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docs: Bluetooth: Add additional notes to BAP related services

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Some of the services have some unique points worth knowing
as both a contributor and an application developer.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>
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Emil Gydesen 2024-03-29 11:41:03 +01:00 committed by Anas Nashif
parent d2dfb8e8d7
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doc/connectivity/bluetooth/bluetooth-le-audio-arch.rst
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@ -264,12 +264,74 @@ defines multiple roles:
* Unicast Server
* Broadcast Source
* Broadcast Sink
* Scan Delegator (not yet implemented)
* Broadcast assistant (not yet implemented)
* Scan Delegator
* Broadcast Assistant
Each role can be enabled individually, and it is possible to support more than
one role.
Notes about the stream control services
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are 3 services primarily used by stream control using the Basic Audio Profile.
Audio Stream Control Service (ASCS)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ASCS is a service used exclusively for setting up unicast streams,
and is located on the BAP Unicast Server device.
The service exposes one or more endpoints that can either be a sink or source endpoint,
from the perspective of the Unicast Server.
That means a sink endpoint is always audio from the Unicast Client to the Unicast Server,
and a source endpoint is always from the Unicast Server to the Unicast Client.
Unlike most other GATT services,
ASCS require that each characteristic in the service has unique data per client.
This means that if a Unicast Server is connected to multiple Unicast Clients,
the Unicast Clients are not able to see or control the endpoints configured by the other clients.
For example if a person's smartphone is streaming audio to a headset,
then the same person will not be able to see or control that stream from their smartwatch.
Broadcast Audio Scan Service (BASS)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
BASS is a service that is exclusively used by the Scan Delegator and Broadcast Assistant.
The main purpose of the service is to offload scanning from low power peripherals to e.g. phones
and PCs.
Unlike ASCS where the data is required to be unique per client,
the data in BASS (called receive states) are (usually) shared among all connected clients.
That means it is possible for a person to tell their headphones to synchronize to a
Broadcast Source using their phone,
and then later tell their headphones to stop synchronizing using their smartwatch.
A Broadcast Assistant can be any device,
and may only support this one role without any audio capabilities.
This allows legacy devices that do not support periodic advertisements or isochronous channels to
still provide an interface and scan offloading for peripherals.
The Bluetooth SIG have provided a guide on how to develop such legacy Broadcast Assistants that can
be found at
https://www.bluetooth.com/bluetooth-resources/developing-auracast-receivers-with-an-assistant-application-for-legacy-smartphones/.
An important note about this guide is that many operating systems (especially on phones),
do not allow generic usage of the BASS UUID,
effectively making it impossible to implement your own Broadcast Assistant,
because you cannot access the BASS.
Published Audio Capabilities Service (PACS)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PACS is used to expose a device's audio capabilities in Published Audio Capabilities (PAC) records.
PACS is used by nearly all roles,
where the Unicast Client and Broadcast Assistant will act as PACS clients,
and Unicast Server and Broadcast Sink will act as PACS servers.
These records contain information about the codec, and which values are supported by each codec.
The values for the LC3 codec are defined by the Bluetooth Assigned numbers
(https://www.bluetooth.com/specifications/assigned-numbers/), and the values for other codecs such
as SBC are left undefined/implementation specific for BAP.
PACS also usually share the same data between each connected client,
but by using functions such as :c:func:`bt_pacs_conn_set_available_contexts_for_conn`,
it is possible to set specific values for specific clients.
The API reference for stream control can be found in
:ref:`Bluetooth Audio <bluetooth_audio>`.