Bluetooth: How it works – A2DP Uncovering

Bluetooth: How it works – A2DP Uncovering


Historical Aspect:

A Long time ago in a previous century in the 1989 year, Two scientists: Dr. NILS Rydbeck from Ericsson Mobile and Dr. JOHAN Ullman develop and use the first the technology which meets the requirements to connect wirelessly a headset to the main radio device. This Short Distance radio link, a few years after this develop was renamed with nowadays extremely growing and very popular trademark name “BLUETOOTH”. BLUETOOTH is so popular as TV or DVD or “Coca-Cola” for example.

The Idea of usage a short Distance radio link for real radio connection between a PC and a mobile phone in 1997 By Jim Kardach.


Jim Kardach was inspired by the tenth-century Danish King-Harald Bluetooth.

He was famous by uniting the_dissonant Danish tribes into a single kingdom.

Jim Kardach was reading Frans G. Bengtsson’s historical novel The Long Ships  about the Vikings and King Harald Bluetooth.


   Bluetooth Symbolics is based on Harald’s initials. Today, Well known Bluetooth logo is a combination between Runic letter ior.svg (Hagall) and Runic letter berkanan.svg (Bjarkan). Just overlap these symbols and you will get the LOGO:



Physical layer:

   The Bluetooth radio physically works in the UHF wave spectrum – more exactly in the ISM semi-band from 2.400 GHz to 2.485 GHz. The main goal on Bluetooth was to replace directly the wired communication interface RS-232 with the wireless one. More Detailed Description of Bluetooth can see here.

   Like many other things on the Earth, it is good to straighten. The Bluetooth also has a need to be standardized and formalized. The organization cares about is named: Bluetooth Special Interest Group (SIG).

SIG defines some Bluetooth standard Versions:

Bluetooth v1.0 and v1.0B

  • Versions 1.0 and 1.0B  have a lot of problems and making manufacturers products non-interoperable. They Using fixed hardware device address (BD_ADDR) in their transmissions till the Connecting process and it deteriorated the work – because was completely inflexible.

Bluetooth v1.1 – IEEE Standard 802.15.1–2002

  • Were fixed many errors in the v1.0B specification,
  • Added features of the Received Signal Strength Indicator (RSSI).

Bluetooth v1.2 – IEEE Standard 802.15.1–2005

  • Allow Faster Discovery and Connection
  • Introduced Flow Control and Retransmission Mode.
  • The data transfer rate is up to 721 kbit/s

Bluetooth v2.0 + EDR

  • The data transfer rate is up to 2.1 Mbit/s
  • EDR – the rate is up to 3.0 Mbit/s


Bluetooth v2.1 + EDR- 2007 

  • some errors were fixed and some improvements were added


Bluetooth v3.0 + HS – 2009

  • The data transfer rate is up to 24 Mbit/s,
  • The new feature AMP (Alternative MAC/PHY)
  • IEEE 802.11 signature.
  • Enhanced Power Control
  • Unicast Connectionless Data


Bluetooth v4.0- 2010

Bluetooth low energy


Bluetooth v4.1– 2013

  • Software update to Bluetooth v4.0
  • added some improving features
  • LTE support
  • Features:
    • Dual Mode and Topology
    • 802.11nLow Duty Cycle Directed Advertising
    • Low Duty Cycle Directed Advertising
    • Limited time to DiscoverFast Data Advertising Interval
    • Fast Data Advertising Interval
    • New update of Audio Architecture for better quality and Wide Band of Speech

Bluetooth v4.2 – 2014

  • were added some key features for IoT
  • IP via Bluetooth Smart devices connection
  • LE Secure Connections


Bluetooth v5 – 2016

  • x4 range,
  • x2 transfer rate
  • x8 bandwidth

of LE Bluetooth

it means transfer rate up to 2Mbit/s of throughput and up to 300m range using LE Bluetooth connection.


Using the Bluetooth devices is commonly connected with using some defined and desired Services (Profiles):

The list is long and we point more interesting (from the author point of view) of them, sorted alphabetically :

  • A2DPAdvanced Audio Distribution Profile
  • ATTAttribute Profile
  • AVRCPAudio / Video Remote Control Profile
  • BIP – Basic Imaging Profile
  • HID – Human Interface Device Profile
  • HFP – Hands-Free Profile
  • HSP – Headset Profile


We pointing the interest one service: The Service Discovery Protocol (SDP)

SDP –  this protocol makes possible the device to discover main services, which are managed and offered by the other hardware device or exactly with their parameters.

If we have a hardware device and remote Bluetooth headset, the hardware device will use the SDP for offer da possibility of connection :

Headset Profile or A2DP or Handsfree Profile  – each service was its unique Identifier (UUID) – UIniversal Unique Identifier.

Advanced Audio Distribution Profile (A2DP)


Now, we will try to explain with a few words the A2DP:

   A2DP or Advanced Audio Distribution Profile describes the manner of streaming the audio from one to another Bluetooth device via Bluetooth connection. This BT Audio Streaming can be 

From mobile to :

  • Headset (Bluetooth Remote LoudSpeaker), Car Audio, Hearing aid or cochlear implant,
  • The Headset profile has abbreviature [HSP], and Hands-Free profile has abbreviature [HFP]. 
  • The A2DP service is mainly designed to single-directionally connection transfer in 2 – stereo channels by BT Host.

Typical Example of A2DP using Device



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