open systems interconnection (OSI)

An international standard for the definition of connections between computers, which allows computers of different makes and architectures to communicate with each other for the transfer of data. The organization committed to the establishment of software and standards to enable computers to fulfil these requirements is the Open Software Foundation (OSF).

The Open Systems Interconnection (usually abbreviated to OSI) was an effort to standardize networking that was started in 1982 by the International Organization for Standardization (ISO), along with the ITU-T.

Prior to OSI, networking was completely vendor-developed and proprietary, with protocol standards such as SNA, Appletalk, NetWare and DECnet. OSI was an industry effort, attempting to get everyone to agree to common network standards to provide multi-vendor interoperability. It was common for large networks to support multiple network protocol suites, with many devices unable to talk to other devices because of a lack of common protocols between them.

The OSI reference model (which actually predates the OSI protocol work, dating to 1977) was the most important advance in the teaching of network concepts. It promoted the idea of a common model of protocol layers, defining interoperability between network devices and software.

However, the actual OSI protocol stack (also known as the X.25 protocol suite)that was specified as part of the project was considered by many to be too complicated and to a large extent unimplementable. Taking the "forklift upgrade" approach to networking, it specified eliminating all existing protocols and replacing them with new ones at all layers of the stack. In addition, the OSI protocols were specified by committees filled with differing and sometimes conflicting feature requests, leading to numerous optional features;

The OSI approach was eventually eclipsed by the Internet's TCP/IP protocol suite. TCP/IP's pragmatic approach to computer networking and two independent implementations of simplified protocols made it a practical standard. For example, the definition for OSI's X.400 e-mail standards took up several large books, while the Internet e-mail (SMTP) definition took only a few dozen pages in RFC-821.

Many of the protocols and specifications in the OSI stack are long-gone or have been superseded, such as token-bus media, CLNP packet delivery, FTAM file transfer, and X.400 e-mail. IS-IS also continues as a network routing protocol used by larger telecommunications companies, having been adapted for use with the Internet Protocol. Many legacy SONET systems still use TARP (TID Address Resolution Protocol - utilizes CLNP and IS-IS) to translate Target Identifier of a SONET node. Often protocols and specifications in the OSI stack remain in use in legacy systems, unless or until such legacy systems are eventually upgraded, replaced or decomissioned. The worst part was that OSI's backers took too long to recognize and accommodate the dominance of the TCP/IP protocol suite.