Bluetooth frame
Bluetooth communication starts with the identification of the devices that want to communicate with each other. With Bluetooth, this identification takes place within two seconds and is based on the exchange of the unique 48-bitBluetooth address. After that, the two devices exchange all the Bluetooth profiles they support with each other. The services and communication types emerge from the Bluetooth profiles. In addition, both communication partners agree on the transmission security and send the required parameters.
Data transmission is packet-oriented with firmly structured data packets consisting of three data fields. The first data field contains the access code, a 68 to 72 bit data field for the access code. It consists of a 4-bit preamble, the 64-bit sync word and the 4-bit trailer. Bluetooth distinguishes between the Channel Access Code( CAC), the Device Access Code( DAC) and the Inquiry Access Code ( IAC). The Access Code is followed by the 54-bit header. It consists of 18 bits that are sent three times for security reasons. The header is followed by the payload, the user data, which has a length of up to 2,745 bits.
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Later developments have added two more data fields between the header and the payload to increase data throughput using Enhanced Data Rate( EDR): The Guard data field and the Sync data field. This EDR data frame is terminated with the trailer.
The individual data fields are not transmitted using the same transmission technique. For example, Bluetooth always transmits the data fields for the access code and the header in Gaussian Frequency Shift Keying( GFSK), while the useful and synchronization data are transmitted in Differential Phase Shift Keying( DPSK) or Differential Quaternary Phase Shift Keying ( DQPSK), depending on the signal-to- noise ratio ( SNR).
In addition to data exchange, Bluetooth also offers real-timevoice communication. The connection-oriented SCO method used for this, Synchronous Connection Oriented Link (SCO), works with time slots and separate channels.