Open Systems Interconnection is a reference model for transmitting messages over a telecommunications network, created by IOS. It comprises seven functions divided into two groups, with each computer in the network able to perform all seven levels of function. The first three layers are used when messages pass through the computer, while the next four are used only when a message passes to or from a user. The physical layer allows hardware to send and receive bits of data, the data link layer knows the transmission protocol and makes sure that the computer follows these rules, and the network layer sends the data to the right computer. If the message is local mail, it is then processed through four other functional layers of Open Systems Interconnection: transport, session, presentation, and application.
Open systems interconnection is a reference model that illustrates how messages should be transmitted over a telecommunications network. Does not include detailed interfaces. Instead, it serves as a guide for network creators so that their products are compatible with those of other creators. Open Systems Interconnection comprises seven functions, known as levels of functions, which 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 over 160 countries that works to create standards to facilitate communication and trade cooperation between countries. Open Systems Interconnection recommendations are codified in section X.200 of the International Telecommunications Union – Telecommunications Standards (ITU-TS).
Every computer in a network should be able to perform all seven levels of function. No program performs all four functions. Instead, they are divided among different computer components. Functions can be performed by the operating system, programs, or 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 destined for the computer it is traversing but is just using the computer to facilitate transport, the messages do not go through the next four levels. These levels are used only when a message passes to or from a user.
The physical layer allows hardware to send and receive bits of data. The data link layer of Open Systems Interconnection knows the transmission protocol and makes sure that the computer follows these rules. Finally, the network layer sends the data to the right computer. Just like an envelope sorter at the post office, it splits messages into outgoing mail destined for other computers and local mail destined for the processing computer.
If the message is local mail, it is then processed through four other functional layers of Open Systems Interconnection. The transport layer makes sure that the complete message has arrived and checks for errors. Like the telephone operator in old movies, the session level coordinates the connection between two computers. Presentationally or syntaxically, data is converted from the format in which it was sent to a machine-readable format. Finally, the application layer takes care of all the little details, from controlling connection speed to ensuring privacy and authorization protections.
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