Fiber Distributed Data Interface System Level Description

This document provides a comprehensive overview of the Fiber Distributed Data Interface (FDDI) standard and Digital Equipment Corporation's implementation of it. FDDI is defined by ANSI as a 100-Megabit-per-second (Mb/s) timed-token ring local area network primarily using fiber optic cable. It is built upon four sublayers: Physical Layer Medium Dependent (PMD), Physical Layer Protocol (PHY), Media Access Control (MAC), and Station Management (SMT), which map to the Physical and Data Link layers of the OSI model.

Digital's implementation emphasizes the "dual ring of trees" network topology as its recommended choice, offering high reliability, scalability, and maintainability. This topology constructs an FDDI ring with concentrators at its core, allowing the network to provide backup physical links, isolate faults, and electronically bypass inactive or defective stations. Key FDDI devices include Single Attachment Stations (SAS), Dual Attachment Stations (DAS), and Concentrators, all of which Digital manufactures to be fully compliant with ANSI standards.

The manual delves into core FDDI concepts such as timed-token protocol, data encoding (4B/5B and NRZ/NRZI), clock synchronization, and ring operation, including fault recovery mechanisms like beaconing and tracing. Digital has also introduced enhancements, such as the Frame Content Independent Stripping (FCIS) algorithm for efficient frame removal and the Ring Purger algorithm to prevent "No Owner Frames" (NOFs) from circulating indefinitely, thereby boosting network robustness and reliability.

Furthermore, the document highlights Digital's sophisticated network management capabilities, which are central to its Enterprise Management Architecture (EMA). These tools enable users to configure, monitor, diagnose faults, manage performance, handle accounting, and ensure security across the extended LAN environment, facilitating effective control and troubleshooting of FDDI networks.