Post on 12-Jan-2016
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Idle Communication PowerIdle Communication Power
Lei Guo, Xiaoning Ding, Haining Wang, Qun Li,
Songqing Chen, and Xiaodong Zhang
ExploitingExploiting
to Improve Wireless Network to Improve Wireless Network Performance and Energy EfficiencyPerformance and Energy Efficiency
Challenges in Wireless System Design
• Energy saving is not easy– Limited battery capacity in wireless devices – High power consumption in wireless communication
• High performance costs energy and fairness – Wireless users demand high throughput, but …– A high throughput device needs less sleep. – A channel allocation mechanism can favor some but
degrade performance of others.
• Can we win both instead of addressing the trade-off?
Power Consumption for Mobile Devices
• Energy consumption
• A simple way to save energy
– Put the WNI into sleep mode when idle (for a 5 V device)
> 50% total energy
up to 10%total energy
high power mode450 mA
low power mode15 mA
802.11 Power Saving Mechanism• Access point
– Buffer data for sleeping stations
– Broadcast beacon with TIM periodically (100 ms)
• Sleeping station– Wake up periodically to
receive beacon– Poll access point to receive
data– Sleep again
Access Point
Internet
Traffic Indication Map (TIM)
sleeping station
wake uppoll
receive data
Observations of IEEE 802.11 Protocol • A client/server model
– Each station independently communicates with AP– AP serves a station one at a time via the channel.
• The saving mode affects TCP traffic– Increasing RTT and decreasing throughput.
• Performance anomaly (Infocom’03)– Non-uniform transfer rates between different stations to
AP due to distance and obstacle condition differences. – A low speed station has low channel utilization rate.
• Waste energy while a station is waiting for its turn.– Idle communication power due to strong dependency
Existing Solutions to address the Limits
• Reducing idle communication power by– Traffic prediction: bounded slowdown (MOBICOM’02)– Self-tuning with application hints (MOBICOM’03)– Limits: case by case, and accuracy can vary.
• Address the performance anomaly– Time-based fairness scheduling: a constant time unit is
given to each device (USENIX 04) – Limits: poorly conditioned devices suffer: fast is faster,
and slow is slower.
Our work: to win both performance and energy
Source of Idle Communication PowerWhile the channel is used by one station, idle communication power is wasted in many other stations
AP
Wireless performance anomaly makes this power waste worse, but also with an opportunity.
Outline
• Motivation and rationale
• System model and algorithms
• System design and implementation
• Performance evaluation
• Conclusion
Multi-hop RelayTo help low channel rate stations to Increase throughput and extend network coverage
AP
X
Multi-hop Relays Leverage Strong Dependency • Slow stations become faster
– Completing the data transfer ahead of the unit time.– Equivalent to move the station closer to AP or improve
the station’s communication condition.
• Faster stations serve as proxies for slow stations– Performance improvement of slow stations reduced the
waste of idle communication powers of fast stations --- shortening the waiting time.
• Effective P2P coordination among stations is the key.
Incentive and Fairness to Fast Stations
• Why not sleep or wait, but proxy/relay for others?– Sleep lowers throughput, and wait wastes energy. – Idle communication energy can be used – The saved time in slow stations should be contributed.
• How much service is fair in a shared radio channel?– A proxy should be paid for its service– For either proxy or client, the throughput and energy
utilization should be improved.
Rationale
• Energy efficiency: what does a user care about?– Energy per second
– Energy per bit: time is energy• Self-incentive multi-hop relay with TBF
– Use channel time to pay the relay service
A win-win solution
Throughput Energy efficiency
Proxy Increase No loss
Client Increase Increase
System Model• Time based fairness in shared radio channel
• Principle of proxy forwarding
– Proxy: throughput idle time energy/bit – Client: channel rate throughput
S1 S2 … Si … Sn
ti = t = 1/n
1 roundidle idle
Sq
Client
Sp
Proxy
S0
AP
Basic Idea of Token-based Channel Scheduling
• A token is a ticket for a data transfer (RX/TX) in one time unit
• AP initially distributes an equal amount of tokens to each station (fairness).
• A pair of RX & TX consumes one token.• Token bucket model to fully use transmission
channel.• Multi-hop forwarding to increase throughput• Incentive rewards to proxies
packets
Token and Token Bucket Modeltokens from AP
Overflow!Re-allocate to other
stations by AP
Token Bucket
Packet Queue
Transmitter 1 token per packet
Multi-hop forwarding
APS1
STA Proxy RateS1 --- R(0,1)S2 --- R(0,2)S3 S2 R(0,3)S4 S2 R(0,4)
S2
S3
S4
Hop Station Rate1 Self R(0,2)2 S3 R(2,3)
S4 R(2,4)
Hop Station Rate1 S2 R(0,2)2 Self R(2,3)
Put Them Together: Selfish Forwarding - SFW
• Proxy discovery and selection– A poorly conditioned client broadcasts a request to relay his
packets
– AP assigns a relaying station for clients based on the game theory (second price auction) to provide fairness for competition among proxy candidates
• Channel scheduling– AP distributes tokens for fairness without any enforcement.
– The replaying actions are determined by token exchanges among stations.
• Multi-hop routing
Implementation
• AP– NetGear MA311 802.11b PCI wireless adaptor– HostAP linux driver version 0.1.3
• Wireless Stations– NetGear MA401 802.11b PCMCIA wireless
adaptor– ORiNOCO linux driver version 0.15rc2
Protocols Compared
• DCF– Most widely used protocol in 802.11b network– Distributed Coordination Function
• TBF– Time-based Fairness (USENIX 2004)
• SFW– Selfish Forwarding
Single Client Experiment
AP
11Mbps
11Mbps
1Mbps
Channel allocation scheme
Channel allocation scheme
Performance Evaluation1 proxy (P), 1 client (Q)
Multi-clients Experiment
AP
11Mbps
1Mbps
Performance Evaluation
Proxy throughput gain
1 proxy, multiple clients
Conclusion
• Idle communication power in TCP sessions
• Energy efficiency metric: task based
• Cooperative relay service– Peer-to-peer– Win-win solution
• System design and prototype implementation
Thank you!Thank you!