Open systems interconnection is a reference model illustrating how messages should be transmitted over a telecommunication network. It does not include detailed interfaces. Instead, it serves as a guide for creators of networks so that their products are compatible with those of other creators. The open systems interconnection includes seven functions, known as function layers, that should be performed when messages are sent over the network.
This reference model was created by the International Organization for Standardization (IOS). The IOS is a group made up of representatives from more than 160 countries that works to create standards to facilitate communication and business cooperation between countries. The recommendations for open systems interconnection are encoded in section X.200 of the International Telecommunications Union - Telecommunications Standards (ITU-TS).
Each computer in a network should be able to perform all seven of the function layers. No one program performs all four functions. Instead, they are divided up between several components of the computer. Functions may be performed by the operating system, the programs, or the network protocols.
The seven layers are divided into two groups. The first three are used whenever messages pass through the computer. These are the physical, data-link, and network layers. If the message is not intended for the computer it is passing through but is just using the computer to help transport, the messages do not pass through the next four layers. Those layers are used only when a message passes from or to a user.
The physical layer allows hardware to send and receive bits of data. The data-link layer of the open systems interconnection is aware of the transmission protocol and makes sure the computer follows those rules. Finally, the network layer sends the data to the right computer. Much like an envelope sorter at the post office, it divides the messages into outbound mail meant for other computers and local mail meant for the computer doing the processing.
If the message is local mail, it is then processed through four more function layers of the open systems interconnection. The transport layer makes sure the full message has arrived and checks for errors. Like the telephone operator in old movies, the session layer coordinates the connection between two computers. At the presentation or syntax layer, data are converted from the format in which they were sent to a format that can be read by the computer. Finally, the application layer takes care of all the little details, from checking the speed of the connection to making sure privacy and authorization protections are in place.
