The OSI Reference Model.

The International Organization for Standardization (ISO), an international engineering organization based in Paris, first published the Open Systems Interconnection (OSI) reference model in 1978. This seven-layer model has become the standard for designing communication methods among network devices since than.

The OSI Reference Model (Open System Interconnection Reference Model) is used as an excellent way to describe and fully understand network communication. The OSI model divides networks into seven functional layers; each of which describes a  specific function or set of functions (specific aspects of network communication) performed when data is transferred between applications across the network and therefore is often called the Seven-LayerSstack. (The OSI Model is a logical model, not a physical one).

In order to memorize the layers and their order, there are two handy memorization tips or acronyms that you can use to remember the layers:

• All People Seem To Need Data Processing, where each word contains the first letter of the layers from the top-down, or you can use 
• Please Do Not Throw Sausage Pizza Away, where each word contains the first letter of the layers from the bottom-up.

The OSI model has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other as well as with users. The bottom four layers define how data is transmitted end to end. 

Below describes the roles and functions of every layer in the OSI reference model:

• Application Layer: This layer interfaces directly with the network-aware applications, giving it access to network resources. Without this layer, no user application would be able to get access to the network.
• Presentation Layer: Encodes the data being sent or received into a generic format that will be understood by both devices. For example, a web browser might receive data in HTML format or a picture in JPG format, which are generic and well understood standards.
• Session Layer: Begins, ends, and manages the sessions between devices.
• Transport Layer: Handles the reliability of the connection and logical separation of applications. For example, if a computer is surfing the Internet with a web browser and at the same time listening to Internet-radio, this layer ensures the correct data arrives to the correct application. In addition, this layer handles flow-control (ensuring one side does not send information faster than the other can receive) and data integrity (ensuring the data is not corrupt). The most common Transport Layer protocol is TCP.
• Network Layer: Provides logical addressing services allowing a device to dictate the source and destination address used for end-to-end communication. This layer is also responsible for routing the packet from its source to its destination. The most common Network layer protocol is IP.
• Data Link Layer: Provides physical addressing services allowing a device to dictate the source and destination address used for local network communication. This layer permits communication between devices connected to the same network. This layer is also responsible for error detection (not correction).
• Physical Layer: Defines the physical standards used for network communication.

It is important to note here that, peer layers form protocol-independent virtual links; each layer in the stack relies on the layers above and below it to operate, yet each operates independently of the others, as if it were having an exclusive conversation with its counterpart layer on the other computer. Each layer on the device is said to have established a virtual link with the same layer on the other device. With all seven virtual links running, a network connection is established, and the two devices are talking as if they were wired directly together.