Bluetooth® Low Energy audio - Stream Management

1. Introduction

The Bluetooth^® LE Audio Stream Management section regroups

• the Basic Audio Profile (BAP)
• the Audio Stream Control Service (ASCS)
• the Published Audio Capability Service (PACS)
• the Broadcast Audio Scan Service (BASS)

These profile and services are used to configure and establish unicast and broadcast audio streams.

2. Basic Audio Profile (BAP)

The Basic Audio Profile defines roles and procedures to establish audio streams with remote devices. It's specified by the Bluetooth^® SIG and the full specification is accessible on their website.

2.1. Roles

The BAP introduces 6 roles:

• Unicast client: Establishes connection to Unicast server, discovers its capabilities and configure the audio stream. This role is used by a smartphone, a laptop, a television etc.
• Unicast server: Advertises its role, exposes its capabilities, accepts the Unicast server to configure the audio stream. This role can be used by a headphone, a speaker, some hearing aids, some earbuds, or even a microphone.
• Broadcast source: establishes a BIS and advertise all the information over extended advertising and periodic advertising.
• Broadcast sink: scan advertising to find the broadcast source, and get synchronized to receive the audio stream
• Scan delegator: exposes its capabilities and wait for a broadcast assistant to receive some information of a broadcast source and commands. The scan delegator also has the broadcast sink role to get synchronized to the selected broadcast source.
• Broadcast assistant: scan advertising to find broadcast source, discovers the scan delegator capabilities. It sends the information of the broadcast source to the scan delegator.

2.2. Unicast Roles

The following diagram illustrates the interaction between the different unicast roles:

2.2.1. Unicast Topologies

The BAP specifies 16 different topologies for the Unicast Roles. They define ways of establishing audio stream between one Central device and one or two Peripheral devices. The following table can be found in the BAP Specification and lists the possible topologies:

Audio Configuration	Legend C------S	Num Servers	Sink ASEs	Source ASEs	Audio Channels per Sink ASE	Min Sink Audio Locations per Server	Audio Channels per Source ASE	Min Source Audio Locations per Server	CISes	Audio Streams
1	-------->	1	1		1				1	1
2	<--------	1		1			1		1	1
3	<------->	1	1	1	1		1		1	2
4	------->>	1	1		2	2			1	1
5	<----->>	1	1	1	2		1		1	2
6(i)	--------> -------->	1	2		1	2			2	2
6(ii)	--------> <--------	2	2		1	1			2	2
7(i)	--------> <--------	1	1	1	1		1		2	2
7(ii)	--------> <--------	2	1	1	1		1		2	2
8(i)	--------> <------->	1	2	1	1	2	1		2	3
8(ii)	--------> <------->	2	2	1	1	1	1	1	2	3
9(i)	<-------- <--------	1		2			1	2	2	2
9(ii)	<-------- <--------	2		2			1	1	2	2
10	<<-------	1		1			2	2	1	1
11(i)	<-------> <------->	1	2	2	1	2	1	2	2	4
11(ii)	<-------> <------->	2	2	2	1	1	1	1	2	4

The arrows in the "Legend" column illustrate the CIS(s) to establish between the devices. One arrow means 1 CIS. The arrow tip indicates the direction of the CIS (unidirectional or bidirectional). When the tip of the arrow is doubled, the CIS transport 2 channels. Topologies with "(i)" define interactions between 1 Central and 1 Peripheral, whereas topologies with "(ii)" define interactions between 1 Central and 2 Peripherals (earbud use-case for example).

For example:

• The configuration 4 defines a unidirectional stereo stream on one CIS to 1 device, which could be applied to transmit media to a headphone.
• The configuration 11(ii) defines two bidirectional CIS to 2 peripheral devices, which could be applied for a telephony use-case using earbuds where the two microphone on the earbuds would be used.

2.2.2. Unicast Codec Configurations

The BAP specifies LC3 codec configurations that can be used to establish audio streams between devices in Unicast mode. The following tables can be found in the BAP Specification and lists the possible configurations:

Low Latency:

Set Name	Sampling Frequency (kHz)	Frame Duration (ms)	SDU Interval (µs)	Framing	Maximum SDU Size (Octets)	RTN	Max Transport Latency (ms)	Presentation Delay (µs)
8_1_1	8	7.5	7500	unframed	26 (27.734 kbps)	2	8	40000
8_2_1	8	10	10000	unframed	30 (24 kbps)	2	10	40000
16_1_1	16	7.5	7500	unframed	30 (32 kbps)	2	8	40000
16_2_1*	16	10	10000	unframed	40 (32 kbps)	2	10	40000
24_1_1	24	7.5	7500	unframed	45 (48 kbps)	2	8	40000
24_2_1	24	10	10000	unframed	60 (48 kbps)	2	10	40000
32_1_1	32	7.5	7500	unframed	60 (64 kbps)	2	8	40000
32_2_1	32	10	10000	unframed	80 (64 kbps)	2	10	40000
441_1_1	44.1	8.163	8163	framed	97 (95.06 kbps)	5	24	40000
441_2_1	44.1	10.884	10884	framed	130 (95.55 kbps)	5	31	40000
48_1_1	48	7.5	7500	unframed	75 (80 kbps)	5	15	40000
48_2_1	48	10	10000	unframed	100 (80 kbps)	5	20	40000
48_3_1	48	7.5	7500	unframed	90 (96 kbps)	5	15	40000
48_4_1	48	10	10000	unframed	120 (96 kbps)	5	20	40000
48_5_1	48	7.5	7500	unframed	117 (124.8 kbps)	5	15	40000
48_6_1	48	10	10000	unframed	155 (124 kbps)	5	20	40000

(*) Mandatory Configuration Unicast Client & Unicast Server

High reliability:

Set Name	Sampling Frequency (kHz)	Frame Duration (ms)	SDU Interval (µs)	Framing	Maximum SDU Size (Octets)	RTN	Max Transport Latency (ms)	Presentation Delay (µs)
8_1_2	8	7.5	7500	unframed	26 (27.734 kbps)	13	75	40000
8_2_2	8	10	10000	unframed	30 (24 kbps)	13	95	40000
16_1_2	16	7.5	7500	unframed	30 (32 kbps)	13	75	40000
16_2_2	16	10	10000	unframed	40 (32 kbps)	13	95	40000
24_1_2	24	7.5	7500	unframed	45 (48 kbps)	13	75	40000
24_2_2	24	10	10000	unframed	60 (48 kbps)	13	95	40000
32_1_2	32	7.5	7500	unframed	60 (64 kbps)	13	75	40000
32_2_2	32	10	10000	unframed	80 (64 kbps)	13	95	40000
441_1_2	44.1	8.163	8163	framed	97 (95.06 kbps)	13	80	40000
441_2_2	44.1	10.884	10884	framed	130 (95.55 kbps)	13	85	40000
48_1_2	48	7.5	7500	unframed	75 (80 kbps)	13	75	40000
48_2_2	48	10	10000	unframed	100 (80 kbps)	13	95	40000
48_3_2	48	7.5	7500	unframed	90 (96 kbps)	13	75	40000
48_4_2	48	10	10000	unframed	120 (96 kbps)	13	100	40000
48_5_2	48	7.5	7500	unframed	117 (124.8 kbps)	13	75	40000
48_6_2	48	10	10000	unframed	155 (124 kbps)	13	100	40000

The configurations are separated between two tables: Low Latency and High Reliability. High Reliability configurations use a higher Retransmission Number (RTN) and higher maximum transport latency to ensure that the audio packets are all correctly transmitter, but induce higher latency due to increased buffering of the audio packets. Most of the configuration uses an "unframed" mode, meaning the spacing of the events on the air will be a multiple of the actual time between two audio Packets (SDU Interval).

2.3. Broadcast Roles

2.3.1. Broadcast Advertisement

A Broadcast Source advertises to be visible by remote Broadcast Sinks. It's advertisement is composed of two different types of advertising:

• Extended Advertising: On advertising channels (37-38-39), it permits to be visible when a Broadcast Sink is scanning
• Periodic Advertising: On connection channels, it contains large packets with information about the stream configuration

2.3.2. Broadcast Topologies

The BAP specifies 3 different topologies for the Broadcast Roles. They define ways of establishing audio stream between one Central device and one or two Peripheral devices. The following table can be found in the BAP Specification and lists the possible topologies:

Audio Configuration	Legend	Audio Channels per BIS	BISes	Audio Streams
12	---)))-->	1	1	1
13	---)))--> ---)))-->	1	2	2
14	---)))-->>	2	1	1

2.3.3. Broadcast Codec Configurations

The BAP specifies LC3 codec configurations that can be used to establish audio streams between devices in Broadcast mode. The following tables can be found in the BAP Specification and lists the possible configurations: Low Latency:

Set Name	Sampling Frequency (kHz)	Frame Duration (ms)	SDU Interval (µs)	Framing	Maximum SDU Size (Octets)	RTN	Max Transport Latency (ms)	Presentation Delay (µs)
8_1_1	8	7.5	7500	unframed	26 (27.734 kbps)	2	8	40000
8_2_1	8	10	10000	unframed	30 (24 kbps)	2	10	40000
16_1_1	16	7.5	7500	unframed	30 (32 kbps)	2	8	40000
16_2_1*	16	10	10000	unframed	40 (32 kbps)	2	10	40000
24_1_1	24	7.5	7500	unframed	45 (48 kbps)	2	8	40000
24_2_1**	24	10	10000	unframed	60 (48 kbps)	2	10	40000
32_1_1	32	7.5	7500	unframed	60 (64 kbps)	2	8	40000
32_2_1	32	10	10000	unframed	80 (64 kbps)	2	10	40000
441_1_1	44.1	8.163	8163	framed	97 (95.06 kbps)	4	24	40000
441_2_1	44.1	10.884	10884	framed	130 (95.55 kbps)	4	31	40000
48_1_1	48	7.5	7500	unframed	75 (80 kbps)	4	15	40000
48_2_1	48	10	10000	unframed	100 (80 kbps)	4	20	40000
48_3_1	48	7.5	7500	unframed	90 (96 kbps)	4	15	40000
48_4_1	48	10	10000	unframed	120 (96 kbps)	4	20	40000
48_5_1	48	7.5	7500	unframed	117 (124.8 kbps)	4	15	40000
48_6_1	48	10	10000	unframed	155 (124 kbps)	4	20	40000

(*) Mandatory Configuration Broadcast Source & Broadcast Sink
(**) Mandatory Configuration Broadcast Sink

High reliability:

Set Name	Sampling Frequency (kHz)	Frame Duration (ms)	SDU Interval (µs)	Framing	Maximum SDU Size (Octets)	RTN	Max Transport Latency (ms)	Presentation Delay (µs)
8_1_1	8	7.5	7500	unframed	26 (27.734 kbps)	4	45	40000
8_2_1	8	10	10000	unframed	30 (24 kbps)	4	60	40000
16_1_1	16	7.5	7500	unframed	30 (32 kbps)	4	45	40000
16_2_1	16	10	10000	unframed	40 (32 kbps)	4	60	40000
24_1_1	24	7.5	7500	unframed	45 (48 kbps)	4	45	40000
24_2_1	24	10	10000	unframed	60 (48 kbps)	4	60	40000
32_1_1	32	7.5	7500	unframed	60 (64 kbps)	4	45	40000
32_2_1	32	10	10000	unframed	80 (64 kbps)	4	60	40000
441_1_1	44.1	8.163	8163	framed	97 (95.06 kbps)	4	54	40000
441_2_1	44.1	10.884	10884	framed	130 (95.55 kbps)	4	60	40000
48_1_1	48	7.5	7500	unframed	75 (80 kbps)	4	50	40000
48_2_1	48	10	10000	unframed	100 (80 kbps)	4	65	40000
48_3_1	48	7.5	7500	unframed	90 (96 kbps)	4	50	40000
48_4_1	48	10	10000	unframed	120 (96 kbps)	4	65	40000
48_5_1	48	7.5	7500	unframed	117 (124.8 kbps)	4	50	40000
48_6_1	48	10	10000	unframed	155 (124 kbps)	4	65	40000

Like the Unicast codec configurations, the Broadcast codec configurations are separated between a Low Latency table and High Reliability table.

3. Audio Stream Control Service (ASCS)

The ASCS is hosted on a Unicast Server to permit remote Unicast Client to configure and establish audio streams between the two devices. It contains the current unicast state of the Unicast Server and its state machine. It's specified by the Bluetooth^® SIG and the full specification is accessible on their website.

4. Published Audio Capability Service (PACS)

The PACS permits to a Unicast Server to expose to remote Unicast Clients and Broadcast Assistants its audio capabilities. It's specified by the Bluetooth^® SIG and the full specification is accessible on their website.

4.1. Published Audio Capability (PAC)

A PACS Server exposes one or multiple Published Audio Capability (PAC) records. These records indicates to a remote PACS Client which audio configuration are supported by the server.

Parameter	Size (octets)	Value
Codec_ID	5	Codec ID, 0x0000000006 for the LC3 Codec
Codec_Specific_Capabilities_Length	1	Length, in octets, of the Codec_Specific_Capabilities value
Codec_Specific_Capabilities	Varies	Sequence of LTVs (Length-Type-Value) of Codec Capabilities
Metadata_Length	1	Length of the Metadata field
Metadata	Varies	Sequence of LTVs (Length-Type-Value) of Metadata

5. Broadcast Audio Scan Service (BASS)

The Broadcast Audio Scan Service is hosted on a Scan Delegator collocated with a Broadcast Sink to communicate with a remote Broadcast Assistant and exchange information and commands about available Broadcast Sources. It's specified by the Bluetooth^® SIG and the full specification is accessible on their website.