dividing a communications system into smaller parts called layers, with each layer adding its own set of special, related functions. Each logical layer has an instance which provides service to the instances at the layer above and requests service from the layer below. The core of OSI Reference Model is a set of seven layers that define the different stages that data must go through to travel from one device to another over a network. Into these layers are integrated the protocol standards developed by the ISO and other standards organization, including the Institute of Electrical and Electronic Engineers (IEEE), American National Standards Institute (ANSI), and the International Telecommunications Union (ITU), formerly known as the CCITT (Comite Consultatif Internationale de Telegraphique et Telephone).

These seven layers are:

Layer 7: Application - This is the layer that actually interacts with the operating system or application specifically written to run over the network whenever a user performs network-related activities. It allows access to network services that support these applications.  Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. Mail, ftp, telnet, DNS, NIS, NFS are examples of network applications.

Layer 6: Presentation - Sometimes called the syntax layer, this layer takes the data provided by the Application layer and converts it into a standard format that the other layers can understand, and vice versa. The original presentation structure used the basic encoding rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serialization of objects and other data structures from and to XML.

Layer 5: Session - Layer 5 is responsible for establishing process-to-process communications between networked hosts or in other words establishing, maintaining and ending communication with the receiving device.

Layer 4: Transport - This layer uses flow control, segmentation and de-segmentation, and error control to manage the reliability of a given link. Flow control means that the Transport layer looks to see if data is coming from more than one application and integrates each application's data into a single stream for the physical network.

Layer 3: Network - This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). Logical protocols, routing and addressing are handled here.

Layer 2: Data – This layer is responsible for communications between adjacent network nodes and provides synchronization for the physical level.  Also, the type of network and the packet sequencing is defined.

Layer 1: Physical - This layer conveys the bit stream through the network at the electrical and mechanical level.  This is the level of the actual hardware. It defines the physical characteristics of the network such as connections, voltage levels and timing.

The short form used to memorize the layer names of the OSI Model is “All People Seem To Need Data Processing”.

The layered approach to network communications gives the subsequent advantages: Reduced intricacy, enhanced teaching/learning, modular engineering, accelerated advancement, interoperable technology, and standard interfaces. The OSI model was created by academicians for academic purposes. Protocols such as TCP/IP, were designed and built around the needs of real users with real problems to solve.