BMAC - Versatile Low Power Media Access for Wireless Sensor Networks

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BMAC - Versatile Low Power Media Access for
Wireless Sensor Networks
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Introduction

• What is BMAC?
• A configurable MAC protocol for WSNs
• Small core
• Factors out higher-level functionality
• Energy efficient
• Goals
• Low Power operation
• Effective collision avoidance
• Simple and predictable
• Small code size and RAM usage
• Tolerable to changing RF/networking conditions
• Scalable to large numbers of nodes

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MAC design approaches

• Traditional (e.g. SMAC)
• Black-box design
• Users pre-configure duty cycle
• Applications rely on S-MAC to adjust its
  operation as things change
• Designed (and optimized) for a set of workloads
  that might not always be applicable
• Minimalistic (BMAC)
• Small core functionality media access control
• RTS/CTS, ACKs, etc are considered higher layer
  functionality (services)
• Applications can turn them on and off
• More flexible and more tunable

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BMAC basics

• Use long preambles (010101010)
• Sleeping nodes will eventually wake up, check the
  channel and detect that it is active (preamble)
  and then begin to listen
• It is required that the preamble be long enough
  so that all nodes will be awake for the data
  transmission.

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BMAC trade-off

• If the time between channel samples is long, then
  the preamble must be very long.
• The preamble must last about as long as the time
  between checking if the channel is busy
• The amount of energy used transmitting long
  preambles depends on how often data is sent.
• E.g., if data must be sent only once a day, it
  might be ok to have a preamble that last an hour,
  since that would allow a very long time between
  checking the channel.
• Letting
• PT be the power used during transmission of a
  preamble
• TP be the duration of the preamble
• R the rate of sending data (pkts/sec)
• Echeck the energy required to check the channel
• Then
• Energy for transmitting
• Energy for receiving
• Energy for forwarding
• Optimal value of TP

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BMAC trade-off

• If the time between channel samples is long, then
  the preamble must be very long.
• The preamble must last about as long as the time
  between checking if the channel is busy
• The amount of energy used transmitting long
  preambles depends on how often data is sent.
• E.g., if data must be sent only once a day, it
  might be ok to have a preamble that last an hour,
  since that would allow a very long time between
  checking the channel.
• Letting
• PT be the power used during transmission of a
  preamble
• TP be the duration of the preamble
• R the rate of sending data (pkts/sec)
• Echeck the energy required to check the channel
• Then
• Energy for transmitting
• Energy for receiving
• Energy for forwarding
• Optimal value of TP

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LPL check interval
Sensor sample period controls the packet rate

• Single-hop application doing periodic data
  sampling
• Sampling rate (traffic pattern) defines optimal
  check interval
• Check interval
• Too small energy wasted on idle listening
• Too large energy wasted on transmissions (long
  preambles)
• In general, its better to have larger preambles
  than to check more often!

Time between listening (checking if the channel
is idle)
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LPL and neighborhood size

• More neighbors more transmissions
• More time spent receiving packets
• Less time left to go to sleep
• To find the best check interval
• Set the reporting rate
• Estimate neighborhood size
• Best result check interval that gives lowest
  effective duty cycle

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Energy vs Latency

• 10-hop network
• Source sends a 100-byte packet every 10 seconds
• SMAC change the default config
• BMAC choose optimal check interval
• SMAC again performs worse...
• Reason sync packets, probability of multiple
  schedules---less time to sleep

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Throughput vs power consumption

• 10 nodes in a neighborhood
• Data must arrive within 10 seconds
• Average power consumption per node
• Low data rates SMAC is better
• Very low duty cycle
• Power vs throughput
• SMAC linear
• BMAC sub-linear
• Reason SMAC duty cycle must increase
• More active periods, more SYNC periods
• BMAC larger preambles at low throughput,
  progressively becoming smaller

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Experimental results throughput

• BMAC is about 4.5 faster than SMAC-unicast
• Not as fast when ACK or RTS/CTS is used
• No combined results...
• Differences less pronounced as of nodes
  increases
• Another issue BMAC has CCA, thus it backs off
  less frequently (and perhaps the backoff timer is
  faster)

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Clear Channel Assessment

• MAC must accurately determine if channel is clear
• Need to tell what is noise and what is a signal
• Ambient noise is prone to environmental changes
• BMAC solution software automatic gain control
• initialization
• Signal strength samples taken when channel is
  assumed to be free so that a noise floor is
  found.
• Check channel
• Samples go in a FIFO queue (sliding window)
• Median of FIFO is added to an EWMA filter. When
  the EWMA filter exceeds a threshold, then it is
  assumed that the channel is active.
• The median filter is known to be robust to
  impulse noise/outliers
• EWMA is not a particularly good way to find a
  change of state.
• A better approach is based on statistical
  change-point detection (see next slide)
• Once noise floor is established, a TX requests
  starts monitoring RSSI from the radio

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Change point detection
If the likelihood ratio is small, then 1. it is
unlikely that what we observed could have occured
and the change, and 2, it is likely that what was
observed occured without the change
likelihood ratio
typical likelihood ratio
typical log-likelihood ratio
But, if the distributions are Gaussian, then this
can be changed into a cummulative summation
if Sk gtThresh, then the change has occurred. This
is the optimal approach in terms of detecting a
change fastest with a given false-alarm rate
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CCA single-sample thresholding vs outlier
detection

• Common approach take single sample, compare to
  noise floor
• Large number of false negatives
• BMAC search for outliers in RSSI
• If a sample has significantly lower energy than
  the noise floor during the sampling period, then
  channel is clear

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CCA results

• 0busy, 1clear
• Packet arrives between 22 and 54 ms
• Single-sample thresholding produces several false
  busy signals

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Low Power Listening

• Goal minimize listen cost
• Principles
• Node periodically wakes up, turns radio on and
  checks channel
• Wakeup time fixed
• Check time variable
• If energy is detected, node powers up in order to
  receive the packet
• Node goes back to sleep
• If a packet is received
• After a timeout
• Preamble length matches channel checking period
• No explicit synchronization required
• Noise floor estimation used to detect channel
  activity during LPL

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Radio powerup sequence of operations

• Goals
• Minimize time radio is on
• Minimize number of times radio gets started
• Minimize sampling time (stage e)

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Conlcusions

• BMAC appears to be better than SMAC
• Easier to tune
• Has better channel assessment
• Doesnt use explicit sync packets
• Doesnt use RTS/CTS/ACK if it doesnt have to
• Is smaller and less complex
• Are large preambles always good?
• Not sure, but
• They do mention cyclic packet transmissions
• They can be turned off
• Enable LPL
• Send first packet, disable LPL
• Send remaining packets in a big burst
• Re-enable LPL