Feed forward rings

Military and other critical (life-threatening/life-saving) systems require performance evaluations from a survivability, reliability, maintainability, and availability (SRMA) perspective. Applying SRMA concepts to commercial token passing ring local area networks (LANs) (e.g., IEEE-802.5, FDDI, SAE-9B) has uncovered several design issues that could impact the utility of these networks in a critical system. This article discloses a new architecture for token passing rings, called “feed forward rings” (FFR), which provides a significant improvement in system survivability, can eliminate “jabber mode” induced ring failures, and provides for passive optical bridging between rings.

The basic “feed forward” concept is that LANs, based on token passing protocols and utilizing multiple, physically separated, fiber optic rings, should have the serial data stream on each ring flowing in the same direction of rotation. This article provides qualitative observations on the performance of current commercial rings and discusses the advantages of several FFR topologies. Implementation issues related to FFRs are also discussed. It is recommended that FFR alternatives to counterrotating rings (CRR) with separated cables be considered for inclusion in LAN standards.