22  LPWAN Fundamentals: Knowledge Checks

Quiz mastery targets are easiest to plan with threshold math:

\[ C_{\text{target}} = \left\lceil 0.8 \times N_{\text{questions}} \right\rceil \]

Worked example: For a 15-question quiz, target correct answers are \(\lceil 0.8 \times 15 \rceil = 12\). If a learner moves from 8/15 to 12/15, score rises from 53.3% to 80%, crossing mastery with four additional correct answers.

In 60 Seconds

These scenario-based quizzes test your ability to apply LPWAN knowledge to real deployment decisions: choosing between LoRaWAN, Sigfox, and NB-IoT based on payload size, message frequency, reliability requirements, and cost constraints. Key skills assessed include link budget calculations, spreading factor trade-offs, and understanding why private LoRaWAN networks become cost-effective at scale.

22.1 Learning Objectives

By the end of this assessment, you will be able to:

  • Select and justify LPWAN technology for real-world deployment scenarios by comparing LoRaWAN, Sigfox, and NB-IoT against payload, frequency, reliability, and cost constraints
  • Calculate total cost of ownership across LPWAN technologies at various device scales and justify the break-even point for private versus operator-managed networks
  • Analyze and diagnose migration risk when an LPWAN operator fails, and construct a transition plan that preserves service continuity
  • Evaluate link budget trade-offs — implement spreading factor selection, assess fade margin adequacy, and distinguish between free-space and real-world path loss

These knowledge checks help you verify your understanding of LPWAN concepts. Each question includes explanations so you learn from any mistakes. Think of them as conversation starters – if you cannot answer a question, it points you back to the section you should review.

22.2 Knowledge Check

Test your understanding of LPWAN concepts with these scenario-based questions.

Quick Check: Distinguish Key LPWAN Properties

Before working through the link budget questions, confirm your mental model of how the three main LPWAN technologies differ on two critical dimensions.

Common Mistake: Assuming LoRaWAN “Unlimited Messages” Means No Restrictions

The Mistake: A developer reads “LoRaWAN has no daily message limit (unlike Sigfox’s 140/day)” and designs a system sending 1 message every 30 seconds (2,880 messages/day), assuming LoRaWAN has truly unlimited capacity.

Why It Fails: LoRaWAN doesn’t have message quotas, but it has duty cycle regulations that are often more restrictive:

EU868 Duty Cycle Analysis (1% limit):
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Allowed airtime: 3,600 seconds/hour × 1% = 36 seconds/hour

Message every 30 seconds = 120 messages/hour
Payload: 20 bytes
Spreading Factor: SF10
Airtime per message: ~370 milliseconds

Required airtime: 120 msgs × 0.37s = 44.4 seconds/hour
Duty cycle used: 44.4 / 3,600 = 1.23%

Result: EXCEEDS 1% limit → Regulatory violation
Penalty: Up to €10,000 fine in Germany, device confiscation, or network shutdown

The Reality of “Unlimited”:

Spreading Factor Airtime (20 bytes) Max msgs/hour (1% DC) Max msg interval
SF7 56 ms 643 msgs/hour Every 5.6 seconds
SF8 103 ms 350 msgs/hour Every 10 seconds
SF9 185 ms 194 msgs/hour Every 18.5 seconds
SF10 370 ms 97 msgs/hour Every 37 seconds
SF11 740 ms 48 msgs/hour Every 75 seconds
SF12 1,319 ms 27 msgs/hour Every 133 seconds

Comparison to Sigfox:

Sigfox limit: 140 messages/day = 5.8 messages/hour (average)
LoRaWAN SF10: 97 messages/hour (16× more than Sigfox!)

BUT: At SF12, LoRaWAN drops to 27 msgs/hour
With 24/7 operation: 27 × 24 = 648 messages/day
Still 4× better than Sigfox, but NOT "unlimited"

What “Unlimited” Actually Means:

  • ✅ No quota enforced by network operator (unlike Sigfox’s 140/day hard cap)
  • ✅ No per-message charges (unlike cellular data plans)
  • ✅ Network server doesn’t artificially throttle you
  • ❌ Still subject to regulatory duty cycle (1% EU868, 36s/hour max airtime)
  • ❌ Still limited by gateway capacity (8 channels × airtime budget)

How to Stay Compliant:

  1. Calculate airtime budget:

    Use LoRa airtime calculator: https://loratools.nl/#/airtime
Example: 50 bytes, SF9, BW125, CR4/5 = 205.8 ms
Max messages: 36,000 ms / 205.8 ms = 174 msgs/hour
Safe design: 80% of limit = 78 msgs/hour (every 46 seconds)

  2. Implement airtime tracking in firmware:

    uint32_t airtime_used_ms = 0;
uint32_t hour_start = millis();

void send_lora_message() {
  if ((millis() - hour_start) > 3600000) {
    // New hour started, reset counter
    airtime_used_ms = 0;
    hour_start = millis();
  }

  uint32_t msg_airtime = calculate_airtime(payload_size, SF, BW);
  if ((airtime_used_ms + msg_airtime) > 36000) {
    // Would exceed 1% duty cycle, defer transmission
    return ERROR_DUTY_CYCLE;
  }

  transmit();
  airtime_used_ms += msg_airtime;
}

  3. Use Adaptive Data Rate (ADR): Network server optimizes SF to use lowest (fastest) SF that maintains link → maximizes message capacity

Key Lesson: “Unlimited” in LPWAN marketing means “no artificial quota” — but physics (duty cycle) and regulations still apply. Always calculate your airtime budget before deploying, especially for high-frequency applications.

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