Partially Overlapping Channels Not Considered Harmful Trevor Reid ECE 256
Dec 30, 2015
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
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Aside: Band-pass Filters
• Only accepts frequencies within a certain range. Everything else is rejected.
802.11(b) Specifications
• Operates at 2.4 GHz• Split into 11 Channels• 22 Mhz Wide• 5 Mhz Separation
Why use them?
• Reduce Contention• 11 partially overlapping channels reduce
contention by a factor of 3.05 compared to using 3 overlapping channels.
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
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.
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.
Wireless Mesh Networks
• Use similar approach to modify existing channel assignments algorithms.
• Using partially overlapped channels shows significant improvements.
So What’s the Catch?
MeNeighbor
Starbucks Free Wifi
Noisy people below you
linksys
linksys
Cute Girl Upstairs
Who Coordinates Channel Assignment?
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.
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