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