OSI Model: Using Open Systems Interconnection to Send and Receive Data

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Instructor: Jill Heaney

Jill has taught college-level business and IT. She has a Doctorate in Business Administration and an M.S. in Information Technology & Leadership.

The Open System Interconnection (OSI) model describes how data is sent and received over a network. It consists of seven layers: the physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer. This lesson discusses how the OSI model works and the role of each layer for transmitting messages.

Open Systems Interconnection (OSI) Model

The Open Systems Interconnection (OSI) model is a suggested standard for communication that was developed by the International Organization for Standardization (ISO). The OSI reference model describes how data is sent and received over a network. This model breaks down data transmission over a series of seven layers. Each layer has a responsibility to perform specific tasks concerning sending and receiving data. All of the layers are needed for a message to reach its destination.

The OSI model gives software developers a standard for developing communication software. The OSI model provides the standard for communication so that different manufacturers' computers can be used on the same network.

The Seven Layers of the OSI Model

These layers are the physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer. Software and hardware can then be developed for each layer separately. However, they must work together to successfully transport a message. Each layer performs a specific function, but all of the layers have one function in common: communicating with the layers above and below them in the model.

The application layer is the software that the end-user interacts with. This is an application like Firefox, Outlook, or Internet Explorer. This layer provides service to applications outside of the OSI model. It performs several functions, including establishing the availability of the communication partner, synchronizing the sending and receiving of applications, establishing agreement on error recovery and data integrity, and determining if sufficient resources exist for the communication to occur.

The presentation layer is concerned with the presentation of data. This layer defines the format the data uses as it is transmitted. It formats the data for the user so that it is readable and the message can be understood. This layer may also compress data for easier transmission or encrypt data for security purposes.

The session layer is responsible for allowing ongoing communication between two parties across the network. It handles the setup of the session, data exchanges, and the end of the session. This layer is responsible for flow control, or defining the rules for communication between two computers. Flow control will prevent too much data from being sent to the receiving computer at one time so it does not become overloaded.

The application layer includes software like Firefox, Internet Explorer, and Outlook
OSI Application Layer

The transport layer, also known as the 'end-to-end layer,' deals with transmission of data between networks. The transport layer ensures that error-free data is given to the user. This layer generates the address for the receiving computer and adds it to the data so that it is sent to the correct destination. It sets priorities for messages and error recovery procedures in the event that an error on the network occurs.

The network layer splits up long messages into smaller bits of data, often referred to as packets. The network layer chooses the route data will take and addresses the data for delivery. It adds a destination address and routing information to enable the packet to travel between nodes on the network.

The data link layer moves information from one computer or network to another computer or network. It performs three specific functions: controls the physical layer and decides when to transmit messages, formats messages indicating where they start and end, and detects and corrects errors that occur during transmission.

While the upper six layers are concerned with software, the physical layer is concerned with hardware. The physical layer provides the physical connection between the computer and network. The physical components may include servers, clients, and circuits.

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