1019 Thread Review: Network Topology and Device Roles

1019.1 Learning Objectives

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

Understand Thread’s Hierarchical Architecture: Comprehend the relationship between Border Routers, Leaders, Routers, and End Devices
Assign Device Roles Correctly: Configure Router, REED, SED, MED, and FED roles based on power source and response requirements
Apply the Device Type Selection Process: Use decision trees to select optimal device types for specific use cases
Calculate Network Capacity: Plan device distributions within Thread’s 32-router and 250-device limits

For Beginners: Understanding Thread Device Roles

Thread networks organize devices into specific roles based on their power source and functionality. Think of it like a postal delivery system:

Border Router = Post office (connects the neighborhood to the outside world)
Leader = Postmaster (manages operations)
Routers = Mail carriers (always working, delivering messages)
End Devices = Mailboxes (receive messages but don’t deliver them)

This chapter explains how to choose the right role for each device in your network.

1019.2 Prerequisites

Required Reading:

Thread Overview - Core Thread concepts
Thread Operation - Network formation
802.15.4 Fundamentals - Physical layer

Technical Background:

Mesh networking concepts
IPv6 addressing basics
Power consumption considerations for IoT devices

Estimated Time: 25 minutes

1019.3 Thread Network Topology Overview

Thread’s hierarchical mesh architecture optimizes battery life and network reliability. The architecture balances three key requirements:

Reliability: Self-healing mesh with no single point of failure
Low Power: Battery-powered devices can sleep for extended periods
IP Connectivity: Native IPv6 addressing for internet integration

1019.3.1 Core Architecture Components

A Thread network consists of several device types working together:

Component	Function	Power Source	Typical Examples
Border Router	IPv6 gateway to internet	Mains (always-on)	Smart hubs, Wi-Fi routers
Leader	Network manager	Mains (always-on)	Any router (elected)
Router	Mesh backbone	Mains (always-on)	Light bulbs, switches, plugs
REED	Router-eligible device	Either	Smart plugs, sensors
SED	Sleepy End Device	Battery	Door sensors, temp sensors
MED	Minimal End Device	Battery	Leak detectors
FED	Full End Device	Mains	Smoke alarms

1019.3.2 Network Hierarchy

The Thread network hierarchy operates as follows:

Internet/Cloud Layer:

Cloud platforms (Matter Controller, mobile apps)
Connectivity via Wi-Fi or Ethernet

Thread Mesh (fd00::/64 Mesh-Local):

Border Router: Gateway providing Thread to IP translation
Leader: Network manager with partition ID assignment
Routers: Mesh backbone forwarding packets (20-40 mA)
End Devices: Children attached to parent routers

Network Limits:

Maximum 32 routers per network
Maximum 250 total devices per network

1019.4 Device Type Selection

Choosing the correct device type is critical for network performance and battery life. The selection depends primarily on power source and response time requirements.

1019.4.1 Decision Process

Use this decision process to select the appropriate Thread device type:

Step 1: Determine Power Source

Battery-powered: Consider SED, MED, or REED
Mains/USB-powered: Consider Router, REED, or FED

Step 2: Evaluate Response Requirements

Minutes acceptable: SED (best battery life)
Seconds needed: MED (balanced)
Instant required: FED or Router

Step 3: Consider Routing Capability

Should route traffic: Router or REED
End device only: SED, MED, or FED

%%{init: {'theme': 'base', 'themeVariables': {'primaryColor': '#2C3E50', 'primaryTextColor': '#fff', 'primaryBorderColor': '#16A085', 'lineColor': '#E67E22'}}}%%
flowchart TD
    START([Thread Device<br/>Type Selection]) --> Q1{Power<br/>source?}

    Q1 -->|Battery| Q2{Response<br/>latency OK?}
    Q1 -->|Mains/USB| Q3{Route<br/>traffic?}

    Q2 -->|Minutes OK| SED[/SED<br/>Sleepy End Device/]
    Q2 -->|Seconds needed| MED[/MED<br/>Minimal End Device/]

    Q3 -->|Yes routing| Q4{Leader<br/>capable?}
    Q3 -->|No end device| FED[/FED<br/>Full End Device/]

    Q4 -->|Yes| ROUTER[/Router<br/>or Leader/]
    Q4 -->|Standby only| REED[/REED<br/>Router Eligible/]

    SED --> S_EX["Door sensors<br/>10+ year battery<br/>60s poll interval"]
    MED --> M_EX["Leak detectors<br/>2-5 year battery<br/>5s poll interval"]
    FED --> F_EX["Smoke alarms<br/>Mains powered<br/>Always responsive"]
    REED --> R_EX["Smart plugs<br/>Becomes router if needed<br/>Network resilience"]
    ROUTER --> RO_EX["Light bulbs<br/>Route mesh traffic<br/>Up to 32 per network"]

    style START fill:#2C3E50,color:#fff
    style SED fill:#7F8C8D,color:#fff,stroke:#2C3E50,stroke-width:3px
    style MED fill:#7F8C8D,color:#fff,stroke:#2C3E50,stroke-width:2px
    style FED fill:#7F8C8D,color:#fff,stroke:#2C3E50,stroke-width:2px
    style REED fill:#16A085,color:#fff,stroke:#2C3E50,stroke-width:2px
    style ROUTER fill:#16A085,color:#fff,stroke:#2C3E50,stroke-width:3px

Figure 1019.1: Thread device type selection decision tree showing how power source and response requirements determine the optimal device role.

1019.4.2 Detailed Device Type Characteristics

Sleepy End Device (SED):

Sleeps 99%+ of time to conserve power
Wakes periodically to poll parent for messages
Poll intervals: 60 seconds to 5 minutes typical
Battery life: 7-10+ years on coin cell
Best for: Door sensors, window sensors, temperature sensors
Trade-off: High latency for incoming messages

Minimal End Device (MED):

Sleeps but polls more frequently than SED
Poll intervals: 5-30 seconds typical
Battery life: 1-5 years depending on poll rate
Best for: Leak detectors, motion sensors needing faster response
Trade-off: Shorter battery life than SED

Full End Device (FED):

Always listening (radio always on)
Zero latency for incoming messages
Requires mains power (battery depletes in days/weeks)
Best for: Smoke alarms, security sensors
Trade-off: Cannot use battery power

Router-Eligible End Device (REED):

Starts as end device
Can promote to router if network needs more routing capacity
Useful for devices with larger batteries or occasional mains power
Best for: Smart plugs, devices that may be plugged in
Trade-off: Uncertain power consumption (depends on promotion)

Router:

Always on, always forwarding
Forms the mesh backbone
Requires mains power (20-40 mA continuous)
Best for: Light bulbs, switches, plugs, smart appliances
Trade-off: Highest power consumption

1019.5 End Device Configuration

End devices attach to routers as children and depend on their parent router for network connectivity.

1019.5.1 End Device Comparison

Device Type	Example	Poll Interval	Battery Life	Power Source
REED	Router-eligible	N/A (can promote)	Varies	Mains/Battery
SED	Door/Window Sensor	60s	10 years	Battery
MED	Leak Detector	5s	2 years	Battery
FED	Smoke Alarm	Always listening	N/A	Mains

1019.5.2 Poll Interval Selection

The poll interval dramatically affects battery life:

Poll Interval	Use Case	Approximate Battery Life (CR2032)
5 minutes	Temperature, humidity	15+ years
60 seconds	Door, window, motion	10-12 years
10 seconds	Leak detector	3-5 years
5 seconds	Fast-response sensor	1-2 years
Always on	Critical sensors	Days-weeks (use mains)

1019.5.3 Parent-Child Relationship

End devices maintain a parent-child relationship with routers:

Attachment: End device discovers and attaches to a parent router
Message Queuing: Parent queues messages for sleeping children
Polling: Child wakes, polls parent for queued messages
Reattachment: If parent fails, child finds new parent automatically

1019.6 Network Capacity Planning

Thread networks have specific limits that affect deployment planning.

1019.6.1 Hard Limits

32 routers maximum: Limited by 5-bit router ID space (2^5 = 32)
250 devices maximum: Protocol design limit per network
Multiple networks: Required for deployments exceeding 250 devices

1019.6.2 Router Placement Guidelines

Optimal router placement ensures good mesh coverage:

Deployment Size	Routers Needed	Coverage Notes
Small home (<1,000 sq ft)	8-12	1 router per 80-125 sq ft
Medium home (1,000-2,500 sq ft)	16-20	1 router per 50-150 sq ft
Large home (2,500+ sq ft)	24-28	Consider multiple networks

1019.6.3 Device Distribution Example

For a 150-device deployment:

Role	Count	Percentage	Function
Border Router	1	0.7%	Internet gateway
Leader	1 (from routers)	N/A	Network management
Routers	20	13%	Mesh backbone
REEDs	10	7%	Backup routing
SEDs	100	67%	Battery sensors
MEDs	15	10%	Fast-response sensors
FEDs	4	2.6%	Critical sensors

1019.6.4 Capacity Planning Formula

Use this formula for initial router estimation:

Recommended Routers = max(16, ceil(Total_Devices / 8))

For example:

50 devices: max(16, ceil(50/8)) = max(16, 7) = 16 routers
200 devices: max(16, ceil(200/8)) = max(16, 25) = 25 routers

1019.7 Knowledge Check

Question: Device Roles

In a Thread network with 200 devices, what is the maximum number that can be routers (always-on devices that forward packets)?

Options:

1. 16
1. 32
1. 64
1. 200 (all devices can be routers)

Answer

Correct: B) 32

Thread Network Limits:

Maximum devices: 250 per network
Maximum routers: 32 per network (fixed limit)
Minimum routers: 16 (recommended for good mesh coverage)

Why 32 Routers Maximum?

Router ID Space: Thread uses 5-bit router ID (part of RLOC addressing). 5 bits = 2^5 = 32 possible values (IDs 0-31).
Mesh Scalability: Each router must maintain routing table. 32 routers = manageable routing complexity.
Design Philosophy: 16-32 always-on routers form stable backbone; 218-234 sleepy end devices attach to routers.

Automatic Router Management: If you try to add 33rd router, it stays as REED (Router-Eligible End Device). If a router fails, REED automatically promotes to router.

Question: Thread Hardware Compatibility

Thread supports “software upgrade” to existing IEEE 802.15.4 hardware (Zigbee chips). What makes this possible, and what are the limitations?

Options:

1. Same PHY/MAC layer (802.15.4); requires firmware update for IPv6 stack and Thread protocols; can’t run Zigbee and Thread simultaneously
1. Thread uses software-defined radio to emulate 802.15.4; any 2.4 GHz radio can run Thread via firmware
1. Thread and Zigbee are interoperable protocols; devices can communicate across both networks after upgrade
1. Only Thread-certified hardware with dual-mode radios can switch; standard 802.15.4 chips cannot be upgraded

Answer

Correct: A) Same PHY/MAC layer (802.15.4); requires firmware update for IPv6 stack and Thread protocols; can’t run Zigbee and Thread simultaneously

Thread and Zigbee share the same physical foundation:

What’s shared (no hardware change needed):

IEEE 802.15.4 PHY: 2.4 GHz radio, 250 kbps, DSSS modulation
IEEE 802.15.4 MAC: CSMA/CA, frame format, addressing

What’s different (firmware change required):

Network Layer: Thread uses IPv6/6LoWPAN; Zigbee uses proprietary network layer
Routing: Thread uses RPL; Zigbee uses AODV/tree routing
Application Layer: Thread uses Matter/CoAP; Zigbee uses ZCL (Zigbee Cluster Library)
Security: Thread uses DTLS + MAC; Zigbee uses different key management

Upgrade process:

Flash new firmware with Thread stack (replacing Zigbee stack)
Commission device to Thread network
Device now operates as Thread, not Zigbee

Limitations:

Not simultaneous: Device runs Thread OR Zigbee, not both (single radio, conflicting network stacks)
No interoperability: Thread devices can’t communicate with Zigbee devices (different protocols above MAC)
Memory requirements: Thread needs ~100-200 KB flash, ~20-50 KB RAM for IPv6 stack - some minimal Zigbee chips lack sufficient memory

Real-world: Many vendors offer dual-firmware chips (selectable at manufacturing or via OTA).

1019.8 Key Concepts

Thread Hierarchy: Border Router > Leader > Routers > End Devices (REED/SED/MED/FED)
32-Router Limit: Enforced by 5-bit router ID space in RLOC addressing
250-Device Limit: Maximum devices per Thread network partition
Device Role Selection: Based on power source, response requirements, and routing needs
Poll Interval Trade-off: Longer intervals = better battery life but higher latency
REED Promotion: Router-eligible devices automatically promote when network needs more routers
Mesh Backbone: Mains-powered routers form always-on network infrastructure

1019.9 Summary

This chapter covered Thread network topology and device roles:

Key Takeaways

Network Architecture:

Thread uses hierarchical mesh with Border Router, Leader, Routers, and End Devices
Border Router provides IPv6 gateway to internet (Wi-Fi/Ethernet)
Leader manages network operations (elected from routers)
16-32 routers form mesh backbone (mains-powered)

Device Types:

Router: Always-on, forwards packets, mains-powered
REED: Can promote to router if needed, backup routing
FED: Always listening, instant response, mains-powered
MED: Polls frequently (5-30s), 1-5 year battery
SED: Polls infrequently (60s-5min), 7-10+ year battery

Capacity Planning:

Maximum 32 routers per network (5-bit router ID)
Maximum 250 devices per network
Recommended 1 router per 6-8 end devices
Multiple networks for deployments >250 devices

Device Selection:

Power source determines if device can be router
Response requirements determine SED vs MED vs FED
Poll interval directly affects battery life

1019.10 What’s Next

Continue to Thread Review: Protocol Stack and Comparison to understand how Thread’s protocol layers work and how Thread compares to Zigbee and integrates with Matter.