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Problem: I am working on a QueueModel in OMNeT++ that involves two input sources generating packets for a server. My aim is to simulate a queueing system and compare the number of packets generated by the simulation against analytical results.

Configuration:

*.source1.arrivalRate = 1 (jobs per second)
*.source2.arrivalRate = 1 (jobs per second)
**.bufferSize = 50
*.server.serviceTimeSource1 = 0.1 (seconds per job for source1)
*.server.serviceTimeSource2 = 1 (seconds per job for source2)
sim-time-limit = 1000000s

With around 30 repetitions, the number of packets generated is 1,032,000, which is roughly 3% higher than my analytical results — an acceptable margin.

Issue: When I increase the arrivalRate from 1 to 10 for each source, the number of packets generated becomes 30-40% higher than the analytical results, which is not acceptable.

Here is a snippet of my code for generating packets in Source1:

void Source1::initialize() {
    sendMessageEvent = new cMessage("sendEvent");
    scheduleAt(exponential(1.0 / par("arrivalRate").doubleValue()), sendMessageEvent);
}
void Source1::handleMessage(cMessage *msg) {
    if (msg == sendMessageEvent) {
        cPacket *packet = new cPacket("job");
        packet->addPar("sourceName").setStringValue("source1");
        send(packet, "out");
        numJobsGenerated++;
        // Other code for logging and visualization
        scheduleAt(simTime() + exponential(1.0 / par("arrivalRate").doubleValue()), sendMessageEvent);
    }
}
void Source1::finish() {
    recordScalar("Source1 Jobs generated", numJobsGenerated);
}

Questions: Is there any obvious reason why increasing the arrivalRate would create such a large discrepancy? Are there any tweaks or fixes that can be applied to make the simulation more accurate? Any help would be greatly appreciated. Thank you!

Waqas Ahmad
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  • Hard to tell without seeing all the related NED files and the INI files. Please provide the Source1.ned file. The C++ code looks ok to me. I assume you expect 10,000,000 messages generated when using 10 for the arrival rate. What is the exact number you received in the simulation instead of that? – Rudi Sep 01 '23 at 08:53

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