Towards Formal Structures for IEEE 802.6

This paper explores the use of a methodology inspired by “Formal Definition Techniques”, or FDTs, for the description of DQDB, Distributed Queue — Dual Bus, a draft standard for an access procedure to be used as a primary mode of transmission for Metropolitan Area Networks. The thesis here is that these “formal structures” are pimarily useful for delineating, discovering and possibly avoiding “extreme” behavior. Extreme behaviour can manifest itself in malfunctioning software, specification incompleteness, logical inconsistencies, and/or unexpected or unforseen actions. Statistical modelling is important for determining expected behaviour but its validity may be compromised if the system does not behave as expected. This paper concentrates on the the small but important part of the access mechanism, the DQDB layer of the draft standard rather than other aspects of the protocol. It is a preliminary analysis that points out some of the power and problems of the approach.