Link-level Measurements from an 802.11b Mesh Network Daniel
Link-level Measurements from an 802.11b Mesh Network Daniel Aguayo (MIT), John Bicket (MIT), Sanjit Biswas (MIT), Glenn Judd (CMU), Robert Morris (MIT) Summary presented by Gary Woo CS 577 1 Outline
Assumptions in neighbor abstraction What is Roofnet? Experiment/Results Hypotheses Cause Conclusion Additional experiments
CS 577 2 Neighbor abstraction Nodes are divided into pairs that can communicate and pairs that can not Examples of usage: Wired networks Obvious physical connection MAC protocol in 802.11 Supported based on S/N and BER relationship
CS 577 3 Roofnet Located in Cambridge, MA next to MIT 38 nodes with roof top antennas CS 577 4
Experiment setup Use 802.11b cards with Prism 2.5 chip set Set to channel 3 (2.422Ghz) Pseudo-IBSS (ad hoc) mode Run early Sunday morning June 6th, 2004 1500 byte 802.11 broadcast for 90 seconds Sequence Number, Arrival Time, Receive Signal Strength Indication (RSSI), Silence values are saved CS 577 5
Results Red = expected Results show that nodes dont adhere to neighbor abstraction CS 577 6 Delivery probability Delivery probability
Delivery probability burstyness Sample (each point is 200 milliseconds) 1 Mbit/s links Sample (each point is 200 milliseconds) 11 Mbit/s links Average loss rate are all about 50% CS 577 7 Loss due to long interference?
Allan Deviation of loss rate measures fluctuations (high means very bursty, low means more smooth) Yellow box shows that around 80% of all pairs have low Allan Deviation of loss rate (around 0.06) CS 577
8 Loss due to distance? Yellow circles are the same size (approximately centered around Sender) There is relation of distance/receiving signal, however not consistent CS 577 9 Loss due to noise?
Broadcast packet delivery probability High signal and low noise should result in delivery Roofnet doesnt show strong correlation Result for 1 Mbit/s Laboratory Roofnet High signal packets still lost
Signal-to-noise ratio (dB) CS 577 10 Experiment packets lost per second Loss due to 802.11 interference? Few interference packets before Non-Roofnet packets observed experiment
per second (before the experiment) If cause is this interference, then the more interference packets the more packets would be drop CS 577 11 So what is the cause? when you have eliminated the impossible, whatever remains, however improbable, must be the truth Sherlock Holmes in The Adventure of the Beryl Coronet (1892) CS 577
12 Reflected signals Buildings and other physical objects reflects signals Weaker duplicated signal are received by nodes after a delay RAKE receiver can suppress reflected copies with delays up to 250 nanoseconds CS 577 13
Experiment for reflected signals Receiver Sender delay attenuation Signal sent to the receiver, and combined with a duplicated version of the original signal (with added delay, and reduce strength) CS 577
14 Results for reflected signals experiment Yellow shows RAKE working Green shows loss if reflected signal is almost as powerful as original the receiver is confused Blue shows intersection with the modulations symbol boundaries CS 577 15
Results for reflected signals experiment (continued) CS 577 16 Cumulative fraction of links Distance between links Link distance (feet or nanoseconds)
Most links have greater than 500 nanosecond delays, RAKE cant handle CS 577 17 Conclusion No clear distinction of links that drop a lot of packets verses ones that receive most Link distance and Signal to Noise ratio has relation but not strong Most likely cause is multi-path fading (reflected signals)
CS 577 18 Varying signal strength 1 Mbit/s The stronger the signal, the higher the probability of being delivered CS 577 19
Throughput compared based on bit rate Shaded portion shows that X Mbit/ s can successfully send more packets than other rates Algorithm should wait until delivering half the packets before reducing bit rate CS 577 20
References and Acknowledgements Author's slides for SIGCOMM 2004 (figures in color were taken from these slides): http://www.pdos.lcs.mit.edu/roofnet/sigcomm-talk.ppt Authors electronic version of paper (other figures and tables were taken from this document): http://www.pdos.lcs.mit.edu/~rtm/papers/p442-aguayo.pdf Roofnets group website: http://www.pdos.lcs.mit.edu/roofnet/index.php CS 577
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