Partially Overlapping Channels Not Considered Harmful
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Partially Overlapping Channels Not Considered Harmful
Trevor ReidECE 256
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Single Channel Spatial Reuse
ab
dc
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What else can we do?
• Utilize the frequency domain• We have all of this space…
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Split the spectrum into Channels
Frequency
Pow
er
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But it’s not perfect
• Can’t get full channel capacity without an ideal band-pass filter
• Instead the channels look more like…
Frequency
Pow
er
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Aside: Band-pass Filters
• Only accepts frequencies within a certain range. Everything else is rejected.
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Partially Overlapping Channels
Frequency
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802.11(b) Specifications
• Operates at 2.4 GHz• Split into 11 Channels• 22 Mhz Wide• 5 Mhz Separation
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Looks Like This… (kind of)
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Why use them?
• Reduce Contention• 11 partially overlapping channels reduce
contention by a factor of 3.05 compared to using 3 overlapping channels.
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How to Model Interference
• I factor: I(i,j) = Amount of overlap between i and j
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Which looks like this
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Transmission Range
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BER
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Take Away
• Partially overlapping channels can reduce contention
• We can model interference between two channels with an I factor
• Interference falls off rapidly with channel separation
• Bit Error Rate falls of rapidly with channel separation
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How Can We Utilize this?
AP
Eric
Stephanie
Jason
Jimmy
Not Going To Help.
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But with many APs
AP3
Stephanie
Trevor
AP1
EricJimmy
AP2
Valentin
Jason
Prof Choudhury
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Channel Assignment Algorithm
• OBJ(T,θ) = Total interference experienced for each client.
• Steps:– Evaluate OBJ(T,θ) – For each AP• Assign to channel that minimizes interference
– Evaluate OBJ(T,θ) again• Stop if nothing changed, otherwise repeat.
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Modify For Multiple Channels
• Instead of calculating interference on only the same channel find channels that interfere (using I-factor).
• Add up interference at client from all interfering channels.
• It just works.
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So does it work?
Yes.By a factor of ~ 2.57
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Wireless Mesh Networks
• Use similar approach to modify existing channel assignments algorithms.
• Using partially overlapped channels shows significant improvements.
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So What’s the Catch?
MeNeighbor
Starbucks Free Wifi
Noisy people below you
linksys
linksys
Cute Girl Upstairs
Who Coordinates Channel Assignment?
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Does this actually perform well?
• We don’t know• Never actually implemented• Don’t know how effective the I-Factor model is
in real world conditions.
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Resources
• http://www.ntia.doc.gov/osmhome/allochrt.pdf
• http://en.wikipedia.org/wiki/File:2.4_GHz_Wi-Fi_channels_%28802.11b,g_WLAN%29.svg
• http://en.wikipedia.org/wiki/File:Bandwidth_2.svg