Bluetooth
Bluetooth is a low cost, low power, short-range radio technology intended to replace cable connections between cell phones, PDAs and other
portable devices. It can clean up your desk considerably, making wires between your workstation, mouse, laptop computer etc. obsolete.
Ericsson Mobile Communications started developing the Bluetooth system in 1994, looking for a replacement to the cables connecting cell phones
and their accessories. The Bluetooth system is named after a Danish Viking king, Harald Blåtand, who united and controlled Norway and Denmark.
The Bluetooth Special Interest Group (SIG) is responsible for further development of the Bluetooth standard. Sony Ericsson,
Intel, IBM, Toshiba, Nokia, Microsoft, 3COM and Motorola are some of the companies involved in the SIG.
Check out http://www.bluetooth.com/ and
http://www.bluetooth.org/ for more information about the Bluetooth SIG and Bluetooth technology.
This document describes the Bluetooth standard version 1.1, the Bluetooth version implemented in most handheld devices at the moment. However, the Bluetooth
1.2 specification is completed and the Bluetooth 2.0 specification is in the works. Devices conforming to these new specifications will
probably show up shortly after the completion of my Master thesis.
This is a quick introduction to the most important parts of the Bluetooth technology. My fellow student Lasse Hægland is writing a Master thesis
about the Bluetooth technology. You will find more in-depth information about Bluetooth on his site.
http://www.geek.no/bluetooth.php?lan=1
You might also want to check out my supervisor's Bluetooth course available at:
http://www.kjhole.com
Bluetooth architecture
The Bluetooth stack is made up of many layers. The lower layers are usually implemented in hardware and the higher layers are usually
implemented in software. The Bluetooth radio is the lowest layer of Bluetooth communication. The Industrial, Scientific and Medical (ISM) band at 2.4 GHz is used for radio
communication. Note that several other technologies use this band as well. Wi-Fi technologies like 802.11b/g and kitchen technologies
like microwave ovens may cause interference in this band.
The Bluetooth radio utilizes a signaling technique called Frequency Hopping Spread Spectrum (FHSS). The radio band is divided into 79
sub-channels. The Bluetooth radio uses one of these channels at a given time. The radio jumps from channel to channel spending
625 microseconds on each channel. Hence, there are 1600 frequency hops per second. Frequency hopping
is used to reduce interference caused by nearby Bluetooth devices or other devices like Wi-Fi stations. Adaptive Frequency Hopping (AFH) is introduced
in the Bluetooth 1.2 specification. This is useful if your device communicates through both Bluetooth and Wi-Fi simultaneously.
The frequency hopping algorithm can then avoid using Bluetooth channels overlapping the Wi-Fi channel in use, hence avoiding interference between your own
radio communications.
Slaves synchronize their frequency hopping with the master using the master's clock and Bluetooth address.
Piconet and scatternet
A piconet is the usual form of a Bluetooth network and is made up of one master and one or more slaves. The device initiating a Bluetooth connection
automatically becomes the master. A piconet can consist of one master and up to seven active slaves. The master device is literally the master
of the piconet. Slaves may only transmit data when transmission-time is granted by the master device, also slaves may not communicate directly with
each other, all communication must be directed through the master.
When connecting two piconets you get a scatternet, with one intermediate node connecting the piconets. This node must time-share, meaning
it must follow the frequency hopping in one piconet at the time. Frequency hopping is not synchronized between piconets, hence the the piconets will
randomly collide on the same frequency.
Role-switching enables two devices to switch roles in a piconet. Consider the following example:
You have two devices A and B. A connects to B, hence A becomes the master of the piconet consisting of A and B.
Then a device C wants to join the piconet. C connects to the master, A. Since C initiated the connection it will automatically become the master of the connection between C and A. We now have two masters, hence we have two piconets. Device A is the intermediate node between these piconets, being the master for B and the slave for C.
A role-switch between A and C will give us one piconet where A is the master and both B and C are slaves. We see that when a new device wants to be part of a piconet we actually need a role-switch to make it happen, else we get a scatternet.
It is important to note that neither version 1.1 nor version 1.2 of the Bluetooth specification define how packets should be routed between piconets.
Bluetooth links
Two types of links are defined in version 1.1 of the Bluetooth specification, Asynchronous ConnectionLess (ACL) links and Synchronous Connection Oriented (SCO) links.
ACL links are intended for data communication.
An ACL link provides error-free transmission of data which means that lost or erronous packets are re-transmitted. No Quality of Service (QoS) guarantee is provided.
The maximum data rate at the application level is around 650 kbps for an ACL link.
SCO links are intended for voice communication. When setting up a SCO link timeslots are reserved for transmission of data, thus
providing a QoS guarantee. Lost or errounous packages are not re-transmitted. This makes sense for voice transmissions. All SCO links operate at 64 kbps.
Bluetooth profiles
Bluetooth profiles provide a well defined set of higher layer procedures and uniform ways of using the lower layers of Bluetooth.
The profiles guide developers on how to implement a given end-user functionality using the Bluetooth system.
The Generic Access Profile (GAP) is the base for all defined Bluetooth profiles. GAP ensures that all devices can successfully establish a baseband link. It
defines all the basic functions a Bluetooth device must have, such as procedures for discovering other Bluetooth devices, link management and use of different
security levels.
The profiles released with the Bluetooth specification version 1.1 are called foundation profiles. These are:
- Generic Access Profile
- Serial Port Profile
- Dial Up Networking Profile
- FAX Profile
- Headset Profile
- LAN Access Point Profile
- Generic Object Exchange Profile
- Object Push Profile
- File Transfer Profile
- Synchronization Profile
- Intercom Profile
- The Cordless Telephony Profile
Using profiles ensure interoperability between different devices from different OEMs. You should be able to buy a cell phone from one vendor and a headset from
another and have them working nicely together assuming that both devices implement the headset profile.
New profiles are defined continuosly. Working groups at the Bluetooth SIG define new profiles when needed.
Bluetooth qualification
New Bluetooth products cannot use the Bluetooth brand for marketing purposes before the products have passed the Bluetooth qualification program. This is to ensure interoperability between Bluetooth devices. When a product has passed this qualification program one may use the Bluetooth brand. Consumers can then be sure that the product will work with other qualified Bluetooth products.
Bluetooth security
Security is important when communicating without wires. The Bluetooth specification includes a security model. Both authentication and link encryption is defined in this security model.
This page was last updated 14. Jul. 2006
Comments and feedback are highly appreciated.
You can reach me at: klings (at) nowires (dot) org