The Four HAP Profile Roles Explained

HAP defines four roles. A single physical device can implement multiple roles simultaneously — for example, a smartphone typically acts as HAUC + HARC + IAC all at once.

HAP Ecosystem Architecture — Who Talks to Whom

The diagram below shows all four HAP roles and their connections to the hearing aid. The HA sits at the centre as the server. CIS streams carry actual audio; GATT operations carry control data; IAS carries alert signals.

Binaural Hearing Aid Set — How Two Aids Work Together

When a user wears two hearing aids, they form a Binaural Hearing Aid Set. HAP handles this using CSIP (Coordinated Set Identification Profile). Both aids share a common SIRK (Set Identity Resolving Key), allowing the HAUC and HARC to discover and recognise them as a matched set. The HAUC then sends separate Left and Right audio channels as two independent CISes — one per aid.

Left vs right is distinguished by the Sink Audio Locations characteristic: the left aid has the Front Left bit set to 1, the right aid has Front Right bit set to 1. A Banded Hearing Aid (single device, both outputs) sets both bits to 1.

HAP Profile Dependency Stack — Full Hierarchy

HAP is a thin orchestrator. It does not reinvent audio streaming, volume control, or microphone management — it relies on this stack of sub-profiles:

HAP Role Conformance Matrix — Quick Reference

This table shows what is mandatory (M), optional (O), conditional (C), excluded (X), or unchanged (—) per HAP role. Study this before building any HAP implementation.

BlueZ Prerequisites — Setting Up for HAP / LE Audio Development

HAP requires Bluetooth Core 5.2+ hardware and BlueZ 5.65 or later. Older BlueZ lacks the ISO socket API needed for CIS and BIS streams. Use the steps below to verify your Linux development machine.

Step 1 — Verify BlueZ Version

# Check installed BlueZ version (need 5.65+ for LE Audio)
bluetoothctl version
# Expected output: Version 5.65 or higher

bluetoothd --version

# Check via package manager on Debian/Ubuntu
dpkg -l bluez | grep bluez

# For latest LE Audio support, build from BlueZ source
# git clone https://git.kernel.org/pub/scm/bluetooth/bluez.git

Step 2 — Verify BT 5.2 Hardware Support

# List Bluetooth adapters
bluetoothctl list

# Show adapter details — look for LE features in output
bluetoothctl show

# Check HCI features at the kernel level
hciconfig hci0 features

# Query LE features via btmgmt (look for "LE ISO Channels" support)
btmgmt info

# Check kernel ISO socket support (needed for CIS/BIS)
cat /boot/config-$(uname -r) | grep -i "BT_LE_L2CAP_ECRED\|BT_HS"

Step 3 — Enable Adapter and Experimental LE Audio Features

# Start and enable bluetoothd service
sudo systemctl start bluetooth
sudo systemctl enable bluetooth

# Power on the adapter
bluetoothctl power on

# Confirm adapter is powered
bluetoothctl show | grep -E "Powered|Pairable|Discoverable"

# Enable experimental features (required for some LE Audio APIs)
# Add to /etc/bluetooth/main.conf under [General]:
sudo tee -a /etc/bluetooth/main.conf << EOF
[General]
Experimental = true
EOF

# Restart to apply
sudo systemctl restart bluetooth

Step 4 — Python D-Bus Setup for BlueZ GATT Scripting

# Install Python D-Bus bindings for BlueZ GATT API
sudo apt-get install -y python3-dbus python3-gi libdbus-1-dev

# Verify BlueZ is reachable over D-Bus
python3 <<EOF
import dbus
bus = dbus.SystemBus()
obj = bus.get_object('org.bluez', '/')
print("BlueZ D-Bus OK:", obj is not None)
EOF

# List all managed BlueZ objects (adapters + bonded devices)
python3 <<EOF
import dbus
bus = dbus.SystemBus()
mgr = dbus.Interface(
    bus.get_object('org.bluez', '/'),
    'org.freedesktop.DBus.ObjectManager'
)
objs = mgr.GetManagedObjects()
for path, ifaces in objs.items():
    if 'org.bluez.Adapter1' in ifaces:
        p = ifaces['org.bluez.Adapter1']
        print(f"Adapter:  {path}  ({p.get('Address', '-')})")
    if 'org.bluez.Device1' in ifaces:
        p = ifaces['org.bluez.Device1']
        print(f"Device:   {path}  ({p.get('Alias', '-')}) paired={p.get('Paired')}")
EOF

Step 5 — Scan for HAP Devices Using the HAS Service UUID

# Hearing Access Service UUID (Bluetooth Assigned Numbers)
# HAS short UUID:   0x1854
# HAS full UUID:    00001854-0000-1000-8000-00805f9b34fb
#
# Key HAS Characteristics:
# Hearing Aid Features:           0x2BDA
# Hearing Aid Preset Control Pt:  0x2BDB
# Active Preset Index:            0x2BDC

# Scan for BLE devices advertising HAS UUID (Python + D-Bus)
python3 <<EOF
import dbus, dbus.mainloop.glib, time
from gi.repository import GLib

HAS_UUID = "00001854-0000-1000-8000-00805f9b34fb"
dbus.mainloop.glib.DBusGMainLoop(set_as_default=True)
bus = dbus.SystemBus()

adapter_props = dbus.Interface(
    bus.get_object('org.bluez', '/org/bluez/hci0'),
    'org.freedesktop.DBus.Properties'
)
adapter = dbus.Interface(
    bus.get_object('org.bluez', '/org/bluez/hci0'),
    'org.bluez.Adapter1'
)

# Filter: only discover devices advertising HAS UUID
adapter.SetDiscoveryFilter({
    'UUIDs': dbus.Array([HAS_UUID], signature='s'),
    'Transport': 'le'
})
adapter.StartDiscovery()
print("Scanning 10s for HAP / Hearing Aid devices...")
time.sleep(10)
adapter.StopDiscovery()

# Report found devices
mgr = dbus.Interface(
    bus.get_object('org.bluez', '/'),
    'org.freedesktop.DBus.ObjectManager'
)
found = False
for path, ifaces in mgr.GetManagedObjects().items():
    if 'org.bluez.Device1' in ifaces:
        p = ifaces['org.bluez.Device1']
        uuids = list(p.get('UUIDs', []))
        if HAS_UUID in uuids:
            print(f"  Found HA: {p.get('Alias','?')}  {p.get('Address','?')}  RSSI={p.get('RSSI','?')}")
            found = True
if not found:
    print("  No HAP devices found.")
EOF

Part 1 Summary