Asynchronous Bandwidth Allocation and Parameter Setting in the Fieldbus Protocol

抄録

In the centralized architecture of the Fieldbus protocol, the synchronous phase executions are being guaranteed timeliness and throughput, by a centralized scheduling scheme on a timely basis. Whereas, the asynchronous phase executions are being served using an emulation of a “circulated” timed token. In such a scheme, the maximum token circulation (i.e., cycle) time is determined by the maximum token holding time parameter within which a station can serve its asynchronous traffic. Furthermore, the maximum token circulation time determines the maximum waiting time an urgent data can incur.
In order to ensure that the maximum delivery bounds of urgent data are not violated, and that the maximum token circulation time is kept within bounds, an asynchronous bandwidth allocation mechanism is proposed. In this approach, first the allocatable asynchronous bandwidth is derived. Then, A modified version of the Normalized Proportional Allocation Scheme (NPAS) is proposed, to determine the bandwidth share of each station (i.e. the maximum token holding time). In case the NPAS is not applicable, due to protocol constraints, a procedure allowing its partial application is proposed.
Analytical and simulation analysis are used to evaluate the proposed approach. The results show that the asynchronous bandwidth allocation scheme and the parameter setting policy can ensure, in one hand, that the delivery delay bounds of urgent messages are not violated, and that the maximum token cycle time is kept within bounds. On the other hand, the results show that parameter tuning can further lead to improve the protocol performance.