The Open Systems Interconnection model (OSI model) is a conceptual model from the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems interconnection."[2] In the OSI reference model, the communications between systems are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.[3]

In comparison, several networking models have sought to create an intellectual framework for clarifying networking concepts and activities,[citation needed] but none have been as successful as the OSI reference model in becoming the standard model for discussing and teaching networking in the field of information technology. The model allows transparent communication through equivalent exchange of protocol data units (PDUs) between two parties, through what is known as peer-to-peer networking (also known as peer-to-peer communication). As a result, the OSI reference model has not only become an important piece among professionals and non-professionals alike, but also in all networking between one or many parties, due in large part to its commonly accepted user-friendly framework.[4]

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The development of the OSI model started in the late 1970s to support the emergence of the diverse computer networking methods that were competing for application in the large national networking efforts in the world (see OSI protocols and Protocol Wars). In the 1980s, the model became a working product of the Open Systems Interconnection group at the International Organization for Standardization (ISO). While attempting to provide a comprehensive description of networking, the model failed to garner reliance during the design of the Internet, which is reflected in the less prescriptive Internet Protocol Suite, principally sponsored under the auspices of the Internet Engineering Task Force (IETF).

The drafters of the reference model had to contend with many competing priorities and interests. The rate of technological change made it necessary to define standards that new systems could converge to rather than standardizing procedures after the fact; the reverse of the traditional approach to developing standards.[12] Although not a standard itself, it was a framework in which future standards could be defined.[13]

OSI had two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols. The OSI reference model was a major advance in the standardisation of network concepts. It promoted the idea of a consistent model of protocol layers, defining interoperability between network devices and software.

OSI was an industry effort, attempting to get industry participants to agree on common network standards to provide multi-vendor interoperability.[18] It was common for large networks to support multiple network protocol suites, with many devices unable to interoperate with other devices because of a lack of common protocols. For a period in the late 1980s and early 1990s, engineers, organizations and nations became polarized over the issue of which standard, the OSI model or the Internet protocol suite, would result in the best and most robust computer networks.[9][19][20] However, while OSI developed its networking standards in the late 1980s,[21][22] TCP/IP came into widespread use on multi-vendor networks for internetworking.

The OSI model is still used as a reference for teaching and documentation;[23] however, the OSI protocols originally conceived for the model did not gain popularity. Some engineers argue the OSI reference model is still relevant to cloud computing.[24] Others say the original OSI model does not fit today's networking protocols and have suggested instead a simplified approach.[25][26]

Although the OSI model is often still referenced, the Internet protocol suite has become the standard for networking. TCP/IP's pragmatic approach to computer networking and to independent implementations of simplified protocols made it a practical methodology.[45] Some protocols and specifications in the OSI stack remain in use, one example being IS-IS, which was specified for OSI as ISO/IEC 10589:2002 and adapted for Internet use with TCP/IP as RFC 1142.

The Open Systems Interconnect (OSI) reference model is the basis of commercially available network service architectures. Other network protocols, developed independently, conform loosely to the model. The TCP/IP Internet Protocol suite is an example.

The OSI reference model is a convenient framework for networking concepts. Basically, data are injected into a network by a sender. The data are transmitted along a communication connection and are delivered to a receiver. To do this, a variety of networking hardware and software must work together.

Industry standards have been or are being defined for each layer of the reference model. Two standards are defined for each layer: one specifies the interface to the services provided by the layer, and the other specifies the protocol observed by the services in the layer. Users of a service interface standard should be able to ignore the protocol and any other implementation details of the layer.

There are many users who use computer networks and are located all over the world. To ensure national and worldwide data communication ISO (ISO stands for International Organization of Standardization.) developed this model. This is called a model for open system interconnection (OSI) and is normally called an OSI model.OSI model architecture consists of seven layers. It defines seven layers or levels in a complete communication system. These seven layers are interconnected to each other. OSI Reference model is explained in detail in another chapter.

OSI (Open Systems Interconnection) is a reference model for how applications communicate over a network. This model focuses on providing a visual design of how each communications layer is built on top of the other, starting with the physical cabling, all the way to the application that's trying to communicate with other devices on a network.

A reference model is a conceptual framework for understanding relationships. The purpose of the OSI reference model is to guide technology vendors and developers so the digital communications products and software programs they create can interoperate and to promote a clear framework that describes the functions of a networking or telecommunications system that's in use.

In the 1970s, technology researchers began examining how computer systems could best communicate with each other. Over the next few years, several competing models were created and published to the community. However, it wasn't until 1984 when the International Organization for Standardization (ISO) took the best parts of competing networking reference models to propose OSI as a way to finally create a framework that technology companies around the world could use as the basis of their networking technologies.

The OSI reference model describes the functions of a telecommunication or networking system, while TCP/IP is a suite of communication protocols used to interconnect network devices on the internet. TCP/IP and OSI are the most broadly used networking models for communication.

Given the growing complexity of computer networks, during the 1970s network researchers proposed various reference models to facilitate the description of network protocols and services. Of these, the Open Systems Interconnection (OSI) model [Zimmermann80] was probably the most influential. It served as the basis for the standardization work performed within the ISO to develop global computer network standards. The reference model that we use in this book can be considered as a simplified version of the OSI reference model 1.

In contrast with OSI, the TCP/IP community did not spend a lot of effort defining a detailed reference model; in fact, the goals of the Internet architecture were only documented after TCP/IP had been deployed [Clark88]. RFC 1122, which defines the requirements for Internet hosts, mentions four different layers. Starting from the top, these are :

Compared to the five layers reference model explained above, the OSI reference model defined in [X200] is divided in seven layers. The four lower layers are similar to the four lower layers described above. The OSI reference model refined the application layer by dividing it in three layers :

The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to describe the functions of a networking system. The OSI model characterizes computing functions into a universal set of rules and requirements in order to support interoperability between different products and software. In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

In computer networks, the bit rate of the physical layer is always expressed in bits per second. One Mbps is one million bits per second and one Gbps is one billion bits per second. This is in contrast with memory specifications that are usually expressed in bytes (8 bits), KiloBytes ( 1024 bytes) or MegaBytes (1048576 bytes). Thus transferring one MByte through a 1 Mbps link lasts 8.39 seconds.

As such, network administrators found installing, configuring, and setting up new equipment in existing networks challenging. Moreover, integrating such devices with outside networks seemed even more complex. With the arrival of the OSI reference model, administrators were able to design efficient network infrastructure where the equipment was capable of communicating with other universal networks.

The OSI model enhances the interoperability between devices used by end users, thereby ensuring smooth communication between different computer networks. Advanced standards such as TCP/IP may replace OSI one day; however, network administrators may continue to use the OSI model to safeguard their computing systems. be457b7860

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