Open systems interconnect

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(Redirected from Open Systems Interconnect)

Starting in 1982, the International Organization for Standardization (ISO) along with the ITU-T initiated 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 stack 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. Its pragmatic approach to computer networking and two independent implementations of simplified protocols made it a practical standard. For example, the definition for 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. It should be noted, however, that over time there have been numerous RFCs which extended the original SMTP definition, so that its complete documentation finally takes up several large books as well.

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. Many legacy SONET systems still use TARP (TID Address Resolution Protocol - utilizes CLNP and IS-IS) to translate Target Identifier of a SONET node.

The collapse of the OSI project in 1996 severely damaged the reputation and legitimacy of the organizations involved, especially ISO. The worst part was that OSI's backers took too long to recognize and accommodate the dominance of the TCP/IP protocol


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