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Feb 24, 2016
Partially Overlapping Channels Not Considered Harmful
Partially Overlapping Channels Not Considered HarmfulTrevor ReidECE 2561Single Channel Spatial ReuseabdcWhat we have talked about so far:Directional AntennasRate adjustmentAll ways of overcoming interference problems
We want parallel communication/spatial reuseHavent found a good way of avoiding contention2What else can we do?Utilize the frequency domainWe have all of this spaceMentioned a channel in my previous slide. Will go into more detail laterEssentially: one channel means everything transmitted on the same frequency.
Butthere are so many different frequency we can transmit on3
FCC Frequency allocations
From 3KHz to 300 GHz
802.11 takes up a small portion. The 2.4 GHz Band 4Split the spectrum into ChannelsFrequencyPowerIdeal model
Lets say we were allotted 0 90 MHz
Split it up into 3 channels0 30 ( 15)30 60 (45)60 90 (75)
Share equal parts of the spectrum
Black lines mark the center frequency for the channel. Important because channels dont really look like this. 5But its not perfectCant get full channel capacity without an ideal band-pass filterInstead the channels look more likeFrequencyPowerHave more of a parabolicish shape to them
Transmitter must emulate a band-pass filter
Center frequency has max power6Aside: Band-pass FiltersOnly accepts frequencies within a certain range. Everything else is rejected.
Takes a low and high frequency, only allows part of signal in this range to pass. Everything else is rejected (kind of)
You have probably built these in a circuit class
Can be done in software too7Partially Overlapping ChannelsFrequencyAreas of overlap:Green and BlueGreen and Red
If communication is going on two partially overlapping channels youre going to see more noise.SNR will decrease. Gonna hurt your latency.
What does this look like for 802.11?
8802.11(b) SpecificationsOperates at 2.4 GHzSplit into 11 Channels22 Mhz Wide5 Mhz Separation9Looks Like This (kind of)
Things to note:
There are 14 channels here In the US we only use the first 11.
The largest subset of nonoverlapping channels is 1 ,6,11
These are what youll usually see used in your wifi router
8 Completely empty channels! Think of all the spectrum utilization were missing out on. 10Why use them?Reduce Contention11 partially overlapping channels reduce contention by a factor of 3.05 compared to using 3 overlapping channels.
Helps with higher layers of network stack
If this is done incorrectly you will actually see a reduction in throughput
(i.e) Dont put everything on channels 1 and 2 11How to Model InterferenceI factor: I(i,j) = Amount of overlap between i and j
Dont want to spend too much time on this.
You are able to discretize the function because 802.11 specifies the channel frequencies to operate on.
Worth noting: This is a fairly simple model12Which looks like this
Falls off very quickly
Confroms to the measured I-Factor very well.
Have someone transmitting on 6, this is what it looks like for interference on all other channels.
Both transmitting on 6 => I Factor = 1Once you get 5 away its 0 (no overlap)13Transmission Range
Take away. If you initially could have same channel transmission of 100m (kind of large). A separation of 3 channels means that that they need to be within a meter now. 14BER
Bit Error Rate
Starts at point where transmission is successful
Distance is between interferer and receiver
Note: ChSep = 3 Is below ambient noise only15Take AwayPartially overlapping channels can reduce contentionWe can model interference between two channels with an I factorInterference falls off rapidly with channel separationBit Error Rate falls of rapidly with channel separation16How Can We Utilize this?APEricStephanieJasonJimmyNot Going To Help.These guys all want to communicate with AP
Can we use multiple channels here?
No. AP can only operate on a single channel at a time. Partial overlap is not going to help here.
Though it would be nice 17But with many APsAP3StephanieTrevorAP1EricJimmyAP2ValentinJasonProf ChoudhuryJason is good example.
We can assign each AP a different example.
This is actually a coloring problem (hence the different colors)
This would work best with 3 non-overlapping channels, but I didnt want to draw another AP18
19Channel Assignment AlgorithmOBJ(T,) = Total interference experienced for each client. Steps:Evaluate OBJ(T,) For each APAssign to channel that minimizes interferenceEvaluate OBJ(T,) againStop if nothing changed, otherwise repeat.
Want to minimize this OBJ(T,theta).X = Set of access points T = set of range sets for each clients (explain using previous slide)K = number of channelsTheta = channel assignment Channel assignment is based on This will eventually converge. (maybe)
No formal proof of this, but its not the authors algorithm so I guess we can let it slide.
This algorithm assumes independent channels (non-overlapping)
20Modify For Multiple ChannelsInstead 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. In the assign to channel that minimizes interference we have to calculate interference for each client associated with an APBut clients are experiencing interference from communications on other channels as well.Which ones? Use a function to find which channels are considered as interferers. Add up the interference caused by them. 21So does it work?Yes.By a factor of ~ 2.57Higher UDP and TCP throughputLess collisionsMuch more consistent collisions
That is so much! Why arent we using it now?!
Thats like if we had 4-5 more non-overlapping channels
Before we talk about why not, cover wireless mesh networks22Wireless Mesh NetworksUse similar approach to modify existing channel assignments algorithms.Using partially overlapped channels shows significant improvements.
Channel AssignmentFlow RoutingLink Flow Scheduling
Hop across clients to best gateway
Weve talked about these before in other papers and the military vehicles in the desert example 23So Whats the Catch?MeNeighborStarbucksFree WifiNoisy people below youlinksyslinksysCute Girl Upstairs Who Coordinates Channel Assignment?Who is going to run that algorithm?In the case of Dukes network it is manageableIn many places it is not
Remember what was said earlier: Poor channel assignment can lead to decreased throughput.
Maybe come up with some fun distributed algorithms like they did in DRAND
How do actual APs choose channel? They just choose 1
You should all go home and change the channelLook up dd-wrt or tomato24Does this actually perform well?We dont knowNever actually implementedDont know how effective the I-Factor model is in real world conditions.
The real works is very different from NS-2
While simple is nice it also kind of worries me.
Keep in mind, 802.11 shares space with Microwaves, bluetooth, etc25Resourceshttp://www.ntia.doc.gov/osmhome/allochrt.pdfhttp://en.wikipedia.org/wiki/File:2.4_GHz_Wi-Fi_channels_%28802.11b,g_WLAN%29.svghttp://en.wikipedia.org/wiki/File:Bandwidth_2.svg