DQDB: an overload cycle analysis of Generalized Bandwidth Balancing with strict priority

Abstract

This paper uses a state-space approach and an analysis of overload cycle constraints to show that a simple extension of Generalized Bandwidth Balancing ( GBWB), can be used to strictly enforce non-preemptive priorities. The extension associates a Bandwidth Balancing Machine and counter with each queue access machine rather than with the node in order to implement priorities. The entire DQDB network is modeled as a large Finite State machine. Overload cycles are those cycles in the state space that correspond to the situation that all nodes attempting to access the bus are attempting to do so continuously. Simple analytical constraints on the node counters allow derivation of the bandwidth sharing amongst the nodes and give insight into how the system structure results in such sharing. GBWB may be viewed as a mechanism that implements “rate-based” priorities that result in predictable apportioning of the bandwidth. In this paper, a system is presented that enforces “access-based” priorities where a single high priority node may prevent all access to the bus of low priority nodes.