BLE Link Layer State Machine Workbench

Step through BLE Link Layer states, roles, scan windows, advertising timing, and broadcast synchronization.

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Interactive BLE Link Layer state machine animation showing Standby, Advertising, Scanning, Initiating, Connection, Synchronization, and Isochronous Broadcasting states with central and peripheral role transitions.
Animation BLE Link Layer State Machine Independent Lab

BLE Link Layer State Machine Workbench

Follow the controller-level states that make BLE feel low power: advertise briefly, scan only during windows, initiate on a connectable packet, then use scheduled connection events. The workbench also shows the later Synchronization and Isochronous Broadcasting states used by periodic advertising and broadcast audio.

Active StateAdvertising
Role OutcomePeripheral candidate
Radio PatternShort TX bursts
Timing RiskDiscovery balanced
1

Separate Device Role From State

Advertising does not mean peripheral yet. The role is assigned when the Link Layer enters Connection.

2

Watch The Radio Duty

Advertising interval and scan window decide how often the radio wakes before any GATT traffic exists.

3

Follow The Connect PDU

A central path moves Scanning -> Initiating -> Connection after a valid connectable advertisement.

4

Include Modern Broadcast Paths

Periodic advertising and broadcast isochronous streams add Synchronization and Isochronous Broadcasting states.

Interactive Animation

State path and packet movement

Step 2 of 5
Controller StateAdvertising
Packet Or WindowADV_IND
What ChangesDiscoverable sensor
BLE Link Layer state machine Seven BLE Link Layer state nodes with a moving packet token and central and peripheral devices. Central Side Phone or gateway listening for adverts Peripheral Side Sensor or beacon advertising events Standby no packets Advertising ADV events Scanning scan windows Initiating connect request Connection central/peripheral Synchronization periodic/BIG RX Iso Broadcasting BIG/BIS TX Primary advertising channels 37 38 39 Scan and connection timing ADV_IND
Diagnosis

What the state means now

ADV_IND
State Purpose Broadcast presence before a connection exists.

The controller sends advertising PDUs on primary advertising channels and may listen for allowed scan or connection responses.

Timing Model Advertising event every about 205 ms

The teaching estimate includes average random advertising delay.

Common Mistake Advertising is not the same as connected.

The role becomes Peripheral only after a connect request creates the connection.

Timing check

The selected scan window is compatible with the scan interval, and the supervision timeout leaves margin for skipped peripheral events.

Quick Reference

Segmented state lookup

ST

Standby

No Link Layer transmit or receive activity. It is the default state and the lowest radio duty point.

AD

Advertising

Sends advertising PDUs in events, usually across primary channels 37, 38, and 39.

SC

Scanning

Listens during scan windows. Passive scan only receives; active scan may send scan requests.

IN

Initiating

Looks for connectable advertisements and sends a connect indication or auxiliary connect request.

CO

Connection

Uses scheduled data-channel events. The initiator becomes Central; the advertiser becomes Peripheral.

SY

Synchronization

Receives a periodic advertising train or broadcast isochronous stream using synchronization information.

IB

Iso Broadcasting

Transmits a Broadcast Isochronous Group made of one or more Broadcast Isochronous Streams.

MI

Multiple Instances

One state machine has one active state, but controllers may support multiple state-machine instances.

Learning Support

Beginner Reading

  • Start with the peripheral scenario to see how a sensor becomes discoverable.
  • Use Step instead of Play when the PDU names are new.
  • Compare passive and active scanning before connecting anything.

Engineering Reading

  • Short advertising intervals and wide scan windows improve discovery but increase radio duty.
  • Connection interval, peripheral latency, and supervision timeout must be designed together.
  • Modern controllers can schedule multiple activities, but support is controller-specific.

Self Check

  • Which transition assigns the Central role?
  • Why can active scan add airtime before a connection exists?
  • When should a product use synchronization instead of a normal connection?
Technical Accuracy Notes

Spec Scope

This workbench teaches the BLE Link Layer state machine, not GATT, L2CAP, application pairing, or OS API states. Synchronization and Isochronous Broadcasting are shown as states entered from Standby, even when scanning or advertising helps discover and announce the broadcast schedule.

Timing Simplification

Discovery time is a teaching estimate based on advertising interval and scan duty. Real discovery also depends on channel order, filters, collisions, PHY support, and scheduling conflicts.

Power Simplification

The page avoids absolute milliwatt claims because current depends on silicon, TX power, PHY, voltage, sleep mode, and firmware scheduling.