robin kravets, 2009 energy conservation in wireless networks
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© R
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Energy Conservation in Wireless Networks
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Power is Everything
Mobile devices are power constrained
Battery powered May be hard to replace or
replenish power Desire long operational
time
How to reduce energy consumption?
Energy Consumption in Wireless Networks
Data communication Transmission of data packets from source to destination
Overhead Data transmission
Have to do this! MAC protocols
Effects all communication Control Packets
Ex: RTS, CTS, ACK Idle devices
Wireless nodes consume energy even if they are not actively transmitting data MAC protocol constantly listens to the channel
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Control Overhead
Medium Access Control Protocol Channel access Contention resolution Ex: IEEE 802.11
Channel acquisition RTS/CTS handshake
medium busy
DIFSDIFScontention window(randomized back-offmechanism)
slot timedirect access if medium is free DIFS
RTS CTS DATA
Sense medium
time
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Device Energy Costs
Base card power Pbase Cost of running the card Fixed, defined by the card
Base transmission power Pxmit Processing costs for sending Fixed, defined by the card
Base reception power Precv Processing costs for reception Fixed, defined by card
Transmit power level Pt Cost for driving the transmitter Tunable
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Device Energy Costs
Idle-time energy consumption Base card costs Time node spends idle Typically node is listening to
the idle channel
Main target of many energy conservation techniques
Idle time
Pba
se
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Device Energy Costs
Transmission-time energy consumption Base card costs Base transmission costs Transmission time
Transmission rate Transmit power level
transmission time
Pba
seP
xmit
Pt
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Device Energy Costs
Receive-time energy consumption Base card costs Base reception costs Reception time
Transmission rate
reception time
Pba
seP
recv
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Example Card Costs
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Cisco Aironet350
Cabletron ORiNOCO11b
Socket LowPower SDIO
Mica2 Mote
Pow
er (W
)
TransmitReceiveIdleSleep
Example Network Energy Consumption
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Pow
er (m
W)
0
500
1000
1500
2000
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0 10 20 30 40 50 60 70 80 90
Time
Pbase
Pxmit
Example Network Energy Consumption
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Pow
er (m
W)
0
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1000
1500
2000
2500
0 10 20 30 40 50 60 70 80 90
Time
Pbase
Idle-Time Energy Conservation
Goal Reduce idle listening costs
Idle 1W Sleep 0.05W
Support communication in the network If all nodes are asleep, no one can communicate!
Approach Allow idle nodes to transition into a low power
sleep state Device suspension
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Idle-Time Energy Conservation Techniques
Device suspension (sleep) Reduced energy consumption Suspended communication capabilities Effects
Buffer overflow Wasted bandwidth Lost messages Breaks broadcast medium
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Communication Device Suspension
Ideal Power Management Sleep whenever there is no data to receive from
the base station Wake up for any incoming receptions
Realistic Goals Adapt the sleep duration to reflect the
communication patterns of the application Remain awake when there is active
communication Otherwise, suspend
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Communication Device Suspension
Problems How can a sender differentiate between a
suspended node and a node that has gone away? Suspended receiver buffer packet Confused sender dropped packet, extra energy
consumption How can a suspended node know there is
communication for it? Wake up too soon waste energy Wake up too late delay/miss packets
Communication Device Suspension
Approach Ensure overlap between sender’s and
receiver’s awake times Protocols
Periodic Resume Triggered Resume MAC-based Approaches
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Periodic Resume
Approach Suspend most of the time Periodically resume to check for pending
communication Communication indications
Out-of-band channel In-band signaling
Protocols Synchronous Asynchronous
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Synchronous Periodic Resume
Approach If all nodes are synchronized, all nodes are
guaranteed to have overlapping awake periods Examples
IEEE 802.11 PSM Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF)
Stations wake up at the same time
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802.11 Synchronization
Infrastructure mode Transmit beacon at start of beacon interval Or at earliest idle time after start
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beacon interval(20ms – 1s)
tmedium
accesspoint
busy
B
busy busy busy
B B B
value of the timestamp B beacon frame
Synchronization
Ad hoc mode Nodes ad random back off to prevent beacon
collisions
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tmedium
station1
busy
B1
beacon interval
busy busy busy
B1
value of the timestamp B beacon frame
station2
B2 B2
random delay
IEEE 8021.11 PSM
Nodes are synchronized and wakeup periodically Each beacon period is broken up into two segments
Ad-hoc Traffic Indication Map (ATIM) Window A node makes an announcement in the ATIM window if it
has a packet for another node The target node responds with an ATIM ACK
Transmission period The sender can transmit the packet until the end of the
beacon period If a node receives no announcements, it goes back to sleep
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IEEE 8021.11 PSM
Infrastructure Mode Traffic Indication Map (TIM)
List of unicast receivers transmitted by AP Delivery Traffic Indication Map (DTIM)
List of broadcast/multicast receivers transmitted by AP Ad hoc Mode
Ad hoc Traffic Indication Map (ATIM) Announcement of receivers by stations buffering frames More complicated - no central AP Collision of ATIMs possible (scalability?)
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B1
B2
IEEE 802.11 PSM – Ad Hoc Mode
awake A Transmit ATIM D Transmit Data
t
Node 1
B Beacon Frame
Node 2
Random Delay
B2
D
d
B1
ATIMwindow beacon interval
A
a
a Acknowledge ATIM d Acknowledge Data
Automatic Power Save Delivery (APSD)
Automatic Power Save Delivery Infrastructure mode Allows setting up scheduled delivery of packets Better power management method When data is sent by a station AP sends back queued
frames Good for synchronous traffic Benefits
The station now does not have to wake up for every beacon Time offset in the beacon interval can be specified so
stations wake up in different parts of the beacon interval to listen
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Synchronous Periodic Resume
Problems Maintaining synchronization in ad hoc networks
is difficult Synchronization overhead may outweigh benefits of
sleeping Throughput is limited by the size of the ATIM
window If the ATIM window is too small, packets get buffered Buffers may eventually overflow
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Asynchronous Periodic Resume
Approach Stay awake longer to guarantee overlap of
awake periods Use beacon messages at the start of awake
periods Some protocols use notification messages
(similar to ATIM) Basic protocol
Overlap is guaranteed if the awake periods are more than half the beacon period
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Asynchronous Periodic Resume
Problem No guarantee that nodes will hear each
other’s beacon or notification messages
time
Beacon Interval
Suspend Period
Node 1
Node 2
Beacon window
Notification Window
Awake Period
Beacon Message
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Asynchronous Periodic Resume
Alternate solution Have a beacon at the beginning and end
of the beacon interval
time
Beacon Interval
Suspend Period
Beacon window
Notification Window
Awake Period
Beacon Message
Node 1
Node 2
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Asynchronous Periodic Resume
Alternate solution Beacon at the beginning of odd periods
Beacon at the end of even periods
time
Beacon window
Notification Window
Awake Period
Beacon Message
Odd Beacon Interval Even Beacon Interval
Odd Beacon Interval Even Beacon Interval
Node 1
Node 2
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Asynchronous Periodic Resume
Problem Nodes stay awake more than half the
time Wastes too much energy!
Solution Do not wake up every beacon interval
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Asynchronous Periodic Resume
Approach 1 Wake up once every T intervals Adds delay up to T length of beacon interval
time
Beacon Interval
Beacon window
Notification Window
Awake Period
Beacon Message
Node 1
Node 2
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ci
ri
Node i is awake
Asynchronous Periodic Resume
Approach 2 Increase number of beacon intervals in cycle (n) Increase number of awake periods (2n – 1 of n2)
cj
Node j is awake
rj
n
n
Both nodes are awake
Delay is determined by where the overlap is (worst case n2)
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Node i is awake
Node j is awake
Asynchronous Periodic Resume
Approach 2 Example: n = 4, n2 = 16, 2n-1 = 7
Two overlapping intervals: delay = n2 - 2
Both nodes are awake
0 1 2 3
4 5 6 7
8 9 10 11
12 13 14 15
0 1 2 3
4 5 6 7
8 9 10 11
12 13 14 15
Node i Node j
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Approach 3 Find a feasible overlapping pattern
Guarantee at least one overlapping interval with every other node
Asynchronous Periodic Resume
Node 2
Node 3
Node 4
Node 5
Node 6
Node 7slot1 2 3 4 5 6 7
awake pattern
Node 1
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Asynchronous Periodic Resume
Approach 3 To prevent buffer overflow, nodes can stay awake longer Delay depends on the number of overlapping intervals
time
Beacon Interval
Beacon window
Awake Period
Beacon Message
Node 1
Node 2
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Asynchronous Periodic Resume
Challenges Reducing time spent awake Reducing delay No support for broadcast
None of the current approaches provide an interval where all nodes are awake
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Triggered Resume
Approach Use a second control channel (second radio)
Sender transmits RTS or beacon messages in control channel
Receiver replies in control channel and turns on main channel
Main channel is only used for data Second channel
Must consume less energy than the main channel Must not interfere with the main channel Ex: RFID, 915Mhz
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Triggered Resume
Protocols Power Aware Multi-Access Protocol (PAMAS)
Shut off device when channel is busy Wake-on-Wireless
Control channel is always active STEM
Control channel is managed similar to IEEE 802.11 PSM
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Triggered Resume
Approach – PAMAS Data channel
Power off radio when data is destined to a different node Control channel
Probe neighbors to find longest remaining transfer
time
Node A
Node B
RTS/CTS
Node C
A CB D Awake/listen Data transmission/receptionE
BUSY
Probe on control channel
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Triggered Resume
Approach – STEM Low duty cycle paging channel to wake up a
neighboring node Use separate radio for the paging channel to
avoid interference with regular data forwarding Trades off energy savings for setup latency
Wakeup plane: f1
Data plane: f2
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Triggered Resume
Approach – STEM
time
Node B - control
Node B - data
Awake/listen Receive and reply
Transmitrequest
Node A – control
Node A - data
Data transmission/reception
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Triggered Resume
Challenges Two radios are more complex than one Channel characteristics may not be the
same for both radios A successful RTS on the control channel
does not guarantee the reverse channel works
A failed RTS on the control channel does not indicate that the reverse channel doesn’t work
MAC-Based Approaches
Break away from straight 802.11 Change the mechanisms in the MAC
layer Allow packet trains Use new notification mechanisms
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MAC-Based Approaches
S-MAC RTS-CTS protocol
Reservations for consecutive fragments Duty cycle
Sleep, wake up, listen, return to sleep Variable sleep time
Tradeoff energy for latency Neighbors exchange schedules
Requires loose synchronization Control overhead may dominate energy savings
Adaptive listening Wake up at end of RTS-CTS-DATA-ACK period to send data Lower latency, save energy
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MAC-Based Approaches
B-MAC Eliminate need to synchronization Put the burden on the sender Sender
Sends a preamble Sends data immediately after preamble
Receiver Wakes up periodically If it hears a preamble, wait for packet transmission
X-MAC Similar to B-MAC Sender sends multiple short preambles and waits for an ACK
between them
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B-MAC
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PREAMBLE
A
B
C
D
E
DATA
t1: Flow 1 data
arrival
DIFS wait
t2: Flow 2 data
arrival
DATA
Channel Polling Intervalt3
Flow1data
Flow2data
PREAMBLE
Energy as a Resource
Energy conservation is still an open problem Must start to consider the applications
and data Trade-offs between application quality
and saving energy
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