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Open systems interconnect

In the 1980s, the International Organization for Standardization (ISO) documented a new effort in networking called Open Systems Interconnect or OSI.

Prior to OSI, networking was completely vendor-developed and proprietary, with standards such as SNA, Decnet, and XNS. OSI was a new 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 model 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 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. This made implementation difficult, and was resisted by many vendors and users with significant investments in other network technologies. In addition, the OSI protocols were specified by committees filled with differing and sometimes conflicting feature requests, leading to numerous optional features. Because so much was optional, many vendors' implementations simply could not interoperate, negating the whole effort.

The OSI approach was eventually eclipsed by the Internet's TCP/IP protocol suite and its simplified pragmatic approach to networking. The Internet approach was to create simple protocols with two independent working implementations in order to be considered a standard. The simplified approach ensured that a standard would be practically implementable. For example, the standards definition for X.400 e-mail took up several large books, while the Internet e-mail (SMTP) definition took only a few dozen pages in RFC-822 and 823.

Most protocols and specifications in the OSI stack are long-gone today, such as token-bus media, CLNP packet delivery, FTAM file transfer, and X.400 e-mail. Only a few still survive, in significantly simplified forms. The X.500 directory structure still survives with significant usage, mainly because the original unwieldy protocol has been stripped away and effectively replaced with LDAP. IS-IS also survives as a network routing protocol used by larger telecommunications companies, having been adapted for use with the Internet protocol.