Part A — Unicast Audio Streaming

The ASE State Machine

Every audio stream in unicast BAP is controlled through an ASE (Audio Stream Endpoint). The ASE is a GATT characteristic on the Unicast Server. Moving an ASE through its states is how a stream gets established, used, and torn down.

Step-by-Step: Establishing a Unicast Stream

BlueZ — Setup ISO Data Path (Transparent Codec, Host-side Codec)

#include <bluetooth/hci.h>
#include <bluetooth/hci_lib.h>

/*
 * Setup ISO Data Path — tells the controller which direction
 * audio flows and which codec is in use.
 * When the LC3 codec runs in the Host (not the controller),
 * we use Coding_Format = 0x03 (Transparent) so the controller
 * just passes raw bytes through without decoding.
 */
void setup_iso_data_path(int hci_fd, uint16_t conn_handle,
                          uint8_t direction)
{
    struct {
        uint16_t conn_handle;
        uint8_t  data_path_dir;   /* 0x00=Input(TX), 0x01=Output(RX) */
        uint8_t  data_path_id;    /* 0x00 = HCI transport */
        uint8_t  codec_id[5];     /* coding format + company + vendor */
        uint32_t controller_delay;
        uint8_t  codec_cfg_len;
    } cmd = {
        .conn_handle      = htobs(conn_handle),
        .data_path_dir    = direction,
        .data_path_id     = 0x00,
        /* codec_id[0] = 0x03 = Transparent (LC3 runs in Host) */
        .codec_id         = { 0x03, 0x00, 0x00, 0x00, 0x00 },
        .controller_delay = 0,
        .codec_cfg_len    = 0,
    };

    /* Send HCI_LE_Setup_ISO_Data_Path (OCF 0x006E) */
    hci_send_cmd(hci_fd, OGF_LE_CTL, 0x006E,
                 sizeof(cmd), (uint8_t *)&cmd);
}

Stopping the Stream — Disable and Release

Part B — Broadcast Audio Streaming

The Broadcast Audio Stream State Machine

Broadcast audio is entirely driven by the Broadcast Source. There is no connection and no GATT involved. The state machine has only three states:

How EA, PA and BIG Fit Together

A Broadcast Source transmits three layers of packets. Each one serves a specific purpose and points to the next:

The BASE Structure — How Stream Parameters Are Announced

The BASE (Broadcast Audio Source Endpoint) is the data structure carried in Periodic Advertising. It describes every stream in a BIG using a three-level hierarchy.

A practical example: a TV broadcasting stereo audio with two language options would have 2 subgroups (English and Spanish), each with 2 BIS (Front Left and Front Right). A total of 4 BIS in one BIG.

How a Broadcast Sink Finds and Receives a Stream

Broadcast Assistant — Helping Power-Constrained Sinks

A hearing aid or a small earbud may not have the power budget to constantly scan for Broadcast Sources on its own. The Scan Delegator role on the sink device allows it to ask a phone (acting as a Broadcast Assistant) to scan on its behalf.

BlueZ — Creating a BIS Broadcast Source

Setting up a Broadcast Source in BlueZ uses ISO sockets with the BT_ISO_BC_SRC flag. The socket bind creates the BIG and the connect creates the BIS channels.

#include <bluetooth/bluetooth.h>
#include <bluetooth/iso.h>

/*
 * BIS Broadcast Source using BlueZ ISO socket API
 * Creates a BIG with one BIS for mono broadcast audio
 * Codec: LC3 16_2_1 (16kHz, 10ms frames, 40 bytes/frame)
 */
int create_bis_source(void)
{
    int fd;

    /* BIS QoS configuration — broadcast version */
    struct bt_iso_qos qos = {
        .bcast = {
            .big            = BT_ISO_QOS_BIG_UNSET,
            .bis            = BT_ISO_QOS_BIS_UNSET,
            .sync_interval  = 6,          /* units of 1.25 ms */
            .packing        = 0x00,       /* sequential */
            .framing        = 0x00,       /* unframed */
            .encryption     = 0x00,       /* no encryption */
            .bcode          = { 0 },      /* broadcast code (used if encrypted) */
            .options        = 0,
            .skip           = 0,
            .sync_timeout   = 0x4000,
            .out = {
                .interval   = 10000,      /* 10 ms SDU interval */
                .latency    = 10,         /* max transport latency ms */
                .sdu        = 40,         /* 40 octets = LC3 16_2 */
                .phy        = BT_ISO_PHY_2M,
                .rtn        = 2,          /* retransmissions */
            },
        },
    };

    struct sockaddr_iso src = {
        .sa_family   = AF_BLUETOOTH,
        .v.bis.dev_id = 0,  /* hci0 */
    };

    fd = socket(AF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_ISO);
    if (fd < 0) { perror("socket"); return -1; }

    setsockopt(fd, SOL_BLUETOOTH, BT_ISO_QOS, &qos, sizeof(qos));

    /* Bind creates the BIG */
    bind(fd, (struct sockaddr *)&src, sizeof(src));

    /* Connect to broadcast address creates the BIS */
    struct sockaddr_iso dst = {
        .sa_family      = AF_BLUETOOTH,
        .v.bis.bdaddr   = *BDADDR_ANY,   /* broadcast target */
    };
    connect(fd, (struct sockaddr *)&dst, sizeof(dst));

    /* Now write() sends LC3-encoded SDUs into the BIS */
    return fd;
}

Summary