Download - 2011 03 15 Wilhelm Kleiminger Mac
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Versatile Low Power Media Access
for Wireless Sensor Networks
SenSys04
Joseph Polastre, Jason Hill, David Culler
Distributed Systems Seminar: Cyber Physical Systems
Wilhelm Kleiminger, Institute of Pervasive Computing
ETH Zurich
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MAC Basics
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Medium Access Control (MAC)
Nodes share a physical medium Interference
Garbage, useless data,
ALOHA 1971 (ALOHAnet) Collision back off and send later
Slotted ALOHA: Discrete timeslots
37% channel utilisation for slotted ALOHA
Does not scale to many nodes
Kerne Erickson
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IEEE 802.11 CSMA/CA
Channel not full duplex Cannot transmit and receive at the same time
Collision AvoidanceListen before send!
Request to send - Clear to send (RTS-CTS) Only allowed to send after receiving a CTS
Solves hidden terminal problem -- other nodes wait
Idle listening
Node continuously checking if channel is clear
Maximise bandwidth, minimise latency
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CSMA/CA: Collision Avoidance
1. sends a message to2. examines the medium
3. sees s transmissionbacks off (2)
4. Eventually, transmits
A
B
C
D
A B
C
C A
C
Note: In ALOHA,
sends first and then checks
if there was a collision
C
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Hidden Terminal Problem
1. sends a message to2. examines the medium (at the same time)
3. starts transmitting
4. receives data from bothinterference!
A CB
A B
C
C
B
RTS/CTS helps to overcome this as C would not start transmitting without first receiving a Clear-To-Send
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Interlude:
MAC in Wireless Sensor Networks I
CSMA-CA?!
Idle listening =
Nodes run on battery
Usually up for months/years Few transmissions (every second)
Idle listening major component of energy consumption
= ++++
A good MAC protocol reduces
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Interlude:
MAC in Wireless Sensor Networks II
Critical issues Low energy consumption
Scalability
Small code size
Zero configuration
What about?
Low latency High bandwidth
Fairness??
Common problems Overhearing
Idle listening
Infrastructural problems Adverse network conditions
Changing network conditions
Faulty nodes
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B-MAC
Implementation
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B-MAC (Polastre et al. 2004)
Carrier Sense based system With some ALOHA sugar (Preambles)
Only link layer protocol (OSI Layer 2)
Organisation, synchronisation, routing build above No hidden terminal support
No message fragmentation
No enforced power policy
But Interfaces
Allow services to tune B-MACs operation
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B-MAC: Clear channel assessment (CCA)
A node wants to transmit
SleepInsert into
FIFO Queue
10 items
Outliers below
noise floor?
Yes
Exponentially
weighted
average
No
TransmittingA valid packet could never have an outlier
significantly below the noise floor
(Polastre et al. 2004)
Sample Signal
Strength
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B-MAC: Low Power Listening (LPL)
Periodic channel sampling
Transmission of preambles Indicate channel is used
Check every 100ms Preamble length at least 100ms long (overhead)
Node wakes up Radio ON
Node sleeps
Activity?
Receiving packet
Check interval triggered
yes
no
timeout*
packet received
* False positive from CCA
CCA
Link LayerACK
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B-MAC: Low Power Listening (LPL)
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Other approaches
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WiseMAC (El-Hoiydi et al. 2004)
MAC for downlink in infrastructure networks Between nodes and access point
Similar to ALOHA
How it works1. Access point learns sampling schedule of nodes
2. AP can predict when to send data for the nodes to receive it
More (energy) efficient than 802.15.4 / ZigBee
Star network topology (no multi hop)
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S-MAC (Heidemann et al. 2002)
MAC Protocol for multi hop sensor networks
How it works Node wakes up (periodically)
listens to the channel (active period) 115ms
SYNC (Synchronising all nodes to a common schedule) Transmission using RTS-CTS
returns to sleep variable time
Duty cycle determined by sleep period
Network Allocation Vector Knowledge about length of transmission = How long can I nap?
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S-MAC: Problems
Protocol is complex
SYNCs can be a problem in a larger network
Higher maintenance cost (neighbours schedules)
Overhead (repeated rounds of synchronisation)
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B-MAC
Results
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LPL check time vs lifetime
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B-MAC in comparison
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Number of hops vs latency
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B-MAC - Recap
Link Layer MAC Protocol (with ACKs) Low Power Listening
Clear Channel Assessment
Modular approach Control interfaces
Already implemented in tinyOS
Free to be used/modified/extended
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References
Polastre J, Hill J, Culler D. Versatile Low Power MediaAccess for Wireless Sensor Networks. 2004.
El-Hoiydi A, Decotignie J-D. WiseMAC: An Ultra Low
Power MAC Protocol for the Downlink of Infrastructure
Wireless Sensor Networks. 2004. Heidemann J, Estrin D. An Energy-efficient MAC Protocol
for Wireless Sensor Networks. 2002.