908 Bluetooth Piconet Architecture

908.1 Learning Objectives

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

Explain the piconet structure with central (master) and peripheral (slave) roles
Understand the 7-device active limit in Classic Bluetooth piconets
Design scatternets to extend network capacity
Identify workarounds for connection limits in IoT deployments
Choose appropriate architectures for different device counts

908.2 Prerequisites

Before diving into this chapter, you should be familiar with:

Bluetooth Overview: Basic understanding of Bluetooth technology
Bluetooth Topologies: Understanding of star, mesh, and broadcast patterns

908.3 Central/Peripheral Model

A Classic Bluetooth piconet uses a central/peripheral topology (legacy terminology: master/slave):

Piconet Structure:

Central (Master): Controls the piconet timing and schedules traffic
Peripheral (Slave): Devices connected to the central
Maximum: 1 central + up to 7 active peripherals

Rules:

Central coordinates all communication
Up to 7 active peripherals per central (Classic BR/EDR)
Communication: Central <-> Peripheral only within a piconet
Central schedules time slots (TDMA)
Direct peripheral-to-peripheral communication is NOT supported inside a piconet

Communication Constraint

Within a Classic Bluetooth piconet, peripherals communicate via the central (legacy: master); direct peripheral-to-peripheral communication is not supported inside that piconet. For peer communication, use a mesh protocol or gateway the traffic at a higher layer.

908.4 The 7-Device Active Limit

Classic Bluetooth Piconet Limit: 7 Active Devices

Classic Bluetooth (BR/EDR) piconets are limited to 7 active peripherals per central. This can be a bottleneck for designs that try to keep many devices connected at the same time.

Why this limit exists:

The 3-bit Active Member Address (AM_ADDR) field in the packet header provides only 7 usable addresses (1-7, with 0 reserved for broadcast).

Why this matters:

If you rely on Classic Bluetooth piconets, you can hit the 7 active device limit quickly
Even with BLE, concurrent connection count is implementation-dependent (some hubs/controllers support only a limited number of simultaneous connections)
Scaling often requires connection scheduling, additional gateways, or a different topology

Workarounds:

Connectionless broadcast: Use advertising for one-to-many updates (where appropriate)
Connection scheduling: Rotate connections or batch data transfers instead of keeping everything connected
Bluetooth Mesh: Managed flooding; address space supports up to ~32k unicast nodes (practical scale depends on airtime)
Choose another protocol: Zigbee/Thread/Wi-Fi when you need large, always-connected device populations

Example: In a patient room with many sensors and actuators, a single always-connected hub design can become a bottleneck. BLE can support more than 7 connections in many implementations, but you still need to plan for update intervals, airtime, and controller limits.

908.5 Scatternet Formation

Scatternets extend Bluetooth capacity by interconnecting multiple piconets through bridge devices.

Scatternet structure:

Piconet 1: Central 1 connected to peripherals A, B, C and also to device D.
Piconet 2: Central 2 connected to peripherals E and F; Central 2 uses the same radio as peripheral D in Piconet 1.
Device D / Central 2 therefore acts as a bridge between the two piconets, forming a scatternet concept.

Scatternet Characteristics:

Bridge devices participate in multiple piconets
Time-division multiplexing between piconets
Increased complexity and latency
Extended total network capacity

908.6 Time Division Multiplexing (TDMA)

Classic Bluetooth uses time-division multiplexing for channel access:

How TDMA works:

Time is divided into 625 microsecond slots
Central transmits in even slots, peripherals in odd slots
Frequency hopping: 1600 hops/second (79 channels)
Central polls each peripheral in sequence

Slot Allocation:

Slot Type	Duration	Direction
Single-slot	625 us	Central -> Peripheral or Peripheral -> Central
3-slot	1875 us	Extended for larger packets
5-slot	3125 us	Maximum for high throughput

908.7 Operational Modes

Bluetooth devices can operate in different power states:

Mode	State	Use Case	Power
Active	Fully awake	Continuous transfer	Highest
Sniff	Periodic wake	Power saving with scheduled activity	Medium
Hold	Temporary sleep	Defined pause period	Low
Park	Deep sleep	Maintain synchronization, minimal activity	Lowest

Mode Selection:

Active: For continuous data streaming (audio, file transfer)
Sniff: For periodic sensor data with predictable intervals
Hold: For temporary pauses (e.g., while user reads notification)
Park: For devices that need to stay discoverable but aren’t actively communicating

908.8 Case Study: Hospital Room Deployment

Scenario: Hospital room with 12 medical sensors requiring connection to a central nursing station:

Heart rate monitor
Blood pressure cuff
Pulse oximeter
Temperature sensor
SpO2 sensor
ECG monitor
IV pump monitor
Respiratory rate sensor
Glucose monitor
Weight scale
Bed position sensor
Call button

Problem Analysis:

Classic BR/EDR piconet: max 7 active peripherals
BLE: Implementation-dependent connection limit
Current sensors: 12 devices
Shortfall: 5+ devices cannot connect simultaneously

Solution Options:

908.8.1 Solution A: Multiple Gateways (Scatternet)

Deploy two Bluetooth hubs, each managing up to 7 devices:

Hub 1: Heart rate, BP, pulse ox, temp, SpO2, ECG, IV pump
Hub 2: Respiratory, glucose, weight, bed position, call button

Pros: All devices always connected, fast alerts Cons: Additional hardware cost (~$50-100), complexity

908.8.2 Solution B: Connection Rotation

Single hub rotates connections based on priority:

Priority Levels:
- High (always connected): Heart rate, ECG, SpO2
- Medium (poll every 30s): BP, temp, glucose
- Low (poll every 5 min): Weight, bed position

Rotation Schedule:
1. Maintain 3 high-priority connections
2. Cycle through 4 medium/low slots
3. Total effective connections: 7

Pros: No additional hardware, $0 cost Cons: Higher latency for low-priority devices (5-60 seconds)

908.8.3 Solution C: BLE Mesh

Deploy mesh-enabled sensors with mains-powered relay infrastructure:

Pros: Scalable to 32,000+ nodes, self-healing Cons: Higher complexity, requires mesh infrastructure ($200+)

Comparison:

Criteria	Scatternet	Rotation	BLE Mesh
Max Devices	14 (2x7)	Unlimited	32,000+
Alert Latency	<1 sec	5-60 sec	<5 sec
Power (sensor)	Medium	Low	Medium
Complexity	High	Low	Very High
Cost	+$50	$0	+$200

Recommended: Solution A (Scatternet) for critical medical monitoring where alert latency is paramount.

908.9 BLE Connection Scaling

While BLE doesn’t have the same 7-device hard limit as Classic Bluetooth, practical limits exist:

Implementation-Dependent Limits:

Mobile phones (iOS/Android): Typically 5-15 concurrent connections
BLE hubs/gateways: Often 20-50 connections
High-end industrial gateways: 100+ connections possible
Nordic nRF52840: Up to 20 connections (software-limited)
ESP32: Typically 3-9 connections (stack-dependent)

Factors Affecting Connection Capacity:

Memory: Each connection requires ~1-2KB RAM for context
Airtime: More connections = less bandwidth per connection
CPU: Connection event processing overhead
Power: More active connections = higher average current

Best Practices for Scaling:

Use advertising where possible: Beacons don’t consume connection slots
Implement connection pooling: Rotate connections to different devices
Optimize connection intervals: Longer intervals = more connections possible
Consider peripheral latency: Allow devices to skip connection events

908.10 Summary

This chapter covered Bluetooth piconet architecture:

Classic Bluetooth piconets support 1 central + 7 active peripherals maximum
The 3-bit AM_ADDR field creates the 7-device limit (addresses 1-7)
Scatternets can extend capacity by bridging multiple piconets
TDMA provides time-slotted channel access with 625us slots
Operational modes (Active, Sniff, Hold, Park) trade off power for responsiveness
BLE connection limits are implementation-dependent, not protocol-defined
Workarounds include multiple gateways, connection rotation, and mesh networking

908.11 What’s Next

The next chapter, Bluetooth Connection Establishment, explains the discovery and connection process including inquiry, paging, advertising, and how BLE’s interactive state machine tool works.