Open Systems Interconnection model (OSI model)

The Open Systems Interconnection (OSI) modelalso called the seven-layer OSI model or the seven-layer modelis a conceptual framework that describes how computers network and communicate with each other. The OSI model defines seven different levels, or layers, that work together to send communication over a network. The model defines the network and describes the way computers transfer packets of information using the different layers. The OSI model, which was first created in the 1980s, has remained a useful framework even as computing and technology have advanced significantly since that time. Some technology professionals have argued that the model should be updated, specifically to add more layers. Some people believe that other layers, such as a security layer, should be standardized and put into the model to make them a vital part of network planning and implementation. The OSI model is used in many different applications, including creating simple networks and creating complicated cloud computing systems. Its broad categories and simple, logical framework have allowed people to apply it to many different situations, even though technology has evolved a great deal since its creation.

Background

The International Organization for Standardization (ISO) first created the OSI model in 1984. The ISOfounded in 1947promotes commercial and industrial standards around the world. The ISO initially created the model so that technology professionals would understand the equipment and design standards needed to create a network. However, the model later became a framework for describing network communication overall. When creating the model, the ISO had to decide how to define each layer and how many layers to include. The organization wanted to create a flexible model that would outlast the technology that was available at the time. The ISO realized that each layer had to perform a well-defined function. Furthermore, the organization wanted to include enough layers so that each had a specific purpose, but not too many layers that the model became confusing or difficult to remember.

Although the OSI is a useful model that was intended to guide people in creating computer networks, computer network architects do not always set up networks that exactly mimic the model. Some networks have fewer layers and still function appropriately. The model is meant to be a guide more than a set of instructions for people creating and using networks. People who create networks are not mandated to follow the model. For some professionals, the framework is better for discussing concepts than for actual network development. Furthermore, organizations other than the ISO have developed models and standards that technology professionals use. For example, the American Institute of Electrical Engineers (AIEE) produces standards for computer manufacturers, and International Telecommunications Union—Telecommunications Sector (ITU-T) creates standards for networks.

Overview

The OSI model includes seven layers, each of which has a different function in the network. The model can be described from layers 1 to 7 or layers 7 to 1, as data process in both directions when sent and received on different devices. The seven layers are application, presentation, session, transport, network, data link, and physical.

Layer 7 is the application layer. Most people consider this layer the “top” layer and describe it first because it is the layer that most users see or interact with. This is the layer in the model that is the closest to the end-user. However, some real-life networks will not include this layer, making a lower level closer to the end-user.

Layer 7 receives information directly from the user, and it is made up of services that are run by the applications that people use. The services in layer 7 verify that the network is available for communication. This layer also allows users to access, retrieve, and manage files on a remote computer. People also use layer 7 to electronically exchange documents. This layer can work with applications such as hypertext transfer protocol (HTTP), post office protocol (POP), domain name service (DNS), and file transfer protocol (FTP).

Layer 6 is the presentation layer. This layer represents the area that prepares and translates data from an application format to a network format. The preparation and translation of the data help to ensure that the data are compatible based on syntax and semantics. It makes sure the data being sent to the next layer will be understandable and useable. One example of layer 6 is when data are encrypted or decrypted so that they can be sent securely. This layer can also take part in data formatting, code conversion, and conversion.

Layer 5 is the session layer. The layer manages the connections between parts of the system. It establishes a connection and maintains it. It also terminates the connection when the communication is completed. Networks use layer 5 when computers or servers need to “speak” to each other. This layer ensures that the computers can send data correctly to each other. At layer 5, data are marked and synchronized so that no data are lost during communication.

Layer 4 is the transport layer. It deals with transferring data between end systems and hosts. The components of the layer determine how much data to send and at what rates to send them. It also determines where the data go. For example, this can determine whether data need to be sent in single paths or parallel paths. Layer 4 ensures quality of service functions and works to resolve errors in the sending of data. One example of layer 4 is the Transmission Control Protocol (TCP), which is built on the Internet Protocol (IP). These protocols are commonly known as TCP/IP.

Layer 3 is the network layer. This layer is involved in forwarding packets of data. This layer channels the packets through different routers. It manages the data delivery between the communicating networks. Layer 3 is unnecessary if computers are communicating over a shared network. Routers and switches are common elements that make up this layer, though switches can also be part of other layers.

Layer 2 is the data link layer. This layer transfers the data across the network by packing and unpacking the data. It can resolve errors that occur in layer 1, the physical layer. Switches are often part of layer 2, though they can also be used in layer 3. This layer includes two sublayers, the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC uses MAC addresses for connecting devices and transmitting data. The LLC establishes and maintains the connections between the communicating devices.

Layer 1 is the physical layer. This is the layer that includes the electrical and physical aspects of the network. This includes cable types, radio frequencies, voltages, and other aspects of the system’s physical and hardware components. Professionals often look to the physical layer first when problems arise with a network. Ensuring that this layer is free of problems will allow the professionals to rule out issues with the physical pieces of the network so they can look at the elements of the other layers.

When data are transferring from one computer to another or from one network to another, the data have to work through all the layers of one system and then all the layers in the connected system. For example, consider data being sent from computer A to computer B. The data start at layer 7 in computer A because that is the layer with which the person sending the information is interacting. The data go through each of the layers to layer 1. They then arrive at layer 1 in computer B. Then, the data move up the layers until they reach layer 7. However, different systems have different layers, so the data will not always work their way through seven separate layers.

Although the seven-layer model has existed since the 1980s, some technology professionals have claimed that the model should be revised to include more layers. Some professionals assert the updated model should include eight to ten layers. One of the biggest concerns about the current model is it does not have a layer devoted to cybersecurity and security culture. Some experts also believe that one layer should address the need for systems to follow regulations and laws. People who support the changes believe that including security and regulations in the model will encourage further measures that ensure security and user safety.

By the mid-2020s, more calls began to emerge over the relevancy of the OSI modelor at least to identify the need for updates to account for newer practices. The emergence of technologies such as software-defined networking (SDN) and network function visualization (NFV) suggested that many functions had now merged into single layers. Others countered that no single model fully depicted how modern networks operated and that OSI remained the best general depiction.  A third idea presented stated while OSI may not accurately illustrate network functions, it still had relevance when designing applications. A consensus was, while not without shortcomings, the OSI model would continue to play a significant role in the future because of its ease in troubleshooting, flexibility, and as a teaching resource.

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