What is the OSI Model?

If you thinking that connecting networks with routers and switches is complicated today, imagine how difficult it was in the past before the International Organization for Standardization (ISO) created the Open Systems Interconnection (OSI) model in the late 1970s. This model made it possible for different vendors to use protocols and create interoperable network devices and applications. As a result, different networks consisting of hardware and software from different vendors could communicate with each other.

Reference models are blueprints that identify and group the processes required for communication. These process groups are called "layers" and the resulting system is known as having a layered architecture.

The OSI model features seven layers:

• Application (layer 7)
• Presentation (layer 6)
• Session (layer 5)
• Transport (layer 4)
• Network (layer 3)
• Data Link (layer 2)
• Physical (layer 1)

Layers 1 through 4 define how data is transmitted while layers 5 through 7 define how applications on networked computers communicate with each other and with end users.

Application Layer (Layer 7) – The Application layer involves users communicating with the computer. This layer is used when network access will be needed soon such as when launching a Web browser to view a website. Even without networking components, you could use a Web browser to view an HTML document on your computer. The Application layer serves as an interface between the software and the Presentation layer down. It offers the software processes for sending information through the protocol stack.

Presentation Layer (Layer 6) – The Presentation layer translates data, formats code, and presents data to the Application layer. This layer serves as a translator. For example, data is often converted into a standard format before transmission. Computers then receive this standardized data and then convert the data back into its original format for presentation.

Session Layer (Layer 5) – The Session layer sets up, manages, and breaks down sessions among Presentation layer participants as well as coordinates system communications and controls dialogue between devices. The Session layer offers several communication modes: simplex, half duplex, and full duplex.

Transport Layer (Layer 4) – The Transport layer creates a data stream out of segments of data. Transport layer Services provide end-to-end data transport services. In addition, they establish logical connections between an internetwork's sending and receiving hosts.

Network Layer (Layer 3) – The Network layer manages device addressing, keeps track of device locations, and transports data between devices. Routers are Network layer devices.

Data Link Layer (Layer 2) – The Data Link layer is responsible for data transmission, flow control, network topology, and error notification. The Data Link layer uses hardware addresses to make sure messages are delivered to the correct device. It also translates Network layer messages into data frames that are transmitted by the Physical layer. These data frames contain a header detailing the both the source and destination addresses.

Physical Layer (Layer 1) – The Physical layer sends and receives bits with a value of either 1 or 0. This layer communicates directly with communication media using a variety of methods such as audio tones or changes in voltage. In order to communicate, protocols are used to explain how the data is encoded, which bit patters should be used, and other qualities unique to the media. The Physical layer specifies what's needed to create, maintain, or break a physical link between systems. The interface between the data communication equipment (which is usually on the service provider's end) and data terminal equipment (which is usually the attached device) is specified in the Physical layer.

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