Deterministic protocols for real-time communication in multiple ...

S. Norden et al. / Computer Communications 22 (1999) 128–136 135effect of the different input parameters, on SR, we canconclude the following:• SR vs Load: In Fig. 2(a), as load increases, there is apredictable fall in SR values for all the three protocolsas more number of messages are generated and morenumber of collisions, thus reducing SR.• SR vs a: In Fig. 3(a), as the laxity factor increases, thedeadline associated with every message increases,resulting in an increase in the number of messagesthat are transmitted successfully, hence increasing SR.• SR vs N: In Fig. 4(a), as the number of nodesincreases, the SR falls. This is the result of an increasein the number of messages, as well as a larger epochtime, which leads to more messages missing theirdeadlines.• SR vs P/Nt: In Fig. 5(a), as P/Nt is decreased, theperformance will fall expectedly, as a smaller ratioimplies a larger overhead as result of notifier transmission,compared to actual packet transmission.An interesting observation from the graphs is that theECU is not always influenced by the SR. Normally, onewould expect that as SR decreases, the ECU will alsodecrease since less number of slots are used for successfultransmission. Sometimes, this relation may not hold wheneven if less number of messages are transmitted, thecombined service times may exceed those of the droppedmessages, which will still increase the ECU (see Fig. 2(a)and (b)). This explanation is supported by the increasingNTL values (in Fig. 2(c)) with increasing load, for all theprotocols, implying that messages with larger service timesare transmitted.5. ConclusionsIn this article, we have identified the need for real-timeprotocols to support multipacket real-time message transmissionwhich is the form of communication in a typicalreal-time system. We have highlighted the inadequacy ofthe existing protocols to support this requirement. As a solutionto the problem, we have proposed protocols with besteffort service for multiple-access networks, namely, theLDCR and MDCR protocols. The MDCR is an extensionof DCR protocol while the LDCR protocol is based on theconcept of message deferment.We have demonstrated the effectiveness of our best effortprotocols through simulation, for a wide range of parametersfor multiple performance metrics, by comparingthem with a protocol (PBCSMA) recently proposed forthe same problem. Our simulation results reveal that theLDCR protocol performs better than the PBCSMA andMDCR for all the three performance metrics – successratio, effective channel utilization, and normalized transmissionlength. We are currently studying the effect of dynamicallyassigning more than one vertex in the CR tree to anode, allowing additional messages to be sent in the sameepoch.References[1] K. Arvind, K. Ramamritham, J.A. Stankovic, Window MAC protocolsfor real-time communication services, Technical Report, Universityof Massachusetts, USA, 1992.[2] J.I. Capetanakis, Tree algorithm for packet broadcast channels, IEEETrans. on Communications 27 (10) (1979) 1476–1484.[3] D.E. Comer, D.L. Stevens, Internet working with TCP/IP-Volume 2,Prentice-Hall International Inc, Englewood Cliffs, NJ, USA, 1994.[4] Intel Corp., CSMA/DCR deterministic access LANs, PresentationViewgraphs, Folsom Microcomputer Group, Feb. 1988.[5] K.H. Kim, C. Serro, Robustness of real-time local area network protocols,Computer Communications 20 (1997) 169–176.[6] L. 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Ramamritham, Virtual time CSMA protocols for hardreal-time communication, IEEE Trans. on Software Engineering 13(8) (1987) 938–952.Samphel Norden obtained his B.Tech degree in Computer Science andEngineering from the Indian Institute of Technology, Madras, in 1998.Currently, he is a research student at the Computer Science Department,University of Washington at St. Louis, USA. His research areas ofinterests are real-time networks and active networks.S. Balaji obtained his B.Tech degree in Computer Science and Engineeringfrom the Indian Institute of Technology, Madras, in 1998.Currently, he is a research student at the Computer Science Department,University of Illinois at Urbana Champaign, USA. His researchareas of interest are parallel and distributed computing and computernetworks.