The Lost Connection Storyboard

JSS 3 MakeCode micro:bit Simulation first | Packet Numbers and Reliable Radio

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Simulation lab Build guide Learn & Story Test model Try a challenge Quick check & Quiz

Simulation lab

Board: BBC micro:bit

⛶ Expand Open MakeCode ↗

Simulator preview

☀️ 80

Change the slider level and observe the 5x5 LED display.

📁 Tools ▴

• Web browser (Chrome, Edge, Firefox)
• Internet connection
• MakeCode (micro:bit) online editor

📖 Learn 🔧 Build </> Code ▷ Test ⚑ Challenge ❓ Quiz

🔧 Build Your Reliable Radio Protocol

1 Set both BBC micro:bits to shared radio channel 42 using 'radio.setGroup(42)'.

2 Structure telemetry packets as formatted strings: 'packet_id#sensor_payload'.

3 Program the receiver to track 'expectedId': if an arriving packet number is greater than expected, flag lost packet!

4 Transmit an automatic Negative Acknowledgment (NACK) requesting retransmission of the missing sequence!

🧩 Block plan / Starter code ▴

radio.setGroup(42)
let packetId = 0
let expectedId = 0

// Transmitter Logic (Button A)
input.onButtonPressed(Button.A, function () {
    packetId += 1
    let payload = "SOIL:45"
    radio.sendString(packetId + "#" + payload)
    basic.showNumber(packetId)
})

// Receiver Logic
radio.onReceivedString(function (receivedString) {
    let parts = receivedString.split("#")
    let id = parseInt(parts[0])
    if (id == expectedId + 1) {
        expectedId = id
        basic.showIcon(IconNames.Yes) // Clean ordered packet
    } else if (id > expectedId + 1) {
        basic.showIcon(IconNames.No)  // Packet loss detected!
        radio.sendString("NACK#" + (expectedId + 1))
    }
})

Sequence numbers allow the receiver to identify dropped packets and trigger automated recovery.

⚗️ Test your model

Test Condition / Action	Expected Result
Packets 1, 2, 3 sent in order	Receiver verifies consecutive sequence; shows Yes checkmark
Packet 2 dropped (simulated)	Packet 3 arrives -> Receiver flags 'No' icon and broadcasts NACK#2
Transmitter receives NACK#2	Transmitter re-sends packet 2 from memory buffer

⚑ Try a challenge

Implement an automated Sender Timeout: if no ACK is received within 2 seconds of sending, retransmit automatically!

❓ Quick check

Why does our wireless protocol attach an incrementing sequence number (1, 2, 3...) to every radio message?

• So the receiver can detect if a packet was dropped in transit and reconstruct the true message order
• To make the radio signal travel twice as far

Check answer

📖 Our Story & Concepts

When sending wireless radio or cellular signals across farmland or school compounds, radio waves reflect off walls, get absorbed by trees, and suffer from interference. Wireless packets get dropped constantly.

If you stream video or download a file over the Internet, how does your phone reconstruct the complete file without corruption? Protocols like TCP (Transmission Control Protocol) stamp every packet with a sequence number. By tracking numbers, receiver devices detect gaps and automatically request missing slices.

</> Full Annotated Code Walkthrough

// Complete Reliable Transport Protocol with Retransmission Buffer
radio.setGroup(42)
let txBuffer: string[] = []
let currentSeq = 0
let ackedSeq = 0

// Sender: Send with Sequence Tracking
input.onButtonPressed(Button.A, function () {
    currentSeq += 1
    let packet = currentSeq + ":TEMP:" + input.temperature()
    txBuffer.push(packet)
    radio.sendString(packet)
    basic.showNumber(currentSeq)
})

// Receiver: Handle In-Order Delivery & NACKs
let expectedSeq = 1
radio.onReceivedString(function (pkt) {
    if (pkt.includes("NACK:")) {
        let missing = parseInt(pkt.split(":")[1])
        // Resend missing packet from buffer
        for (let item of txBuffer) {
            if (parseInt(item.split(":")[0]) == missing) {
                radio.sendString(item)
            }
        }
    } else {
        let seq = parseInt(pkt.split(":")[0])
        if (seq == expectedSeq) {
            expectedSeq += 1
            basic.showIcon(IconNames.Happy)
        } else if (seq > expectedSeq) {
            basic.showIcon(IconNames.Sad)
            radio.sendString("NACK:" + expectedSeq)
        }
    }
})

Sequence numbers allow the receiver to identify dropped packets and trigger automated recovery.

▷ Expected Results & Troubleshooting

Test Condition / Action	Expected Result
Packets 1, 2, 3 sent in order	Receiver verifies consecutive sequence; shows Yes checkmark
Packet 2 dropped (simulated)	Packet 3 arrives -> Receiver flags 'No' icon and broadcasts NACK#2
Transmitter receives NACK#2	Transmitter re-sends packet 2 from memory buffer

⚑ Extension Challenges & Blueprints

⚑ Extension Challenges

• Level 1: Display packet latency (time between transmission and ACK) in milliseconds.
• Level 2: Build a 5-packet queue buffer that sends packets in bursts.
• Level 3: Implement checksum error detection to verify that packet payload bytes were not corrupted in transit.

❓ Check Your Understanding

Question 1: Why does our wireless protocol attach an incrementing sequence number (1, 2, 3...) to every radio message?

• So the receiver can detect if a packet was dropped in transit and reconstruct the true message order
• To make the radio signal travel twice as far
• Because radio waves only carry even numbers
• To change the frequency channel

Question 2: The receiver receives packet 1, packet 2, and then packet 4. What should the protocol do?

• Ignore packet 4 and turn off the screen
• Detect that packet 3 is missing and broadcast a retransmission request (NACK 3)
• Pretend packet 3 never existed
• Delete the entire program

Question 3: Which built-in radio feature of the BBC micro:bit is used to simulate dual-device wireless communications?

• radio.setGroup() and radio.sendValue()
• music.setVolume()
• pins.digitalReadPin()
• input.acceleration()

Question 4: What is this protocol concept called in the architecture of the global Internet?

• TCP (Transmission Control Protocol) packet sequencing
• HTML styling
• CSS typography
• Battery management

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