Transcript Versatile Low Power Media Access for Wireless Sensor Networks

B-MAC: Versatile Low Power
Media Access for Wireless
Sensor Networks
SenSys ’04
Reminder: Proposal Presentation
on Monday, March 5
• Prepare powerpoint slides to motivate your project
and show the overall approach to tackling the problem
• Come at least 10 minutes before the class begins
– Give your slides to the tech staff
– Lean how to use the presentation tool etc.
• Teams & presentation order:
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Mike & Vic (+ Brian)
Chris & Yan
Dahee & Chao
Surabh
Mehmet
Design goals of B-MAC
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Low power operation
Effective collision
Simple implementation, small code & RAM size
Efficient channel utilization at low & high
data rates
• Reconfigurable by network protocols
• Tolerant to changing RF/Networking
conditions
• Scalable to large numbers of nodes
Small, configurable MAC
• Export control to higher services to support
wide variety of WSN workloads
– WSNs are supposed to support various applications
• S-MAC (discussed in the previous class) is
more than a link layer protocol
– Drawbacks of S-MAC
• Scalability: A node may have to remember many schedules
and wake up accordingly
• MAC layer may not be the best place for sleep scheduling
& synchronization
Adaptive, reconfigurable MAC
• Adaptive bidirectional interface for
WSN applications
– Reconfigure the MAC protocol based on the
current workload
– Identify the best parameters for an
arbitrary low power WSN applications at
compile or run time & estimate the
application’s lifetime
B-MAC Design
• Really simple
– CSMA via CCA (Clear Channel Assessment)
& backoff
– Low power listening via Preamble
– Acknowledgment
• Enable/disable anything above and allow
to build anything on top of the
configured B-MAC!
B-MAC Interfaces
Clear Channel Assessment
• Effective collision avoidance
• Find out whether the channel is idle
– If too pessimistic: waste bandwidth
– If too optimistic: more collisions
• Key observation
– Ambient noise may change significantly depending
on the environment
– Packet reception has fairly constant channel
energy
• Software approach to estimating the noise
floor
• Take a signal sample when the channel is assumed to
be free
– Right after a packet is transmitted or when no valid data is
received
– Take exponential moving average (EMA) of the median signal
strength
• Works as a low pass filter
• Smoothed idle signal level Sm(t) = a * S(t) + (1 - a) * Sm(t-1)
– Sm(t): EMA at time t
– S(t-1): Signal strength of ambient noise at t
– Sm(t-1): EMA at time t-1
– It contrasts to common threshold-based methods in which
only a single sample is taken
– Resilient to time-varying ambient noise
CCA vs. Threshold techniques
Idle
CCA dynamically
adjusts threshold
CCA finds channel
busy/idle status with
high accuracy
• Threshold: waste channel utilization
• CCA: Fully utilize the channel since a valid
packet could have no outlier significantly below
the noise floor
• CCA can be turned on/off
– If turned off, a schedule-based protocol, e.g., SMAC, can be implemented atop B-MAC
• If turned on, initial channel backoff when
sending a message
• B-MAC does not set the backoff time, but
signals an event to the higher service that
sent the packet
• The higer level service may return an initial
backoff time or ignore the event
– If ignored, use a short random delay
Low Power Listening: Preamble Sampling
• Preamble is not a packet but a physical layer RF pulse
– Minimize overhead
Sender
Preamble
Send data
S
R
Receiver
Preamble sampling
Active to receive a message
|Preamble| ≥ Sampling period
Optional link layer ACK
• If enabled, ACK is sent immediately after receiving a
unicast packet
• Overall, B-MAC is easier to implement than S-MAC
– No RTS/CTS
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Is this always good?  or ?
We know RTS/CTS can reduce hidden/exposed node problem
You may have to implement RTS/CTS on your own...
Simple but not very friendly
– No synchronization
• No need for a schedule table in S-MAC
• But periodic sleep & wake-up is a good approach to energy saving
Modeling Lifetime
• Monitoring applications
• E = Esleep + Elisten + Ed + Erx + Etx
• Given #nodes in the neighborhood, BMAC can
estimate the network lifetime
• Lifetime tl = 1/E * Cbatt * V * 60 * 60
Derivation of Lifetime
• Ed = td * cdata * V where td = tdata * r
• Etx = ttx * ctxb * V where ttx = r * (Lpreamble + Lpacket) *
ttxb
• Erx = trx * crxb * V where trx ≤ n * r * (Lpreamble + Lpacket)
* trxb
– r * sum_i=1^n ( children (i) + 1 )
– Lpreamble ≥ ti / trxb
Derivation of Lifetime (Cont’d)
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Esample = 17.3 uJ
Elisten ≤ Esample * 1/ti
tlisten = (tr_init + tr_on + trx/tx + tsr) * 1/ti
tsleep = 1 – trx – ttx – td – tlisten
Esleep = tsleep * csleep * V
Lifetime tl = 1/E * Cbatt * V * 60 * 60
Network Parameters
• Scientists may determine the physical location of the nodes &
ideal sampling rate
– Compute the parameters to get the best lifetime that BMAC can achieve
– Best LPL check interval is the lowest line at a given network
density in the following graph
If n=60, 25ms is
best
If n = 20, 50ms check
interval is optimal
Experiments
• Compare BMAC to S-MAC & T-MAC
– T-MAC is similar to S-MAC, but a receiver
goes to sleep if it does not receive any
message
– B-MAC & S-MAC: implemented in TinyOS
– TMAC: simulated in Matlab
TinyOS Implementation
• B-MAC does not need timestamp
Packet transmission time vs.
Checking frequency
• More frequent checking of the radio
– Shorter transmission time
– More energy consumption
LPL check interval
Channel utilization
• Place n nodes equidistant from a receiver
• Increase n to increase load
• BMAC relies on higher level services to send data
the traffic pattern
according to
• Consider hidden node problem too
• For example, after a packet is sent to the parent, nodes in the same
cell wait for a certain amount of time for the parent to forward the
packet up the tree Always work? More difficulty for developing
sensing applications?
Energy per byte
End-to-end latency
Contention-Free Protocols
Classic Protocols
• TDMA (Time Division Multiple Access)
– A node can sleep when it is not its turn to
send or receive
• FDMA (Frequency Division Multiple
Access)
• CDMA (Code Division Multiple Access)
• No node within two hops can use the
same slot to avoid the hidden node
problem
Optimal channel assignment
• Achieve contention-free communication
using the minimum number of channels
• The problem of assigning a minimum
number of channels for an arbitrary
graph is NP-hard
– Develop efficient heuristics
– Centralized approaches do not scale
Stationary MAC and Startup
• Local synchronization only
• Starting phase
– Handshaking on a common control channel
– Each link utilizes a unique random
frequency or CDMA frequency hopping code
– Assume there are sufficiently many
frequencies or codes
– Periodically use the slot
BFS/DFS-based scheduling
• Breadth-first or depth-first traversals
of a data gathering tree
• Every single node is given a slot
• BFS might provide more chances for
aggregation
• DFS may transmit individual data more
quickly
• Global synchronization required
Reservation-based synchornized
MAC (ReSync)
• TDMA is not flexible enough to allow the traffic from
each node to change over time
• ReSync provides more flexibility
• Each node maintains an epoch based on its local time
(or can be synchronized with nearby neighbors)
– Select a regular time in each epoch to send a short intent
message
• Probability of collisions is low since the intent is very short
– Listen long enough to learn when the neighbor is sending the
intent
– The intended receiver wakes up at the corresponding time to
receive the message
– No RTS/CTS
• Data transmissions are scheduled randomly
Traffic-adaptive medium access
(TRAMA)
• Distributed TDMA for flexible &
dynamic scheduling of time slots
• Divide time epochs into a set of short
signaling slots followed by a set of
longer transmission slots
• Key components
– Neighbor protocol (NP)
– Schedule exchange protocol (SEP)
– Adaptive election algorithm (AEP)
Neighbor Protocol
• Nodes exchange one-hop neighbor info
during the random access signaling slots
– Ensure the slots are long enough to allow all
nodes to get consistent two-hop neighbor
info
Schedule exchange protocol
• Each node publishes its schedule during
the last winning slot in each epoch
• Use bitmaps to indicate the intended
unicast or multicast recipients
• Sleep when not required to transmit or
receive
Adaptive election algorithm
• Hash function based on node IDs and
time
– Ensure there’s a unique ordering of node
priorities within any two-hop region at each
time
– A node transmits iff it has the highest
priority among the two hop neighbors at
the moment
– Sophisticated slot reuse protocol
Summary
• MAC protocols in WSNs
– Arbitration for access to the wireless channel
– Energy conservation
• B-MAC is a nice building block for diverse applications and
workloads
– TDMA, for example, can be built on top of it
• Energy savings
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Preamble
S-MAC like periodic sleep & wake-up
Adaptive listening in S-MAC & T-MAC
D-MAC & DESS try to minimize the E2E delay, while using sleep
modes
• TDMA
– No idle listening
– Higher complexity distributed algorithms or tight synchronization
required
Questions?