Digital Equipment Corporation (DEC) has given support to network connectivity for networks of its own and networks of other vendors also. DEC has its own architecture known as Digital Network Architecture (DNA) the product that implements the architecture is known as DEC net. The DNA was developed during 1974, to provide communication facility between various DEC systems.
DNA is designed as a peer-to-peer network. No node on the network is the master or a controller. When all nodes follow proper protocol, any DEC net node can communicate with any other node. DNA is flexible and copes with changes in network hardware and software. The major design issues of DEC net are the peer-to-peer design and the network administration. Polling is performed on the network nodes to obtain information about links, performance, bottlenecks, delays, etc.
To get proper network configuration, DEC chronologically introduced different phases of DEC nets.
Each phase of DEC net specifies the operating systems supported by that phase, communication hardware supported, and new features introduced in each phase. Each higher-level phase has downward compatibility to the immediate lower level. Phase mechanism allows DEC net to operate with a variety of other vendor's software products. The following are the phases of DEC net evolved until early 90s.
Phase I Introduced in 1976.It used few nodes via asynchronous communication and supported few operating systems. No big standard was emerged during that time. Hence, DEC net phase I was not conforming to any standard.
Phase II This phase has total conformance with OSI reference model. Routing was done manually.
It supported a number of other operating systems.
Phase III Enhanced version of DEC net with full routing capability, file transfer facility, and task-to-task communication facility. Additional features are:
- • Ability to talk to other networks
- • Remote login.
- • Remove file manipulations, using send and receive messages.
Phase IV Introduced in the year 1984. It has better conformance to 051. It supports Ethernet, MSDOS and Ultrix (A version of Unix developed by DEC). It supports non-DEC hardware and operating systems.
Phase V Announced in 1987. It has got full 051 compatibility. In addition to the 051 compatibility, it supports new features such as distributed naming service, distributed file services, file transfer management, and Virtual Terminal Protocol (VTP), etc.
DNA phase V architecture is similar to 0SI model. DNA layers communicate with each other to provide desired communication facility. Each layer follows a different type of protocol. Users can interact with the top layer through application programs. Layered DNA model is shown in Figure.
The session control layer and the transport layers are responsible for routing of packets between DNA nodes. The three lower layers perform physical network functions and provide necessary interfaces to hardware. To maintain the functionality of the network, the DEC net maintains two databases. They are the permanent database and the volatile database. The permanent database contains static information about the network nodes, interfaces, and the status of the Program after node initialization. This static information database is loaded into the memory whenever the DEC net is initialized. The volatile database is used to record the information that dynamically changes during the network running time. Information such as adjacent node status, routing status, etc., falls in this category. The volatile database exists until the network is alive. It is erased when the network is down or crashes. An administrator can directly modify both tables. DEC net gateways permit access to other networks such as SNA and X.25-based networks.