Low Energy Audio – Basic
One-Way Unicast Audio
Bluetooth® Informational Publication▪

Version: v1▪

Version Date: 2024-06-11▪

Prepared By: Generic Audio Working GroupIf you have any questions or comments on this informational publication or Bluetooth
LE Audio specifications, please submit a ticket: https://www.bluetooth.com/leaudiospec-feedback/

Abstract:
This document gives an introduction to Bluetooth LE Audio by explaining the basics of a use case wherestereo one-way audio is transmitted from one device to another.

Bluetooth SIG ProprietaryLow Energy Audio – Basic One-Way Unicast Audio / Informational Publication
Version History
Version NumberDate(yyyy-mm-dd)

Comments

v1

2024-06-11

Approved by the Bluetooth SIG Board of Directors.

Acknowledgments
NameCompany

Asbjørn SæbøNordic Semiconductor ASA

Georg Dickmann

Sonova AG

Nick Hunn

GN Hearing A/SAndrew Estrada

Sony Group Corporation

Oren HaggaiIntel Corporation

Chris White

Dolby Laboratories, Inc.

Bjarne Klemmensen

Demant A/S

Bluetooth SIG Proprietary

Page 2 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication
This document, regardless of its title or content, is not a Bluetooth Specification as defined in the Bluetooth Patent/Copyright
License Agreement (“PCLA”) and Bluetooth Trademark License Agreement. Use of this document by members of Bluetooth
SIG is governed by the membership and other related agreements between Bluetooth SIG Inc. (“Bluetooth SIG”) and its
members, including the PCLA and other agreements posted on Bluetooth SIG’s website located at www.bluetooth.com.
THIS DOCUMENT IS PROVIDED “AS IS” AND BLUETOOTH SIG, ITS MEMBERS, AND THEIR AFFILIATES MAKE NO
REPRESENTATIONS OR WARRANTIES AND DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING ANYWARRANTY OF MERCHANTABILITY, TITLE, NON-INFRINGEMENT, FITNESS FOR ANY PARTICULAR PURPOSE, THAT THE
CONTENT OF THIS DOCUMENT IS FREE OF ERRORS.
TO THE EXTENT NOT PROHIBITED BY LAW, BLUETOOTH SIG, ITS MEMBERS, AND THEIR AFFILIATES DISCLAIM ALL
LIABILITY ARISING OUT OF OR RELATING TO USE OF THIS DOCUMENT AND ANY INFORMATION CONTAINED IN THISDOCUMENT, INCLUDING LOST REVENUE, PROFITS, DATA OR PROGRAMS, OR BUSINESS INTERRUPTION, OR FOR
SPECIAL, INDIRECT, CONSEQUENTIAL, INCIDENTAL OR PUNITIVE DAMAGES, HOWEVER CAUSED AND REGARDLESS OF
THE THEORY OF LIABILITY, AND EVEN IF BLUETOOTH SIG, ITS MEMBERS, OR THEIR AFFILIATES HAVE BEEN ADVISEDOF THE POSSIBILITY OF SUCH DAMAGES.
This document is proprietary to Bluetooth SIG. This document may contain or cover subject matter that is intellectual
property of Bluetooth SIG and its members. The furnishing of this document does not grant any license to any intellectual
property of Bluetooth SIG or its members.
This document is subject to change without notice.
Copyright © 2024 by Bluetooth SIG, Inc. The Bluetooth word mark and logos are owned by Bluetooth SIG, Inc. Other third party brands and names are the property of their respective owners.

Bluetooth SIG Proprietary

Page 3 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

Contents
1

Introduction ..................................................................................................................................... 5
1.1
1.2

2

3

Scope ...................................................................................................................................... 5
MSC conventions..................................................................................................................... 6

One-way audio................................................................................................................................. 7
2.1
2.2
2.3
2.4
2.5
2.6

Initial conditions ....................................................................................................................... 8
Establishing an ACL connection ............................................................................................. 10
Codec and CIG configuration ................................................................................................. 12
QoS configuration .................................................................................................................. 14
Enabling ASEs and setting up CISes...................................................................................... 16
Starting streaming .................................................................................................................. 18

2.7
2.8

Ending streaming and cleaning up ......................................................................................... 20
Considerations on setup latency............................................................................................. 22

References..................................................................................................................................... 23

Bluetooth SIG Proprietary

Page 4 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

1 Introduction
This document is intended as a first introduction to unicast1 Low Energy (LE) Audio. It gives an overview
of a simple scenario where stereo one-way audio is transferred from one device to another.
A Message Sequence Chart (MSC), subdivided into multiple parts, is used to explain the steps for theBluetooth LE Audio use case of a user listening to music from one device (e.g., a PC) on another device(e.g., a headset) connected to the first one. While each of the LE Audio specifications addresses only
individual aspects of a given use case, this document shows how the concepts from multiple LE Audio
specifications are combined.
The content of this document is based upon the following specifications:•

Basic Audio Profile (BAP) [1]•

Published Audio Capabilities Service (PACS) [2]•

Audio Stream Control Service (ASCS) [3]•

Common Audio Profile (CAP) [4]These specifications are considered required reading for anyone implementing LE Audio. In addition,
Bluetooth SIG publishes other informational documents to help understand the specifications.
This is not a Bluetooth specification, therefore, the established Bluetooth SIG specification language
conventions for use of the words shall, shall not, must, should, should not, may, and can do not apply
to this document.

1.1

Scope

The purpose of this document is to introduce the basics of LE Audio by showing one possible
implementation using the features within Bluetooth LE Audio specifications. The MSC used in thisdocument shows many of the concepts of LE Audio specifications including the:•Use of Context Types [5]•Setup of data paths•

Configuration of Audio Stream Endpoints (ASEs)•

Use of BAP [1] Announcements

The solution shown is not the only possible solution; developers can use alternative approaches based onother considerations.

1«Unicast» means that the audio data are transferred over a connection between the devices, as
opposed to “broadcast” transfer, where the audio data are not transferred over a connection.
Bluetooth SIG Proprietary

Page 5 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

1.2

MSC conventions

The MSC in this document uses the conventions illustrated in Figure 1.1.

Figure 1.1: MSC conventions•

Descriptions that refer to the in-progress process are shown in yellow boxes that span across therelevant devices. The same type of box is also used for the initial conditions.

•

PACS [2] characteristics, with relevant values, are shown in rounded white boxes.

•

ASE characteristics, with the state and relevant other values, are shown in rounded grey boxes.

•

Advertisements are shown as dashed arrows starting from the transmitters. The direction of thearrows does not imply that the advertisement is received by any specific device.

•

BAP procedures (or other sets of steps that together form a larger logical operation or process) aresurrounded by large light-gray boxes. A tag in the upper-left corner of each box describes theprocedure.

•Some parts of the MSC, where the details are not important to the topic of this document, are shown
abstracted as white block arrows.

•

To refer to GATT characteristic X, <<X>> is used. For example, as in <<Sink PAC>>, which refers to
the PACS Sink PAC characteristic.

•

The steps of the MSC are simply numbered according to the order in which they occur in the MSC;
the numbering does not imply that this is the only possible ordering of the steps.
Bluetooth SIG Proprietary

Page 6 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

2 One-way audio
The MSC in Figure 2.1 through Figure 2.7 shows an overview of one-way unicast transmission of audio
from a computer to a headset. (Instead of a computer, the transmitting device could be, for example, atablet or a smartphone, and instead of a headset, the receiving device could be e.g., a loudspeaker. The
steps in the MSC would still be the same.)
The MSC is simplified to show only the fundamental operations and parameters required to set up theaudio transmission. Additional operations and parameters, such as using media control operations forcontrolling a media player and details on the use of Context Type values are out of scope for thisdocument.
The MSC begins with the situation where the two devices have previously paired and bonded but are notconnected. It is also possible to start from an already-connected state; in that case, Steps 7 through 22
shown in the MSC would not apply.
As shown in the MSC, the computer in this setup is a CAP Initiator. The computer is also:•

A BAP Unicast Client•

A Generic Attribute Profile (GATT) Client•

In the Generic Access Profile (GAP) Central role•

In the BAP Audio Source audio role

The MSC also shows that the headset in this setup is a CAP Acceptor. The headset is also:•

A BAP Unicast Server•

A GATT Server•

In the GAP Peripheral role•

In the BAP Audio Sink audio role

Bluetooth SIG Proprietary

Page 7 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

2.1

Initial conditions

Figure 2.1: One-way unicast streaming from a computer to a headset (Part 1 of 7)

Bluetooth SIG Proprietary

Page 8 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication
Figure 2.1 shows the initial conditions:•

The computer and the headset are bonded but not connected. The computer has previously
discovered the services on the headset and subscribed to notifications for the service characteristics.

•

The computer is ready for a connection for LE Audio – it is in the CAP Ready for Audio related
Peripheral (RAP) mode (see Section 8.2 in [4]). It is therefore scanning for announcements.

•

The headset exposes two sink ASEs. This corresponds to BAP Audio Configuration 6(i). In BAP, thisconfiguration is mandatory to be supported by Unicast Servers in the Audio Sink role that expose
more than one audio location. The configuration is also mandatory to be supported by Unicast Clientsin the Audio Source role. This configuration can, for example, be used both for banded headphones
and for LE Audio devices representing a pair of physical devices (e.g., a pair of earbuds represented
by one of the earbuds).

•

The headset is announcing that it is available.

Bluetooth SIG Proprietary

Page 9 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

2.2

Establishing an ACL connection

Figure 2.2: One-way unicast streaming from a computer to a headset (Part 2 of 7)

Bluetooth SIG Proprietary

Page 10 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication
Figure 2.2 shows the following steps:•

Some external input (e.g., a button press from a user) starts a media player.

•

The computer now has a need for a sink for media. It is in the CAP Immediate Need for Audio related
Peripheral (INAP) mode (see Section 8.2.2 in [4]). It knows from the announcements that the headset
is available for this and connects to the headset. (This illustrates the “unconnected model” possible
under LE Audio, where a device does not connect until there is a need to transfer audio.)•

The headset notifies the computer of any changes to the sink PAC record and the state of the ASEs
since the computer was last connected to the headset. (See [2] and [3] for details on when it ismandatory to send such notifications.) Alternatively, the computer may read these characteristics
from the headset.

•

As a result, the computer is aware of the current state of the ASEs and of the PACS characteristics ofthe headset.

Bluetooth SIG Proprietary

Page 11 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

2.3

Codec and CIG configuration

Figure 2.3: One-way unicast streaming from a computer to a headset (Part 3 of 7)

Bluetooth SIG Proprietary

Page 12 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication
Figure 2.3 shows the following steps:•

The computer performs Configure Codec operations for ASE 1 and ASE 2. Some of the codec
configuration values are sent as LTV structures (see Section 6.12.5 in [6]).

•The headset transitions the ASEs to Codec Configured state and notifies the new state and theassociated parameters.

•

The computer then proceeds to configure the Connected Isochronous Group (CIG). This may also bedone earlier (implementation specific) but must be done before proceeding to QoS configuration.

Bluetooth SIG Proprietary

Page 13 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

2.4

QoS configuration

Figure 2.4: One-way unicast streaming from a computer to a headset (Part 4 of 7)

Bluetooth SIG Proprietary

Page 14 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication
Figure 2.4 shows the following steps:•

The computer configures the QoS settings.

•

The headset notifies the new state (QoS Configured) and the associated parameters.

•

After the successful Config QoS operation, the Connected Isochronous Streams (CISes) identified in
the Config QoS operation are now considered bound to the ASEs.

•

Note that the QoS parameters in Steps 36 to 39 are the ones written by the computer host in Step 34.
The actual QoS parameters chosen by the controller may still differ from these (see Volume 4, Part E,
Section 7.8.97 in [7]).

Bluetooth SIG Proprietary

Page 15 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

2.5

Enabling ASEs and setting up CISes

Figure 2.5: One-way unicast streaming from a computer to a headset (Part 5 of 7)

Bluetooth SIG Proprietary

Page 16 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication
Figure 2.5 shows the following steps:•

The computer enables the ASEs. At this point, the headset knows both the configuration of, and theuse case (context) for, the CISes connected to its ASEs. At this point, the headset may also drop
other CISes to free up capacity.

•

The headset notifies the new state (Enabling) and the associated parameters.

•

The computer proceeds to set up the required CISes. This triggers CIS setup on both sides, ending
when the host on each side has received the two HCI_LE_CIS_Established events, one for each CIS.
The CIS setup may also be done earlier, but if so, the headset may reject, not knowing the use casefor the CISes. By doing the CIS setup here, after the Enable operation has been accepted, thecomputer can be confident that the headset will not reject the CISes. (See Section 5 in [1] for details
on CIS setup.)•

After the CISes are established, they are considered coupled to the ASEs.

Bluetooth SIG Proprietary

Page 17 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

2.6

Starting streaming

Figure 2.6: One-way unicast streaming from a computer to a headset (Part 6 of 7)

Bluetooth SIG Proprietary

Page 18 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

Figure 2.6 shows the following steps:•

The headset sets up the ISO data path for each of the CISes to route data from the controller to where it will be rendered.

•

The headset initiates the Receiver Start Ready operation, transitioning the ASEs to the Streaming state.

•

The headset notifies the new state (Streaming) and the associated parameters.

•

The computer sets up the ISO data path for each of the CISes to route data from where it is coming to the controller.

•

Audio is now flowing from the computer to the headset.

Bluetooth SIG Proprietary

Page 19 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

2.7

Ending streaming and cleaning up

Figure 2.7: One-way unicast streaming from a computer to a headset (Part 7 of 7)

Bluetooth SIG Proprietary

Page 20 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication
Figure 2.7 shows the following steps:•

Some external input (e.g., a button press from a user) stops the media player.

•

The computer no longer has a need to stream audio and disables the ASEs.

•

The headset decouples the ASEs from the CISes and notifies the new state.

•

The computer and the headset disconnect the CISes (indicated by the block arrow) and remove theCIG.

•

The ACL is disconnected. The computer may also choose to not disconnect, depending upon the usecase or other circumstances.

•

The headset transitions the ASEs to Codec Configured state. The headset may also transition the
ASEs further, to Idle state (via the Releasing state), by doing an autonomous Release operation.

•

The headset may start the announcements again (if it stopped them in the first place).

Bluetooth SIG Proprietary

Page 21 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

2.8

Considerations on setup latency

The number of GATT transactions characterize the setup latency. This section summarizes the number of
GATT transactions.
For the example in this document, from the point in time where an ACL connection is established until the
point in time at which audio transmission starts:•

There are 3 GATT transactions from the computer to the headset (Codec configuration, Quality ofService (QoS) configuration, and enabling of ASEs, with corresponding result notifications).

•

There are from 8 to 13 additional notifications from the headset to the computer, depending upon
whether the state of the sink PAC record and the ASEs has changed since the last connection.

Bluetooth SIG Proprietary

Page 22 of 23

Low Energy Audio – Basic One-Way Unicast Audio / Informational Publication

3 References[1]

Basic Audio Profile, Version 1.0.1 or later[2]

Published Audio Capabilities Service, Version 1.0.1 or later[3]

Audio Stream Control Service, Version 1.0[4]

Common Audio Profile, Version 1.0[5]

Low Energy Audio – Context Types and Availability, https://www.bluetooth.com/bluetoothresources/low-energy-audio-context-types-and-availability[6]Bluetooth Assigned Numbers, https://www.bluetooth.com/specifications/assigned-numbers[7]

Bluetooth Core Specification, Version 5.4 or later

Bluetooth SIG Proprietary

Page 23 of 23