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BabbleSim and Zephyr

In the Zephyr project we use the Babblesim simulator to test some of the Zephyr radio protocols, including the BLE stack, 802.15.4, and some of the networking stack.

BabbleSim is a physical layer simulator, which in combination with the Zephyr bsim boards can be used to simulate a network of BLE and 15.4 devices. When we build Zephyr targeting a bsim board we produce a Linux executable, which includes the application, Zephyr OS, and models of the HW.

When there is radio activity, this Linux executable will connect to the BabbleSim Phy simulation to simulate the radio channel.

In the BabbleSim documentation you can find more information on how to get and build the simulator. In the nrf52_bsim, nrf5340bsim, and nrf54l15bsim boards documentation you can find more information about how to build Zephyr targeting these particular boards, and a few examples.

Types of tests

Tests without radio activity: bsim tests with twister

The bsim boards can be used without radio activity, and in that case, it is not necessary to connect them to a physical layer simulation. Thanks to this, these target boards can be used just like native_sim with twister, to run all standard Zephyr twister tests, but with models of a real SOC HW, and their drivers.

Tests with radio activity

When there is radio activity, BabbleSim tests require at the very least a physical layer simulation running, and most, more than 1 simulated device. Due to this, these tests are not build and run with twister, but with a dedicated set of tests scripts.

These tests are kept in the tests/bsim/ folder. The and scripts contained in that folder are used by the CI system to build the needed images and execute these tests in batch.

See sections below for more information about how to build and run them, as well as the conventions they follow.

There are two main sets of tests:

  • Self checking embedded application/tests: In which some of the simulated devices applications are built with some checks which decide if the test is passing or failing. These embedded applications tests use the bs_tests system to report the pass or failure, and in many cases to build several tests into the same binary.

  • Test using the EDTT tool, in which a EDTT (python) test controls the embedded applications over an RPC mechanism, and decides if the test passes or not. Today these tests include a very significant subset of the BT qualification test suite.

More information about how different tests types relate to BabbleSim and the bsim boards can be found in the bsim boards tests section.

Test coverage and BabbleSim

As the nrf52_bsim and nrf5340bsim, and nrf54l15bsim boards are based on the POSIX architecture, you can easily collect test coverage information.

You can use the script tests/bsim/ to generate an html coverage report from tests.

Check the page on coverage generation for more info.

Building and running the tests

See the NRF52 simulated board (BabbleSim) page for setting up the simulator.

The scripts also expect a few environment variables to be set. For example, from Zephyr’s root folder, you can run:

# Build all the tests

# Run them (in parallel)
RESULTS_FILE=${ZEPHYR_BASE}/myresults.xml \
   SEARCH_PATH=${ZEPHYR_BASE}/tests/bsim \

Or to build and run only a specific subset, e.g. host advertising tests:

# Build the Bluetooth host advertising tests

# Run them (in parallel)
RESULTS_FILE=${ZEPHYR_BASE}/myresults.xml \
   SEARCH_PATH=${ZEPHYR_BASE}/tests/bsim/bluetooth/host/adv \

Check the help for more options and examples on how to use this script to run the tests in batch.

After building the tests’ required binaries you can run a test directly using its individual test script.

For example you can build the required binaries for the networking tests with

WORK_DIR=${ZEPHYR_BASE}/bsim_out ${ZEPHYR_BASE}/tests/bsim/net/

and then directly run one of the tests:



Test code

See the Bluetooth sample test for conventions that apply to test code.

Build scripts

The build scripts simply build all the required test and sample applications for the tests’ scripts placed in the subfolders below.

This build scripts use the common compile.source which provide a function (compile) which calls cmake and ninja with the provided application, configuration and overlay files.

To speed up compilation for users interested only in a subset of tests, several compile scripts exist in several subfolders, where the upper ones call into the lower ones.

Note that cmake and ninja are used directly instead of the west build wrapper as west is not required, and some Zephyr users do not use or have west, but still use the build and tests scripts.

Test scripts

Please follow the existing conventions and do not design one-off bespoke runners (e.g. a python script, or another shell abstraction).

The rationale is that it is easier and faster for the maintainers to perform tree-wide updates for build system or compatibility changes if the tests are run in the same manner, with the same variables, etc..

If you have a good idea for improving your test script, please make a PR changing all the test scripts in order to benefit everyone and conserve homogeneity. You can of course discuss it first in an RFC issue or on the babblesim discord channel.

Scripts starting with an underscore (_) are not automatically discovered and run. They can serve as either helper functions for the main script, or can be used for local development utilities, e.g. building and running tests locally, debugging, etc..

Here are the conventions:

  • Each test is defined by a shell script with the extension .sh, in a subfolder called test_scripts/.

  • It is recommended to run a single test per script file. It allows for better parallelization of the runs in CI.

  • Scripts expect that the binaries they require are already built. They should not compile binaries.

  • Scripts will spawn the processes for every simulated device and the physical layer simulation.

  • Scripts must return 0 to the invoking shell if the test passes, and not 0 if the test fails.

  • Each test must have a unique simulation id, to enable running different tests in parallel.

  • Neither the scripts nor the images should modify the workstation filesystem content beyond the ${BSIM_OUT_PATH}/results/<simulation_id>/ or /tmp/ folders. That is, they should not leave stray files behind.

  • Tests that require several consecutive simulations (e.g, if simulating a device pairing, powering off, and powering up after as a new simulation) should use separate simulation ids for each simulation segment, ensuring that the radio activity of each segment can be inspected a posteriori.

  • Avoid overly long tests. If the test takes over 20 seconds of runtime, consider if it is possible to split it in several separate tests.

  • If the test takes over 5 seconds, set EXECUTE_TIMEOUT to a value that is at least 5 times bigger than the measured run-time.

  • Do not set EXECUTE_TIMEOUT to a value lower than the default.

  • Tests should not be overly verbose: less than a hundred lines are expected on the outputs. Do make use of LOG_DBG() extensively, but don’t enable the DBG log level by default.