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Reducing pessimism in CAN response time analysis
Publication Type:
Student Thesis
Abstract
This thesis investigates the level of pessimism in the traditional
schedulability analysis for the Controller Area Network (CAN).
Specifically, we investigate the effects of considering bit-stuffing
distributions instead of worst case bit-stuffing. This allows us to
obtain bus utilisation values more close to reality. On the other
hand, since our analysis is based on assumptions concerning
distributions of stuff-bits, our response times will only be met
with some probability.We introduce a model and some methods, that relax the pessimism of the
worst case analysis, and we show the effect of our methods by
considering both an artificial traffic model and samples of real CAN
traffic. Also, we propose a simple coding scheme that substantially
reduces the number of stuff-bits in the considered real traffic.Delay variations (jitter) in computations and communications cause
degradation of performance in control applications. There are many
sources of jitter, including variations in execution time and bus
contention. By introducing some restrictions when using CAN, such as a
small reduction of available frame priorities, we are able to reduce
the number of stuff-bits in the worst case. We also combine this with
the methods mentioned above that reduces the number of stuff-bits in
the data part of the frame. We show the actual penalty introduced by
forbidding priorities and we show the overall improvement by using
these techniques together in a small case study.------------------------------------------------------------ Svenska ------------------------------------------------------------Examensarbetet utreder graden av pessimism i traditionell schemaläggningsanalys för CAN-bussen (även kallad Controller Area Network). Arbetet fokuserar pÃ¥ användandet av bit-stuffing-distributioner istället för traditionella värsta falls scenarion. Genom att använda distributioner kan vi erhÃ¥lla en analytisk utnyttjandegrad närmare den verkliga, dvs. dÃ¥ pessimismen i analysen är lägre Ã¥terspeglar de analytiska värdena mer de verkliga. Ã
andra sidan, dÃ¥ vÃ¥r analys är baserad pÃ¥ antaganden om distributionernas utseenden, kommer de erhÃ¥llna bara att hÃ¥llas med en viss grad av sannolikhet.I examensarbetet introducerar vi en modell samt nÃ¥gra metoder som lättar pÃ¥ pessimismen i den traditionella analysen. Vi pÃ¥visar effekten av vÃ¥ra metoder genom att använda bÃ¥de modellerad trafik samt âriktigâ trafik uppsamlad hos en av vÃ¥ra industriella partners. Vi föreslÃ¥r en metod för kodning av meddelanden som avsevärt minskar antalet stuff-bits.Variationer (jitter) i beräkningstider samt kommunikation orsakar en degradering av prestanda i reglersystem. Det finns mÃ¥nga orsaker till jitter, till exempel variationer i exekveringstider samt trafik pÃ¥ meddelandebussen. Genom att införa smÃ¥ begränsningar vid användandet av CAN, som till exempel förbjudandet av vissa utvalda meddelandeidentifierare, kan vi minska antalet stuff-bitar avsevärt. I rapporten visar vi även kostnaden av att förbjuda dessa meddelandeidentifierare. Vi visar även den totala prestandaökningen vid användandet av alla dessa metoder i ett exempel.
Bibtex
@mastersthesis{Nolte383,
author = {Thomas Nolte},
title = {Reducing pessimism in CAN response time analysis},
month = {November},
year = {2002},
publisher = {MRTC},
url = {http://www.es.mdu.se/publications/383-}
}