DECNET TECHNICAL SUMMARY PDP-11

This document is a Technical Summary of DECnet for PDP-11 systems, produced by Digital Equipment Corporation's Distributed Data Processing Group.

It aims to provide an understanding of DECnet, its implementation, and the associated PDP-11 hardware and software used to build computer networks.

Key aspects of the document include:

1. DECnet Overview:

   • Definition: DECnet is a collection of software products that extend DIGITAL operating systems (specifically for PDP-11, but also PDP-8 and DECsystem-10) to form computer networks.
   • Network Types: It enables the creation of various networks for purposes such as communications (data movement, cost reduction), resource sharing (expensive hardware, databases), and distributed computing (coordinating independent systems without a central "host" CPU).
   • Key Functions: These include inter-program communication, inter-system resource sharing (files and devices), down-line loading of operating systems and programs, remote program development, inter-system file transfer, and a high-level language interface for user applications.

2. Digital Network Architecture (DNA):

   • DECnet's design is structured into layered protocols for modularity and flexibility.
   • DDCMP (Digital Data Communications Message Protocol): The physical link layer protocol that handles link traffic control, error detection, and recovery over various types of communication links (synchronous, asynchronous, parallel, full-duplex, half-duplex, point-to-point, and multipoint).
   • NSP (Network Services Protocol): The logical link layer that manages logical connections between programs, message routing, and general network supervision (e.g., detecting node failures).
   • DAP (Data Access Protocol): The highest layer protocol that provides convenient access to remote files and devices within the network.

3. Software System Components:

   • DECnet supports several PDP-11 operating systems, including the RSX-11 family (RSX-11S, RSX-11M, RSX-11D), RT-11, RSTS/E, and IAS. The document provides details on each OS's capabilities, such as multi-tasking, real-time operation, timesharing, and file management.

4. Network Building Blocks:

   • DIGITAL offers "building block" systems for users to implement networks in phases. Examples include:
     • DECcomm 600: A multi-terminal system (RSX-11D based) for transaction processing, offering an upgrade path to full DECnet.
     • Remote Computer Systems: Packages (RSX-11M/D, RSTS/E with 2780 emulation) for data transmission to IBM host systems and local processing.
     • IBM Front End System Base: A PDP-11 based system with an IBM 360/370 interface for programmable front-end processing (note: its software is not directly DECnet-supported).

5. Communications Hardware:

   • The document also provides information for selecting PDP-11 processors and communication interfaces. It details various PDP-11 processor models (PDP-11/04 to PDP-11/70) and their characteristics.
   • It describes asynchronous and synchronous communication links and interfaces (e.g., DL11, DJ11, DH11 for asynchronous; DU11, DQ11, DV11 for synchronous), discussing their features, throughput considerations, and how to configure systems with them.
   • It also covers modem selection and special adapters for various connection types.

In essence, the document serves as a comprehensive guide for understanding and implementing DECnet-based distributed data processing solutions using PDP-11 systems, covering both software architecture and supporting hardware.