Bluetooth (IEEE 802.15.1) WPAN

1. Overview and Core Philosophy

Bluetooth is a short-range, low-power, low-cost wireless technology designed to interconnect portable and stationary devices, eliminating the cables connecting peripherals (keyboards, mice, headphones, mobile phones).

Standardized by the Bluetooth Special Interest Group (SIG) and adopted under IEEE 802.15.1, it forms a Wireless Personal Area Network (WPAN).

Physical Characteristics:

Frequency Band: Operates in the license-free $2.4\text{ GHz}$ ISM (Industrial, Scientific, Medical) band ( $2.402 - 2.480\text{ GHz}$ ).
Modulation & Channeling: Slices the band into 79 RF channels, each with a bandwidth of $1\text{ MHz}$ .
Frequency Hopping (FHSS): Employs Frequency Hopping Spread Spectrum, hopping $1{,}600\text{ times per second}$ across all 79 channels. This rapid hopping makes Bluetooth extraordinarily immune to interference from Wi-Fi routers and microwave ovens sharing the 2.4 GHz band.

2. Network Topologies: Piconet and Scatternet

Bluetooth organizes devices into two hierarchical topologies:

  A. PICONET (1 Master + Up to 7 Active Slaves)
                 [ Master ]
                /    |    \
        [ Slave 1 ] [ Slave 2 ] [ Slave 3 ] ... (Up to 7 Active Slaves)
        (•) [ Parked Slave ] (Up to 255 Parked Slaves)

  B. SCATTERNET (Interconnected Piconets)
      Piconet 1                                 Piconet 2
    [ Master 1 ]                              [ Master 2 ]
       /    \                                    /    \
   [Slave] [ Bridge Node ] ═════════════════════+    [Slave]
           (Slave in Piconet 1 AND
            Master or Slave in Piconet 2)

1. Piconet

The basic architectural unit of a Bluetooth network.
Contains 1 Master node and up to 7 Active Slave nodes (8 active devices maximum).
Master Role: Sets the clock timing and frequency hopping sequence for the entire piconet.
Slave Role: Synchronizes its clock to the master. Communication is strictly Master-to-Slave or Slave-to-Master; slaves can never communicate directly with each other!
Parked Nodes: A piconet can also support up to 255 Parked Slaves (synchronized in low-power sleep mode, awaiting an activation command from the master).

2. Scatternet

Formed when two or more independent piconets overlap in physical space.
A device can act as a Bridge Node: serving as a Slave in one piconet while acting as the Master (or a Slave) in an adjacent piconet.
The bridge node uses time-division multiplexing to participate in each piconet during alternating time slices, relaying packets across piconet boundaries.

3. Bluetooth Protocol Stack Architecture

+---------------------------------------------------------------+
| APPLICATION PROFILES (A2DP Audio, HFP Handsfree, HID Keyboard)|
+---------------------------------------------------------------+
| MIDDLEWARE / PROTOCOLS: RFCOMM (Serial), SDP (Discovery), OBEX|
+---------------------------------------------------------------+
| L2CAP: Logical Link Control & Adaptation Protocol             |
| - Segmentation and Reassembly (SAR), Multiplexing, QoS        |
+---------------------------------------------------------------+
| BASEBAND LAYER: TDD Framing, 625 μs time slots, Addressing    |
+---------------------------------------------------------------+
| RADIO LAYER: Physical 2.4 GHz RF, 79 channels, FHSS (1600/s)  |
+---------------------------------------------------------------+

Key Sublayer Functions:

Radio Layer: Specifies the air interface, transceivers, frequency bands ( $2.4\text{ GHz}$ ), and 1600 hops/sec FHSS.
Baseband Layer:
- Manages physical channels and links.
- Uses Time-Division Duplexing (TDD): Time is divided into $625\text{ }\mu\text{s}$ time slots. Master transmits in even-numbered slots ( $0, 2, 4\dots$ ), and the addressed slave replies in the subsequent odd-numbered slot ( $1, 3, 5\dots$ ).
- Link Types:
  - SCO (Synchronous Connection-Oriented): Circuit-switched, dedicated voice link (fixed slots, no retransmission).
  - ACL (Asynchronous Connection-Less): Packet-switched, best-effort data link (uses ARQ retransmissions).
L2CAP (Logical Link Control and Adaptation Protocol):
- Provides Segmentation and Reassembly (SAR) (slices large $64\text{ KB}$ user packets into baseband-sized chunks).
- Multiplexes multiple upper-layer protocols (RFCOMM, SDP) over a single baseband link.
Bluetooth Profiles: Defines application-specific behavior:
- A2DP (Advanced Audio Distribution Profile) for stereo music headphones.
- HID (Human Interface Device) for mice and keyboards.
- HFP (Hands-Free Profile) for in-car calling.

4. Bluetooth Classic vs. Bluetooth Low Energy (BLE)

Parameter	Bluetooth Classic (BR/EDR)	Bluetooth Low Energy (BLE / Smart)
Primary Goal	High-throughput continuous streaming	Ultra-low power consumption (Years on coin cell)
RF Channels	79 channels ( $1\text{ MHz}$ wide)	40 channels ( $2\text{ MHz}$ wide)
Active Slaves	Maximum 7 active per piconet	Virtually unlimited
Connection Setup	Slow ( $100\text{ ms} - \text{seconds}$ )	Instantaneous ( $< 3\text{ ms}$ )
Typical Use Cases	Wireless Headphones, Car Audio	Smartwatches, Heart-rate monitors, Apple AirTags

5. Key Exam Questions & Takeaways

How many active devices can participate in a single Bluetooth Piconet?
- Maximum 8 active devices: 1 Master and up to 7 Active Slaves. (Up to 255 additional devices can be kept in parked/dormant state).
What is a Scatternet?
- An interconnected network created when a device participates in two or more overlapping Piconets (acting as a slave in one and a master/slave in another) to bridge data traffic between them.
Why does Bluetooth use Frequency Hopping (FHSS) at 1600 hops/second?
- To avoid packet collisions and radio interference within the crowded $2.4\text{ GHz}$ ISM band shared with Wi-Fi networks, cordless telephones, and microwave ovens.
What is the function of the L2CAP sublayer?
- L2CAP performs protocol multiplexing, segmentation and reassembly (SAR) of large packets, and Quality of Service (QoS) negotiation between the upper applications and the lower baseband layer.

Bluetooth