1. FRACTEL: Building (Rural) Mesh Networks with Predictable Performance On the Feasibility of the Link Abstraction in (Rural) Mesh Networks Bhaskaran Raman, Kameswari Chebrolu IIT Bombay Dattatraya Gokhale, Sayandeep Sen Work done at IIT Kanpur Presentation at AirTight Networks, Pune, 22 Dec 2007

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

3. FRACTEL Goals Support a variety of applications: –HTTP/FTP –Voice over IP –Video-conferencing based –Real-time applications particularly important in rural setting Quality of Service is necessary Scalable operation: –Deployment for a few hundred nodes in a district

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

5. Long Distance Networks (LDNs) "Long-Distance 802.11b Links: Performance Measurements and Experience'', Kameswari Chebrolu, Bhaskaran Raman, and Sayandeep Sen, MOBICOM 2006 Link abstraction holds

6. 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

7. Link Abstr.: DGP, Roofnet, FRACTEL Typical link distances Network architecture Environme nt Multipath effects SNR or RSSI External interference Link abstra ction 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 LACNs Mostly < 500 m Would like to avoid tall towers Rural, campus, residential To be determined

8. 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

9. FRACTEL Measmt. Study: IITK Source: Google Maps

10. FRACTEL Measmt. Study: Amaur Source: Google Maps

11. 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

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

13. MIT Roofnet Data: 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 Roofnet: Noise band as high as 16dB; Max noise = -75dBm Our expts: Noise band ~ 2 dB, Max Noise = -94 dBm 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 ?

15. Understanding Interference Does interference increase the noise level reported by the card? Can packet loss be related to the number of foreign packets seen? Can the reported noise level be used to gauge the level of interference? Can we estimate the link performance based on the average measured noise floor?

16. 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

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

18. Can packet loss be related to foreign packets seen? 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

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

20. Can the reported noise level be used to gauge the level of interference? –Instantaneous noise levels show variability –Noise levels reported differ from known level P4: On this H/W, gauging level of interference is error prone

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

22. Conclusion so far External interference can cause high loss rates (from our expts) Intermediate loss rates in Roofnet possibly due to interference and not multi-path Estimating level of interference from noise floor (for use in routing etc) is difficult

23. 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

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

25. Operating near the threshold Village Location Cannot distinguish between links with loss rates between 0-100% Cannot use routing metrics based on ETX or WCETT

26. Design Implications: Link Abstraction Allows us to plan links with predictable performance. Simplifies higher layer protocol design considerably RSSI Threhold -79dBm RSSI variation 3-4dB Modified RSSI Threshold Value -75dBm

27. Design Implication: Routing Routing metrics (ETX and WCETT) that tend to distinguish between links with intermediate loss rates are unstable Opportunistic routing (EXOR) cannot give performance guarantees Gauging interference can be error prone in interference-aware routing

28. Design Implications: 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

29. Discussion Multipath and delay spread: –Not a likely factor in rural areas –May or may not be the culprit in urban areas –Roofnet extrapolated multipath measurements done in 900 MHz cellular band! Lesson: wireless measurements  pay attention to detail, double-check Open questions: –802.11g and 802.11a? –Future of unplanned deployments?

30. 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/http://www.cse.iitb.ac.in/~br/