232 SPECTRUM-AWARE DISTRIBUTED CHANNEL ASSIGNMENT FOR COGNITIVE RADIO
Distributed Dynamic Channel Selection in Chaotic Wireless Networks
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Transcript of Distributed Dynamic Channel Selection in Chaotic Wireless Networks
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Distributed Dynamic Channel Selection in Chaotic Wireless
NetworksBy:
Matthias Ihmig and Peter Steenkiste
Presented by:James Cialdea and Ken Breeman
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Overview
Wireless networking problem: Channel Allocation
A proposed solution:Dynamic Channel Selection
Proposed algorithm explained. Simulations using OPNET Conclusions
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Problem:
Wireless networking is popular Many users in contention for a finite amount of bandwidth per channel
Most users do not properly configure their devices to maximize throughput
Even if they do, the situation can change at any time
Wireless Channel Selection Issues Include: High density of APs
Unpredictable, Dynamic load
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Solution:
Dynamically select channel based on current load conditions
Problems: What devices should calculate load in an
unmanaged space? How should load be calculated? How often should load be calculated? What is the best way to switch channels on the fly?
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Algorithm Overview
Monitoring Component Monitor load on current channel
Evaluation Component Compare current channel load against a threshold
Trigger switch if this channel is no good
Change threshold accordingly
Channel Switching Component Set up the hardware for new channel
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Algorithm
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Channel Switching Coordination
No Coordination The AP simply switches channels, expecting the
client to loose connectivity and reconnect AP-Client Coordination
The AP tells the client what channel to switch to and when (SWITCH message)
This is supported by 802.11k AP-AP Coordination
The AP tells other APs what channel it plans to switch to to avoid congestion (HOLD message)
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Where To Calculate Network Load
Because this is an unmanaged, decentralized system:
Devices are not distributed across the space evenly No one device can be guaranteed to hear all other
devices No one device can be trusted to calculate fairly for
all devices All devices must calculate load for their particular
space and situation
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Which metric?
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Calculating Network Load
Dynamic nature of traffic Dynamic number of clients
Dynamic bandwidth demand
Channel Utilization Many spikes – can cause false triggers
Transmit Queue Length Easy to measure
Highly variable – many false triggers
MAC/Packet Delay Transmit queue and channel contention time measured
Most attractive – very steady readings
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OPNET Simulation
Utilizes both SWITCH for AP-Client coordination and HOLD for AP-AP coordination
Comparison: All APs and clients on a single channel All APs and clients on non-overlapping channels APs set to dynamically configure channel and
coordinate with clients and other APs
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Simulation Result
Single channel performance was poor – as expected
Manually configured on non-overlapping channels provided the most throughput – as expected
Dynamic configuration provided about 5% less throughput than the manually configured non-overlapping channel setup
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Simulation Result
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Tuning the Algorithm: Coordination
Removing Client-AP coordination – for legacy clients – reduces performance around the channel switch by a constant factor based on rescan/reconnect time
Remove the SWITCH capability About 7% delay was gained and 2% throughput
was lost Removing AP-AP coordination – the HOLD
capability – reduces the global throughput by about 3%
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Tuning the Algorithm: Parameters
Load Average The load is calculated using a moving average to
negate small changes Shrinking the averaging interval
Causes the system to converge on the maximum global throughput quickly
Causes excessive channel switching Increasing the averaging interval
Slower convergence Less time wasted switching channels
Same is true of the channel hold time t_HOLD
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Tuning the Algorithm: Parameters (contin)
thres_min The threshold for channel switching Set to about 50% of channel capacity Higher values increase convergence time Lower causes increased number of channel
switches
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Tuning the Algorithm: Parameters (contin)
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Conclusions
A decentralized algorithm that works. Delay based metrics are suitable. 95% throughput compared to manual config. Handles varying usage.
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Works Cited
Article: Distributed Dynamic Channel Selection in Chaotic Wireless Networks
by Matthias Ihmig and Peter Steenkiste
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Questions?