Friday, November 23, 2012

Notes - Bluetooth

The following are notes from Computer Networks written by Tanenbaum 5th edition.
  • developed by Ericsson, IBM, Nokia, Intel, and Toshiba in a SIG(Special Interest Group)
  • bluetooth utilizes pairing to securely transfer data between mobile devices
Bluetooth Architecture
  • piconet a master node with up to seven slave nodes
  • multiple piconets can be connected into a scatternet
  • up to 255 parked nodes in a set, master set these devices into a low power state to reduce drain on batteries
  • at its heart is a TDM system
Bluetooth Applications
  • most network protocols provide channels
  • Bluetooth provides applications called profiles
    • 6 are for audio and video
    • human interface profile such as keyboards and mice
    • profiles for networking
      • personal area network profile, ad hoc network or remote access to another network through an AP
    • profiles for higher level info exchange
The Bluetooth Protocol Stack
  • loose group into layers does not follow OSI or TCP/IP or the 802 model
  • layer description here
  • bottom layer is the physical radio layer
  • link control is baseband layer similar to MAC sublayer with some physical layer
  • two protocols to use link control
    • link manager
      • establishment channels, pairing and encryption, QoS
      • below the line implemented on the chip
    • L2CAP(Logical Link Control Adaptation Protocol)
      • frames variable length messages and provides reliability as needed
      • many protocols use this
  • Top layer is where applications are located
  • profiles are vertical boxes because they define a slice of the protocol stack
The Bluetooth Radio Layer
  • radio layer moves bis from master to slave, operating range 10 meters
    • uses adaptive frequency hopping
    • 3 forms of modulation used to send bits
      • frequency shift keying to send a 1 bit symbol every microsecond
      • 2 or 3 bit enhanced rates introduced in 2.0 used for only data portion of frame
The Bluetooth Link Layers
  • link control or baseband layer close to MAC
  • turns the raw bit stream into frames and defines formats
  • piconet defines 625 microsecond time slots
    • frames 1, 3, 5 slots long
    • overhead of 126 bits for access code and header, settling time of 250-260 microseconds per hop to allow for stability
    • 5 slot frame much more efficient than 1 slot frame
  • link manager sets up channels called links using pairing procedure, configured with same PIN(Personal Identification Number)
  • SCO (Synchronous Connection Oriented)
    • used for real time data
    • fixed slot in each direction
  • ACL (Asynchronous Connectionless Link) 
    • used for packet switched data
  • sent over L2CAP layer
    • accepts packets of up to 64KB from upper layers breaks them into frames
    • handles multiplexing/demultiplexing of multiple packet sources
    • handles error control and retransmission
The Bluetooth Frame Structure
  • defines several formats, two types of frames
  • Header field
    • Adress identifies which of the 8 active devices we can go for
    • type identifies ACL, SCO, poll, or null
    • Flow assert by slave when buffer is full
    • ack is for acknowledgements
    • Sequence is to number the frames for transmission, since its stop and wait protocol only need one bit
    • 8 bit checksum
    • entire header repeated 3 times for error correction, if all 3 same received, if not majority option is accepted
  • data field has its own formatting

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