Open Systems Interconnect

Open Systems Interconnection

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, according to its proponents, networking was largely 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. However while OSI developed its networking standards, TCP/IP came into widespread use on multivendor networks, while below the network layer, both Ethernet and token ring played much the same role.

The OSI reference model (which actually predates the OSI protocol work, dating to 1977) was a major 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.


The actual OSI protocol suite that was specified as part of the project was considered by many to be too complicated and inefficient, 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. Even demands by the USA for OSI support on all government purchased hardware did not save the effort.

Beyond their objections to the Byzantine protocol suite itself, OSI opponents generally contended that the very OSI standardization process represented little more than institutional unwillingness on the part of the ISO and ITU-T to admit that vendor-neutral standards might exist that had not been developed and ratified by their own particular processes (an example of the "Not Invented Here" phenomenon). Much bad blood arose between these standards organizations and the IETF, the Internet standards body, as a result of this dispute. The most vitriolic opponents of the OSI suite at times made the further claim that, far from being a "vendor neutral" standard, OSI represented an attempt by minor, or diminishing, players in the networking and computer industries to recover by government fiat market share that they were rapidly losing to the proponents of TCP/IP through fair competition.

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. 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 took up several large books as well. Furthermore, the X.400 standard contained so many optional format choices that early implementations in France and Germany were unable to parse each other's messages.

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. Some still survive, often in significantly simplified forms. The X.500 directory structure still remains with significant usage, mainly because the original unwieldy protocol has been stripped away and effectively replaced with LDAP. 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 decommissioned.

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 suite. The financial damage done to Japan and Europe (where Internet deployment was delayed by years) is difficult to estimate.

See also


Further reading

  • Marshall T. Rose, The Open Book (Prentice-Hall, Englewood Cliffs, 1990)
  • David M. Piscitello, A. Lyman Chapin, Open Systems Networking (Addison-Wesley, Reading, 1993)
  • Andrew S. Tanenbaum, Computer Networks, 4th Edition, (Prentice-Hall, 2002) ISBN 0-13-066102-3

External links

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