DECnet

DECnet
Name	DECnet
Developer	Digital Equipment Corporation
Introduced	0 1975
Based on	DNA (Digital Network Architecture)
Influenced	OSI model, Microsoft Windows, TCP/IP

DECnet. It is a comprehensive suite of network protocols created by the Digital Equipment Corporation (DEC), first introduced in 1975. The system was built upon a proprietary, layered architecture known as DNA (Digital Network Architecture), which presaged later open standards. DECnet enabled robust communication and resource sharing primarily among DEC's own family of PDP and later VAX minicomputer systems, becoming a dominant network in academic, engineering, and industrial settings throughout the 1980s.

Overview

DECnet was designed to provide transparent file access, task-to-task communication, and network management across a distributed computing environment. Its development was driven by the need to connect DEC's popular PDP-11 systems and later the powerful VAX series. The protocol suite saw widespread adoption in institutions like CERN, where it was used to control major experiments, and within the United States Department of Defense. It competed with contemporary systems from IBM, namely Systems Network Architecture (SNA), and later with the emerging TCP/IP protocol suite.

Architecture

The foundation of the system was DNA (Digital Network Architecture), a layered model that closely mirrored the later OSI model with eight distinct layers. These layers included physical control, data link control, routing, end-to-end communication, and session control, among others. This structured approach facilitated modular development and interoperability between different DEC hardware, from the DECsystem-10 to the MicroVAX. Key architectural concepts included adaptive routing and a hierarchical network addressing scheme that allowed for extensive, scalable networks.

Phases and versions

Development occurred in distinct phases, each adding significant capabilities. Phase I, released in 1975, supported basic communication between two PDP-11 systems. Phase II expanded the network node limit and introduced file transfer capabilities. Phase III, in 1980, added support for Ethernet and adaptive routing. The most significant, Phase IV, became synonymous with DECnet and was widely deployed; it supported 64,449 nodes and full integration with Ethernet as its primary data link. The final major version, DECnet/OSI or Phase V, integrated TCP/IP and OSI protocol stacks, aligning with open standards.

Protocols and technology

The suite implemented numerous proprietary protocols that operated across the DNA layers. For routing, it used the DECnet Routing Protocol, a distance-vector protocol. Upper-layer services included the Data Access Protocol (DAP) for file transfer and the Network Information and Control Exchange (NICE) protocol for network management. For terminal access, it employed CTERM. At the physical and data link layers, it utilized standard technologies like Ethernet (using its own framing), DDCMP for serial links, and later supported FDDI and X.25 networks.

Applications and usage

DECnet enabled a wide range of distributed applications that were advanced for their time. It was integral to the VAXcluster system, creating tightly coupled multiprocessor systems. Users could transparently access files on remote nodes using commands like `COPY`. The ALL-IN-1 office automation suite and the DECnet-DOS product for IBM PC compatibles extended its reach. Major scientific and telecommunications users, including the United States Navy and many universities, relied on it for laboratory automation, industrial control, and academic research networks.

Legacy and influence

While largely supplanted by TCP/IP by the late 1990s, its influence on modern computing is profound. Its network management concepts informed the design of SNMP. The layered DNA model provided a real-world implementation that influenced the development of the OSI model. Elements of its design and utilities were incorporated into later Microsoft Windows networking subsystems. Furthermore, the extensive deployment of DECnet at pivotal institutions like CERN helped establish the culture of networked computing that led to the Internet.

Category:Digital Equipment Corporation Category:Network protocols Category:Computer networking