Distributed Topology Construction of Bluetooth PANs

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Distributed Topology Construction of Bluetooth
PANs

• Theodoros Salonidis, Pravin Bhagwat
• Presented by Sudarshan Srinivasan

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Introduction

• Bluetooth wireless technology for establishing
  ad hoc networks between devices such as cell
  phones, PDAs, identification badges and cameras.
• Allows for low-power low-bandwidth short-range
  communication.

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Introduction - 2
Bluetooth does not use a broadcast mechanism of
communication like 802.11 It divides the medium
into a number of channels. How?
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Frequency Hopping

• Bluetooth uses frequency hopping in the physical
  layer.

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Advantages of Frequency Hopping

• Advantages
• Can operate in noisy radio frequency envts.

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Advantages of Frequency Hopping-2

• Allows a number of devices to coexist in a small
  area and form independent Personal Area Networks

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Challenges Because of FH

• Devices within communication range of each other
  need to synchronize their frequency hopping
  patterns in order to communicate.
• Interconnection of devices should be efficient
  and should result in fully connected networks.

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Bluetooth Terminology

• Scatternet A mobile ad hoc network formed by
  devices that wish to communicate with each other.
• Piconet A unit in a scatternet which consists
  of 1 master and several slaves using the same
  FHS.
• Master A node which determines the FHS and
  controls access of other nodes to the shared
  medium.

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Bluetooth Terminology 2

• Slave A node in the piconet which synchronizes
  with the master and has its access to the medium
  shared by the master
• Bridge A node which connects two piconets and
  transfers packets across piconets (usually points
  of bottleneck in communication)

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Different Possible Configurations

• For a given physical distribution of devices, the
  devices can be connected into different
  scatternets with different properties.
• Some useful properties network diameter, bridge
  connectivity, devices per piconet, routing
  complexity.

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Different Configurations
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Different Scatternet Formation Algorithms

• BTCP Salonidis, Bhagwat et al UMD first
  attempt
• Bluetooth Scatternet Formation Algorithm Ching
  Law and Kai-Yeung Siu MIT - dynamic network
  conditions
• Algorithm contructing scatternets with a tree
  structure Godfrey Tan, Hari Balakrishnan - MIT

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Goals of Scatternet Formation Algorithms

• Connectedness
• Efficiency of communication
• Avoiding overloading of bridges Opposing
• Reducing network diameter goals
• Minimize number of piconets

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Asymmetric Link Formation Protocol

• Two roles sender, receiver
• The sender goes through two phases
• Inquiry Sender discovers receivers within
  communication range
• Paging Sender establishes a link with the
  receivers
• Corresponding Scan states for the receiver

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Asymmetric Link Formation Inquiry Phase

• Sender is in Inquiry phase. Receiver is in
  Inquiry Scan phase.
• Receiver listens on a broadcast channel
  (frequency sequence). Shifts slowly between
  frequencies so that receiver can catch up.
• Sender switches faster between frequencies.

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Asymmetric Link Formation Inquiry Phase

• 111111222222333333444444111111
• 334411223344...
• ------
• Frequency Synchronization delay
• Sender eventually catches up with receiver and
  sends it an IAC pkt. Receiver backs off for
  random interval listens again. Again after FS,
  sender catches up.

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Asymmetric Link Formation Inquiry Phase - 2

• Sender sends the IAC packet again. Receiver
  responds with FHS packet contains receivers
  address (for deriving DAC), receivers clock
  value
• Receiver enters Page Scan mode. Sender enters
  Page mode

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Asymmetric Link Formation Paging Phase

• Sends a DAC packet on receivers listening
  frequency.
• Receiver responds with a DAC informs it that
  its ready.

Are you ready?
Yup! Go on
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Asymmetric Link Formation Paging Phase

• Sender sends FHS packet.
• Receiver uses this to find the masters FHS and
  becomes its slave. Acks FHS with DAC.
• Total time 2 FS RB
• FS and RB are random variables

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Problem with Asymmetric Protocol

• Requires a priori determination of roles sender
  and receiver.
• This may not be possible in an environment where
  devices join dynamically.

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Symmetric Link Formation Protocol

• Every device alternates between listener and
  sender roles (inquiry and inquiry scan), spending
  a random amount of time in each role.
• Two devices can communicate during overlapping
  opposite phases.

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Estimating Setup Delay

• Determine cdf and pdf of the merged schedule
  process X given that the two nodes alternate
  independently according to an identical
  distribution Z.

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BTCP

• Reasonable set of goals
• Algorithm works in three stages
• Distributed coordinator election
• Role determination
• Actual connection

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BTCP Coordinator Election

• By a voting process
• Each node has a VOTES variable initially set to
  1, alternates between INQUIRY and INQUIRY SCAN.
• When two nodes discover each other, they compare
  VOTES. The one with the higher count wins. If the
  counts are equal, the tie is broken based on
  Bluetooth address.

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BTCP Coordinator Election -2

• The winner gets all the FHS packets that the
  loser had won, and the losers FHS.
• The loser removes all FHSs and enters the PAGE
  SCAN mode.
• Finally, the last man standing becomes the
  coordinator.
• PROBLEM!!! How long should the last node try to
  connect to other nodes before deciding that it is
  the coordinator?

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BTCP Role Determintion

• The elected coordinator chooses P (including
  itself) out of the N nodes in the system to
  become masters, where P is

36 - Magic number ??
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BTCP Role Determination

• Coordinator has the FHSs of the to-be masters.
• It prepares a SlaveList and BridgeList for each
  master list of FHSs of slaves and bridges for
  each master.
• It then forms a temporary piconet with the chosen
  masters and sends them the lists.

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BTCP Actual Connection Establishment

• Each new master pages the slaves in the SLAVELIST
  and establishes a connection with them.
• When a node which is designated a bridge receives
  a page message from one of its masters, it waits
  for the second one before sending them both
  CONNECTED mesgs.

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Performance of BTCP

• Two metrics
• Connection delay
• Probability of protocol correctness
• The two are somewhat opposing a large
  ALT_TIMEOUT will result in a large connection
  delay but better correctness, and vice versa

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Performance of BTCP

• Protocol timeout efficiency (correctness
  criterion) first increases with timeout interval
  and then attains steady state.
• Tactual Tideal ALT_TIMEOUT

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Assumptions made by BTCP

• All nodes are within communication range of each
  other (workaround suggested, not followed up)
• There are at most 36 devices that wish to come
  together in one scatternet
• Once the scatternet is formed, there are no
  additions or removals in the network

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Cons of the Protocol

• Centralized control after coordinator election
• Difficult to determine ALT_TIMEOUT time after
  which a node assumes it is the coordinator. It is
  set when voting begins, and is reset when each
  time it wins a 1-to-1 confrontation

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Conclusions

• First attempt at topology creation for Bluetooth
  networks
• Addressed a novel problem