OSI is a standard description or for how messages should be transmitted between any two points on a network. The OSI model defines seven layers that take place at each end of a communication. It is also valuable as a single reference view of communication that lets everyone have a common ground for education and discussion. Developed by representatives of major computer companies beginning in 1983, OSI was made to be a detailed specification of interfaces. Instead, the committee decided to make a common reference model for which others could make detailed interfaces, that could become standards. OSI was officially adopted as an international standard by the International Organization of Standards (ISO).The main idea in OSI is that the process of communication between two end users in a network can be divided into layers, with each layer adding its own set of special, related functions. Each user is at a computer equipped with these seven layers of function. So, in a given message between users, there will be a flow of data through each layer at one end down through the layers in that computer and, at the other end, when the message arrives, another flow of data up through the layers in the receiving computer. The actual programming and hardware that makes these seven layers of function is usually a combination of the computer OS, applications, TCP/IP and the software and hardware that enable you to put a signal on one of the lines attached to your computer. OSI divides communication into seven layers. The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are passed on to another host. The seven layers are: The application layer (Layer 7) this is the layer in which communication partners are identified, and user authentication and privacy are considered. This layer is not the application itself, although some applications may perform application layer functions. The presentation layer (Layer 6), this is a layer, usually part of an operating system, that converts incoming and outgoing data from one format to another.The session layer (Layer 5), this layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection. The transport layer (Layer 4), this layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-control. It ensures complete data transfer. The network layer (Layer 3), 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). The network layer does routing and forwarding. The data link layer (Layer 2), this layer provides error control and synchronization for the physical level and does bit-stuffing for strings of 1's in excess of 5. It makes transmission protocol knowledge and management. The physical layer (Layer 1), this layer moves the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier. This is basically what the OSI Model is and the descriptions of it s layers. Without the OSI model today, large corporations would still rely on the paper and pen method of conveying information, and most importantly, there wouldn t be a T1 line running at PHS for Internet.