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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

DECnet is Digital Equipment Corporation's name for the set of hardware and software products that implement DNA (Digital Network Architecture), a protocol suite that is very similar in structure to the OSI protocol model, except that an additional layer exist under the application layer for network management. DEC (Digital Equipment Corporation) was purchased in 1998 by Compaq Computer Corporation.

DECnet was first announced in the mid 1970s along with the introduction of the DEC VAX 11/780. It was originally designed for parallel interfaces that connected nearby systems (within 30 feet). In 1980, Digital, Xerox, and Intel announced Ethernet as a way to interconnect computer systems. The protocol layers of DECnet (which later influenced the OSI protocol stack) worked well in implementing the signaling and access scheme for nodes attached to Ethernet. DECnet also defines communication networks over FDDI (Fiber Distributed Data Interface) metropolitan area networks, and wide area networks that use private or public data transmission facilities.

DECnet has evolved over the years in phases. DECnet Phase III implemented many advanced networking features, including adaptive routing that could detect link failures and reroute traffic as necessary. DECnet Phase IV, introduced in 1982, provided various enhancements, including the following:

  • A virtual terminal that let users log on to a remote node
  • Support for up to 64,000 nodes (1,023 nodes in 63 areas)
  • Implementation of RIP (Routing Information Protocol), a distance-vector routing algorithm
  • An IBM SNA (Systems Network Architecture) gateway

DECnet Phase IV is still the most widely used of the DECnet releases, even through DECnet Phase V was announced in 1987. Phase 5 provided full compliance with the OSI model and backward compatibility to Phase IV. In Phase V, networks are broken up into routing domains that provide more flexibility in management than the backbone structure of Phase IV networks. The size of node addresses was increased to accommodate the domain number where a node exists.

In 1991, DEC announced ADVANTAGE-NETWORKS, a strategy that adds support for other protocols, backing away from its total commitment to OSI in Phase V. Most important, DEC began supporting TCP/IP and the ability to build multiprotocol backbones that can transport DECnet, TCP/IP, and OSI data. For example, users can transmit data between TCP/IP applications using OSI transport protocols or between OSI applications using TCP protocols.

DECnet interconnects Digital PDP and VAX minicomputers systems as well as desktop personal computers and workstations. Fundamental to DECnet was the now common idea of interconnecting machines that have their own processing power, rather than connecting terminals to a central system.

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