Sending data from Arduino - part 1

Part 1: as a first approach, let’s try the simplest way to send data using UDP packets.

It’s not important at the moment: software or networking performance, throughput, etc. This is a first test to check the basics, there is a lot of room for improvements in the code, protocol, approach, which will be done in future iterations and based on the discoveries done in this test.

The experiment

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Packet contents

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Each packet contains a string, composed of 2 values separated by coma and with \n at the end:

• value of an incremental counter
• the value of micros()

For example:

and so on.

Why micros()? Because I want to experiment with a way to have a timestamp as part of the message.

Why use a string? Because running netcat in the Raspberry Pi will allow to easy check what we received. This is the same reason why I added an \n at the end.

Running the experiment: Arduino point of view

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While sending the packets from the Arduino, the status is written to the serial console, and this is what I got:

(...)
stats | iter=1797000 | err=9285 | msg/sec=507
stats | iter=1798000 | err=9285 | msg/sec=507
stats | iter=1799000 | err=9285 | msg/sec=507
stats | iter=1800000 | err=9285 | msg/sec=507
ENDED | Sent 1800001 UDP packets in 3545431 ms. with 9285 errors

Running the experiment: Raspberry Pi point of view

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On the Raspberry Pi, netcat was listening for those UDP packets and the output was redirected to a file:

$ netcat -u -s 0.0.0.0 -p 4545 -l > netcat-output.txt

Once finished, the last lines of netcat-output.txt were:

$ tail -n5 netcat-output.txt 
1799996,3579146918
1799997,3579148832
1799998,3579150780
1799999,3579152790
1800000,3579154958

How the errors are counted?

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The return value of WiFiUDP.endPacket() is used: that method returns 0 if there was some error. See https://www.arduino.cc/en/Reference/WiFiNINAUDPEndPacket for mor information.

On the Raspberry Pi, we can easilly check how many packets were actually received:

$ wc -l netcat-output.txt
1790712 netcat-output.txt

Results

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Errors in percentages

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Next steps

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• Refactor Arduino code
• Send data in binary format instead of strings
• Experiment with retries
• Experiment buffering data and send multiple data points per UDP message
• Experiment with different WiFi configuration

Appendix: snippet of Arduino code

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void floodUdp(int iters) {
    int ret;
    int ratio;
    counter = 1;
    errors = 0;
  
    Serial.println("Flooding...  :D");
    udpStart = millis();
    for (; iters == -1 || counter <= iters; counter++) {
        if (counter % 1000 == 0) {
            ratio = (counter) / ((millis() - udpStart) / 1000);
            Serial.println(str + "stats | iter=" + counter + " | err=" + errors + " | msg/sec=" + ratio);
        }
        if (sendUdpToServer(serverHostname) != 1) {
            errors++;
        };
    }
    udpEnd = millis();
  
    Serial.println(str + "ENDED | Sent " + counter + " UDP packets in " + (millis() - udpStart) + " ms. with " + errors + " errors");
}

int sendUdpToServer(char* ip) {
    if (Udp.beginPacket(ip, 4545) != 1) {
        Serial.println("[UDP] Udp.beginPacket() failed");
        return -1;
    }
  
    String(counter).toCharArray(buf, 100);
    Udp.write(buf);
    Udp.write(sep);
    String(micros()).toCharArray(buf, 100);
    Udp.write(buf);
    Udp.write(nl);
  
    if (Udp.endPacket() != 1) {
        Serial.print("e");
        return -1;
    }
  
    return 1;
}

Appendix: Wifi configuration

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Configuration of hostapd running in Raspberry Pi:

# --------------------------------------------------------------------
# 2 Ghz (Arduino Nano 33 IoT)
# --------------------------------------------------------------------

interface=wlanrpi0
hw_mode=g
channel=10
ieee80211d=1
country_code=US
ieee80211n=1
wmm_enabled=1
ssid=********
auth_algs=1
wpa=2
wpa_key_mgmt=WPA-PSK
rsn_pairwise=CCMP
wpa_passphrase=********

macaddr_acl=1
accept_mac_file=/etc/hostapd/accept_mac.txt