implications of power control in wireless networks: a quantitative study ioannis broustis, jakob...
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Implications of Power Control in Wireless Networks: A Quantitative Study
Ioannis Broustis, Jakob Eriksson, Srikanth V. Krishnamurthy, Michalis FaloutsosDepartment of Computer Science and Engineering
University of California, Riverside{broustis, jeriksson, krish, michalis}@cs.ucr.edu
PAM 2007
The problem
Increasing the transmission power may:• Increase throughput, due to increased signal quality• Increase interference levels in neighboring links
Conflict in providing the best network-wide throughput• Need to identify the possible interference scenarios • For these scenarios, need to examine:
Whether power control can help alleviate interference Whether fairness is affected by power control Whether the traditional Virtual Carrier Sensing (RTS/CTS exchange) can
co-exist with power control
In this work…
We conduct a set of experiments on an indoor wireless testbed• We focus on the interference between pairs of links
We identify 3 interference scenarios:• Overlapping
Neither power control nor RTS can improve performance
• Hidden-terminal Power control essential for fairness RTS with power control degrades performance!
• Potentially disjoint Power control increases performance significantly RTS with power control, however, results in lower throughput here as well
Experimental set-up
Indoor wireless testbed• Nodes: 15 Soekris net4826 • Wireless: EMP-8602-6G Atheros AR5006• MadWifi driver• Linux kernel v2.6, mounted over NFS• 802.11a for avoiding external interference
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Methodology
In our experiments we activate two links at a time• We observe the achieved throughput by each of the two links, for
different power levels
• Fully-saturated UDP traffic iperf measurement tool 30-sec back-to-back 1500-byte packets
• Power is maintained at a constant level during each 30-sec experiment After the end of the experiment, we vary the powers and start a new 30-sec
session Power levels: sequentially from 1 dBm to 16 dBm
Exhaustive search of different transmission power combinations (162)
• We conduct experiments with and without RTS/CTS exchange
Types of interference behavior
Overlapping case• The two links always contend; power control not helpful
The channel between the two senders is typically better than the channel between sender and receiver
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Types of interference behavior (ii)
Hidden-terminal case• Most of the links under investigation belong to this category• Senders cannot sense each others’ tranmissions: PCS fails
This is not a problem if the strength of the desired signal is significantly higher than that of the interfering signal
Competition between signal andinterference
Fairness is consistently better along a diagonal (after a certainpower level)
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Types of interference behavior (iii)
Potentially disjoint case• Here, power control can greatly improve performance
May enable simultaneous transmissions 15-16: 9 dBm 22-31: 6 dBm
If power is different, the throughput is lower
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Use of RTS/CTS
We repeat the experiments, with RTS enabled
Overlapping case with RTS• Worsened, due to the RTS/CTS transmission
overhead
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Without VCS With VCS
Use of RTS/CTS (ii)
Hidden-terminal case with RTS• Consistently underperforms the plain vanilla CSMA in all considered
scenarios! Overall throughput is considerably lower Fairness is also affected to a large degree !
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Without VCS With VCS
Use of RTS/CTS (iii)
Hidden-terminal case with RTS• A case where fairness is improved
However, RTS leads to large reduction in overall throughput
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Without VCS With VCS
Use of RTS/CTS (iv)
Potentially disjoint case with RTS• RTS/CTS has a negative impact here as well
While the overall throughput is still higher than in the isolated link capacity, it is still lower than the achieved with the PCS alone
The regime of powers for achieving spatial reuse is also reduced More precise power control is now required
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Without VCS With VCS
Conclusion
Proper power control is beneficial in wireless deployments• The topology determines if power control will help
Power control improves throughput in potentially disjoint case, and fairness in hidden-terminal case
• RTS/CTS cannot coexist with power control, while it degrades performance in indoor settings
Power control holds great promise for improving the performance of indoor wireless networks
Potential extensions of this work:• Import higher complexity, by activating more than two links at a time• Vary the sensing threshold as well
Questions?
Thank you