Link-level Measurements from an 802.11b Mesh Network Daniel Aguayo John Bicket Sanjit Biswas Glenn...

Link-level Measurements from an 802.11b Mesh NetworkDaniel Aguayo John Bicket Sanjit Biswas Glenn Judd † Robert Morris

M.I.T. Computer Science and Artificial Intelligence Laboratory{aguayo, jbicket, biswas, rtm}@csail.mit.edu

† Carnegie Mellon [email protected]

Devin Barillari - 600.647 - Advanced Topics in Wireless Networks

802.11b networks in an wide urban environment:


The Experimental SetupEffect of distance

Effect of SNR (Signal vs. Noise Ratio)

Effect of transmit bit-rate

Interference from other 802.11b sources

Effect of Multi-path

Output power

•802.11b Laptops

•+8 db omni directional roof mounted antenna

•Channel 3 (2.422Ghz)

•Prism wireless cards in “pseudo-IBSS” Ad-Hoc mode to prevent network partitioning

•Each node transmits as all other nodes receive and record each successful frame.



The Experimental Setup

Effect of distanceEffect of SNR (Signal vs. Noise Ratio)




Output power

Figure 5: These maps show the delivery probabilities from three senders to all other nodes. The sender is marked S, and each receiver is indicated by a circle with radius proportional to the fractionof packets it received. There is a correlation to distance but it is not always consistent.




Effect of distance

Effect of SNR Effect of transmit bit-rate



Output power

•In ideal conditions there is a 3db difference in signal strength that gives 10% to 90% loss rates.

•Many nodes have mid-range loss rates, Does this mean there are many nodes with a signal in this 3db range? No.

•In real conditions the 3db range is much greater due to obstacles and attenuation.

Ideal Emulated Conditions Observed Real-World Conditions




Effect of distance


Effect of transmit bit-rateInterference from other 802.11b sources


Output power

•Lower bitrates are usually more robust.

•A higher bitrate may have better data throughput with loss rates above 50%.

•Performance at low bitrates are no indication of performance at higher bitrates.



•Much of the “other source” noise is due to beacons.

•Channel 6 is very busy (usually the default configuration for commercial products.


Effect of distance



Interference from other 802.11b sourcesEffect of Multi-path

Output power




Effect of distance



Interference from other 802.11b

Effect of Multi-pathOutput power

•Multipath differs greatly from an Small indoor area to a wide outdoor urban area.

•Larger Distances introduce delays longer then multipath filters cannot identify. (rake < 250.10-9 s)

•No loss occurs if reflection has more then 4db weaker.

Δt = 10 nanosecondsΔd = 3 Meters

Indoor

Sender

Receiver

Δt = 1 microseconds Δd = 300 Meters

Outdoor

Sender

Receiver




Effect of distance



Interference from other 802.11b

Effect of Multi-pathOutput power




Effect of distance





Output power

•Quadrupeling the output power will increase effective radius by around 2 times.

•The effect of varying the transmit power level on the delivery probability, for 1 Mbit/s.

•For example, raising the power level from 10 to 40 milliwatts almost doubles the number of nodes that have delivery probabilities of 40% or more.




Effect of distance





Output power Conclusion:

Links with intermediate levels of loss are the common case; there is no clear distinction between “working” and “non-working” links. Link distance and S/N ratio do have an effect on loss rates, but the correlation is weak.

Link-level Measurements from an 802.11b Mesh Network Daniel Aguayo John Bicket Sanjit Biswas Glenn...

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