Wireless Sensor Networking: Understanding Radio, MAC, & Routing Protocols

1. Wireless Sensor Networking(Understanding the radio, MAC, & routing protocols) Romit Roy Choudhury

2. Sensor Networking – Why ?? • Data Collection – A basic need • Will the volcano erupt? Need temperature/gas signatures • How much Global Warming? Need ocean current data • How many enemy tanks crossed? • Human monitoring possible/feasible ? • Often risky, impenetrable, costly, … • But science has collected data for centuries … • Manual (wired) placement, periodic human visits • Wireless data transmitters • Community accepted barriers/defiiencies

3. New Opportunities • Device miniaturization • Moore’s law • Processors envisioned as smart dust • Innovations in wireless communication • Low power communication • Antenna sizes smaller with high frequency Device + RF + sensors - A new breakthrough: Scattered sensor motes self-organize themselves forming a network. Sensed data aggregated, processed, and transported to base station. Low risk, low cost, and heavy penetration

4. Plethora of Applications

5. Plethora of Challenges • Devices • Reducing energy consumption • Heavy programming constraints (16 KB RAM) • Wireless Radio Network • Reliable low power communication • Medium access control (MAC) • Network wide energy conservation • Routing • Aggregation, compression, suppression • …

6. Today’s Talk • Understanding the wireless channel • The departure from wireline • The key challenges • Medium access control • Protocol design • Energy-awareness (coordinated sleeping) • Routing • Unicast (Diffusion) • Broadcast (Gossip)

7. The Wireless Channel

8. Many Motivations for Wireless • Unrestricted mobility / deployability • Unplugged from power outlet • Significantly lower cost • No cable layout, service provision • Low maintenance • Ease • Direct communication with minimum infratructure

9. From Links to Networks • Variety of architectures • Single hop networks • Multi-hop networks

10. Internet The Wireless Future …

11. No Free Lunch • Numerous challenges • Channel fluctuation • Lower bandwidth • Limited Battery power • Disconnection due to mobility • Security • …

12. Question Is … Can’t we use the rich “wireline” knowledge ? In solving the wireless challenges

13. The Answer Wireless channel: A dispersive medium The PHY and MAC layer completely dissimilar The whole game changes

14. On Our Agenda • Quick Glimpse • Medium Access Control • Wired • Wireless • The emergence of 802.11 • Evolution of sensor network MAC protocols • Energy awareness

15. Medium Access Control

16. The Channel Access Problem • Multiple nodes share a channel • Pairwise communication desired • Simultaneous communication not possible • MAC Protocols • Suggests a scheme to schedule communication • Maximize number of communications • Ensure fairness among all transmitters A B C

17. The Trivial Solution • Transmit and pray • Plenty of collisions --> poor throughput at high load A B C

18. The Simple Fix Don’t transmit • Transmit and pray • Plenty of collisions --> poor throughput at high load • Listen before you talk • Carrier sense multiple access (CSMA) • Defer transmission when signal on channel A B C Can collisions still occur?

19. CSMA collisions spatial layout of nodes Collisions can still occur: Propagation delay non-zero between transmitters When collision: Entire packet transmission time wasted note: Role of distance & propagation delay in determining collision probability

20. CSMA/CD (Collision Detection) • Keep listening to channel • While transmitting • If (Transmitted_Signal != Sensed_Signal)  Sender knows it’s a Collision  ABORT

21. 2 Observations on CSMA/CD • Transmitter can send/listen concurrently • If (Sensed - received = null)? Then success • The signal is identical at Tx and Rx • Non-dispersive The transmitter can DETECT if and when collision occurs

22. Unfortunately … Both observations do not hold for wireless Leading to …

23. Wireless Medium Access Control C D A B Signal power SINR threhold Distance

24. Wireless Media Disperse Energy A cannot send and listen in parallel C D A B Signal power Signal not same at different locations SINR threhold Distance

25. Collision Detection Difficult • Signal reception based on SINR • Transmitter can only hear itself • Cannot determine signal quality at receiver

26. Calculating SINR B A C

27. Red < Blue = collision Red signal >> Blue signal C D X A B Signal power SINR threhold Distance

28. Important: C has not heard A, but can interfere at receiver B C is the hidden terminal to A C D X A B Signal power SINR threhold Distance

29. Important: X has heard A, but should not defer transmission to Y Y X is the exposed terminal to A C D X A B Signal power SINR threhold Distance

30. A Project Idea! C D X A B Signal power SINR threhold Sensitivity threshold Distance

31. A Project Idea! Do not transmit in this region Will this solve the wireless MAC problem? C D X A B Signal power SINR threhold T Sensitivity threshold Distance

32. The Emergence of 802.11 • Wireless MAC proved to be non-trivial • 1992 - research by Karn (MACA) • 1994 - research by Bhargavan (MACAW) • Led to IEEE 802.11 committee • The standard was ratified in 1999

33. RTS = Request To Send CTS = Clear To Send IEEE 802.11 with Omni Antenna M Y S RTS D CTS K

34. IEEE 802.11 with Omni Antenna silenced M Y silenced S Data D ACK silenced X K silenced

35. But is that enough?

36. RTS CTS RTS/CTS • Does it solve hidden terminals ? • Assuming carrier sensing zone = communication zone E F A B C D E does not receive CTS successfully  Can later initiate transmission to D. Hidden terminal problem remains.

37. Hidden Terminal Problem • How about increasing carrier sense range ?? • E will defer on sensing carrier  no collision !!! RTS E F CTS A B C D Data

38. Hidden Terminal Problem • But what if barriers/obstructions ?? • E doesn’t hear C  Carrier sensing does not help RTS E F CTS A B C D Data

39. Exposed Terminal • B should be able to transmit to A • RTS prevents this E RTS CTS A B C D

40. Exposed Terminal • B should be able to transmit to A • Carrier sensing makes the situation worse E RTS CTS A B C D

41. Thoughts ! • 802.11 does not solve HT/ET completely • Only alleviates the problem through RTS/CTS and recommends larger CS zone • Large CS zone aggravates exposed terminals • Spatial reuse reduces  A tradeoff • RTS/CTS packets also consume bandwidth • Moreover, backing off mechanism is also wasteful The search for the best MAC protocol is still on. However, 802.11 is being optimized too. Thus, wireless MAC research still alive

42. Questions?

43. Energy-Awareness • 802.11 optimizes for throughput/latency • Energy savings is second priority • Unattended sensor networks • Operate on AA batteries • Yet, expected to last for months or years • Energy-awareness is the key • Throughput and latency is secondary

44. An Energy-Efficient MAC Protocol for Wireless Sensor Networks (S-MAC) Wei Ye, John Heidemann, Deborah Estrin

45. Major source of energy waste • Collision • Overhearing • Control Overhead • Idle Listening • Listening to possible traffic that is not sent • 50%-100% energy drain compared with receiving

46. Avenues to Reduce Energy Consumption (1) Periodic listen and sleep (2) Collision avoidance (3) Overhearing avoidance (4) Message passing

47. (1) Periodic Listen and Sleep • The main idea • Put nodes to sleep periodically • Called “Duty Cycles” • However, ensure that sleep/wake-up is synchronous

48. B Listen/Sleep Schedule Assignment Choosing Schedule (1) Synchronizer • Listen for a mount of time • If hear no SYNC, select its own SYNC • Broadcasts its SYNC immediately Listen A Sleep Go to sleep after time t Listen for SYNC Broadcasts Follower • Listen for a mount of time • Hear SYNC from A, follow A’s SYNC • Rebroadcasts SYNC after random delay td Listen Go to sleep after time t- td Sleep td Broadcasts

49. A Listen/Sleep Schedule Assignment Choosing Schedule (2) • B receives A’s schedule and rebroadcast it. 2. Hear different SYNC from C 3. Adapt both schedules Listen Sleep Go to sleep after time t1 Listen for SYNC Broadcasts Listen B Sleep td Broadcasts Only need to broadcast once Nodes only rarely adopt multiple schedules Listen C Sleep Go to sleep after time t2 Listen for SYNC

50. Keeping Clocks in SYNC • SYNC packets must not collide • Reserve separate time window for SYNC transmission