Assessment of the priority mechanism in the Fieldbus data link layer

The behavior of the priority mechanism is evaluated with reference to the tokens CiT and DeT provided for in the DLL (data link layer) protocol of the Fieldbus architecture. The use of priorities in the access mechanism is assessed in terms of throughput and access delay. Two policies are considered for allocation of the bandwidth between the high- and low-priority channels. The first provides for a static subdivision of the time available for high- and low-priority acyclic traffic (by use of the DeT token). The second allows dynamic allocation of the available bandwidth between the two priorities (by use of the CiT token). Examination of the throughput for the two policies shows that the same values are only obtained when the first policy allocates at least 80% of the available bandwidth to high-priority traffic. This, however, systematically penalizes low-priority traffic, even when the amount of high-priority traffic is lower than the LAS (link active scheduler) has estimated it to be through polling operations. The second policy, being self-adapting, allows bandwidth allocation to be optimized. The same considerations can be made for access delay.<
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