T – Lohi: A New Class of MAC Protocols for Underwater Acoustic Sensor Networks

1. T – Lohi: A New Class of MAC Protocols for Underwater Acoustic Sensor Networks Credit: borrowed from University of Delaware 20083127 Kim Jae Hong

2. Agenda • Challenges of Medium Access Control (MAC) • About the paper • Flavors of T-Lohi • Protocol correctness • Simulation results • Why T-Lohi? • Conclusion

3. Challenges of Medium Access Control (MAC) • Wireless MACs • Lack of ability to detect collisions (hence use of CSMA/CA) • Inconsistent view of network (hidden and exposed terminal problem) • Underwater sensornets (UWSN) – shared acoustic medium • Magnifies wireless bandwidth limitations • Transmit energy costs (transmission is expensive, 1:125) • Acoustic propagation latencies

4. This paper is about – Tone Lohi • Focus of paper – energy efficient, stable, fair and throughput efficient MAC protocol – Tone Lohi (Lohi – slow in Hawaiian) • First part describes – unique characteristics of high latency acoustic medium access. • Space-time uncertainty • Deafness conditions • Second part proposes – T-Lohi for UWSN • Exploits propagation latency for channel reservation • Exploits space-time uncertainty to detect and count contenders

5. Space-time uncertainty • In RF networks, channel state estimated at transmit time, as propagation delay is minimum. • Large propagation delay -> we need to consider location of receiver and transmitter time – space-time uncertainty. • Helps detect and count contenders.

6. Clear Channel Assessment • Clear Channel Assessment (CCA) – sampling of medium for activity. • Not perfect – delay between sensing the channel and beginning the transmission. • Extension – sample for entire slot and send after a clear slot. • Drawbacks – high latency, collisions (same slot selected), degrade efficiency, energy loss.

7. Spatial Unfairness • Channel becomes clear earlier at nodes closer to the transmitter. • Therefore, 2 nodes can monopolize the channel, even if transmitter is not allowed to contend in next slot. • Solution – distributed random backoff.

8. Contender Detection and Counting • Contender Detection (CTD) – listening to channel after sending your tone. • Space-time uncertainty -> contender counting {iff} tones are short relative to contention round and larger propagation delay. • Wireless transceivers are half-duplex -> node unable to receive completely when transmitting ->deafness

9. Tone Detection • Tone detection requires energy accumulation over time Tdetect > symbol detection time for data -> deafness while transmitting tone leading to failure to hear other tone. • Conditions of deafness – NOZ < Tdetect ---- (1) • NOZ = Ttone - (ttx,B + Ttone – tr x ,A->B) Where, tr x ,A->B is the time when B receives A’s tone. ttx,B = ttx,A TA,B < Tdetect From (1) • Ddeaf = Tdetect * Vsound ----- (3) Bidirectional deafness • (ttx,B - ttx,A) - TA,B < Tdetect --- (4) • (ttx,A - ttx,B ) + TA,B < Tdetect ----(5) • (4) and (5) are unidirectional deafness conditions

10. Tone Detection • Time Difference of Transmission (TDT) = ttx,B - ttx,A • Time Difference of Location (TDL) = TA,B • Generalized Deafness Condition (GDC) • |TDT – TDL| < Tdetect ---(6)

11. T-Lohi • Tone Reservation – Each frame consists of series of CRs that conclude with one node reserving the channel and sending data. • Nodes send a short tone and listen for CR. If one node contends-> successful. Many nodes -> detect contention -> each backoffs ->try again in later CR->extends RP. • CR is long enough for CTD and CTC. • Data Transfer – general modem receivers and host CPU is off, activated when a tone is detected by low power wake up receiver. • Data is preceded by a contention tone ->enables a node to distinguish between contention indicator and actual data.

12. T-Lohi

13. T-Lohi Flavors • Synchronized T-Lohi (ST-Lohi) – each contention round is synchronized. CRST = Ʈmax + Ttone Ʈmax worst case one way propagation delay Ttone – tone detection time • Tones are sent at beginning of CR -> arrive before end and detected. • Can decide backoff policy at start of every new frame -> depending on number of contenders.

14. T-Lohi Flavors

15. ST-Lohi • BackOff Algorithm ΔT = propagation delay relative to the start of the slot. SAI = 1- ΔT/ CRST Nodes already contended are prioritize by setting the variable didCntd, over other nodes. Nodes having higher SAI are likely to wait an extra slot. ST-Lohi BackOff (FCC, didCntd, SAI ) 1:if didCntd=true then 2: return |(random[0,1] +SAI).FCC| 3:else 4: return|(random[0,1] + SAI).2FCC| 5:end if

16. Conservative Unsynchronized T-Lohi (cUT) • Nodes contend anytime -> worst case CRcUT = 2Ttone + 2 Ʈmax to observe the channel. • C transmits at tc. • Worst case contender A transmits at -> tc + Ttone + Ʈmax - ε , just beforeit hears C’s transmission. • tc + Ttone + 2 Ʈmax - ε -> A’s tone arrives at C. • Cannot determine FCC

17. Aggressive UT-Lohi (aUT-Lohi) • cUT-Lohi ’s long contention reduces throughput. • aUT- Lohi cuts the duration of its contention round to CRaUT = Ttone+ Ʈmax • Does not account for worst case timings and results in either in tone detection or tone-data collision or data-data collision. • From fig- Tone-data collision near C, however, A gets C’s tone and hence backoffs, and C assumes it won and transmits the data. Node A receives C’s data.

18. Protocol correctness • Tone-data collision and data-data collision can lead to incorrect channel reservation  solved by higher contention. • Tone-data collision – TDT < (TDL + Ttone) – interferer B transmits before A’s tone is detected by B. This is superset of deafness condition. ---(7)

19. Protocol correctness • Data-Data collision • In ST-Lohi, as a result of bidirectional deafness  Nodes are closer than Ddeaf. • In aUT-Lohi, as a result of pseudo-bidirectional deafness (deafness conditions + tone-data collision). condition (6) and (7) High Contention (from fig) – Adding a contender to two deaf nodes contending for frame, would break the deafness.

20. Performance Evaluation • Simulation parameters: 300*400m area for a fully connected network, acoustic modem with 500m range. Data rate 8kb/s and packet length 650bytes. • Packet transmission duration Ptx = 650ms • Tone detection = 5ms • Channel utilization = Ptx/(Ptx + CR) – ratio of data to frame length. • µ = Ptx/CR

21. Throughput as load varies • ST-Lohi close to maximum theoretical utilization for offered load less than 0.5packet/sec. • For higher load(0.5-1 pkt/s), 50% of maximum utilization. Longer Reservation period (1.6 to 3.3 contention rounds). • For load > 1 pkt/s, ST-Lohi is stable.

22. Channel Utilization of Three T-Lohi Flavors • All three are efficient at low load and stable at high load  CTD and CTC. • cUT has a saturation capacity, about 2/3 of aUT longer contention rounds. • aUT has higher utilization than ST  ST delays all access attempts to start of next slot.

23. Energy Efficiency • Relative energy overhead for the three T-Lohi protocols for an 8 node network

24. Impact of Deafness and Aggression • cUT experiences no collision at any offered load -> long CR. • ST-Lohi has very few packet losses, high variability. • aUT, collisions increase with network load  pseudo-bidirectional deafness (2 pkts are lost)

25. Correcting the Loss • Adding more contenders can reduce the packet loss in aUT.

26. Correcting the Loss • For ST-Lohi, for 2 node network, few packets lost with high variance (topology dependent bidirectional deafness). • For 4 node network, deafness condition is broken by extra contender.

27. Why T-Lohi • The existing terrestrial RF-based MAC protocols do not cater the special needs of high latency acoustic networks. • Space-time uncertainty. • Energy efficiency. • The satellite MACs considers the large propagation delay. • They have abundant bandwidth and also do not consider energy efficiency. Hence need for special MAC protocol for underwater sensor networks.

28. Conclusion • Simulation results show: • ST-Lohi is most energy efficient protocol, within 3% of optimal energy. • aUT achieves highest throughput, ~ 50% channel utilization. • cUT provides the most robust packet delivery with almost no packet loss. • All three flavors exhibit efficient channel utilization, stable throughput and excellent energy efficiency.

29. Modem Hardware