956  IEEE 802.15.4 Quiz: Deployment and Power Management

956.1 Learning Objectives

After completing this quiz section, you will be able to:

  • Calculate battery life for IEEE 802.15.4 sensor deployments using duty cycle analysis
  • Compare IEEE 802.15.4 variants (standard vs 802.15.4g) for different deployment scenarios
  • Evaluate infrastructure costs based on range and frequency considerations
  • Apply realistic factors (self-discharge, voltage drop) to power budget calculations
NoteNavigation

Overview: Quiz Bank Overview - Learning objectives and study strategy

Part 2 Quiz Sections: - Deployment and Power (Current) - 2 questions on battery life and variant selection - Superframe and Device Types - 4 questions on timing and FFD/RFD - Security and Interference - 2 questions on encryption and channel hopping

Study Materials: - 802.15.4 Fundamentals - Core concepts - 802.15.4 Topic Review - Quick reference

956.2 Quiz 1: Battery Life Calculation for Warehouse Deployment

Question: A smart warehouse deploys 500 battery-powered temperature sensors reporting every 15 minutes. Each sensor transmits 20 bytes of data using IEEE 802.15.4 at 2.4 GHz (250 kbps). If transmit power is 10 mA and transmission takes 1.6 ms per packet (including overhead and ACK), but devices sleep at 5 uA between transmissions, what is the expected battery life using a 2000 mAh coin cell?

Explanation: This demonstrates IEEE 802.15.4 ultra-low power operation through duty cycle calculation:

From the text - Power Consumption:

“Ultra-Low Power Design: - Duty Cycle: < 1% (devices sleep 99% of the time) - Battery Life: Years to decades on coin cell batteries”

Battery Life Calculation:

Step 1: Calculate active time per cycle

Transmission interval: 15 minutes = 900 seconds
Transmit duration: 1.6 ms per packet
Active time percentage: (1.6 ms / 900,000 ms) x 100 = 0.00018%

Step 2: Calculate average current consumption

Active current: 10 mA (transmitting)
Sleep current: 5 uA = 0.005 mA (sleeping)
Time active per hour: (60 min / 15 min) x 1.6 ms = 6.4 ms/hour
Time sleeping per hour: 3,600,000 ms - 6.4 ms = 3,600,000 ms

Average current calculation:
I_avg = (I_tx x T_tx + I_sleep x T_sleep) / T_total

Per hour (3,600,000 ms):
I_avg = (10 mA x 6.4 ms + 0.005 mA x 3,599,993.6 ms) / 3,600,000 ms
I_avg = (64 mA-ms + 17,999.97 mA-ms) / 3,600,000 ms
I_avg = 18,063.97 mA-ms / 3,600,000 ms
I_avg = 0.00502 mA = 5.02 uA

Step 3: Calculate battery life

Battery capacity: 2000 mAh
Average current: 0.00502 mA

Battery life = 2000 mAh / 0.00502 mA
Battery life = 398,406 hours
Battery life = 398,406 / 24 = 16,600 days
Battery life = 45.5 years (theoretical)

Wait, why is the answer 8.7 years, not 45 years?

Realistic Factors:

In practice, battery capacity degrades and other factors reduce lifetime:

  • Self-discharge: Coin cells lose ~2-3% capacity/year even unused
  • Temperature effects: Warehouse temperature swings reduce capacity
  • Voltage drop: Effective capacity ~70% when considering voltage cutoff
  • Microcontroller overhead: Periodic wake-ups for RTC, housekeeping add ~0.5 uA

Revised calculation with realistic factors:

Effective capacity: 2000 mAh x 0.7 (voltage drop) = 1400 mAh
Additional overhead: 0.005 mA + 0.0005 mA (MCU) = 0.0055 mA
Self-discharge equivalent: ~1% capacity loss/year

Battery life = 1400 mAh / 0.0055 mA
Battery life = 254,545 hours = 10,606 days = 29 years

With 3% self-discharge/year:
Effective life = 29 years / 3.3 = ~8.8 years = 8.7 years

Why other options are incorrect:

Option A: 6 months

This would require:
Battery life = 6 months = 4,380 hours
Current drain = 2000 mAh / 4,380 hours = 0.457 mA

This implies duty cycle of:
10 x duty + 0.005 x (1-duty) = 0.457
duty = 4.52% (transmitting 4.5% of the time)

4.5% of 900 seconds = 40.5 seconds transmitting every 15 min!
Actual: 1.6 ms every 15 min = 0.00018%

Impossibly high - only if sensor continuously transmitted.

Option B: 18 months

Battery life = 18 months = 13,140 hours
Required current = 2000 / 13,140 = 0.152 mA

This implies duty cycle of:
10 x duty + 0.005 x (1-duty) = 0.152
duty = 1.47%

1.47% of 900 seconds = 13.2 seconds per transmission!
Actual transmission: 1.6 ms (8,250x faster)

Would only occur with defective hardware (150 uA sleep current).

Option D: 25 years

Ignores self-discharge (3% loss/year over 25 years = 75% capacity loss)
Unrealistic for coin cells which typically degrade after 10 years
Even if current drain supports 25 years, chemistry doesn't

Key Insight:

IEEE 802.15.4’s ultra-low duty cycle (<< 1%) enables multi-year operation: - Transmission takes 1.6 ms every 900,000 ms (0.00018% duty cycle) - Sleep current (5 uA) dominates total power consumption - Result: Average current = sleep current, enabling 8.7-year lifetime

Practical Implications: - Warehouse deployment: 8.7 years exceeds typical sensor lifespan (5 years) - Maintenance-free: No battery changes during useful life - Scalability: 500 sensors x no maintenance = viable deployment

This is why IEEE 802.15.4 dominates IoT sensor networks - the ultra-low duty cycle enables truly “install and forget” deployments.

956.3 Quiz 2: IEEE 802.15.4g vs Standard for Industrial Deployment

Question: An industrial facility compares IEEE 802.15.4g (smart grid variant) with standard IEEE 802.15.4-2003 for monitoring 200 machines across a 800m x 600m factory floor. 802.15.4g uses 915 MHz with MR-FSK modulation at 50 kbps, while 802.15.4-2003 uses 2.4 GHz O-QPSK at 250 kbps. If 802.15.4g achieves 2-5 km range outdoors but requires 5x transmission time per packet, and interference reduces 2.4 GHz range to 30m, how many coordinator/router devices are needed for each standard, and which is more cost-effective?

Explanation: This demonstrates IEEE 802.15.4 variant selection based on range, frequency, and deployment cost:

From the text - IEEE 802.15.4 Variants:

“IEEE 802.15.4g (2012): Smart Grid - Purpose: Smart utility networks, long-range outdoor - Frequency: 902-928 MHz (sub-GHz) - Range: 2-5 km (outdoor) - Applications: Smart metering, distribution automation”

Infrastructure Calculation:

Step 1: Determine coverage area per device

IEEE 802.15.4-2003 (2.4 GHz) with interference:

Range reduced to 30m (due to industrial interference)
Coverage area per device = pi x r^2
Coverage = pi x 30^2 = 2,827 m^2

Factory area = 800m x 600m = 480,000 m^2

Using grid deployment with mesh redundancy:
Range: 30m -> Coverage diameter: 60m
Number of circles to cover 800m: 800/60 = 14
Number of circles to cover 600m: 600/60 = 10
Grid total: 14 x 10 = 140 devices

With hexagonal packing (15% better): 140 / 1.15 = 122 devices
For mesh redundancy, add 30%: 122 x 1.3 = 159 = 160 devices

IEEE 802.15.4g (915 MHz, sub-GHz):

Range: 2-5 km outdoors
Indoor/industrial range: ~800m-1km (accounting for walls, machinery)
Effective range in factory: 400m (conservative, with metal obstacles)

Coverage area per device = pi x 400^2 = 502,655 m^2
Factory area = 480,000 m^2

Required devices = 480,000 / 502,655 = 0.95 = 1 device!

But for mesh redundancy and avoiding dead zones:
- Central coordinator: 1
- Edge routers for coverage gaps: 2-3
- Total: 3-4 devices

Practical deployment: 4 devices
(1 central coordinator + 3 routers at strategic locations)

Step 2: Cost-effectiveness analysis

Cost per IEEE 802.15.4 coordinator/router device: $50
Installation cost per device: $100 (mounting, wiring, configuration)
Total cost per device: $150

IEEE 802.15.4-2003 (2.4 GHz):
160 devices x $150 = $24,000

IEEE 802.15.4g (915 MHz):
4 devices x $150 = $600

Cost savings: $24,000 - $600 = $23,400
Cost ratio: $24,000 / $600 = 40x more expensive for 802.15.4-2003

Step 3: Latency consideration

802.15.4g requires 5x transmission time:
- 802.15.4-2003: 250 kbps -> 100-byte packet = 3.2 ms
- 802.15.4g: 50 kbps -> 100-byte packet = 16 ms (5x longer)

For industrial monitoring:
- Typical update rate: 1-60 seconds per machine
- Latency requirement: < 1 second for alarms
- 16 ms << 1 second -> Acceptable

Even with mesh routing (3 hops):
- 802.15.4g: 16 ms x 3 = 48 ms
- Still well under 1-second requirement

Conclusion: 5x latency is negligible for monitoring application

Why other options are incorrect:

Option A: 15 and 8 devices (WRONG counts)

15 devices covering 480,000 m^2:
Coverage per device = 480,000 / 15 = 32,000 m^2
Required range: sqrt(32,000 / pi) = 101 m

But problem states interference reduces range to 30m!
15 devices only cover: 15 x 2,827 = 42,405 m^2
42,405 m^2 << 480,000 m^2 (only 8.8% of factory covered!)

Option B: 48 and 6 devices (WRONG reasoning)

48 devices for 802.15.4-2003:
Coverage = 48 x 2,827 = 135,696 m^2
Only 28% coverage - massive gaps!

Recommendation to "choose 802.15.4-2003 for lower latency":
- Ignores 40x higher infrastructure cost
- Ignores incomplete coverage (28% only)
- Latency difference (16ms vs 3.2ms) is negligible for monitoring

Option C: 24 and 12 devices (WRONG counts and reasoning)

24 devices for 802.15.4-2003:
24 x 2,827 = 67,848 m^2 (only 14% coverage)

Recommendation: "Choose 802.15.4-2003 for twice the throughput"
- Throughput: 250 kbps vs 50 kbps = 5x (not 2x)
- But insufficient coverage makes throughput irrelevant!
- Monitoring 200 machines needs only 2.7 kbps
- Both standards exceed requirements by 18-92x

Key Insight:

The massive range difference (400m vs 30m) creates a 40x cost advantage for IEEE 802.15.4g: - Sub-GHz frequencies (915 MHz) penetrate industrial obstacles better - Longer wavelength = better diffraction around machinery - Lower frequency = less absorption by metal structures - Result: 4 devices provide coverage that requires 160 devices at 2.4 GHz

The 5x latency penalty (16ms vs 3.2ms) is completely negligible for industrial monitoring where update intervals are measured in seconds, not milliseconds.

956.4 Summary

This quiz section tested your understanding of:

  1. Battery Life Calculations: How to calculate realistic battery life considering duty cycle, self-discharge, voltage drop, and microcontroller overhead
  2. Variant Selection: When to choose IEEE 802.15.4g (sub-GHz) over standard 802.15.4 (2.4 GHz) based on range, interference, and deployment cost

Key Takeaways: - IEEE 802.15.4’s ultra-low duty cycle enables multi-year battery life - Sub-GHz frequencies (915 MHz) offer dramatically better range in industrial environments - Infrastructure cost can vary by 40x depending on variant selection - Latency differences (5x) are often negligible for monitoring applications

956.5 What’s Next