906 Bluetooth Network Architecture

Piconets, Scatternets, and Power Classes

networking

wireless

bluetooth

topology

piconet

Author

IoT Textbook

Published

January 19, 2026

Keywords

bluetooth, piconet, scatternet, master-slave, power class, network topology

906.1 Learning Objectives

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

Describe the piconet topology and master-slave relationship
Explain the 7-device active slave limitation and its implications
Compare Bluetooth power classes and their range characteristics
Understand scatternet formation for larger networks
Design Bluetooth network architectures for IoT applications

906.2 Introduction

Bluetooth networks use a specific topology called a piconet - a small network with one master device and up to seven active slave devices. Understanding this architecture is essential for designing IoT systems that effectively use Bluetooth connectivity.

This chapter explores the fundamental network structures, power classes that determine range, and how scatternets can extend connectivity beyond the basic piconet limitations.

For Beginners: Master and Slave Devices

In Bluetooth, one device acts as the “boss” (master) and controls when other devices (slaves) can talk. It’s like a classroom where the teacher (master) calls on students (slaves) to speak.

Key Rules: - Only 1 master per network - Up to 7 active slaves can “speak” - Slaves can only talk when the master allows them - Slaves cannot talk directly to each other

906.3 Piconet Architecture

A piconet is the fundamental Bluetooth network topology:

906.3.1 Structure

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flowchart TD
    subgraph PICONET["Piconet (Max 7 Active Slaves)"]
        MASTER[Master<br/>Controls timing<br/>Assigns slots]
        S1[Slave 1<br/>AMA: 001]
        S2[Slave 2<br/>AMA: 010]
        S3[Slave 3<br/>AMA: 011]
        S4[Slave 4<br/>AMA: 100]
        S5[Slave 5<br/>AMA: 101]
        S6[Slave 6<br/>AMA: 110]
        S7[Slave 7<br/>AMA: 111]

        MASTER --- S1
        MASTER --- S2
        MASTER --- S3
        MASTER --- S4
        MASTER --- S5
        MASTER --- S6
        MASTER --- S7
    end

    subgraph PARKED["Parked Slaves (Up to 255)"]
        P1[Parked 1<br/>PMA: 00000001]
        P2[Parked 2<br/>PMA: 00000010]
        PN[... up to 255]
    end

    MASTER -.->|"Can unpark"| P1
    MASTER -.->|"Can unpark"| P2

    style MASTER fill:#E67E22,stroke:#2C3E50,stroke-width:3px,color:#fff
    style S1 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S2 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S3 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S4 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S5 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S6 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S7 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style P1 fill:#7F8C8D,stroke:#2C3E50,stroke-width:1px,color:#fff
    style P2 fill:#7F8C8D,stroke:#2C3E50,stroke-width:1px,color:#fff
    style PN fill:#7F8C8D,stroke:#2C3E50,stroke-width:1px,color:#fff

Figure 906.1: Piconet structure showing master device with 7 active slaves (3-bit AMA addressing) and parked slaves in standby.

906.3.2 Key Characteristics

Feature	Value	Description
Active Slaves	7 maximum	3-bit Active Member Address (AMA)
Parked Slaves	255 maximum	8-bit Parked Member Address (PMA)
Master	1 only	Controls timing and slot allocation
Addressing	3-bit AMA	001-111 for active slaves

906.3.3 Why Only 7 Active Slaves?

The limitation comes from the 3-bit Active Member Address (AMA) in the packet header:

3 bits = 2³ = 8 possible addresses (000-111)
Address 000 is reserved for broadcast
Addresses 001-111 available for 7 active slaves

7-Device Limitation Impact

This is a fundamental protocol constraint, not a hardware limitation. If your IoT application requires more than 7 simultaneously active devices:

Use BLE instead: Modern BLE supports 20+ concurrent connections
Deploy multiple piconets: Use scatternet topology
Use Bluetooth Mesh: Supports 32,000+ nodes
Time-division approach: Rotate which devices are active

906.4 Master-Slave Communication

In a piconet, all communication flows through the master:

906.4.1 Time Division Multiplexing (TDM)

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sequenceDiagram
    participant M as Master
    participant S1 as Slave 1
    participant S2 as Slave 2

    rect rgb(44, 62, 80)
    Note over M,S2: Slot 0 (Even) - Master TX
    M->>S1: Data to Slave 1
    end

    rect rgb(22, 160, 133)
    Note over M,S2: Slot 1 (Odd) - Slave TX
    S1->>M: Response from Slave 1
    end

    rect rgb(44, 62, 80)
    Note over M,S2: Slot 2 (Even) - Master TX
    M->>S2: Data to Slave 2
    end

    rect rgb(22, 160, 133)
    Note over M,S2: Slot 3 (Odd) - Slave TX
    S2->>M: Response from Slave 2
    end

    Note over M,S2: Each slot = 625 µs

Figure 906.2: Bluetooth TDM showing master transmitting in even slots, slaves responding in odd slots.

906.4.2 Communication Rules

Master controls channel access using TDM
Master transmits in even time slots (0, 2, 4, …)
Slaves respond in odd time slots (1, 3, 5, …)
Slaves can ONLY transmit after being addressed by master

Slaves Cannot Communicate Directly

In a piconet, Slave A cannot send data directly to Slave B. All communication must route through the master:

Correct: Slave A → Master → Slave B (two hops)

Impossible: Slave A → Slave B (direct)

For direct peer-to-peer communication, use BLE Mesh or establish separate point-to-point connections.

906.5 Power Classes and Range

Bluetooth devices are categorized into power classes that determine transmission range:

906.5.1 Power Class Comparison

Class	Max Power	Typical Range	Use Cases
Class 1	100 mW (20 dBm)	~100m	Industrial, warehouse scanners
Class 2	2.5 mW (4 dBm)	~10m	Smartphones, headphones, most IoT
Class 3	1 mW (0 dBm)	~1m	Ultra-low power wearables

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flowchart LR
    subgraph C1["Class 1 (100 mW)"]
        R1["~100m Range"]
        U1["Industrial<br/>Warehouse<br/>Long-range sensors"]
    end

    subgraph C2["Class 2 (2.5 mW)"]
        R2["~10m Range"]
        U2["Smartphones<br/>Headphones<br/>Most IoT devices"]
    end

    subgraph C3["Class 3 (1 mW)"]
        R3["~1m Range"]
        U3["Wearables<br/>NFC alternatives<br/>Ultra-low power"]
    end

    style C1 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style C2 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style C3 fill:#7F8C8D,stroke:#2C3E50,stroke-width:2px,color:#fff

Figure 906.3: Bluetooth power classes with typical ranges and use cases.

906.5.2 BLE 5.0 Extended Range

BLE 5.0 introduced Coded PHY for extended range:

PHY Mode	Data Rate	Range	Use Case
1M PHY	1 Mbps	~50m	Standard BLE
2M PHY	2 Mbps	~30m	High throughput
Coded (S=2)	500 kbps	~200m	Extended range
Coded (S=8)	125 kbps	~400m+	Long range IoT

906.6 Scatternet Topology

When applications require more than 7 devices or need inter-piconet communication, scatternets connect multiple piconets:

906.6.1 Scatternet Structure

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flowchart TD
    subgraph PN1["Piconet 1"]
        M1[Master 1]
        S1A[Slave A]
        S1B[Slave B]
        BRIDGE[Bridge Device<br/>Slave in PN1<br/>Master in PN2]

        M1 --- S1A
        M1 --- S1B
        M1 --- BRIDGE
    end

    subgraph PN2["Piconet 2"]
        S2A[Slave C]
        S2B[Slave D]
        S2C[Slave E]

        BRIDGE --- S2A
        BRIDGE --- S2B
        BRIDGE --- S2C
    end

    style M1 fill:#E67E22,stroke:#2C3E50,stroke-width:3px,color:#fff
    style BRIDGE fill:#2C3E50,stroke:#E67E22,stroke-width:3px,color:#fff
    style S1A fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S1B fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S2A fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S2B fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S2C fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff

Figure 906.4: Scatternet topology with bridge device connecting two piconets, enabling communication between networks.

906.6.2 Bridge Device Roles

A bridge device can participate in multiple piconets by:

Slave-Slave: Slave in both piconets (time-division participation)
Master-Slave: Master in one piconet, slave in another
Time-sharing: Alternates between piconets

Scatternet vs Mesh

Feature	Scatternet	BLE Mesh
Topology	Multiple connected piconets	Managed flooding
Scale	~20-50 devices	32,000+ devices
Complexity	High (manual bridging)	Lower (automatic routing)
Use Case	Legacy systems	Modern building automation

For new IoT deployments requiring many devices, BLE Mesh is recommended over scatternet.

906.7 Operating Modes and Power States

Bluetooth devices can operate in different power states to balance responsiveness with power consumption:

906.7.1 Power States

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stateDiagram-v2
    [*] --> Active
    Active --> Sniff: Periodic data needed
    Active --> Hold: Scheduled bursts
    Active --> Park: Low power standby

    Sniff --> Active: Data to transmit
    Hold --> Active: Timer expires
    Park --> Active: Master unparks

    note right of Active: Full communication<br/>Highest power
    note right of Sniff: Listens periodically<br/>Medium power
    note right of Hold: Preset intervals<br/>Low power
    note right of Park: Maintains sync only<br/>Lowest power

Figure 906.5: Bluetooth power states transitioning between Active, Sniff, Hold, and Park modes.

State	Description	Power	Wake-up Time
Active	Full communication	Highest	Immediate
Sniff	Listens at intervals	Medium	Fast
Hold	Scheduled burst mode	Low	Predetermined
Park	Synchronized standby	Lowest	Slow (~2s)

906.7.2 BLE Connection States

BLE uses a simpler connection state model:

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stateDiagram-v2
    [*] --> Standby

    Standby --> Advertising: Start advertising
    Standby --> Scanning: Start scanning
    Standby --> Initiating: Connect request

    Advertising --> Connection: Connection received
    Advertising --> Standby: Stop advertising

    Scanning --> Initiating: Device found
    Scanning --> Standby: Stop scanning

    Initiating --> Connection: Connection established
    Initiating --> Standby: Timeout

    Connection --> Standby: Disconnect

    note right of Advertising: Peripheral role<br/>Broadcasting availability
    note right of Scanning: Central role<br/>Looking for devices
    note right of Connection: Data exchange<br/>GATT operations

Figure 906.6: BLE connection state machine showing transitions between Standby, Advertising, Scanning, Initiating, and Connection states.

906.8 Connection Parameters

BLE connection parameters significantly impact power consumption and latency:

906.8.1 Key Parameters

Parameter	Range	Description
Connection Interval	7.5ms - 4s	Time between connection events
Slave Latency	0 - 499	Number of events slave can skip
Supervision Timeout	100ms - 32s	Max time without communication

906.8.2 Parameter Impact

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quadrantChart
    title Connection Interval Tradeoffs
    x-axis Low Latency --> High Latency
    y-axis Low Power --> High Power
    quadrant-1 Gaming, Real-time
    quadrant-2 Not recommended
    quadrant-3 Ideal for sensors
    quadrant-4 Battery drain

    "7.5ms interval": [0.1, 0.9]
    "50ms interval": [0.3, 0.6]
    "200ms interval": [0.5, 0.4]
    "1000ms interval": [0.8, 0.2]

Figure 906.7: Connection interval tradeoffs between latency and power consumption.

Application	Recommended Interval	Slave Latency
Game controller	7.5-15ms	0
Fitness tracker	100-200ms	4-10
Environmental sensor	1000-4000ms	10-20
Beacon	Advertising only	N/A

906.9 Inline Knowledge Check

Question 1: What is the maximum number of active slaves per piconet?

7 active slaves per master in a piconet. Master uses 3-bit Active Member Address (AMA) for slaves: 3 bits = 2³ = 8 addresses (0-7), but 0 is reserved for broadcast. Additional slaves can be “parked” (inactive, low power state) with up to 255 parked slaves allowed.

Question 2: Can slaves communicate directly with each other in a piconet?

No, not directly. In a piconet, all communication must go through the master. Master controls channel access using Time Division Multiplexing (TDM), transmitting in even time slots while slaves respond in odd slots. A slave can ONLY transmit after being addressed by master.

Question 3: What is the typical range of BLE Class 2 devices?

~10 meters for Class 2 devices (2.5 mW transmit power). Class 1 (100 mW) reaches ~100m for industrial applications, Class 2 (2.5 mW) reaches ~10m for smartphones and most IoT devices, and Class 3 (1 mW) reaches ~1m for ultra-low power wearables. BLE 5.0+ with Coded PHY can extend range to 400m+.

906.10 Summary

This chapter covered Bluetooth network architecture:

Piconet Topology: One master controls up to 7 active slaves using TDM
AMA Addressing: 3-bit address limits active slaves; 8-bit PMA for parked slaves
Master-Slave Rules: All communication flows through master; slaves cannot communicate directly
Power Classes: Class 1 (~100m), Class 2 (~10m), Class 3 (~1m)
Scatternets: Bridge devices connect multiple piconets for larger networks
Power States: Active, Sniff, Hold, and Park modes for power management
Connection Parameters: Interval and latency tuning balances power vs responsiveness

906.11 What’s Next

Continue to BLE Protocol Stack and GATT to learn about the BLE protocol architecture, Generic Attribute Profile (GATT), and how services and characteristics enable data exchange between BLE devices.