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

6. 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

7. 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

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

9. 802.11 BSS  Basic Service Set (BSS) --- a basic LAN  Infrastructure BSS  Independent BSS (Ad Hoc LAN) Access point

10. 802.11 ESS  Extended Service Set (ESS) Distributed System

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

14. Comparison of Bluetooth to 802.11b ParameterBluetooth802.11b Bandwidth1 Mbps11 Mbps Range10 meters100 meters ProfilesAlmost unlimitedAP, STA Current consumption60mA300mA AudioPCM channelsvoice/ 802.3 Cable replacementSerial, USB, Audio802.3 Circuit cost (9/2001)$11.00$46.00 Ad hoc networkingmulti-hopsingle-hop

16. Bluetooth or 802.11?

17. 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

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

19. 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.

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

21. 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

22. 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

23. Tightly-Coupled Sensor Array

24. Artificial Retina

25. Smart Clothing & Wearable Computing  Smart Underwear  Smart Eyeglasses  Smart Shoes  …

26. Berkeley Smart Dust  bi-directional communications  sensor: acceleration and ambient light  11.7 mm 3 total circumscribed volume  4.8 mm 3 total displaced volume

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

28. 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

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

30. Is IEEE 802.11 Scalable?

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

32. 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.

33. 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. window beacon interval

34. 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. faster adopts 12:0112:00 slower not adopts 12:01 12:02 12:01

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

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

37. Performance of TSF

38. 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) 1n1n Prob <

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

40. 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 faster C(x) +1 x slower C(x) -1

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

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

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

44. Compatible with current TSF  Suppose some nodes do not implement the new protocol.

45. Performance of Modified TSF

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

47. What’s Next?  IBSS: single-hop  MANET: multihop ?? transmission range

48. Time Synchronization in 802.11-based MANET

49. 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)

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

51. Dominating Set  A set of nodes that covers the entire graph. connected dominating set

52. Constructing CDS  Many existing algorithms.  Layer 3 algorithms – useful for routing, useless for our purpose.

53. A New CDS Algorithm  Embedded in TSF (time sync function)  Node exchanging info via beacons  Overhead: 3 bits per beacon (550 bits)  Assumption: unique fastest clock window beacon interval

54. DS, Bridges, Covered, Uncovered Nodes DS

55. Constructing a DS: basic idea  Initially, DS contains a single node.  The fastest node enters DS.  Bridges keep entering DS until no more bridges. DS

56. Example  Fastest node enters DS.  Bridges keep entering DS until no more bridges.

57. Design Issue #1  How to recognize the fastest node, bridges, DS nodes, covered nodes, uncovered nodes thru beacons? SA Timestamp Beacon

58. Design Issue #2  How to minimize the number of bridges entering DS?

59. Design Issue #3  Cope with topology change and node mobility. B A A B

60. Design Issue #4  How to merge two subnets? Easy & hard. ?

61. Design Issue #5: MANET Formation  How to form a MANET from scratch? ?

62. Another way of MANET formation ?

63. Assumptions  Formation: MANET initiated by a single node.  Connectivity: MANET remains connected.

64. Summary of Design Issues 1. How to recognize the fastest node and bridges? 2. How to control the number of bridges entering DS? 3. How to cope with topology change and node mobility? 4. How to merge subnets?

65. Initialization Rule 1:  Let the starting node enter the DS.

66. Rule 2:  A node x recognizes itself as the fastest if T(beacons) < T(x) for the last k received beacons.  The fastest enters DS Am I the Fastest? 1:00 12:01 3:45 8.16 1.00 1.01 7:591.01 0:59 1:33 1:32 1:31 1:30 1.00 10:01 1:35

67. Solution for Design Issue #1  How to recognize fastest node and bridges, DS nodes, covered nodes, uncovered nodes thru beacons? SA Timestamp Beacon

68. Adding Bridges to DS Rule 3:  In each beacon interval, let bridge i enter DS with probability P(i).  Desired properties of P(i)? DS

69. Does it construct a CDS? R1. The starting node enters DS. R2. The fastest node enters DS. R3. Each bridge enters DS with a probability. DS, yes. CDS, not necessarily.

70. How to make it connected?  Gateway: a covered node receiving a beacon from a with a far smaller timing. Rule 4:  Let gateways enter DS. 12:05 12:04 12:03 12:32 12:30 12:20

71. How fast can gateways be recognized?  Depends on the drift rate difference between fastest node and A.  The higher the drift rate, the easier and faster to recognize gateways. A

72. Is the resulting DS always connected?  Not necessarily  Not a problem as far as clock sync is concerned.

73. What if we do need a connected DS?  Is it possible to always construct a CDS using only beacons? Yes.

74. A problem: entrance only, no exit. R1. The starting node enters DS. R2. The fastest node enters DS. R3. Each bridge enters DS with a probability. R4. Each gateway enters DS.

75. Exit Rules R1. The starting node enters DS. R2. The fastest node enters DS. R3. Each bridge enters DS with a probability. R4. Each gateway enters DS. R2’. If no longer the fastest, leaves the DS.

76. Exit Rules R1. The starting node enters DS. R2. The fastest node enters DS. R3. Each bridge enters DS with a probability. R4. Each gateway enters DS. R3’ & R4’. Leaves DS after a random amount of time.

77. Maximum Clock Drift – 802.11b vs. DS-based 802.11b DS-based

78. Summary  Proposed: a DS-based clock sync protocol  By-product: an algorithm for constructing DS.  DS: mostly connected, occasionally not.  What’s next?

79. Constructing Connected Dominating Sets in Mobile Ad Hoc Networks

80. Requirements  Nodes exchange info via beacons.  Info added to beacons is fixed in size and as little as possible. SA Time Beacon (~550 bits)

81. Assumption  MANET remains connected

82. Constructing Connected DS  Important to recognize gateways.  Use time difference between components to recognize gateways. DS

83. Dilemma  Time synchronization: Good if every node relays time info. Good if clocks are (almost) identical in accuracy.  Constructing CDS: Good if nodes in DS only relay time info. Good if clocks are quite different in accuracy.

84. Resolving the Dilemma

85. What If the Initiator Crashes?

86. What if MANET gets disconnected?