Scenario: A manufacturing facility has 200 production machines that generate sensor data. Each machine produces 500 readings/second (temperature, vibration, pressure). The facility must decide how to distribute processing across edge, fog, and cloud tiers.
Given Data:
- 200 machines × 500 readings/sec = 100,000 readings/sec total
- Each reading = 20 bytes
- Raw data rate = 2 MB/sec = 5.2 TB/month
- Cloud bandwidth cost = $0.08/GB
- Edge compute (per machine): NVIDIA Jetson Nano, 5W, $99
- Fog server: Dell Edge Gateway, 25W, $2,500
- Safety-critical shutdown latency requirement: <10ms
- Anomaly detection latency requirement: <100ms
Step 1: Calculate cloud-only cost:
Monthly data: 5.2 TB
Cloud bandwidth: 5,200 GB × $0.08/GB = $416/month
Cloud processing (compute): $800/month (estimated)
Total cloud-only: $1,216/month = $14,592/year
Step 2: Apply three-tier rule:
- Edge tier: Safety-critical shutdown (<10ms) — must process locally at each machine
- Fog tier: Cross-machine anomaly detection (10-100ms) — aggregate from 200 machines
- Cloud tier: Historical analysis and ML training (>100ms) — use aggregated summaries
Step 3: Design edge processing:
Each edge node:
- Monitors own machine's 500 readings/sec
- Applies simple threshold rules (temp > 85°C → shutdown)
- Transmits only anomalies (5% of data) to fog
- Latency: 2-5ms local processing ✓ Meets <10ms requirement
Step 4: Design fog aggregation:
Fog server collects from 200 edge nodes:
- Receives 5% anomaly data = 100 KB/sec
- Runs correlation analysis (vibration spike + temp rise = bearing failure)
- Sends hourly summaries to cloud (1 MB/hour = 720 MB/month)
- Latency: 50ms aggregation + correlation ✓ Meets <100ms requirement
Step 5: Calculate three-tier cost:
Edge hardware: 200 × $99 = $19,800 (one-time)
Fog hardware: 1 × $2,500 = $2,500 (one-time)
Monthly cloud bandwidth: 720 MB × $0.08/GB = $0.06/month
Monthly cloud storage (summaries): $50/month
Three-tier TCO (Year 1): $19,800 + $2,500 + ($0.06 + $50) × 12 = $23,000
Three-tier TCO (Year 2+): $600/year
Cloud-only TCO (Year 1): $14,592
Three-tier TCO (Year 1): $23,000
Payback period: Year 1 loss = $8,408
Year 2+ savings: $14,592 - $600 = $13,992/year
Break-even: 8,408 / 13,992 = 0.6 years (7.2 months into Year 2)
Result: Three-tier architecture costs more in Year 1 but saves $14K/year after break-even (18 months). Over 5 years, saves $47,560 compared to cloud-only while meeting latency requirements that cloud-only cannot achieve.
Key Lesson: Don’t evaluate edge-fog-cloud economics on bandwidth alone. The real value is meeting latency requirements that pure cloud cannot satisfy — safety-critical shutdown at <10ms is impossible with 80-200ms cloud round-trip time.