Bluetooth: Connection Subrating

The Bluetooth® Connection Subrating sample showcases the effect of the LE Connection Subrating feature on the duty cycle of a connection.

Requirements

The sample supports the following development kits:

Hardware platforms

PCA

Board name

Board target

nRF54L15 PDK

PCA10156

nrf54l15pdk

nrf54l15pdk/nrf54l15/cpuapp

nRF54L15 DK

PCA10156

nrf54l15dk

nrf54l15dk/nrf54l15/cpuapp

nRF54H20 DK

PCA10175

nrf54h20dk

nrf54h20dk/nrf54h20/cpuapp

nRF5340 DK

PCA10095

nrf5340dk

nrf5340dk/nrf5340/cpuapp/ns nrf5340dk/nrf5340/cpuapp

nRF52 DK

PCA10040

nrf52dk

nrf52dk/nrf52832

nRF52840 DK

PCA10056

nrf52840dk

nrf52840dk/nrf52840

You can use any combination of the development kits mentioned above in your testing setup.

Additionally, the sample requires a connection to a computer with a serial terminal for each of the development kits.

Overview

This sample transmits data in bursts between two development kits to measure the transmission latency using different subrating parameters.

Subrating provides a mechanism for indicating that only a specific subset of connection events will be actively used by the connected devices, with the radio not being used in other connection events. Connection events are actively used when the peripheral listens for the event or sends data in the event. A subrated connection can therefore have a short ACL connection interval but still exhibit a low duty cycle so that power is conserved.

When a higher bandwidth is needed, the subrate request procedure allows for connection parameter updates to be made with minimal delay.

To measure the transmission latency from application layers, a GATT Latency service BT_UUID_LATENCY is included to compute the time spent. When the sender writes its timestamp to the BT_UUID_LATENC_CHAR characteristic of the receiver, the Latency service of the receiver will automatically reply back. Whenever the sender receives a response, it will use its current time and the corresponding timestamp written before to estimate the round-trip time (RTT) of a writing characteristic procedure (see Bluetooth Core Specification: Vol 3, Part F, 3.4.5 Writing attributes).

GATT Attributes

Type

UUID

Property

Primary service

BT_UUID_LATENCY

Read only

Characteristic

BT_UUID_LATENCY_CHAR

BT_GATT_CHRC_WRITE, BT_GATT_PERM_WRITE

By default, the following connection parameters are used:

Default parameter values

Parameter

Value

Connection interval

8 units (10 ms)

Subrate factor

1 or 10

Continuation number

0 or 1

Building and running

This sample can be found under samples/bluetooth/subrating in the nRF Connect SDK folder structure.

To build the sample, follow the instructions in Building an application for your preferred building environment. See also Programming an application for programming steps and Testing and optimization for general information about testing and debugging in the nRF Connect SDK.

Note

When building repository applications in the SDK repositories, building with sysbuild is enabled by default. If you work with out-of-tree freestanding applications, you need to manually pass the --sysbuild parameter to every build command or configure west to always use it.

Testing

After programming the sample to both development kits, test it by performing the following steps:

  1. Connect to both kits with a terminal emulator (for example, nRF Connect Serial Terminal). See Testing and optimization for the required settings and steps.

  2. Reset both kits.

  3. In one of the terminal emulators, type c to start the application on the connected board in the central role.

  4. In the other terminal emulator, type p to start the application in the peripheral role.

  5. Observe that the kits establish a connection.

  6. The sample will alternate between the following configurations:

    • No subrating

    • Subrating with a factor of 10, continuation number 0

    • Subrating with a factor of 10, continuation number 1

  7. Observe the difference in the duty cycle of the connection:

    • With no subrating, the link will always use a fast connection interval of 10ms.

    • With a subrate factor of 10 and no continuation events, the link will always use a slower effective connection interval of 100ms.

    • With a subrate factor of 10 and one continuation event, the link will use a fast connection interval of 10ms when data needs to be sent, and switch to a low effective connection interval of 100ms otherwise.

The sample demonstrates the ability for the peripheral to request changes to the subrating parameters. This behavior can be changed by setting #define SUBRATE_INITIATOR_ROLE to BT_CONN_ROLE_CENTRAL in the sample.

Sample output

The result should look similar to the following output:

  • For the central:

    *** Booting nRF Connect SDK v2.7.99-9539f0a1a59b ***
*** Using Zephyr OS v3.6.99-766b306bcbe8 ***
Starting Bluetooth Subrating example
I: SoftDevice Controller build revision:
I: 0f 3f c8 4a 7e 8d b6 7a |.?.J~..z
I: 64 7f 04 47 8b 3c 4b ae |d..G.<K.
I: 95 16 0d 96             |....
I: HW Platform: Nordic Semiconductor (0x0002)
I: HW Variant: nRF52x (0x0002)
I: Firmware: Standard Bluetooth controller (0x00) Version 15.51263 Build 3062726218
I: Identity: EA:9C:4D:3A:2F:A8 (random)
I: HCI: version 5.4 (0x0d) revision 0x1224, manufacturer 0x0059
I: LMP: version 5.4 (0x0d) subver 0x1224
Bluetooth initialized
Choose device role - type c (central) or p (peripheral):
Central. Starting scanning
Scanning successfully started
Filters matched. Address: FD:AA:B8:5A:36:E2 (random) connectable: 1
Connected as central
Service discovery completed
Subrate parameters changed: Subrate Factor: 10 Continuation Number: 0
Peripheral latency: 0 Supervision timeout: 0x01f4 (5000 ms)
Subrate parameters changed: Subrate Factor: 10 Continuation Number: 1
Peripheral latency: 0 Supervision timeout: 0x01f4 (5000 ms)
Subrate parameters changed: Subrate Factor: 1 Continuation Number: 0
Peripheral latency: 0 Supervision timeout: 0x01f4 (5000 ms)

  • For the peripheral:

    *** Booting nRF Connect SDK v2.7.99-9539f0a1a59b ***
*** Using Zephyr OS v3.6.99-766b306bcbe8 ***
Starting Bluetooth Subrating example
I: SoftDevice Controller build revision:
I: 0f 3f c8 4a 7e 8d b6 7a |.?.J~..z
I: 64 7f 04 47 8b 3c 4b ae |d..G.<K.
I: 95 16 0d 96             |....
I: HW Platform: Nordic Semiconductor (0x0002)
I: HW Variant: nRF52x (0x0002)
I: Firmware: Standard Bluetooth controller (0x00) Version 15.51263 Build 3062726218
I: Identity: FD:AA:B8:5A:36:E2 (random)
I: HCI: version 5.4 (0x0d) revision 0x1224, manufacturer 0x0059
I: LMP: version 5.4 (0x0d) subver 0x1224
Bluetooth initialized
Choose device role - type c (central) or p (peripheral):
Peripheral. Starting advertising
Advertising successfully started
Connected as peripheral
Service discovery completed
Subrate parameters changed: Subrate Factor: 1 Continuation Number: 0
Peripheral latency: 0 Supervision timeout: 0x01f4 (5000 ms)
Simulating burst of data.
Response round-trip time: 16174 us
Response round-trip time: 16845 us
Response round-trip time: 16845 us
Response round-trip time: 16845 us
Response round-trip time: 16845 us
Response round-trip time: 16876 us
Response round-trip time: 16845 us
Response round-trip time: 16845 us
Response round-trip time: 16876 us
Response round-trip time: 16845 us
Subrate parameters changed: Subrate Factor: 10 Continuation Number: 0
Peripheral latency: 0 Supervision timeout: 0x01f4 (5000 ms)
Simulating burst of data.
Response round-trip time: 186218 us
Response round-trip time: 196777 us
Response round-trip time: 196746 us
Response round-trip time: 196777 us
Response round-trip time: 196746 us
Response round-trip time: 196746 us
Response round-trip time: 196777 us
Response round-trip time: 196746 us
Response round-trip time: 196777 us
Response round-trip time: 196746 us
Subrate parameters changed: Subrate Factor: 10 Continuation Number: 1
Peripheral latency: 0 Supervision timeout: 0x01f4 (5000 ms)
Simulating burst of data.
Response round-trip time: 96069 us
Response round-trip time: 16845 us
Response round-trip time: 16845 us
Response round-trip time: 16845 us
Response round-trip time: 16845 us
Response round-trip time: 16845 us
Response round-trip time: 16845 us
Response round-trip time: 16845 us
Response round-trip time: 16876 us
Response round-trip time: 16845 us

References

For more information about LE Connection Subrating, see the following chapters of the Bluetooth Core Specification:

  • Vol 6, Part B, 4.5.1 Connection events

  • Vol 6, Part B, 5.1.19 Connection Subrate Update procedure

  • Vol 6, Part B, 5.1.20 Connection Subrate Request procedure

Dependencies

This sample uses the following nRF Connect SDK libraries:

This sample uses the following sdk-nrfxlib libraries:

In addition, it uses the following Zephyr libraries:

  • include/console.h

  • Kernel Services:

    • include/kernel.h

  • include/sys/printk.h

  • include/zephyr/types.h

  • API:

    • include/bluetooth/bluetooth.h

    • include/bluetooth/conn.h

    • include/bluetooth/gatt.h

    • include/bluetooth/hci.h

    • include/bluetooth/uuid.h

    • include/bluetooth/scan.h

    • include/bluetooth/gatt_dm.h