Robcast: A singlehop reliable broadcast protocol for WSNs

1
Robcast A singlehop reliable broadcast protocol
for WSNs

• Murat Demirbas
• Srivats Balachandran
• University at Buffalo, SUNY

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Singlehop reliable(?) broadcast

• Reliable broadcast is important
• Safety (consistency) reasons Sensor/actuator
  devices coordinating regulator valves should take
  consistent decisions to prevent a malfunction
• Performance (goodput) reasons Unreliable
  broadcast wastes the bandwidth
• Hidden terminal problem may account for loss of
  50 of traffic
• Hidden terminal problem occurs when two senders
  are not in range of each other (carrier sensing
  fails), but are in range of a common receiver

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RTS-CTS achieves reliable unicast

• Sender transmits a Request to Send (RTS)
• Destination node replies with a Clear To Send
  (CTS)
• Any other node receiving the CTS frame should
  refrain from sending data for a given time
  (solves hidden terminal)
• RTS/CTS provides virtual carrier sensing

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RTS-CTS fails for broadcast

• Direct application sacrifices reliability
• CTSs collide at the initiator making it
  impossible to know if all neighbors are ready to
  receive
• CTS from any neighbor is used as sufficient for
  data transmission (BSMA)
• Reliable application sacrifices efficiency
• Individual RTS-CTS handshake is performed with
  each neighbor before data is transmitted (BMMM)

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Our contribution Robcast

• Robcast addresses the singlehop broadcast problem
  both efficiently and reliably
• Robcast uses bandwidth efficiently
• Bandwidth is not statically allocated among nodes
  as in TDMA
• Robcast is an on-demand access protocol, nodes
  contend for the channel
• Robcast is useful for standardization efforts in
  WSNs
• SP focuses on singlehop broadcast as narrow-waist
  for standardization
• Two main ideas are
• Collision detection at the receiver-side is
  employed for feedback collection
• Although positive feedback do not compose,
  negative feedback compose

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Receiver-side collision detection (RCD)

• Carrier sensing at the transmitter-side is
  impossible
• Carrier sensing at the receiver-side is feasible
  and is widely used for CSMA
• RCD is achievable using the same basic techniques
  as CSMA
• Channel activity can be differentiated from noise

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RCD using CC2420 radio

• Received Signal Strength Indicator (RSSI) based
  collision detection
• requires additional processing on the processor,
  unreliable
• Cyclic Redundancy Check (CRC) based collision
  detection
• reliable, but only works when preamble and packet
  frame are received
• Clear Channel Assessment (CCA) based collision
  detection
• can detect collisions even when the preambles are
  not detected
• CCA signal is generated only after a transmission
  is scheduled to the radio, so we manipulate the
  radio to perform CCA also in the idle state

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Robcast protocol

• Nodes are synchronized to maintain rounds with 3
  phases
• This structure facilitates RCD-based feedback
  collection by providing context
• A collision at the RTS phase means multiple
  initiators are trying to transmit
• A collision at the NCTS phase means multiple
  nodes are reporting conflicts
• Robcast avoids any collisions in the DATA phase
• Positive feedback (CTS) do not compose, yet
  negative feedback (NCTS) do compose nicely

j
RTS
NCTS
DATA
k
l
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Robcast protocol

• INITIATOR
• Bcast (RTS)
• Listen for NCTS or collision
• If no NCTS or collision
• Bcast (DATA)
• Else backoff for some rounds

• RECEIVERS
• Listen for RTS or collision
• If collision Bcast(NCTS)
• Listen for DATA if any

RTS
NCTS
DATA
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Robcast protocol

• Scenario 1 success

• Scenario 2 deferral

A
C
B
A
C
B
Phase DATA
Phase NCTS
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Extensions

• Reducing idle listening
• Sleep through the rest of the round if no
  activity is scheduled
• Data phase may account for around 90 of the
  total time
• Multiple round-length packets
• Explained in the paper

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Our previous work on BEMA

• BTMA
• Use a control channel for signaling busy for
  ongoing transmissions
• Separate radio infeasible for WSNs
• BEMA
• Synchronized rounds, leader is elected based on
  duration of BUSY signals
• Double power transmissions during CONTROL phase
• Locked nodes always transmit BUSY signals
• Requires a more finer-granularity RCD
• Eliminates hidden terminals
• DATA collisions still occur due to equal length
  of BUSY signal
• Susceptible to obstacle arrangements

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Simulations

• Prowler
• A MATLAB-based, event driven simulator for WSNs
• Simulates a Rician fading model
• Setup
• 5x5 grid of motes
• With varying number of nodes transmitting data

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Goodput
Goodput data/ settling time
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Loss of messages
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Energy consumption
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Concluding remarks

• Reliable broadcast is significant for many
  applications
• Robcast achieves lightweight reliable broadcast
  assuming synchronized rounds
• RCD is used for singlehop collaborative feedback
  collection
• Future work is to investigate synchronization
  free solutions to reliable broadcast

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www.cse.buffalo.edu/demirbas
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Our ongoing workLightweight singlehop
collaboration

• The idea is to use receiver-side collision
  detection (RCD) for lightweight singlehop
  collaborative feedback
• Applications in filtering false-positives, leader
  election, clustering, etc
• Pollcast Does P hold for the neighborhood?
• Nodes where P holds answer simultaneously
• Initiator uses RCD to detect whether there are
  answers
• Initiator can followup with a broadcast

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Generalization to counting

• Initiator uses pollcasts to approximate of
  nodes P holds
• Each node has a 0.5 probability of voting at each
  round they only vote once in their lifetime
• Expected number of voters is halved in each round
• Expected number of such rounds for n nodes is
  logn
• Bayesian inference is useful for categorizing the
  result
• Applications in in-network classification of
  intruders soldier, car, tank

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Assurance ?

• poll vote is enough for best-effort primitives,
  but how do we achieve reliable (or eventually
  reliable) primitives
• in the absence of synchronization, how do we
  decide whether poll is heard by all nodes in the
  neighborhood ?
• poll veto vote approach where a veto is sent
  following any RCD leads to veto storms
• channel reservation based approaches may be
  useful for reliability
• Or, is it better to use the lightweight primitive
  and build assurance on top of this primitive ?
• if you stand back fast enough, no one notices you
  fell down