What is the OSI Stack

Defines OSI and its history. Describes the seven layers of the stack.

OSI means Open Systems Interconnection. It refers to the layered architecture used in computer communication design. The architecture was developed by the ISO (International Organization for Standardization). It defines the way communication is passed from one layer to another in computer systems, ultimately passing through seven different layers. The simplified representation of the OSI stack shows neighboring layers communicating with one another, through specific entities within them.

In the OSI stack the layers that interact with a human are called the higher layers. They are given the conventional levels of 5, 6 and 7. Below these are the four other layers that normally don’t interact with humans. They are called the lower layers. Normal interaction between these layers is initiated by a human from the seventh layer in one computer system, goes down to the lowest level, interfaces with another computer system, and rises up the layers of this second computer system up to the seventh, which then provides the interface with another human. A brief description of these layers follows.

The seventh layer is also known as the application layer. It operates closest to a human user. It interacts with the application that interfaces between the person and the computer system. However, the application being used directly by the user is outside the scope of the seventh OSI layer. The seventh level identifies communication partners, governs resource usage, and basically ensures quality of communication. File transfer protocols (FTP), Simple Mail Transfer Protocol (SMTP) and Telnet are examples of applications contained entirely within this layer.

The sixth layer is also called the presentation layer. It primarily determines how a computer presents its information e.g. the type of characters or file formats to use. In this way, it resolves problems that would otherwise emanate from different file encryption and origins. It then communicates with the fifth layer, also called the session layer. This layer establishes a communication session and basically organizes data into packets.

The fourth layer, called the transport layer, governs reliability and timely delivery of data to the higher layers from the lower ones. This is accomplished through data flow control, package segmentation and error control. The Transmission Control Protocol is an example of this layer. The layer is adjacent to the third one, called the network layer. The network layer provides virtual circuits which provide switching and routing for the data from one node to the next.

The second layer is also called the data link layer. In it, the logical organization of data bits transported over a medium is defined. Data gets encoded into a valid signaling format for the first layer, called the physical layer. An example of the data link layer is the HDLC (High level data link) protocol. Finally, the physical layer conveys the data stream (bits) in form of electrical impulses or radio waves at the mechanical level. Hence the cables and cards used in computer communications are part of this layer.