David Campbell

Bluetooth

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

            Bluetooth is the name given to a new technology using short-range radio links, intended to replace the cable(s) connecting portable and/or fixed electronic devices.  In essence, Bluetooth is taking cables that connect one device to another.  Bluetooth got its name from Harald Blaatand “Bluetooth” II.

            This new technology is intended to make our lives simpler by allowing our devices to work together.  Bluetooth is an always-on, short-range radio initially developed by Swedish mobile-phone maker Ericsson in 1994 as a way to let laptop computers make calls over a mobile phone.    

Bluetooth radio is using a hopping scheme to make the link from one device to another.  This process is called frequency hopping in which the spectrum is accomplished by spreading in 79 hops displaced by 1 MHz, starting at 2.402 GHz and finishing at 2.480 GHz. 

Slave #2

 

Slave #3

 

Master

 

Slave #1

 
Two or more Bluetooth devices using the same channel form a piconet.  Every device is in standby mode before a piconet connection is made. In each piconet, there is one master and one or more slaves.  The hopping sequence is unique in the piconet; the phase in the hopping sequence is determined by the Bluetooth clock of the master.  The device that sends out the transmission signal automatically becomes the master.  Once a radio joins a piconet, it is assigned a three-bit Active Member Address this allows more radios on the piconet to address it. After a piconet has eight active radios the master puts one of the active radios in park. The parked radio stays connected with group for eight-bit Passive Member Address.  The combination of the two member groups allows for up to two hundred and fifty six radios.


            A scatternet is when two piconets have overlapping coverage.  The two piconets operate on their own frequencies and the ones that overlap use time division multiplexing. 

The transmitter characteristics of Bluetooth are classified into three power classes: 1, 2, and 3, which are based on a nominal antenna power of 0dBm.  Power class 1 is designed for long range devices (100m) that have a max output power of 20 dBm.  Power Class 2 id designed for ordinary range (10m), with a max output power of 4 dBm.  Last, Power Class 3 is designed for short range devices (10cm), with a max output power of 0 dBm.  Sniff mode is when a master can communicate with devices during specific sniff designated areas.  A slave that does not receive asynchronous packets and listens only to determine if it should become active again is known as being in a state of hold. 

            Bluetooth is divided into layers which are: Radio, Baseband, LMP, HCI, L2CAP, RFCOMM, and SDP.  The radio layer defines the requirements for a Bluetooth transceiver operating system in 2.4 GHz ISM band.  The Baseband layer describes the specification of the Bluetooth Link Controller which carries out the baseband protocols and other low-level link routines.  The Baseband layer is responsible for the synchronization and transmission between different Bluetooth units in a piconet.  The Link Manager Protocol (LMP) is used by the Link Managers for link setup and control.  The Host Controller Interface (HCI) provides a command interface to the Baseband Link Controller and Link Manager, and accessto hardware status and control registers.  Logical Link Control and Adaptation Protocol (L2CAP) supports higher level protocol multiplexing, packet segmentation and reassembly and conveying of quality of service information.  The RFCOMM protocol provides emulation of serial ports over the L2CAP protocol.  It is based on the ETSI standard TS 07.10.  The Service Discovery Protocol (SDP) provides a means for applications to discover which services are provided by or available through Bluetooth device.  It also allows applications to determine the characteristics of those available services.  SDP provides a way to determine what Bluetooth services are available to a device.  A SDP service can provide information, perform an action, or control a resource. Each service belongs to a service class which defines all attributes by id. 

            All Bluetooth devices must support the generic access profile (GAP) that defines device discovery connection procedures, and security levels.

            As far as security for the Bluetooth, Authentication, Encryption, and Key Management are required. Authentication involves the user providing a (PIN) Personal Identification Number that is translated into a 128-bit link key that can be authenticated in a one or two way direction.  

            The original idea for Bluetooth was to replace cords and wires, and transfer the information to the computer or phone.  The originators soon realized that more was possible with Bluetooth; if information can be transmitted between a computer and printer why not transmit data from a mobile phone to a printer, or even a printer to a printer?

References

Bluetooth Official Info Site (www.bluetooth.com)

IBM (www.research.ibm.com/Bluetooth)