773 Network Topology Selection and Decision Framework

773.1 Learning Objectives

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

Apply Decision Frameworks: Use structured criteria to select appropriate topologies
Compare Performance Metrics: Understand latency, bandwidth, and scalability tradeoffs
Calculate Costs: Estimate infrastructure costs for different topology choices
Match Topology to Application: Select optimal topology for specific IoT use cases

773.2 Prerequisites

Topologies Introduction: Physical vs logical topology concepts
Topology Types: Understanding of star, mesh, ring, bus, tree characteristics

773.3 Topology Decision Framework with Specific Numbers

IoT Topology Selection Matrix

Use this framework to choose the right topology for your IoT deployment based on quantified requirements.

773.3.1 Performance Comparison Table

Metric	Star (Wi-Fi)	Mesh (Zigbee)	Star (LoRaWAN)	Tree (Hierarchical)
Max Range	100m	10m per hop, extends with hops	5-15 km	Varies (wired backbone unlimited)
Typical Latency	10-50ms	50-200ms (multi-hop)	1-10 seconds	1-100ms (depends on tiers)
Bandwidth	50-600 Mbps	250 kbps	0.3-50 kbps	1 Gbps+ (wired tiers)
Devices Supported	50-250	65,000	1,000 per gateway	Unlimited (hierarchical scaling)
Power (Avg Device)	500 mA	15 mA	10 mA	Varies (usually powered)
Cost per Node	$20-50	$5-15	$10-30	$50-200 (includes switch/gateway)
Failure Tolerance	0% (hub fails = all down)	30-40% node loss	0% (gateway down)	Gateway redundancy available
Setup Complexity	Low (1-2 hours)	Medium (4-8 hours)	Low (2-4 hours)	High (days to weeks)

773.3.2 Decision Flowchart

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graph TD
    Start[IoT Deployment Decision] --> Q1{Data Rate?}

    Q1 -->|> 1 Mbps<br/>Video/Audio| Wi-Fi_Star[Star: Wi-Fi]
    Q1 -->|< 250 kbps<br/>Sensor Data| Q2{Coverage Area?}

    Q2 -->|< 100m<br/>Single Building| Q3{Reliability Critical?}
    Q2 -->|> 1 km<br/>Wide Area| LoRaWAN_Star[Star: LoRaWAN]

    Q3 -->|Yes<br/>99.9%+ Uptime| Zigbee_Mesh[Mesh: Zigbee/Thread]
    Q3 -->|No<br/>Simple Setup| Wi-Fi_Star

    Q4{Number of Devices?}
    LoRaWAN_Star --> Q5{Battery Powered?}
    Q5 -->|Yes| LoRaWAN_Star
    Q5 -->|No<br/>AC Powered| Q6{Latency Requirement?}

    Q6 -->|< 100ms| Wi-Fi_Star
    Q6 -->|> 1 second| LoRaWAN_Star

    Zigbee_Mesh --> Final1[Mesh Topology<br/>Self-healing, low power]
    Wi-Fi_Star --> Final2[Star Topology<br/>High speed, simple]
    LoRaWAN_Star --> Final3[Star Topology<br/>Long range, low power]

    style Start fill:#2C3E50,stroke:#16A085,color:#fff
    style Q1 fill:#16A085,stroke:#2C3E50,color:#fff
    style Q2 fill:#16A085,stroke:#2C3E50,color:#fff
    style Q3 fill:#16A085,stroke:#2C3E50,color:#fff
    style Q5 fill:#16A085,stroke:#2C3E50,color:#fff
    style Q6 fill:#16A085,stroke:#2C3E50,color:#fff
    style Final1 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Final2 fill:#E67E22,stroke:#2C3E50,color:#fff
    style Final3 fill:#E67E22,stroke:#2C3E50,color:#fff

773.4 Use Star Topology When…

Choose Star if 3+ of these apply:

Small deployment (<50 devices)
Good line-of-sight to central location
High bandwidth needed (>1 Mbps per device)
Simple setup is priority (install in <2 hours)
AC power available at central hub
Budget < $50/device
Downtime acceptable (99% uptime OK)

Real-world examples: - Home automation: 15 devices, Wi-Fi router hub, $30/device - Office conference rooms: 10 sensors per room, PoE switch hub - Parking lot: LoRaWAN gateway covers 500 spaces, 5 km range

Cost breakdown (50-device star):

Hub/Gateway:        $200-500
Devices (50 × $30): $1,500
Installation:       $500
Total:              $2,200-2,500 ($44-50/device)

773.5 Use Mesh Topology When…

Choose Mesh if 3+ of these apply:

High reliability required (>99.9% uptime)
Physical obstacles (walls, metal equipment)
Large deployment (>50 devices)
Battery-powered sensors
Expansion expected (easy to add nodes)
Self-healing critical (remote/unmanned location)
Low data rate acceptable (<250 kbps)

Real-world examples: - Smart building: 200 sensors, Zigbee mesh, batteries last 3-5 years - Industrial monitoring: 500 nodes, Thread mesh, survives 40% node failure - Smart agriculture: 100 soil sensors, mesh extends range across farm

Cost breakdown (200-device mesh):

Coordinator/Hub:          $100
Router nodes (20 × $15):  $300  (Powered devices that relay)
End devices (180 × $10):  $1,800 (Battery sensors)
Installation:             $1,000
Total:                    $3,200 ($16/device)

773.6 Use Tree (Hierarchical) Topology When…

Choose Tree if 3+ of these apply:

Very large deployment (>1,000 devices)
Multi-building campus or city
Mix of wired and wireless
Tiered data processing (edge/fog/cloud)
Departmental/geographic organization
High-speed backbone needed
Professional IT infrastructure available

Real-world examples: - University campus: Fiber backbone, Wi-Fi per building, sensors per floor - Smart city: Fiber to neighborhoods, LoRa gateways, streetlight sensors - Factory complex: Ethernet backbone, wireless mesh per production area

Cost breakdown (2,000-device tree):

Core switches (3 × $5,000):        $15,000
Distribution switches (20 × $500): $10,000
Edge devices (2,000 × $20):        $40,000
Fiber installation:                $30,000
Total:                             $95,000 ($47.50/device)

773.7 Specific Numbers for Common IoT Applications

Application	Recommended Topology	Typical Scale	Cost/Device	Battery Life	Key Metric
Smart Home	Star (Wi-Fi)	10-30 devices	$30	N/A (powered)	Simplicity
Building Automation	Mesh (Zigbee)	100-500 devices	$15	3-5 years	Reliability 99.9%
Smart City Lights	Star (LoRaWAN)	1,000-50,000	$25	5-10 years	Range 5-15 km
Industrial Monitoring	Mesh (Thread)	200-2,000	$20	2-4 years	Uptime 99.99%
Campus Network	Tree (Hybrid)	5,000-50,000	$50	N/A (powered)	Scalability
Agriculture	Star (LoRaWAN) + Mesh	50-500	$35	2-5 years	Coverage 10+ km
Healthcare	Tree (Wired + Wi-Fi)	500-5,000	$75	N/A (critical)	Latency <100ms
Retail Stores	Mesh (BLE)	50-200	$10	1-2 years	Cost <$15/device

773.8 Bandwidth vs Range Trade-offs

The Fundamental IoT Triangle - You Can Pick Two:

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graph TD
    subgraph "The IoT Topology Triangle"
        A[Long Range<br/>5-15 km LoRaWAN]
        B[High Bandwidth<br/>50+ Mbps Wi-Fi]
        C[Low Power<br/>2-10 years battery<br/>Zigbee/Thread]

        A -.->|Can't have<br/>both| B
        B -.->|Can't have<br/>both| C
        C -.->|Can't have<br/>both| A
    end

    A --> Example1[LoRaWAN Star:<br/>Long Range + Low Power<br/>BUT: 50 kbps only]
    B --> Example2[Wi-Fi Star:<br/>High Bandwidth + Short Range<br/>BUT: Power hungry]
    C --> Example3[Zigbee Mesh:<br/>Low Power + Medium Range<br/>BUT: 250 kbps limit]

    style A fill:#E67E22,stroke:#2C3E50,color:#fff
    style B fill:#16A085,stroke:#2C3E50,color:#fff
    style C fill:#2C3E50,stroke:#16A085,color:#fff
    style Example1 fill:#7F8C8D,stroke:#2C3E50,color:#fff
    style Example2 fill:#7F8C8D,stroke:#2C3E50,color:#fff
    style Example3 fill:#7F8C8D,stroke:#2C3E50,color:#fff

Key takeaway: No topology is perfect. Choose based on your top 2 priorities, accept the limitation on the 3rd.

773.8.1 Technology Range-Power Comparison

This chart shows how common IoT technologies compare across range and power consumption - two critical factors in topology selection:

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quadrantChart
    title Range vs Power Consumption
    x-axis Short Range --> Long Range
    y-axis Low Power --> High Power
    quadrant-1 High Power, Long Range
    quadrant-2 High Power, Short Range
    quadrant-3 Low Power, Short Range
    quadrant-4 Low Power, Long Range

    Wi-Fi: [0.25, 0.85]
    Bluetooth LE: [0.15, 0.20]
    Zigbee Mesh: [0.35, 0.25]
    Thread Mesh: [0.35, 0.30]
    LoRaWAN: [0.90, 0.15]
    Cellular NB-IoT: [0.85, 0.55]
    LTE-M: [0.80, 0.65]
    Wi-Fi HaLow: [0.55, 0.45]

Reading the Chart: - Bottom-Right (Quadrant 4) is ideal for battery-powered wide-area: LoRaWAN dominates - Bottom-Left (Quadrant 3) is best for short-range battery sensors: BLE, Zigbee, Thread - Top-Left (Quadrant 2) trades power for bandwidth: Wi-Fi for video/high-throughput - Top-Right (Quadrant 1) cellular options for wide-area with higher power budget

Topology Implications: - Star topologies work best with technologies at the extremes (Wi-Fi center, LoRaWAN edge) - Mesh topologies extend the effective range of Quadrant 3 technologies (Zigbee, Thread) - Hybrid topologies combine technologies from different quadrants

773.9 Understanding Checks: Real IoT Scenarios

Understanding Check: Smart Factory Deployment

Scenario: You’re deploying 500 sensors across a 200,000 sq ft manufacturing facility. The factory has: - Heavy machinery causing RF interference - Metal walls and equipment blocking signals - Critical safety requirements (99.9% uptime needed) - 24/7 operations with $50,000/hour downtime cost

Think about: Why would you choose mesh topology over star for this deployment?

Key Insights:

Self-Healing = Uptime: Mesh networks automatically route around failed nodes
- Star topology: Hub failure = 100% network down = $50,000/hour loss
- Mesh topology: Can survive 30-40% node failures with no downtime
- Real number: Zigbee mesh maintains connectivity even if 100 out of 500 sensors fail
RF Penetration: Metal obstacles block Wi-Fi signals
- Star (Wi-Fi): Direct line-of-sight to hub required, dead zones behind metal equipment
- Mesh (Zigbee/Thread): Messages hop around obstacles through neighboring sensors
- Real number: Mesh reduces “dead zones” by 90% compared to star in industrial environments
Scalability: 500 sensors overwhelm single hub
- Star (Wi-Fi): Single access point supports ~50 devices before congestion
- Mesh (Zigbee): Each router node extends capacity, supports 65,000 nodes/network
- Real number: Mesh handles 10x more devices per area than star

Decision Rule:

Use MESH when:
- Reliability > 99% required
- Physical obstacles (metal, concrete)
- Large number of devices (>50)
- Long-term deployment (installation cost amortized)

Use STAR when:
- Simple setup is priority
- Open space with good line-of-sight
- Small number of devices (<20)
- Temporary deployment

Understanding Check: Smart City Streetlights

Scenario: A city wants to network 10,000 streetlights across 50 square miles for: - Remote on/off control - Energy monitoring - Maintenance alerts (bulb failures) - Budget: $2M for networking equipment

Think about: Why would LoRaWAN star topology beat Wi-Fi mesh for this application?

Key Insights:

Range vs Density Trade-off:
- Wi-Fi mesh: 100m range → Need 10,000 devices as routers → Expensive
- LoRaWAN star: 5-15 km range → Need only 10-20 gateways → Cost-effective
- Real numbers: LoRaWAN gateway covers 500 streetlights, Wi-Fi AP covers 5-10
Bandwidth Requirements:
- Streetlight data: ~100 bytes/minute (on/off status, power consumption)
- LoRaWAN: 50 kbps sufficient for this low-bandwidth application
- Wi-Fi: Overkill - paying for 100+ Mbps you don’t need
- Real number: LoRaWAN costs $50/device, Wi-Fi mesh costs $200/device
Power Consumption:
- Streetlights already have power, but reducing consumption saves money
- LoRaWAN: 10-50 mA average → $2/year electricity per device
- Wi-Fi: 500-1000 mA average → $20/year electricity per device
- Real numbers: LoRaWAN saves $180,000/year on electricity for 10,000 devices

Decision Rule:

Use STAR (LoRaWAN) when:
- Low data rate (<50 kbps)
- Wide area coverage (>1 km²)
- Battery-powered devices
- Cost-sensitive deployment

Use MESH (Wi-Fi/Zigbee) when:
- High data rate (video, audio)
- Indoor/dense deployment
- Reliability > range
- Power available

Understanding Check: Hospital Patient Monitoring

Scenario: A 500-bed hospital needs real-time patient monitoring: - Heart rate, oxygen, temperature sensors - Patients move between rooms - Life-critical data (missed reading = patient death) - 1-second update rate required

Think about: Why would you use a hybrid topology (wired star backbone + wireless mesh access)?

Key Insights:

Latency Layering:
- Critical path: Sensor → Mesh (Wi-Fi) → Wired Star → Monitoring Station
- Wireless mesh: 10-50ms latency (acceptable for 1-second updates)
- Wired backbone: 1-5ms latency (ensures fast central processing)
- Real number: Hybrid achieves 50-100ms end-to-end vs 200ms pure wireless
Mobility Support:
- Patients move: Room 301 → Radiology → Room 405
- Mesh topology: Seamless handoff between access points
- Star topology alone: Would need AP in every room (expensive)
- Real number: Mesh reduces AP count by 60% while maintaining coverage
Reliability Tiers:
- Wired backbone: 99.999% uptime (fiber optic, redundant paths)
- Wireless mesh: 99.9% uptime (self-healing, but RF interference possible)
- Real number: Hybrid achieves 99.99% uptime (critical for life-safety)

Decision Rule:

Use HYBRID (Wired + Wireless) when:
- Critical reliability AND mobility
- High throughput AND wide coverage
- Tiered latency requirements
- Hospital, industrial, campus networks

Architecture Pattern:
Level 1: Wired fiber backbone (reliability)
Level 2: Wired switch per floor/building (aggregation)
Level 3: Wireless mesh access layer (mobility + coverage)

773.10 Topology Selection Checklist

Before choosing a topology, ask:

Question	Guides You Toward
How many devices?	<20: Star, 20-100: Mesh, >100: Hierarchical
Battery or powered?	Battery: Mesh (low power), Powered: Star (simplicity)
How critical is uptime?	Mission-critical: Mesh/Dual-ring, Normal: Star
Indoor or outdoor?	Outdoor/large area: Mesh (range), Indoor/small: Star
Do I have skilled staff?	No: Star (simple), Yes: Mesh acceptable
What’s my budget?	Low: Star, Moderate: Partial mesh, High: Full mesh
Bandwidth needs?	High (video): Wi-Fi star, Low (sensors): Zigbee mesh

Golden Rule: Choose the SIMPLEST topology that meets your requirements—complexity is the enemy of reliability!

773.11 Summary

Performance comparison shows tradeoffs between range, latency, bandwidth, and cost
Star topology is best for small deployments, high bandwidth, or simple setup needs
Mesh topology excels for reliability-critical, obstacle-heavy, or large-scale deployments
Tree topology suits enterprise-scale, multi-building deployments with professional IT support
Hybrid approaches combine wired backbone reliability with wireless mesh flexibility
The IoT triangle (range, bandwidth, power) means you can only optimize for two of three

773.12 What’s Next

Continue to Topology Failure Scenarios and Pitfalls to understand how different topologies behave when things go wrong, common deployment mistakes, and strategies for building resilient networks.