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An Energy Efficient MAC Protocol for Wireless Sensor Networks “S-MAC”Wei Ye, John Heidemann, Deborah Estrin
Presentation: Deniz ÇokusluMay 2008
Outline Motivation S-MAC Experiments Results Conclusion Questions
Motivation WSN’s are battery operated, disposable(!) devices Energy consumption is an important issue Communication dominates most of the energy
consumption Medium Access is very critical
Motivation Medium Access Control (MAC)
Allows accessing transmission media Should avoid collisions
TDMA Reserves time slices to nodes (Time Synchronization)
CDMA Transmission is coded into signals (Extra computation)
FDMA Nodes are assigned to different frequencies (Complex
Xmitters) *CSMA
Media is shared (Collisions)
Motivation Contention-based protocols
MACA + RTS/CTS are used - Hidden & Exposed Terminal Problems
T-MAC + RTS/CTS, TimeOut - Early Sleeping
Preamble sampling Periodic sleeping, Long Preamble to wake up receiver
B-MAC Low Power Listening
PAMAS Use of Control Channel (RTS/CTS/Busy Tone)
Motivation IEEE802.11
RTS/CTS Fragmentation support CTS only reserves medium for the first fragment ACK of the first fragment reserves medium for the
second and so forth.
Overhearing problems Long delays because of the fragmentation faults Idle listening
Challenges *Energy efficiency *Scalability *Fairness Latency Throughput Bandwidth utilization
Sources of Energy Waste in WSNs Collisions
Causes retransmission of packets Overhearing
Receiving others’ packets Control packet overheads
Forms some part of the transmission Idle listening
Keeping the receiver powered on while the node is idle
Consumes %50 - %100 of energy required for receiving
S-MAC
Contributions of S-MAC Periodic listen/sleep
Nodes listens and sleep periodically Reduces idle listening
Collision and overhearing avoidance Uses RTS/CTS and carrier sense Puts a node into sleep if its neighbors are communicating Eliminates overhearing problem
Message passing Divides the message into fragments and send them in burst Reduces control overhead
Periodic Listen / Sleep Sleep some time
Turn off the radio Set the timer
Wake up Listen to see if there is a request for
communication
Requires periodic synch among neighbors Relative timestamps Small time slots
Periodic Listen / Sleep Synchronization
Neighbor nodes exchange their sleep/listen schedules
Two neighbor nodes may have different schedules
All nodes know their neighbors’ schedule When a node wants to send a message, it waits
until receiver wakes up If multiple nodes want to send, they contend the
media using RTS/CTS
Periodic Listen / Sleep Synchronization
Each node maintains a schedule table Choosing schedule
Listens if any neighbor broadcast a schedule If it receives a schedule, it uses as its own schedule
(follower node) and broadcasts it as a SYNC message If no schedule is received, choose the schedule
randomly and broadcasts it as a SYNC message (synchronizer node)
If the node receives a schedule after selecting its own, it uses both (multiple schedule)
Periodic Listen / Sleep Maintaining Synchronization
Periodically send SYNC packets SYNC packets are small, and includes sender’s next
sleep time Listen interval is divided into two:
Listening SYNC Listening RTS
If a node wants to send a SYNC or Data Wait until receiver wakes Start carrier sense and sense till random time If no xmission, send RTS and then data
Collision and Overhearing Avoidance Collision Avoidance
Carrier sense, RTS and CTS are used There is a “duration” field in all packets If a node receives a packet destined to another
node, it records the duration into Network Allocation Vector (NAV)
This value is decremented when the NAV timer is fired
If this value is greater than 0, it means that the medium is busy (virtual carrier sense)
Collision and Overhearing Avoidance Collision Avoidance
Physical carrier sense is performed at the physical layer
If both physical and virtual carrier sense indicates that medium is free than node uses the medium
Message Passing Deals with transfering long messages
efficiently Options
Long Messages: + Low control overhead - High cost of retransmitting in case of errors
Fragmented Messages: + Low cost of retransmission - High control overhead
Message Passing S-MAC Approach:
Small fragments of messages Cure to control overhead:
Send fragments burstly Use only one RTS/CTS packet for the whole message All fragments are confirmed by ACK messages
If any fragment is failed: Extend the reserved transmission time Retransmit the current fragment Hidden terminals learn the extension from the ACK packets Extensions are limited to a pre-defined number
S-MAC Trade-offs Delays inherent to multi-hop contention
based network protocols (i.e. IEEE802.11) Carrier sense delay Backoff delay Transmission delay Propagation delay Processing delay Queueing delay
Additional delay of S-MAC Sleep delay
S-MAC Trade-offs Avg sleep delay
A cycle consisting of sleep and listen is called a frame
Tframe = Tlisten + Tsleep
Ds = Tframe / 2
Energy saving
Es =Tsleep / Tframe
Sleep SleepListen
Frame
Implementation Rene Motes are used
Transmission rate: 19,2 Kbps Energy consumption:
Receiving/Idle: 4,5 mA Transmitting: 12 mA Sleeping: 5 μA
Motes use TinyOS Standard Packets in TinyOS
Header: 6B Payload: 30B CRC: 2B Total: 38B
Implementation Additional packet type: Control Packets
(RTS, CTS, ACK) Header: 6B Payload: NA (Benefit: 30 Bytes) CRC: 2B Total: 8B
Implementation 3 MAC Modules are implemented for
comparison Simplified IEEE802.11 DCF Message Passing with overhearing avoidance S-MAC
Implementation Simplified IEEE802.11
Carrier Sense Random duration
Backoff and retry Random backoff (unlike standard IEEE802.11
Exponential) RTS/CTS/DATA/ACK packet exchange Fragmentation support (Burst mode) Nodes are either in listen/receive or transmitting
mode
Implementation Message Passing with overhearing
avoidance Reduces control overhead Eliminates overhearing No periodic sleep, therefore no additional delay to
IEEE802.11 Node goes to sleep if its neighbors are
communicating
Implementation S-MAC
Listen time: 300 mSec Sleep time: 1000 mSec Schedule update (SYNC): 10 frames (13 sec)
Experiments Used Scenario:
Messages:A C DB C E
Energy consumption is measured Inter arrival period of messages varies between 1s – 10s
Ex. If period is 5s then a message is generated in every 5 sec by each source node
Experiments Used Scenario:
Each source generates 10 messages Each message consists of 10 fragments Total of 200 packets are exchanged in each
experiment In the higher traffic scenario inter-arrival: 1s, the
wireless media is fully utilized
Experiments Calculations of energy consumption:
Time to pass packets is measured Percentage of Listen – Sleep is measured Consumption is calculated by multiplying the time
with the required energy Receive/Listen: R = 13,5 mW Transmit: T = 24,75 mW Sleep: S = 15 μW
Total = (R * Tlisten) + (T * Ttransmit) + (S * Tsleep)
Experiments
Measured energy consumption in the source nodes
Experiments
Measured percentage of time that the source nodes in the sleep mode
Experiments
Measured energy consumption in the intermediate node
Conclusions S-MAC is an energy efficient mac protocol
compared to IEEE802.11 Trade-off between energy consumption and
latency can be easily tuned Experiments show that the aimed results are
satisfied
Acknowledgements This work is supported by:
NSF under grant ANI-9979457 as the SCOWR project (http://robotics.usc.edu/projects/scowr/)
DARPA under grant DABT63-99-1-0011 as the SCADDS project (http://www.isi.edu/scadds/) and under contract N66001-00-C-8066 as the SAMAN project (http://www.isi.edu/saman/) via the Space and Naval Warfare Systems Center San Diego
Questions ...