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FRACTEL Deployment

FRACTEL: Building (Rural) Mesh Networks with Predictable Performance On the Feasibility of the Link Abstraction in (Rural) Mesh Networks. Dattatraya Gokhale (Indian Navy), Sayandeep Sen (U. Wisconsin-Madison) , Kameswari Chebrolu (IIT Bombay), Bhaskaran Raman (IIT Bombay) Work done at IIT Kanpur

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FRACTEL Deployment

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  1. FRACTEL: Building (Rural) Mesh Networks with Predictable PerformanceOn the Feasibility of the Link Abstraction in (Rural) Mesh Networks Dattatraya Gokhale (Indian Navy),Sayandeep Sen (U. Wisconsin-Madison),Kameswari Chebrolu (IIT Bombay),Bhaskaran Raman (IIT Bombay) Work done at IIT Kanpur Presentation at Infocom 2008, Phoenix, Apr 2008

  2. Local-Gateway: gateway to LDN Alampuram Tetali Point-to-Multi-Point 802.11 link-sets using Sector Antennas Kasipadu LACN: Local-ACcess Network at one of the villages (desired, not deployed) Point-to-Point 802.11 Links with Directional Antennas Pippara Ardhavaram Kesavaram Jalli Kakinada 19 Km Korukollu Polamuru IBhimavaram Jinnuru Tadinada 19.5 Km Cherukumilli Bhimavaram Juvvalapalem Lankala Koderu LDN: Long-Distance Network (deployed, in the Ashwini project) Landline: wired gateway to the Internet FRACTEL Deployment wiFi-based Rural data ACcess & TELephony Need to support real-time apps.

  3. Link doesn’t exist. Steep change in Error Rate Link Exists Negligible error rates Link Abstraction: Background • Link abstraction: • Either link exists or does not • That is, 0% packet reception, or ~100% • Abstraction holds in wired networks • Understanding link behaviour has implications on • Network Planning, Protocol Design, Application Design Packet Error Rate (%) Average RSSI (dBm)

  4. DGP, Roofnet, FRACTEL LACN Typical link distances Network architecture Environment Multipath effects SNR or RSSI External interference Link abstraction Long-distance mesh networks (e.g. DGP) Up to few tens of kms High gain directional & sector antennas on tall towers or masts Rural setting studied in depth Effect not apparent Has strong correlation with link quality Affects links performance Valid Rooftop mesh networks (e.g. Roofnet) Mostly < 500 m Mostly omni-directional antennas on rooftops Dense urban setting studied in-depth Reported as a significant component Not useful in predicting link quality Reported as not significant Not valid FRACTEL LACNs Mostly < 500 m Would like to avoid tall towers Rural, campus, residential To be determined To be determined To be determined To be determined

  5. DGP, Roofnet, FRACTEL LACN Typical link distances Network architecture Environment Multipath effects SNR or RSSI External interference Link abstraction Long-distance mesh networks (e.g. DGP) Up to few tens of kms High gain directional & sector antennas on tall towers or masts Rural setting studied in depth Effect not apparent Has strong correlation with link quality Affects links performance Valid Rooftop mesh networks (e.g. Roofnet) Mostly < 500 m Mostly omni-directional antennas on rooftops Dense urban setting studied in-depth Reported as a significant component Not useful in predicting link quality Reported as not significant Not valid FRACTEL LACNs Mostly < 500 m Would like to avoid tall towers Rural, campus, residential To be determined To be determined To be determined To be determined

  6. DGP, Roofnet, FRACTEL LACN Typical link distances Network architecture Environment Multipath effects SNR or RSSI External interference Link abstraction Long-distance mesh networks (e.g. DGP) Up to few tens of kms High gain directional & sector antennas on tall towers or masts Rural setting studied in depth Effect not apparent Has strong correlation with link quality Affects links performance Valid Rooftop mesh networks (e.g. Roofnet) Mostly < 500 m Mostly omni-directional antennas on rooftops Dense urban setting studied in-depth Reported as a significant component Not useful in predicting link quality Reported as not significant Not valid FRACTEL LACNs Mostly < 500 m Would like to avoid tall towers Rural, campus, residential To be determined To be determined To be determined To be determined

  7. DGP, Roofnet, FRACTEL LACN Typical link distances Network architecture Environment Multipath effects SNR or RSSI External interference Link abstraction Long-distance mesh networks (e.g. DGP) Up to few tens of kms High gain directional & sector antennas on tall towers or masts Rural setting studied in depth Effect not apparent Has strong correlation with link quality Affects links performance Valid Rooftop mesh networks (e.g. Roofnet) Mostly < 500 m Mostly omni-directional antennas on rooftops Dense urban setting studied in-depth Reported as a significant component Not useful in predicting link quality Reported as not significant Not valid FRACTEL LACNs Mostly < 500 m Would like to avoid tall towers Rural, campus, residential To be determined To be determined To be determined To be determined

  8. DGP, Roofnet, FRACTEL LACN Typical link distances Network architecture Environment Multipath effects SNR or RSSI External interference Link abstraction Long-distance mesh networks (e.g. DGP) Up to few tens of kms High gain directional & sector antennas on tall towers or masts Rural setting studied in depth Effect not apparent Has strong correlation with link quality Affects links performance Valid Rooftop mesh networks (e.g. Roofnet) Mostly < 500 m Mostly omni-directional antennas on rooftops Dense urban setting studied in-depth Reported as a significant component Not useful in predicting link quality Reported as not significant Not valid FRACTEL LACNs Mostly < 500 m Would like to avoid tall towers Rural, campus, residential To be determined To be determined To be determined To be determined

  9. DGP, Roofnet, FRACTEL LACN Typical link distances Network architecture Environment Multipath effects SNR or RSSI External interference Link abstraction Long-distance mesh networks (e.g. DGP) Up to few tens of kms High gain directional & sector antennas on tall towers or masts Rural setting studied in depth Effect not apparent Has strong correlation with link quality Affects links performance Valid Rooftop mesh networks (e.g. Roofnet) Mostly < 500 m Mostly omni-directional antennas on rooftops Dense urban setting studied in-depth Reported as a significant component Not useful in predicting link quality Reported as not significant Not valid FRACTEL LACNs Mostly < 500 m Would like to avoid tall towers Rural, campus, residential To be determined To be determined To be determined To be determined

  10. DGP, Roofnet, FRACTEL LACN Typical link distances Network architecture Environment Multipath effects SNR or RSSI External interference Link abstraction Long-distance mesh networks (e.g. DGP) Up to few tens of kms High gain directional & sector antennas on tall towers or masts Rural setting studied in depth Effect not apparent Has strong correlation with link quality Affects links performance Valid Rooftop mesh networks (e.g. Roofnet) Mostly < 500 m Mostly omni-directional antennas on rooftops Dense urban setting studied in-depth Reported as a significant component Not useful in predicting link quality Reported as not significant Not valid FRACTEL LACNs Mostly < 500 m Would like to avoid tall towers Rural, campus, residential To be determined To be determined To be determined To be determined

  11. Experimental Methodology • Two kinds of environment • Five locations on campus, One village location • One transmitter, Multiple receiver positions • Broadcast 6K 1400 byte pkts with 20ms gap. • Hardware same as in DGP study, Roofnet study

  12. Results: Err Rate vs RSSI • For Interference free positions • If RSSI > ThresholdError rates are low and stable • For Interference prone positions • Intermediate error rates Data Rate: 1Mbps

  13. MIT Roofnet: Conclusions • No correlation between SNR and Error Rate • Loss rate high at high SNR • No correlation between lost packets and foreign packets observed • Introducing delay spread causes high loss rates Multipath induced delay spread and not interference is the culprit

  14. MIT Roofnet data: Fresh Analysis Data Rate: 1Mbps, Average RSSI > -80 dBm, 80% >Error Rate > 20%

  15. MIT Roofnet data: Fresh Analysis Noise band as high as 16 dB, Ours ~ 2 dB Data Rate: 1Mbps, Average RSSI > -80 dBm, 80% >Error Rate > 20%

  16. MIT Roofnet data: Fresh Analysis Roofnet Max Noise = -75 dBm, Ours = -94 dBm Data Rate: 1Mbps, Average RSSI > -80 dBm, 80% >Error Rate > 20%

  17. MIT Roofnet data: Fresh Analysis Data Rate: 1Mbps, Average RSSI > -80 dBm, 80% >Error Rate > 20% What is the cause of increased noise level? Multipath does not cause high noise level Is it Interference ?

  18. Understanding Interference • Does interference on affect noise levels ? • Can pkts. loss be related to no. of foreign pkts seen? • Can reported noise level be used to gauge the level of interference? • Can we estimate the link performance based on the average measured noise floor?

  19. Understanding Interference • Does interference on affect noise levels ? • Does pkts. loss correlate with foreign pkts. ? • Use reported noise level to gauge level of interf.? • Estimate the link perf. based on the avg. measured noise floor?

  20. Controlled Interference Expt • Experimental Setup • A and B Hidden Nodes to one another • B’s power fixed at -75 dBm • A’s power varied: -90, -85, -80, -75 dBm

  21. Does Interference affect noise levels? Controlled Experiment Roofnet Avg Received RSSI from A is -85 dBm and B is -75 dBm Noise extends right up to -65 dBm P1: Interference causes noise level to be high and variable

  22. Can packet loss be related to foreign packets seen? Hidden node, receiver outside of interferer’s reception range As far as B’s packets are concerned: B’s loss = 18.3%; A’s loss = 99.2%  4 foreign pkts /sec P2: Packet loss high even though number of observed foreign packets low

  23. IN RANGE Non-hidden-node case NODE ‘A’ NODE ‘B’ NODE ‘R’ P3: Packet loss can be low even though number of observed foreign packets is high Support Roofnet’s observation but disprove conclusion P2 P3

  24. Can the reported noise level be used to gauge the level of interference?

  25. Can the reported noise level be used to gauge the level of interference? Instantaneous noise levels show variability Large Noise Band

  26. Can the reported noise level be used to gauge the level of interference? • Noise levels reported differ from known level Actual Interference Perceived Interference P4: On this H/W, gauging level of interference is error prone

  27. Can link performance be estimated based on average noise floor? From Roofnet Data P5: It is not possible to estimate the link quality based on reported noise floor

  28. Operating near the RSSI threshold Variable Loss Rates Village Location

  29. Operating near the RSSI threshold Variable Loss Rates No Interference Village Location

  30. Operating near the RSSI threshold Variable Loss Rates No Interference Village Location What is the reason behind Intermediate error rates ?

  31. Operating near the RSSI threshold RSSI Below Thresh. Packet Error Rate (%) Village Location Average RSSI (dBm)

  32. Operating near the RSSI threshold RSSI Above Thresh. Packet Error Rate (%) Average RSSI (dBm) Village Location

  33. Operating near the RSSI threshold Small variation in RSSI  large variation in error rates Village Location Cannot distinguish between links with loss rates between 0-100% Cannot use routing metrics based on ETX or WCETT (going to be unstable)

  34. Design Implications • Link abstraction: • Absence of external interference  can plan links • Routing: • Metrics to distinguish between links with intermediate loss rates (e.g. ETX and WCETT) are likely unstable • Gauging interference can be error prone in interference-aware routing • MAC: • CSMA/CA will lead to unpredictable loss rates, due to self-interference and hidden node cases

  35. Conclusion • Multipath and delay spread: • Not a likely factor in rural areas • May or may not be the main culprit in urban areas • Roofnet: extrapolates 900 MHz multipath measmts. • Lesson: double-check measurements, analysis • Open questions: • 802.11g and 802.11a? • Future of unplanned deployments? • For further information on FRACTEL: • http://www.cse.iitb.ac.in/~br/

  36. Backup slides

  37. Operating near the RSSI threshold • In the absence of external interference • Intermediate error rates due to operation in steep region • Small variation in RSSI  large variation in error rates Village Location • Cannot distinguish between links with loss rates between 0-100% • Cannot use routing metrics based on ETX or WCETT

  38. Interference free locations: RSSI Stability • Short-term stability (2-min expt): Mostly within 3-4 dB Hardware Quirk Person standing near antenna Band: Difference between the 95%-ile and 5%-ile values of RSSI

  39. Long-term Stability • Band variation depends on environment but within 4dB in most cases

  40. Design Implication: Link Abstraction • Absence of external interference  can plan links with predictable performance. • Simplifies higher layer protocol design considerably RSSI threshold -79dBm Modified RSSI Threshold Value -75dBm RSSI variation 3-4dB

  41. Design Implication: MAC • CSMA/CA unsuitable in multihop mesh networks • External interference can worsen CSMA/CA performance • Roofnet data indicates considerable sources at interference range but not reception range • RTS/CTS will not help in this setting

  42. FRACTEL: Ongoing Work • Scope for TDMA-based mesh network • Lots of theory research, little in systems • Systems issues in TDMA-based networks: • Interference mapping • Synchronization • Schedule dissemination, dynamic scheduling • Scaling • For further information: • http://www.cse.iitb.ac.in/~br/

  43. Long Distance Networks (LDNs) Link abstraction holds Measurements on the DGP network, Kanpur, India "Long-Distance 802.11b Links: Performance Measurements and Experience'', Kameswari Chebrolu, Bhaskaran Raman, and Sayandeep Sen, MOBICOM 2006

  44. Local Access Networks (LACNs) • Prior Studies: MIT Roofnet study • Outdoor WiFi mesh, Boston/Cambridge area • Most links have intermediate loss rates, between 0% and 100% • Multi-path (not external interference) is a major cause of losses • No Link Abstraction! • Work around: design of appropriate routing metrics

  45. Experimental Methodology: Environment • Two kinds of environment • Five locations on campus • One village location • Link lengths: 150-400m • One transmitter position • Up to 6 receiver positions • Good – Avg. RSSI ≈ -70 dBm • Medium – Avg. RSSI ≈ -75 dBm • Bad – Avg. RSSI ≈ -80 dBm

  46. FRACTEL Measmt. Study: IITK Dense buildings, academic area, student dormitories, campus housing, several trees Source: Google Maps

  47. Understanding Interference • Does interference on affect noise levels ? • Can pkts. loss be related to no. of foreign pkts seen? • Can reported noise level be used to gauge the level of interference? • Can we estimate the link performance based on the average measured noise floor?

  48. FRACTEL Measmt. Study: Amaur Dense buildings, 2-3 storey tall Source: Google Maps

  49. Experimental Methodology • Hardware • Laptops with Senao 2511CD Plus 802.11b PCMCIA cards • Antennas • Sector Antenna – 17 dBi • Omni Directional Antenna – 8 dBi • Software • Linux – kernel 2.6.11 • Modified HostAP driver – ver 0.4.9 • Each experiment broadcasts 6000 1400 byte pkts with 20ms gap at 4 different data rates Hardware same as in DGP study, Roofnet study

  50. Controlled Interference Expt • Experimental Setup • A: 1400-byte packets, 2ms interval, 11Mbps • B: 1300-byte packets, 2ms interval, 11Mbps • B’s power fixed at -75 dBm • A’s power varied: -90, -85, -80, -75 dBm

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