Partially Overlapping Channels Not Considered Harmful
Trevor ReidECE 256
Single Channel Spatial Reuse
ab
dc
What else can we do?
• Utilize the frequency domain• We have all of this space…
Split the spectrum into Channels
Frequency
Pow
er
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
Aside: Band-pass Filters
• Only accepts frequencies within a certain range. Everything else is rejected.
Partially Overlapping Channels
Frequency
802.11(b) Specifications
• Operates at 2.4 GHz• Split into 11 Channels• 22 Mhz Wide• 5 Mhz Separation
Looks Like This… (kind of)
Why use them?
• Reduce Contention• 11 partially overlapping channels reduce
contention by a factor of 3.05 compared to using 3 overlapping channels.
How to Model Interference
• I factor: I(i,j) = Amount of overlap between i and j
Which looks like this
Transmission Range
BER
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
How Can We Utilize this?
AP
Eric
Stephanie
Jason
Jimmy
Not Going To Help.
But with many APs
AP3
Stephanie
Trevor
AP1
EricJimmy
AP2
Valentin
Jason
Prof Choudhury
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.
So does it work?
Yes.By a factor of ~ 2.57
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
