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

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

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

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

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.

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.

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.

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

Tone Detection Tone detection requires energy accumulation over time T detect > symbol detection time for data -> deafness while transmitting tone leading to failure to hear other tone. Conditions of deafness – NOZ < T detect ---- (1) NOZ = T tone - (t tx,B + T tone – t r x,A->B ) Where, t r x,A->B is the time when B receives A’s tone. t tx,B = t tx,A T A,B < T detect From (1) D deaf = T detect * V sound (3) Bidirectional deafness (t tx,B - t tx,A ) - T A,B < T detect --- (4) (t tx,A - t tx,B ) + T A,B < T detect ----(5) (4) and (5) are unidirectional deafness conditions

Tone Detection Time Difference of Transmission (TDT) = t tx,B - t tx,A Time Difference of Location (TDL) = T A,B Generalized Deafness Condition (GDC) – |TDT – TDL| < T detect ---(6)

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.

T-Lohi

T-Lohi Flavors Synchronized T-Lohi (ST-Lohi) – each contention round is synchronized. CR ST = Ʈ max + T tone Ʈ max worst case one way propagation delay T tone – 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.

T-Lohi Flavors

ST-Lohi BackOff Algorithm  Δ T = propagation delay relative to the start of the slot. SAI = 1- Δ T/ CR ST 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).2 FCC | 5:end if

Conservative Unsynchronized T-Lohi (cUT) Nodes contend anytime -> worst case CR cUT = 2T tone + 2 Ʈ max to observe the channel. C transmits at t c. Worst case contender A transmits at -> t c + T tone + Ʈ max - ε, just before it hears C’s transmission. t c + T tone + 2 Ʈ max - ε -> A’s tone arrives at C. Cannot determine FCC

Aggressive UT-Lohi (aUT-Lohi) cUT-Lohi ’s long contention reduces throughput. aUT- Lohi cuts the duration of its contention round to CR aUT = T tone + Ʈ max 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.

Protocol correctness Tone-data collision and data-data collision can lead to incorrect channel reservation  solved by higher contention. Tone-data collision – TDT < (TDL + T tone ) – interferer B transmits before A’s tone is detected by B. This is superset of deafness condition. ---(7)

Protocol correctness Data-Data collision –In ST-Lohi, as a result of bidirectional deafness  Nodes are closer than D deaf. –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.

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 P tx = 650ms Tone detection = 5ms Channel utilization = P tx /(P tx + CR) – ratio of data to frame length. µ = P tx /CR

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.

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.

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

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)

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

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.

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.

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.

Modem Hardware