Open System Interconnect (OSI) is an International Organization for Standardization (ISO) layered architecture standard that defines a framework for implementing protocols in seven layers.
The seven layers in the OSI model are represented as a top-down list in the following illustration:
Layers 7, 6, 5, and 4 work at the host (computer) level and are sometimes called host layers, whereas layers 3, 2, and 1 are related to transmission through the media and are called media layers:
- Layer 7: This is the application layer that provides application services that are required for application processes. Interaction between the user and the host application is facilitated in this layer. All functionalities in this layer are oriented toward applications.
- Layer 6: This is the presentation layer that manages the way information or data is encoded or represented. Hence, this layer transforms the data for applications.
- Layer 5: This is the session layer, and the primary purpose of this layer is to manage communications between two computers.
- Layer 4: This is used to maintain the integrity and validity of the data that is being transported, and it is known as the transport layer.
- Layer 3: This is called the network layer, and it ensures that the proper route is established for transporting the data.
- Layer 2: This is the data link layer that ensures node-to-node validity of the data that is being transmitted.
- Layer 1: This deals with electrical and mechanical characteristics of the data and is called the physical layer.
OSI mnemonics are used to remember the order of the seven layers. The following are a couple of the most popular OSI mnemonics:
- Top to bottom: All People Seem To Need Data Processing
- Bottom to top: Please Do Not Take Sales People's Advice
Transmission Control Protocol / Internet Protocol (TCP/IP) is an Internet Protocol suite on which most of the Internet and commercial networks run. There are two important protocols in it, Transmission Control Protocol (TCP) and Internet Protocol (IP) based on which this suite is named.
The original TCP/IP reference model consists of four layers that are purely related to Internet communications. These four layers are the application layer, the transport layer, the network/internet layer, and the data link layer.
The following diagram illustrates the four-layered TCP/IP model:
