Before diving into GAP itself, it helps to understand where it sits in the bigger picture. Every Bluetooth profile depends on one or more other profiles beneath it. GAP sits at the very bottom — every other profile depends on it directly or indirectly.

The dependency rule is simple: if Profile A uses parts of Profile B, then Profile A is dependent on Profile B. In the diagram below, an inner box means “depends on the outer box”.

For example: Hands-Free Profile (HFP) depends on Serial Port Profile (SPP), which in turn depends on GAP. So HFP sits inside SPP, which sits inside GAP.

Bluetooth Profile Dependency Tree (Figure 4.14):

Inner boxes depend on outer boxes. All profiles ultimately depend on GAP (outermost box).

For BR/EDR (Basic Rate / Enhanced Data Rate) Bluetooth devices, GAP defines that every device must include at minimum these five components in its protocol stack:

Beyond the mandatory stack, GAP also defines the procedures and behaviours for:

GAP has three main goals that it achieves across all Bluetooth implementations:

GAP defines procedures for two different Bluetooth technologies — classic Bluetooth (BR/EDR) and Bluetooth Low Energy (BLE). It recognises three types of devices:

The BR/EDR procedures are covered in this guide. The BLE-specific procedures are covered separately in the Low Energy chapters.

GAP defines exactly how Bluetooth parameters must be represented on user interfaces, in manuals, and in advertising. This matters because if every manufacturer uses different names and formats for the same thing, users get confused. GAP standardises the terminology.

BlueZ — Reading and Setting These Parameters:

# Show the Bluetooth Device Address (BD_ADDR) of your adapter
hciconfig hci0 | grep "BD Address"
# Output example: BD Address: 00:AB:CD:EF:12:34

# Read local device name
hciconfig hci0 name
# Or using bluetoothctl:
bluetoothctl show | grep "Name"

# Set device name (Bluetooth Device Name)
hciconfig hci0 name "MyLinuxDevice"
# Or in bluetoothctl:
bluetoothctl system-alias "MyLinuxDevice"

# Read Class of Device
hciconfig hci0 | grep "Class"
# Output example: Class: 0x3c0104  (Computer - Desktop, Service: Capturing)

# Decode Class of Device value
# Bit fields: Service Class (bits 23-13), Major Device (bits 12-8), Minor Device (bits 7-2)
# Example: 0x3c0104 = Computer (Major), Desktop (Minor), Capturing+Audio (Service)

GAP defines three categories of modes. Each category controls a different aspect of how the device behaves with respect to other Bluetooth devices. Every mode can be toggled — you choose which state your device is in at any given time.

4.10.2.1 — Discoverability Modes (Inquiry-related)

These modes control whether your device can be found by other devices scanning for Bluetooth devices nearby. The process of finding devices is called Inquiry.

4.10.2.2 — Connectability Modes (Paging-related)

These modes control whether another device can connect to your device. The process of initiating a connection is called Paging. A device in PAGE_SCAN state is listening for incoming page requests.

4.10.2.3 — Bondable Modes (Pairing-related)

Bonding is the process of pairing where a passkey is exchanged and a bond is created — a long-term link key stored on both devices. This bond means you do not need to re-enter the passkey every time you reconnect.

Summary — All Three Mode Categories Together:

BlueZ — Controlling All Three Modes:

# === DISCOVERABILITY MODES ===

# Set device to General Discoverable mode (GIAC)
hciconfig hci0 piscan          # enables both Page Scan and Inquiry Scan
hciconfig hci0 iscan           # Inquiry Scan only (discoverable, not connectable)

# Set device to Non-Discoverable
hciconfig hci0 noscan

# In bluetoothctl (modern interface):
bluetoothctl discoverable on    # General Discoverable
bluetoothctl discoverable off   # Non-Discoverable

# Limited Discoverable mode (LIAC) - timeout after N seconds
bluetoothctl discoverable-timeout 60   # visible for 60 seconds then hides


# === CONNECTABILITY MODES ===

# Make device connectable (enables PAGE_SCAN)
hciconfig hci0 pscan            # Page Scan only
bluetoothctl pairable on

# Make device non-connectable
hciconfig hci0 noscan

# Check current scan modes
hciconfig hci0 | grep "UP"


# === BONDABLE (PAIRING) MODES ===

# Enable pairing (Bondable mode)
bluetoothctl pairable on

# Disable pairing (Non-Bondable mode)
bluetoothctl pairable off

# Set pairing timeout
bluetoothctl pairable-timeout 120    # bondable for 120 seconds

# View current adapter settings
bluetoothctl show
# Shows: Discoverable, Pairable, Powered, etc.

Idle mode procedures are actions a Bluetooth device takes when it is not currently busy with an active connection. The name idle is slightly misleading — these procedures can also run while a device is already connected, for example to discover a second device and form a scatternet.

The main idle mode procedures are: Inquiry, Name Discovery, and Bonding.

Inquiry is the process of scanning for Bluetooth devices in the local vicinity. Devices are mobile — they move in and out of range constantly. Inquiry gives you a live snapshot of what devices are currently reachable.

When a device completes an inquiry, it gets back a list of discovered devices. For each discovered device, the inquiry provides:

Two Types of Inquiry

Inquiry Flow — What Happens Step by Step:

BlueZ — Running Inquiry / Scanning:

# General Inquiry — discover nearby Bluetooth devices (8 second scan)
hcitool scan
# Output example:
# Scanning ...
#   00:1A:7D:DA:71:13  MyHeadset
#   68:ED:43:25:0E:99  Samsung Galaxy S22

# General Inquiry with more details (RSSI, Class of Device, clock offset)
hcitool inq
# Output:
# Inquiring ...
#   00:1A:7D:DA:71:13  clock offset: 0x0abc  class: 0x240404

# Limited Inquiry (LIAC) — only devices in limited discoverable mode
hcitool inq --iac liac

# Scan with Extended Inquiry Response (EIR) data
hcitool scan --extended

# Using bluetoothctl (modern recommended method)
bluetoothctl scan on
# Then: [NEW] Device 00:1A:7D:DA:71:13 MyHeadset
bluetoothctl scan off

# Get info about a specific discovered device
bluetoothctl info 00:1A:7D:DA:71:13
# Shows: Name, Alias, Class, Icon, Paired, Trusted, Blocked, Connected, UUIDs

Here is a complete quick-reference showing how GAP relates to the most commonly implemented Bluetooth classic profiles and what each requires:

# =============================================
# 1. ADAPTER / DEVICE INFORMATION (BD_ADDR, Name, CoD)
# =============================================

# List all Bluetooth adapters
hciconfig -a

# Show adapter details: BD_ADDR, name, class, features
hciconfig hci0

# Get adapter info via bluetoothctl
bluetoothctl show

# Read remote device name (Name Discovery procedure)
hcitool name 00:1A:7D:DA:71:13

# Remote Name Request via bluetoothctl
bluetoothctl info 00:1A:7D:DA:71:13


# =============================================
# 2. DISCOVERABILITY AND CONNECTABILITY
# =============================================

# Enable both Inquiry Scan and Page Scan (discoverable + connectable)
hciconfig hci0 piscan

# Inquiry Scan only (discoverable but not connectable)
hciconfig hci0 iscan

# Page Scan only (connectable but not discoverable)
hciconfig hci0 pscan

# Disable all scanning (non-discoverable, non-connectable)
hciconfig hci0 noscan

# Set discoverable via bluetoothctl (simpler, recommended)
bluetoothctl discoverable on
bluetoothctl discoverable off
bluetoothctl pairable on
bluetoothctl pairable off


# =============================================
# 3. INQUIRY (DEVICE DISCOVERY)
# =============================================

# Run a General Inquiry (GIAC) - 10 second scan
hcitool scan

# Run Inquiry with RSSI values
hcitool rssi 00:1A:7D:DA:71:13

# Inquiry with all returned details
hcitool inq

# Extended inquiry (gets device name in inquiry packet if EIR supported)
hcitool scan --extended --flush

# Continuous scan in bluetoothctl
bluetoothctl scan on
# Press Ctrl+C or:
bluetoothctl scan off


# =============================================
# 4. BONDING AND PAIRING
# =============================================

# Pair with a device (initiates bonding)
bluetoothctl pair 00:1A:7D:DA:71:13

# Trust a device (so it auto-connects)
bluetoothctl trust 00:1A:7D:DA:71:13

# Remove a bond (unpair)
bluetoothctl remove 00:1A:7D:DA:71:13

# List all paired/bonded devices
bluetoothctl paired-devices

# Check if a device is bonded
bluetoothctl info 00:1A:7D:DA:71:13 | grep "Paired"


# =============================================
# 5. CONNECTION ESTABLISHMENT
# =============================================

# Connect to a paired device
bluetoothctl connect 00:1A:7D:DA:71:13

# Disconnect
bluetoothctl disconnect 00:1A:7D:DA:71:13

# Check active connections
hcitool con

# Low-level connection test (ping)
l2ping 00:1A:7D:DA:71:13