983 Zigbee Network Topologies and Device Roles

Understanding star, tree, and mesh configurations with Coordinator, Router, and End Device roles

983.1 Learning Objectives

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

Distinguish between Zigbee star, tree, and mesh network topologies
Explain the roles and responsibilities of Coordinator, Router, and End Device
Determine which device role is appropriate for different hardware configurations
Design network topologies that balance coverage, reliability, and power efficiency
Calculate the number of routers needed for adequate mesh coverage

983.2 Introduction

Zigbee supports three network topologies: star, tree, and mesh. Each topology offers different tradeoffs between simplicity, range, and reliability. Understanding these topologies and the device roles within them is essential for designing effective Zigbee networks.

For Beginners: Network Topologies Explained

Imagine a company with offices and employees:

Star topology: Everyone reports directly to the CEO (simple but CEO is overloaded)
Tree topology: Employees report to managers, managers report to CEO (hierarchical)
Mesh topology: Everyone can talk to multiple colleagues (many paths, very resilient)

In Zigbee networks, devices similarly can connect in different patterns. The choice affects how messages travel and what happens when a device fails.

983.3 Network Topologies

983.3.1 Star Topology

In a star topology, all devices communicate directly with the central Coordinator:

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graph TB
    C[Coordinator<br/>0x0000]
    E1[End Device<br/>0x0001]
    E2[End Device<br/>0x0002]
    E3[End Device<br/>0x0003]
    E4[End Device<br/>0x0004]
    E5[End Device<br/>0x0005]

    C --- E1
    C --- E2
    C --- E3
    C --- E4
    C --- E5

    style C fill:#E67E22,stroke:#2C3E50,stroke-width:3px,color:#fff
    style E1 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style E2 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style E3 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style E4 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style E5 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff

Figure 983.1: Star topology with all devices connecting directly to the Coordinator

Characteristics:

Aspect	Star Topology
Hop count	Always 1 hop
Latency	Lowest (10-50ms)
Range	Limited to Coordinator range (10-30m)
Single point of failure	Coordinator
Complexity	Simplest
Best for	Small deployments, single room

Advantages: - Lowest latency (single hop) - Simplest to deploy and debug - Predictable message delivery times

Disadvantages: - Limited range (all devices must reach Coordinator) - Coordinator is single point of failure - No redundancy in communication paths

983.3.2 Tree Topology

Tree topology adds hierarchical structure with Routers between End Devices and the Coordinator:

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graph TB
    C[Coordinator<br/>0x0000]
    R1[Router<br/>0x0001]
    R2[Router<br/>0x0002]
    E1[End Device<br/>0x0011]
    E2[End Device<br/>0x0012]
    E3[End Device<br/>0x0021]
    E4[End Device<br/>0x0022]

    C --> R1
    C --> R2
    R1 --> E1
    R1 --> E2
    R2 --> E3
    R2 --> E4

    style C fill:#E67E22,stroke:#2C3E50,stroke-width:3px,color:#fff
    style R1 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style R2 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style E1 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style E2 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style E3 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style E4 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff

Figure 983.2: Tree topology with hierarchical routing through Routers

Characteristics:

Aspect	Tree Topology
Hop count	Variable (1-5 typical)
Latency	Medium (50-200ms)
Range	Extended through Routers
Single point of failure	Parent Router
Complexity	Medium
Best for	Buildings with clear floor/room structure

Advantages: - Extended range through multiple hops - Efficient addressing (hierarchical) - Clear parent-child relationships

Disadvantages: - Single path to Coordinator (no redundancy) - Router failure orphans all children - Limited self-healing capability

983.3.3 Mesh Topology

Mesh topology allows multiple routing paths between any two devices:

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graph TB
    C[Coordinator<br/>0x0000]
    R1[Router<br/>0x0001]
    R2[Router<br/>0x0002]
    R3[Router<br/>0x0003]
    E1[End Device<br/>0x0011]
    E2[End Device<br/>0x0021]

    C --- R1
    C --- R2
    R1 --- R2
    R1 --- R3
    R2 --- R3
    R1 --- E1
    R3 --- E2

    style C fill:#E67E22,stroke:#2C3E50,stroke-width:3px,color:#fff
    style R1 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style R2 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style R3 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style E1 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style E2 fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff

Figure 983.3: Mesh topology with multiple redundant routing paths

Characteristics:

Aspect	Mesh Topology
Hop count	Variable (1-10+)
Latency	Higher (100-500ms)
Range	Maximum (multi-hop)
Redundancy	Multiple paths
Complexity	Highest
Best for	Large deployments, reliability-critical

Advantages: - Self-healing: automatic rerouting when paths fail - Maximum range through multi-hop routing - High reliability through redundant paths - Load balancing across multiple routes

Disadvantages: - Higher latency due to multi-hop routing - More complex debugging - Increased network traffic for route maintenance

983.4 Device Roles

Zigbee defines three device roles, each with specific capabilities and power profiles:

983.4.1 Coordinator (ZC)

The Coordinator is the network controller and trust center:

Responsibilities: - Forms the network (selects PAN ID and channel) - Acts as Trust Center for security - Maintains device address assignments - Stores binding and group tables - Always mains-powered (cannot sleep)

Hardware Requirements: - Mains power (USB, wall adapter, PoE) - Sufficient memory for device tables - Typically a hub device (SmartThings, Hubitat, Zigbee2MQTT)

Example Coordinators:
- SmartThings Hub (1,000 devices)
- Hubitat Elevation (128 devices)
- Zigbee2MQTT with CC2652P (200 devices)
- ConBee II USB adapter (100 devices)

Key Point: There is exactly ONE Coordinator per Zigbee network. Losing the Coordinator means the network cannot accept new devices and may lose critical state.

983.4.2 Router (ZR)

Routers extend the mesh network by forwarding messages:

Responsibilities: - Relay messages between devices - Allow End Devices to join as children - Maintain neighbor and routing tables - Always powered (cannot sleep)

Hardware Requirements: - Mains power (plugged-in devices) - Examples: smart bulbs, smart plugs, powered sensors

Determining Router Capability:

Power Source	Device Type	Router Capable?
Wall outlet	Smart plug	Yes
Light socket	Smart bulb	Yes
24V wired	Thermostat	Yes
Battery	Door sensor	No
Battery	Motion sensor	No
Solar	Outdoor sensor	Usually No

983.4.3 End Device (ZED)

End Devices are leaf nodes that cannot route messages:

Responsibilities: - Send and receive messages through parent Router - Sleep most of the time to conserve power - Poll parent periodically for pending messages

Hardware Requirements: - Battery or mains power - Limited memory (no routing tables) - Examples: door sensors, motion sensors, temperature sensors

Power Management:

End Device Power States:
1. Deep Sleep: 1-5 µA (99% of time)
2. Wake: Check for work (10ms)
3. Poll: Ask parent for messages (15ms)
4. Transmit: Send sensor data (10ms)
5. Return to sleep

Result: 5-10 year battery life on CR2450 coin cell

983.4.4 Device Role Comparison

Feature	Coordinator	Router	End Device
Power	Always on	Always on	Sleeps
Routing	Yes	Yes	No
Children	Yes	Yes (20-50)	No
Network per	1 only	Many	Many
Memory	High	Medium	Low
Battery life	N/A (mains)	N/A (mains)	5-10 years

983.5 Designing Network Topology

983.5.1 Coverage Calculation

To determine router placement for mesh coverage:

Indoor range (conservative): 10-15 meters
Outdoor range (open area): 30-100 meters

Coverage area per router (indoor): ~150 m² (10m radius)
Overlap requirement: 30% for redundancy

Example: 200 m² apartment
- Minimum routers: 200 / 150 = 2 routers
- With redundancy: 3-4 routers recommended

983.5.2 Router Density Guidelines

Deployment	Area per Router	Redundancy Factor
Apartment	150-200 m²	1.3x
House	200-300 m²	1.5x
Office	300-500 m²	2.0x
Warehouse	500-1000 m²	2.5x
Industrial	Custom survey	3.0x

983.5.3 Topology Selection Guide

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flowchart TD
    Q1{All devices within<br/>30m of Coordinator?}
    Q2{Reliability-critical<br/>application?}
    Q3{More than 20<br/>devices?}

    STAR[Use Star Topology]
    TREE[Use Tree Topology]
    MESH[Use Mesh Topology]

    Q1 -->|Yes| Q2
    Q1 -->|No| MESH
    Q2 -->|No| STAR
    Q2 -->|Yes| Q3
    Q3 -->|No| TREE
    Q3 -->|Yes| MESH

    style STAR fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style TREE fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style MESH fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff

Figure 983.4: Decision tree for selecting Zigbee network topology

983.6 Smart Home Example

A typical 2-story home deployment illustrates device role assignment:

Inventory:
- 1 SmartThings Hub (Coordinator)
- 10 Smart bulbs (Routers)
- 1 Smart thermostat (Router)
- 8 Door/window sensors (End Devices)
- 4 Motion sensors (End Devices)
- 2 Smart plugs (Routers)

Device Role Summary:
- Coordinator: 1 (hub)
- Routers: 13 (bulbs + thermostat + plugs)
- End Devices: 12 (sensors)

Router:End Device ratio: 13:12 (excellent coverage)

Why this works well: - Smart bulbs installed throughout provide mesh backbone - Every room has at least one router - End devices (sensors) always within range of multiple routers - If one bulb is off at wall switch, alternate routes available

983.7 Common Mistakes

983.7.1 Mistake: All Battery Devices

Bad deployment:
- 1 Coordinator (hub in basement)
- 20 door sensors (all battery-powered end devices)
- 0 routers!

Result:
- Only sensors within 10-15m of hub work
- Far sensors drop offline constantly
- No mesh - just a very limited star

Fix: Add mains-powered devices (smart plugs, bulbs) as routers between the Coordinator and distant sensors.

983.7.2 Mistake: Switching Off Router Bulbs

Problem:
- User turns off bedroom smart bulb at wall switch
- Bulb loses power
- Router goes offline
- Bedroom sensor loses parent
- Automations stop working

Fix: - Use smart switches instead of wall switches - Smart switch sends Zigbee “off” command - Bulb stays powered as router, just light is off

983.8 Summary

This chapter covered Zigbee network topologies and device roles:

Star Topology: Simple, lowest latency, limited range
Tree Topology: Extended range, hierarchical structure
Mesh Topology: Maximum reliability, self-healing, highest complexity
Coordinator: One per network, forms PAN, trust center
Router: Extends mesh, always powered, forwards messages
End Device: Leaf node, battery-powered, sleeps to conserve power

Key design principles: - Match device role to power source (battery = End Device, mains = Router) - Maintain adequate router density for coverage - Plan for redundancy in reliability-critical applications - Consider mesh topology for deployments over 20 devices

983.9 What’s Next

In the next chapter, Zigbee Routing and Self-Healing, we explore how AODV routing enables automatic path discovery and mesh self-healing when devices fail.

Related Chapters

Zigbee Protocol Stack - Layered architecture
Zigbee Routing and Self-Healing - AODV and mesh recovery
Zigbee Network Formation - Joining process