OSI model

OSI model
Name	OSI model
Caption	Layered conceptual framework for network communication
Developer	International Organization for Standardization
Introduced	1984
Type	Network architecture model

OSI model The OSI model is a seven-layer conceptual framework developed to standardize and describe functions of telecommunication and computing systems. It provides a common language for engineers and organizations to interoperate, design protocols, and teach networking concepts across institutions such as the International Organization for Standardization, International Electrotechnical Commission, European Computer Manufacturers Association, United Nations forums and academic curricula at institutions like Massachusetts Institute of Technology and Stanford University. The model influenced protocol development at corporations including IBM, Cisco Systems, Hewlett-Packard, and standards bodies such as the Institute of Electrical and Electronics Engineers and Internet Engineering Task Force.

Overview

The architecture defines seven abstraction layers that separate concerns from physical transmission to application semantics, enabling modular design and testing practices used by companies like Xerox and research projects at Bell Labs and DARPA. It was presented alongside competing efforts such as the Department of Defense (United States) networking model and has shaped curricula at universities including University of California, Berkeley and Carnegie Mellon University. The model’s terminology appears in technical documentation from vendors like Microsoft and Novell, in training from certification bodies such as CompTIA and EC-Council, and in regulatory discussions at European Union agencies.

Layers

Each layer addresses specific functions and interfaces with adjacent layers; many protocols and implementations map to one or more layers described in standards from organizations such as ISO and the ITU-T. From lowest to highest:

- Layer 1 — Physical Layer: deals with hardware signaling, connectors, and transmission media used in systems by Western Electric and cabling standards from International Electrotechnical Commission committees; examples include specifications adopted in deployments by Bell System and manufacturers like Siemens. - Layer 2 — Data Link Layer: handles framing, MAC addressing, and link control implemented in technologies such as Ethernet, Token Ring, and ATM, with vendors like 3Com and Sun Microsystems producing hardware. - Layer 3 — Network Layer: provides routing and logical addressing used in protocols such as those developed in the Internet Protocol Suite work influenced by DARPA research and implemented by organizations like Cisco Systems and Juniper Networks. - Layer 4 — Transport Layer: ensures end-to-end communication, flow control, and reliability with protocols analogous to Transmission Control Protocol and User Datagram Protocol developed through IETF working groups and deployed by operating systems from Apple and Microsoft. - Layer 5 — Session Layer: manages sessions and dialogues, referenced in distributed computing frameworks from Sun Microsystems and standards like OSI Presentation and Session protocols used in middleware from Oracle Corporation. - Layer 6 — Presentation Layer: handles data representation, encryption and compression with standards such as ASCII, Unicode, and encryption suites standardized by National Institute of Standards and Technology and used in products by RSA Security. - Layer 7 — Application Layer: provides network services to end-user applications; examples include services standardized by IETF and implemented in software by Google, Mozilla, SAP SE, and enterprise suites from Microsoft.

History

Work leading to the model emerged from international standardization efforts and networking research in the 1970s and early 1980s involving committees at International Organization for Standardization and collaborations among national bodies like British Standards Institution and Deutsches Institut für Normung. The model was formalized during meetings attended by delegates from France, United Kingdom, United States Department of Commerce, and industry representatives from AT&T and Siemens. It was published in the 1984 ISO documentation and debated alongside initiatives such as the ARPANET spin-offs and protocol designs from Xerox PARC and the University College London research groups.

Implementation and Usage

Although few modern products implement the model as discrete layers, its conceptual framework guides protocol stacks and vendor documentation at Cisco Systems, Hewlett-Packard Enterprise, and cloud providers like Amazon (company), Microsoft Azure, and Google Cloud. Network certification programs from Cisco's training arm and academic syllabi at institutions such as Imperial College London and ETH Zurich teach the layered approach. Standards derived from ISO work inform national regulations in jurisdictions such as European Commission rulemaking and procurement policies in ministries of Japan and Germany.

Comparison with TCP/IP Model

The OSI model is often contrasted with the TCP/IP model developed from ARPANET and codified by the IETF. TCP/IP’s four- or five-layer pragmatic architecture used in implementations by BSD and UNIX System V maps unevenly to OSI layers; for example, TCP/IP application and presentation concerns are conflated into the application layer of TCP/IP stacks implemented by Linux distributions and server software from Apache Software Foundation. Network engineers from vendors such as Juniper Networks and researchers at University of Cambridge routinely translate concepts between the models during protocol design and interoperability testing.

Criticisms and Limitations

Critics from industry and academia such as groups at MIT and commentators in publications like IEEE Communications Magazine note that the model is prescriptive yet impractical for modern protocol design, because many protocols span multiple layers and real-world systems developed by Google and Facebook violate strict layering. Others point to the model’s lack of guidance on security and performance tuning compared with operational frameworks from NIST and contemporary practices in data centers operated by Equinix and hyperscalers. Nonetheless, the model remains a pedagogical and contractual reference in standards work by ISO and professional education offered by organizations such as ACM.

Category:Computer network architectures