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Introduction to Wireless Ad Hoc and Sensor Networks: From IEEE 802.11 to Berkeley Motes

Introduction to Wireless Ad Hoc and Sensor Networks: From IEEE 802.11 to Berkeley Motes. Ten-Hwang Lai Ohio State University. Outline. Wireless LANs Ad Hoc Networks IEEE 802.11 Bluetooth Berkeley Motes. Wireless LANs. IEEE 802.11 Bluetooth HiperLan (Europe). History of IEEE 802.11.

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Introduction to Wireless Ad Hoc and Sensor Networks: From IEEE 802.11 to Berkeley Motes

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  1. Introduction toWireless Ad Hoc and Sensor Networks:From IEEE 802.11 to Berkeley Motes Ten-Hwang Lai Ohio State University

  2. Outline Wireless LANs Ad Hoc Networks IEEE 802.11 Bluetooth Berkeley Motes

  3. Wireless LANs IEEE 802.11 Bluetooth HiperLan (Europe)

  4. History of IEEE 802.11 • 802.11 standard first ratified in 1997 • 802.3 LAN emulation • 1 & 2 Mbps in the 2.4 GHz band • Two high rate PHY’s ratified in 1999 • 802.11a: 6 to 54 Mbps in the 5 GHz band • 802.11b: 5.5 and 11 Mbps in the 2.4 GHz band

  5. The Beat Goes On • 802.11d: new support for 802.11 frames • 802.11c: support for 802.11 frames • 802.11e: QoS enhancement in MAC • 802.11f: Inter Access Point Protocol • 802.11g: 2.4 GHz extension to 22 Mbps • 802.11h: channel selection and power control • 802.11i: security enhancement in MAC • 802.11j: 5 GHz globalization

  6. Can Bluetooth Compete with 802.11? • IEEE 802.11 already has been widely accepted. • What’s Bluetooth chance of success stacking against 802.11?

  7. 802.11 BSS Basic Service Set (BSS) --- a basic LAN Infrastructure BSS Independent BSS (Ad Hoc LAN) Access point

  8. 802.11 ESS Extended Service Set (ESS) Distributed System

  9. Bluetooth Piconet & Scatternet Master Master Slaves Slaves S M Piconet Master Slaves Scatternet

  10. Comparison of Bluetooth to 802.11b Parameter Bluetooth 802.11b Bandwidth 1 Mbps 11 Mbps Range 10 meters 100 meters Profiles Almost unlimited AP, STA Current consumption 60mA 300mA Audio PCM channels voice/802.3 Cable replacement Serial, USB, Audio 802.3 Circuit cost (9/2001) $11.00 $46.00 Ad hoc networking multi-hop single-hop

  11. Bluetooth or 802.11?

  12. Can Bluetooth Compete with 802.11? • IEEE 802.11 already has been widely accepted. • What’s Bluetooth chance of success stacking against 802.11? Answer: ? 802.11 --- WLAN Bluetooth-- WPAN

  13. Ad Hoc Networking • BT Scatternet --- multihop? • 802.11 --- single hop? Master Slaves S M Master Slaves

  14. BT Scatternet Formation • Problem: design a protocol that given a set of bluetooth nodes organizes the nodes into a scatternet. • Still an interesting research problem.

  15. A Sensor Node Memory (Application) Processor Network Interface Actuator Sensor

  16. Berkeley Mote: a sensor device prototype • Atmel ATMEGA103 • 4 Mhz 8-bit CPU • 128KB Instruction Memory • 4KB RAM • RFM TR1000 radio • 50 kb/s • Network programming • 51-pin connector • Analog compare + interrupts

  17. Berkeley DOT Mote • Atmel AVR 8535 • 4MHz • 8KB of Memory • 0.5KB of RAM • Secondary store • Low power radio • Power consumption • Active 5mA • Standby 5μA

  18. Tightly-Coupled Sensor Array

  19. Artificial Retina

  20. Smart Clothing & Wearable Computing • Smart Underwear • Smart Eyeglasses • Smart Shoes • …

  21. Berkeley Smart Dust • bi-directional communications • sensor: acceleration and ambient light • 11.7 mm3 total circumscribed volume • 4.8 mm3 total displaced volume

  22. Is IEEE 802.11 Suitable for Supporting Large-Scale Multihop Ad Hoc Networks? Ten-Hwang Lai Ohio State University

  23. Approach to the Problem • Now: single-hop, small-scale • Future: multi-hop, large scale? Single-hop, Small-scale Single-hop, Large-scale Multi-hop,Large-scale

  24. Topics • Is IEEE 802.11 (single-hop) scalable? • Time sync in multihop ad hoc networks. • Constructing connected dominating sets by way of clock synchronization.

  25. Is IEEE 802.11 Scalable?

  26. Problem Statement • Can 802.11 support a large-scale ad hoc network? • Large scale – say, a few hundred nodes

  27. 802.11 Timers (Clocks) • Timer: 64 bits, ticking in microseconds. • Accuracy: within + 0.01%, or +100 ppm. • Time synchronization needed for: • Frequency hopping • Power-saving mode • ∆ = max tolerable difference between clocks.

  28. 802.11’s Time Sync Function (I) • Time divided into beacon intervals, each containing a beacon generation window. • Each station: • waits for a random number of slots; • transmits a beacon if no one else has done so. • Beacon: several slots in length. beacon interval window

  29. 802.11’s Time Sync Function (II) • Beacon contains a timestamp. • On receiving a beacon, STA adopts beacon’s timing if T(beacon) > T(STA). • Clocks move only forward. 12:01 12:02 12:01 12:00 12:01 faster slower adopts not adopts

  30. Problems with 802.11’s TSF • Faster clocks synchronize slower clocks. • Equal opportunity for nodes to generate beacons. 1:16 1:17 1:18 1:19 1:21 1:23 1:21 1:22 1:23 1:25 1:28 1:31 1:18 1:18 1:18 1:19 1:21 1:23 1:23 1:23 1:23 1:25 1:28 1:31 1:10 1:11 1:12 1:13 1:14 1:15 1:13 1:13 1:13 1:13 1:14 1:15 +3 +4 +5 +6 +7 +8 +3 +4 +5 +6 +7 +8

  31. The Out-of-Sync Problem When the number of stations increases • More beacon contention • Fastest station send beacons less frequently Stations out of sync

  32. Performance of TSF

  33. How to fix it? • Desired properties: simple, efficient, and compatible with current 802.11 TSF. • Causes of out-of-sync • Unidirectional clocks • Equal beacon opportunity • Single beacon per interval • Beacon contention (collision) 1 n Prob <

  34. Improve fastest station’s chance • Let the fastest station contend for beacon generation more frequently than others.

  35. Adaptive Clock Sync Protocol • Station x participates in beacon contention once every C(x) intervals. • Initially, C(x) =1. Always, 1 < C(x) < Cmax. • Dynamically adjust C(x): x x C(x) +1 faster C(x) -1 slower

  36. Once the protocol converges Fastest station, C(x) =1 Other stations, C(x) = Cmax (Cmax= ?)

  37. What if the fastest node leaves the IBSS? • The previously second fastest now becomes the fastest. Its C(x) will decrease to 1.

  38. What if a new fastest node enters the IBSS? • The previously fastest now no longer the fastest. Its C(x) will increase to Cmax.

  39. Compatible with current TSF • Suppose some nodes do not implement the new protocol.

  40. Performance of Modified TSF

  41. Summary • Showed: the IEEE 802.11 Timing Sync Function (TSF) is not scalable. • Proposed: a simple remedy compatible with the current TFS.

  42. What’s Next? • IBSS: single-hop • MANET: multihop ?? transmission range

  43. Time Synchronization in 802.11-based MANET

  44. Out-of-sync problem in MANETs • More sever than in IBSS because of hidden terminals. • Recall: causes of out-of-sync • Unidirectional clocks • Equal beacon opportunity • Single beacon per interval • Beacon contention(collision)

  45. Select a subset of nodes to generate beacons more frequently than the rest. What subset? Basic Idea fastest node+ (connected) dominating set

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