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Link Layer State Machine for BLE Devices using Stateflow

This example shows how to model link layer state machine to establish a connection between Bluetooth® low energy (BLE) devices using the Communications Toolbox™ Library for the Bluetooth Protocol and Stateflow®. The working mechanism of the link layer is described in terms of a state machine consisting of these five states: Standby, Advertising, Scanning, Initiating, and Connection. Based on these states, the Bluetooth devices can either be Advertisers, Scanners, Initiators, Master or Slave. This example presents a model to demonstrate the connection establishment process between a Master and a Slave through transitions between different states of the link layer state machine. The simulation results display the time taken to establish a connection between a Master and a Slave. Moreover, this example also provides plots showing the variation in connection establishment time as a function of configuration parameters such as advertising interval and interference.

BLE Stack

The Bluetooth core specification [ 1 ] includes a low energy (LE) version for low-rate wireless personal area networks, referred as BLE or Bluetooth Smart. The BLE stack consists of: Generic Attribute Profile (GATT), Attribute Protocol (ATT), Security Manager Protocol (SMP), Logical Link Control and Adaptation Protocol (L2CAP), Link Layer (LL) and Physical layer (PHY). BLE was added to the Bluetooth Special Interest Group (SIG) [ 1 ] standard for low energy devices generating small amount of data such as notification alerts used in applications like home automation, health-care, fitness, and Internet of Things (IoT).

BLE operates in the 2.4 GHz ISM band at 2400 - 2483.5 MHz. It uses 40 RF channels and each channel is 2 MHz wide.

BLE classifies these 40 RF channels into three advertising channels (channel indices: 37, 38, 39) and 37 data channels (channel indices: 0 - 36). Advertising channels are mainly used for creating connection between the BLE devices by transmitting advertising packets, scan request/response packets and connection indication packets. Data channels are mainly used after connection establishment for exchanging data packets.

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BLE Link Layer State Machine Model

The model in this example demonstrates the connection establishment process between two BLE devices, where one device acts as a Master and the other acts as a Slave. The Master device starts scanning for the advertising packets on the advertising channels. The Slave device starts sending advertising packets on advertising channels. After selecting an advertiser, the Master initiates a connection with it using the 'Connection indication' packet. The connection is established when the Slave receives this packet. Thereafter, the Master and Slave devices can exchange the data packets.

Link Layer States

The link layer state machine is implemented using Stateflow® as shown below.

The link layer maintains a state machine consisting of these five states:

  1. Standby : This is the default state in the link layer. The device does not send or receive packets in this state. It may leave the Standby state to enter one of the Advertising, Scanning or Initiating states.

  2. Advertising : In this state, the device transmits advertising packets in periodic intervals called advertising events. These devices are called advertisers.

  3. Scanning : In this state, the device listens for advertisers on the advertising channels and transits to the Standby state for selecting an advertiser to initiate a connection.

  4. Initiating : In this state, the device listens for a specific advertiser and initiates a connection with it. The device will move to Connection state after receiving the advertising packet from the selected advertiser. These devices are called initiators.

  5. Connection : In this state, the device transmits data packets in periodic intervals called connection events.

After the connection is established, the initiator becomes the Master (Central) device and the advertiser becomes the Slave (Peripheral) device.

Link Layer Events

An advertising device transmits advertising packets on the three advertising channels in a cyclic manner (starting from channel index 37). Each cycle of advertising is called an advertising event. The same procedure is used by the scanning or the initiating device that listens on the three advertising channels in a cyclic manner called scan event.

A connected device changes to a new data channel for every connection event. A connection event is a time frame for exchanging a sequence of data packets between two connected devices, which repeats at regular intervals. All the packets within a connection event are transmitted on the same data channel. A new connection event uses a new data channel. Two alternate channel selection algorithms are specified by the Bluetooth core specification (refer Section 4.5.8, Part-B, Vol-6 of [ 1 ]). These algorithms are used to select the data channel for each connection event.

This figure illustrates the connection establishment process between two BLE devices using link layer events:

Model Configuration

The link layer state machine model demonstrated in this example use these two library blocks: Device and Wireless Medium.

You can update the device configuration through the mask parameters of the Device block in the model. Based on the selected GAP role (Central/ Peripheral), the mask will be updated with relevant properties.

You can also configure the interference statistically for each advertising channel (37, 38 and 39) in the Wireless Medium block.

Simulation Results

The results obtained in this simulation are:

  • Connection establishment time:

This model outputs the time taken, in milliseconds, for establishing the connection. This time value is saved to a base workspace variable timeTakenToEstablishConnection.

  • Channel hopping sequence:

The sequence of channel hops used by the Master and Slave devices is separately shown in a scope diagram.

  • Packet exchange between devices

The packets exchanged between the Master and the Slave are captured in a PCAP format file with the name BLEPacketExchange.pcap. This PCAP file can be opened with any third-party packet analyzing software.

  • Diagnostic log for device operations

You can enable or disable the message log using the settings of the Device block.

Further Exploration

In the simulation, you can vary the intervals and the interference to observe the variation of the connection establishment time. The simulation results are shown below:

  • Simulation results without interference:

This plot shows how the connection establishment time varies with the advertising interval. Here, scan window is 10 milliseconds with a scan interval of 100 milliseconds i.e. 10% of the active period in scanning.

Run helperBLEPlotConnectionTimeWithoutInterference script (long run simulation) for generating the above results.

  • Simulation results with interference:

This plot shows how the average connection establishment time varies with different levels of interference. The plot also shows the variation for multiple values of advertising intervals. Here, scan window is 100 milliseconds with a scan interval of 100 milliseconds i.e. 100% of active period in scanning. These connection establishment time values are averaged over 100 simulation runs for different values of advertising intervals and interference levels.

Run helperBLEPlotConnectionTimeWithInterference script (long run simulation) for generating the above results.

Apart from advertising interval, the connection establishment time varies with respect to multiple parameters such as scan interval, scan window, scan duration, advertising duration and interference. You can further explore by varying any of these parameters. Both the above simulations are run with scan and advertising duration of 1 and 10 seconds respectively.

Conclusion

This example illustrates a link layer state machine model to establish a connection between two BLE devices namely: Master and Slave. This model gives detailed information about different link layer states and events. The state transition diagrams presented in this example clearly explain the process of connection establishment between the Master and the Slave. The derived simulation results display the time taken to establish a connection between the BLE devices. Furthermore, the derived plots also show that the connection establishment time varies with the configuration parameters such as advertising interval and interference.

Appendix

The example uses these features:

The example uses these helpers:

Selected Bibliography

  1. Bluetooth® Technology Website. "Bluetooth Technology Website | The Official Website of Bluetooth Technology." Accessed July 8, 2020. https://www.bluetooth.com/.

  2. "Development/LibpcapFileFormat - The Wireshark Wiki." Accessed July 8, 2020. https://wiki.wireshark.org/Development/LibpcapFileFormat.

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