The OSI Model was created based on recommendations from the International Organization for Standardization (ISO) in 1980, which started expanding on the DoD model in the late 1970s. The current standard was published in 1996. The official title for the model is the ISO OSI (Open Systems Interconnection) Reference Model since it describes or relates to connecting systems that are open for communication with other systems. In the model, the functions of the communication system are standardized by categorizing them into abstract layers. The functions which are similar are grouped into the same layer and provide services to the layers above their existing layer.
What Does the OSI Model Do?
The OSI model depicts how data communications should take place. It splits the functions or processes into seven groups that are described as layers. When protocols or other standards are developed by other organizations such as the American National Standards Institute (ANSI), Institute of Electrical and Electronic Engineers (IEEE), and the International Telecommunications Union (ITU) formerly known as the CCITT (Comite Consultatif Internationale de Telegraphique et Telephone), they are placed into a layer of the model to help with communication protocol integration and conceptual understanding. The majority of major network and computer vendors, large commercial entities, and governments support the use of the OSI model. Each of the layers of the OSI model is intended to function with those above and below it respectfully within the model definition.
The OSI model defines standards for:
- The way in which devices communicate between each other.
- The means used to inform devices when to send data and when not to transmit data.
- The methods which ensure that devices have a correct data flow rate
- The means used to ensure that data is passed to, and received by the intended recipient.
- The manner in which physical transmission media is arranged and connected.
What Are the Seven Layers of the OSI Model?
The OSI model is made up of seven layers which are presented as a stack. Each of the layers of the OSI model has a numerical level or layer, and a plain text descriptor. Data which is passed over the network moves through each layer.
The seven layers of the OSI model are:
- Application Layer – layer 7
- Presentation Layer – layer 6
- Session Layer – layer 5
- Transport Layer – layer 4
- Network Layer – layer 3
- Data-Link Layer – layer 2
- Physical Layer – layer 1
A common mnemonic used to remember the OSI model layers starting with the seventh layer (Application) is: “All People Seem to Need Data Processing.” The lower two layers of the model are normally implemented through software and hardware solutions, while the upper five layers are typically implemented through the use of software only.
Each layer of the OSI model has its own unique functions. The process of sending data is typically started at the Application layer, is sent through the stack to the Physical layer, and then over the network to the recipient. Data is received at the Physical layer, and the data packet is then passed up the stack to the Application layer.
Different protocols operate at the different layers of the OSI model. Each layer of the OSI model has its own protocols. TCP and IP are collectively called the protocol stack or the network/transport protocols. This is due to the protocols operating at the Network and Transport layers to make it possible for computers to communicate. A protocol stack, r stack, is a group of protocols which are arranged in layers to enable communication. In the protocol stack, each layer provides services to the layer above it; and each layer also receives services from the layer beneath it. For two computers to partake in communications, each computer has to be running the same protocol stack. They can however have different operating systems.
The published “advantages” of the OSI Model are: enhanced learning/teaching, reduced intricacy, modular engineering, interoperable technology, accelerated advancement, and standard interface definitions. Unfortunately; however, the OSI Model has not been found to map well to real world networking implementations or issues as the technical world has evolved. It is the most recognized model; however, and is still often used to describe networking protocols, gear, problems, and solutions.
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