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Link-level Measurements from an 802.11b Mesh Network

Link-level Measurements from an 802.11b Mesh Network Daniel 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 University glennj@cs.cmu.edu.

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Link-level Measurements from an 802.11b Mesh Network

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  1. Link-level Measurements from an 802.11b Mesh Network Daniel 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 University glennj@cs.cmu.edu Devin Barillari - 600.647 - Advanced Topics in Wireless Networks

  2. 802.11b networks in an wide urban environment: 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 • 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. Devin Barillari - 600.647 - Advanced Topics in Wireless Networks

  3. 802.11b networks in an wide urban environment: 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 fraction of packets it received. There is a correlation to distance but it is not always consistent. 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 Devin Barillari - 600.647 - Advanced Topics in Wireless Networks

  4. 802.11b networks in an wide urban environment: • 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. 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 Ideal Emulated Conditions Observed Real-World Conditions Devin Barillari - 600.647 - Advanced Topics in Wireless Networks

  5. 802.11b networks in an wide urban environment: 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 • 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. Devin Barillari - 600.647 - Advanced Topics in Wireless Networks

  6. 802.11b networks in an wide urban environment: 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 • Much of the “other source” noise is due to beacons. • Channel 6 is very busy (usually the default configuration for commercial products. Devin Barillari - 600.647 - Advanced Topics in Wireless Networks

  7. 802.11b networks in an wide urban environment: • 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. 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-path Output power Δt = 10 nanoseconds Δd = 3 Meters Δt = 1 microseconds Δd = 300 Meters Sender Sender Receiver Receiver Indoor Outdoor Devin Barillari - 600.647 - Advanced Topics in Wireless Networks

  8. 802.11b networks in an wide urban environment: 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-path Output power Devin Barillari - 600.647 - Advanced Topics in Wireless Networks

  9. 802.11b networks in an wide urban environment: 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. Devin Barillari - 600.647 - Advanced Topics in Wireless Networks

  10. 802.11b networks in an wide urban environment: 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. Devin Barillari - 600.647 - Advanced Topics in Wireless Networks

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