link-level measurements from an 802.11b mesh network daniel aguayo john bicket sanjit biswas glenn...
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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:
Devin Barillari - 600.647 - Advanced Topics in Wireless Networks
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.
802.11b networks in an wide urban environment:
Devin Barillari - 600.647 - Advanced Topics in Wireless Networks
The Experimental Setup
Effect of distanceEffect of SNR (Signal vs. Noise Ratio)
Effect of transmit bit-rate
Interference from other 802.11b sources
Effect of Multi-path
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.
802.11b networks in an wide urban environment:
Devin Barillari - 600.647 - Advanced Topics in Wireless Networks
The Experimental Setup
Effect of distance
Effect of SNR Effect of transmit bit-rate
Interference from other 802.11b sources
Effect of Multi-path
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
802.11b networks in an wide urban environment:
Devin Barillari - 600.647 - Advanced Topics in Wireless Networks
The Experimental Setup
Effect of distance
Effect of SNR (Signal vs. Noise Ratio)
Effect of transmit bit-rateInterference from other 802.11b sources
Effect of Multi-path
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.
802.11b networks in an wide urban environment:
Devin Barillari - 600.647 - Advanced Topics in Wireless Networks
•Much of the “other source” noise is due to beacons.
•Channel 6 is very busy (usually the default configuration for commercial products.
The Experimental Setup
Effect of distance
Effect of SNR (Signal vs. Noise Ratio)
Effect of transmit bit-rate
Interference from other 802.11b sourcesEffect of Multi-path
Output power
802.11b networks in an wide urban environment:
Devin Barillari - 600.647 - Advanced Topics in Wireless Networks
The Experimental Setup
Effect of distance
Effect of SNR (Signal vs. Noise Ratio)
Effect of transmit bit-rate
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
802.11b networks in an wide urban environment:
Devin Barillari - 600.647 - Advanced Topics in Wireless Networks
The Experimental Setup
Effect of distance
Effect of SNR (Signal vs. Noise Ratio)
Effect of transmit bit-rate
Interference from other 802.11b
Effect of Multi-pathOutput power
802.11b networks in an wide urban environment:
Devin Barillari - 600.647 - Advanced Topics in Wireless Networks
The Experimental Setup
Effect 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
•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.
802.11b networks in an wide urban environment:
Devin Barillari - 600.647 - Advanced Topics in Wireless Networks
The Experimental Setup
Effect 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 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.