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

Packet contents

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

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

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?

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

Errors in percentages

Next steps

• 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

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

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