A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks

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1 : An Adaptve, Anycast MAC Protocol for Wreless Sensor Networks Hwee-Xan Tan and Mun Choon Chan Department of Computer Scence, School of Computng, Natonal Unversty of Sngapore {hweexan, Abstract Energy constrants n wreless sensor nodes necesstate the desgn and development of energy-effcent MAC protocols to arbtrate access to the shared communcaton medum. Whle there exsts a plethora of sensor MAC protocols, these protocols do not ndvdually vary the duty-cycle of each sensor accordng to local connectvty status, to maxmze energy savngs. In ths paper, we propose - an Adaptve, Anycast MAC protocol for low-powered wreless sensor networks. It utlzes: () a random wakeup schedule, such that each node can ndependently and randomly wakeup n each cycle wthout coordnaton and tme synchronzaton; () adaptve duty-cycles based on network topology; and () adaptve anycast forwarders selecton, whch allows each node to transmt to any member n ts forwardng set. There are two key adaptve mechansms n : () each node vares ts duty-cycle and set of forwardng nodes such that energy consumpton can be locally mnmzed for a gven local delay performance objectve; () nodes cooperatvely reduce the duty-cycles requred of ther forwardng nodes, dependng on local network connectvty. By allowng nodes to operate wth dfferent duty-cycles and forwardng sets, acheves better energy-latency tradeoffs and extends node lfetme substantally, whle provdng good end-to-end latency. I. INTRODUCTION Advancements n wreless networkng and technology have led to the prolferaton of tny computng and sensng devces that are capable of performng collaboratve tasks such as tactcal survellance and envronmental montorng. These network elements communcate wth one another va multhops wthout centralzed control and are densely deployed to maxmze sensng coverage. However, the nherent nature of wreless sensor networks, such as ntermttent connectvty and energy constrants, poses challenges to ther deployment and operaton. In partcular, the severe energy lmtaton of sensor nodes has receved much focus n the research communty. To reduce energy consumpton and prolong network lfetme, sensor networks are usually duty-cycled. Each node remans n low-power sleep mode most of the tme, and wakes up perodcally to sense for channel actvtes. The Medum Access Control (MAC) layer s responsble for arbtratng access to the shared medum n a far and effcent manner. Typcally, sensor MAC protocols ncorporate wakeup schedules nto the medum access control operaton, such that nodes need not montor the channel contnuously for communcaton. Performance studes [1][2][3][4][5][6][7][8] show that whle wakeup schedules are effectve n reducng energy consumpton n sensor networks due to the sporadc characterstcs of sensor traffc, the delay ncurred by watng for the next-hop forwardng node to be awake, vz. sleep latency, can be qute large. For example, a 1% duty-cycle can potentally reduce the energy consumpton of a network by 99% when no traffc s beng generated. However, the expected per-hop sleep latency of a packet s 5% of the cycle perod. The wakeup schedule s a key component n the desgn of a duty-cycled MAC to reduce energy consumpton. Synchronous schemes such as S-MAC [2][3], T-MAC [4], D-MAC [5] and R-MAC [6] requre nodes to synchronze wth each other, whch can be complex and expensve especally n large multhop networks wth clock drfts, low duty-cycles and transent lnk qualtes. Reducton n sleep latency s thus acheved at the expense of substantal control overhead. Asynchronous schemes such as B-MAC [7], [1] and C-MAC [8] rely on preambles to coordnate access to the channel and do not requre synchronzaton. Whle such asynchronous schemes are energy-effcent, they stll spend sgnfcant energy on dle lstenng and preamble samplng. The end-to-end latency can also be large (long sleep latency). In ths paper, we propose, an asynchronous Adaptve, Anycast MAC protocol for low-powered sensor networks. It utlzes () a random wakeup schedule, such that each node can ndependently and randomly select ts wakeup schedule wthout coordnaton and tme synchronzaton; () adaptve duty-cycles based on network topology; and () adaptve anycast forwarders selecton, whch allows each node to transmt to any member to ts forwardng set and effectvely reducng expected sleep latency. There are two key adaptatons n : () each node adaptvely vares ts duty-cycle and set of forwardng nodes such that energy consumpton s locally mnmzed for a delay performance objectve; and () nodes cooperatvely reduce the duty-cycles requred of ther forwardng nodes, dependng on local network connectvty. By explotng the redundancy of dense network deployments as well as combnng random schedules and anycast mechansms, nodes can operate wth dfferent duty-cycles and forwardng sets to reduce energy consumpton. We compare wth and the optmal protocol n [9] (hereafter referred to as for brevty) whereby all nodes use the same duty-cycle. Our evaluaton shows that acheves better energy-latency trade-offs and extends node lfetme substantally whle provdng good end-to-end latency. The rest of ths paper s organzed as follows. Secton II dscusses related work. Sectons III and IV detal the basc and adaptve (forwarder and duty-cycle selecton) components

2 2 of respectvely. We evaluate the performance of A 2 - MAC n Secton V and conclude n Secton VI. II. RELATED WORK In the poneer work on sensor MAC protocols, Ye et al [2] dentfes the man sources of energy consumpton n any contenton-based MAC protocol as: () collson; () overhearng; () control packet overhead; and (v) dle lstenng. It s hghlghted that n sensor networks wthout energy awareness, nodes expend most of ther energy n dle lstenng due to the sporadc nature of data traffc. Consequently, subsequent works on sensor MAC protocols always ncorporate some form of wakeup schedulng such that nodes do not reman awake throughout the entre network lfetme but wakeup at ntervals for communcaton and to check for channel actvty. Wakeup mechansms can be broadly classfed as: () ondemand; () synchronous; and () asynchronous. Sensor MAC protocols that make use of on-demand wakeup mechansms [1] requre out-of-band sgnalng (usng a low power rado) n order to wake up the nodes n tme for data recepton. In synchronous wakeup (or scheduled rendezvous) schemes such as S-MAC [2][3], T-MAC [4], D-MAC [5] and R- MAC [6], nodes wakeup durng the same desgnated tme slots to communcate, thus effectvely reducng dle lstenng. However, tght tme synchronzaton and pre-negotaton of schedules are necessary, whch ncurs hgh overheads. In asynchronous wakeup schemes such as B-MAC [7] and [1], the schedules of the sender(s) and the recever(s) are decoupled, thereby removng the need for any synchronzaton. Nodes wake up perodcally to check for any channel actvty - a technque commonly known as LPL (Low Power Lstenng). If channel actvty s detected, the node remans awake to receve the ncomng packet; otherwse, the node resumes sleepng. These asynchronous MAC protocols are uncast n nature and use the same duty-cycle for each node. Anycast MAC schemes [8][9], whereby a transmttng node sends a packet to any one of the members wthn a forwardng set, have been proposed n the context of sensor networks. A key dfference between and many exstng anycast protocols s that the latter uses the same duty-cycle for all nodes, whle vares duty-cycles on a per-node bass, leadng to better energy-latency trade-offs. Fnally, [11] studes the mpact of unrelable communcaton lnks on data forwardng n duty-cycled networks. One nterestng result as clamed by the authors s that opportunstc loopng can potentally reduce the overall delay. However, ways to desgn the wakeup schedule or adapt the duty-cycle are not presented, but assumed to be gven nputs to the problem. A. Basc Mechansm III. PROTOCOL DETAILS The wakeup schedule of s based on an asynchronous slot model. The schedule n each cycle s dvded nto: () actve (lstenng) slots, n whch nodes wakeup and Fg. 1. S f 2 S ms nactve (sleep) slot actve (lstenng) slot (1, 1, 2ms) random wakeup functon wth 3 unsynchronzed nodes. DATA t t t9 t t1 t2 t3 7 8 t4 Fg. 2. t t 1 nactve (sleep) mode t 2 t 3 t 4 t 5 t 6 t 7 t 8 Packet slot t 3 of S P P P P wakes slot t 2 of A P A D actve (lstenng) mode Data transfer n between 2 unsynchronzed nodes. montor the channel for actvtes (analogous to LPL n asynchronous MAC schemes); and () nactve (sleep) slots, n whch nodes swtch to low-powered sleep mode by default. We consder a random wakeup schedulng functon represented as a (v, α, τ) desgn, where v s the total number of slots per cycle (chosen to acheve a sleep latency constrant); α v s the number of actve lstenng slots per cycle; and τ s the duraton of each slot n the duty-cycle such that cycle length s v.τ. At the begnnng of each cycle, each node selects α out of v slots to be actve n, such that ts actve/awake probablty n any slot s α v. Ths selecton s done randomly and ndependently of other nodes to elmnate coordnaton and synchronzaton overheads, as well as provde ease of adaptaton of duty-cycles. In the (1, 1, 2ms) random wakeup functon of Fgure 1, nodes S, and f 2 each selects 1 actve slot out o avalable slots wthn a cycle, and the slots of each node may be unsynchronzed wth one another. Compared to synchronous schedules, the number of actve one-hop neghbors durng an arbtrary tme slot n s reduced, whch effectvely mnmzes collsons and reduces overhearng. However, the default actve slots of each node are unlkely to overlap, partcularly when duty-cycle s low. Hence, uses a probng mechansm to guarantee communcaton between the transmtter and ts forwarder (f one exsts) wthn a sngle cycle perod. In Fgure 2, S wakes up at (ts) slot t 1 accordng to ts wakeup functon n Fgure 1 and resumes sleepng at t 2. Upon a packet arrval at t 3, S swtches to actve (lstenng) mode and contnuously probes ts neghbors usng short preambles P n every subsequent slot. Probng termnates when S receves a preamble acknowledgement A P at t 6 from a forwarder that s awake. Transmsson completes when S transmts data to durng t 7 and receves a correspondng data acknowledgement A D n t 8. The probng for actve neghbors does not ncur addtonal delays or overheads as compared to exstng asynchronous MAC protocols, as all such protocols have to transmt preambles up to duraton of a cycle perod to guarantee commu-

3 3 S f 2 S t A t A strobed preambles P P P DATA early ACK A preambles as probes P P DATA A metrc that ndcates the progress made by each forwarder. Examples of such metrcs nclude hopcount to destnaton, geographcal dstance [13] and ETX [14]. In ths paper, we use the Maxmum Forward Progress (MFP) [15] routng metrc, whch forwards packets based on geographcal locatons. Only neghbors wth postve progress (closer to the snk than the transmtter) are consdered elgble forwarders. As no state nformaton s requred n MFP, ths allows us to study the performance of wthout routng overheads. f 2 Fg. 3. A P A D Protocol detals of and. ncaton between nodes. Fgure 3 llustrates behavors of X- MAC and upon a packet arrval at tme t A at S. Due to the uncast nature of, f 2 cannot forward packets for S even though t wakes up before, as s the desgnated forwarder for S. In contrast, acheves shorter delays and ncurs less overheads usng anycast. When collsons of A P s occur due to the presence of multple awake forwarders that detect P and transmt ther A P s at the same tme, each forwarder backoffs for a randomly chosen nterval before attemptng to retransmt ts A P. When there s only one forwardng node, behaves smlarly to. B. Advantages of Combnng Anycast wth Random Schedules Besdes path dversty and multpath routng [8][12][13], the anycast mechansm can provde robustness to ntermttent lnk connectvty and latency reducton n a duty-cycled MAC. 1) Robustness to Intermttent Lnk Connectvty: The transent characterstcs of the physcal layer leads to ntermttent lnk connectvty. Typcal MAC protocols attempt multple retransmssons across the same temporally-broken lnk before a lnk falure s ascertaned and an alternatve routng path s utlzed. Wth anycast, a node can dynamcally select ts forwarder based on prevalng lnk condtons and provde robustness to ntermttent connectvty. 2) Reducton n Latency: In contrast to synchronzed schemes where actve slots are congregated together, enables packets to be transmtted across multple hops n one cycle. Consderng the (1, 1, 2ms)-desgn n Fgure 1, a packet can be transmtted from S to n slots t 5 and t 6 (of S), and to f 2 n slots t 6 and t 7 (of ). As the actve slots n a cycle are randomly chosen, the average sleep latency T (measured n slots) of node before any one of ts forwarders s awake s dependent on v and α, computed by: v T = j F α j + 1, (1) where F s the forwardng set of ; and α j s the awake probablty (duty-cycle) requred of forwarder j F by. C. Interacton wth Routng Protocol s nter-operable wth any routng protocol that provdes: () a set of canddate forwardng nodes; and (2) a IV. ADAPTATION IN The prmary objectve of s to reduce the dutycycles (and energy consumpton) of nodes, n order to extend network connectvty and coverage, and subject to a desred delay constrant d max. In ths secton, we descrbe the two key adaptaton components of, vz. forwarder selecton and duty-cycle selecton, that help to acheve ths objectve. The canddate set ℵ of an arbtrary node s the set of onehop neghbors wth postve progress towards the destnaton (snk). ℵ can be learnt va a smple neghbor dscovery scheme durng network ntalzaton. For each canddate node j ℵ, d j denotes the progress made by when t transmts ts packet to the snk va j and α j (, v] denotes the duty-cycle requred of j by (n a cycle wth v slots). The forwardng set F ℵ s the set of neghbors wthn the canddate set that are selected to forward packets from to the snk. (d j F jα j) We defne D = to be the average per-hop α j j F progress. The correspondng average per-hop rate of progress s gven by V = D T, where T s the sleep latency n Equaton 1. Generally, the ncluson of more forwarders decreases T ; however, ncluson of forwarders wth small progress (d j ) decreases the average progress D and rate of progress V. The mnmum average per-hop rate of progress requred to satsfy the delay constrant d max s V mn = D max d max, where D max s the maxmum dstance from any node to the snk. Then, the selecton process n node has to fnd the set of forwarders F and ther assocated duty-cycles α j j F such that: () V V mn to meet the rate of progress and delay constrants; () maxmum duty-cycle requred of each forwarder (max α j ) s mnmzed, to prolong network connectvty and coverage. A. Forwardng Set and Duty-Cycle Selecton We frst present two lemmas that are useful n the selecton process for the forwardng set and duty-cycle of each node. Lemma 1: Let the set of canddate nodes ℵ of node be sorted n descendng order of progress, from 1 to ℵ. The optmal set of forwarders F opt() F that mnmzes max j ℵ α j s the frst n forwarders wth the largest progress. Lemma 2: To meet the rate of progress constrant V mn, max j F α j s mnmzed among all forwarders ff ther assocated duty-cycles are the same,.e. α j = α k j, k F. The proofs for these two lemmas can be found n [16], and they provde gudelnes on how the forwardng set and duty-cycle of each node should be selected to acheve the

4 4 α j Node 1 Node 2 mnmum value of α j (%) β=.5% β=.5% β=.5% no. of forwarders n no. of forwarders n Fg. 4. Computaton of forwardng sets and duty-cycles. Fg. 5. α j versus n for two dfferent nodes. Fg. 6. Mnmum α j requred for varyng n. objectves of,.e. to reduce duty-cycles, as well as extend connectvty and coverage, subject to a delay constrant. Intally, all the nodes n the canddate set ℵ are sorted n descendng order of progress. Each canddate node s then (ϕ) ncrementally added nto the forwardng set F. We use F to denote the forwardng set contanng the frst ϕ nodes n ℵ wth the most progress, where 1 ϕ ℵ. For each forwardng set F (ϕ) ℵ, we compute the mnmum α j needed to ensure that V V mn. The forwardng set wth the smallest α j s consdered to be optmal for node. We llustrate the selecton process usng the example n Fgure 4 wth V mn = 2 and v = 1. Node 1 has three canddate nodes such that ℵ 1 = {3, 4, 5}, wth correspondng progresses d 13 = 1, d 14 =.9 and d 15 =.2. The mnmum dutycycles requred for the three forwardng set combnatons are α 13 = 1, α 13 = α 14 =.5526 and α 13 = α 14 = α 15 =.619 usng the forwardng sets {N3}, {N3, N4} and {N3, N4, N5} respectvely, as llustrated n Fgure 5. Hence, the mnmum duty-cycle s obtaned when only {N3, N4} are used as forwarders, resultng n α 13 = α 14 = Smlarly, node 2 has three canddate nodes such that ℵ 2 = {4, 5, 6}, wth progresses d 24 = 1, d 25 =.75 and d 26 =.5. The duty-cycles requred are α 24 = 1, α 24 = α 25 =.6429 and α 24 = α 25 = α 26 =.5556 respectvely for the forwardng sets {N4}, {N4, N5} and {N4, N5, N6}. In ths case, all three forwarders should be used to obtan the mnmum duty-cycle. As the duty-cycles requred of forwarder 4 by nodes 1 and 2 are dfferent, the fnal duty-cycle of node 4 s gven by α 4 = max{α 14, α 24 } = We note that when j F α j s small, the maxmum sleep latency T for node can be large. T can be bounded by ensurng that the probablty of havng no forwarders actve throughout a cycle wth v tme slots s less than a specfed QoS threshold β ( < β < 1), such that: P n = [ (1 α j v )]v β. (2) j F Fgure 6 llustrates the mnmum average α j (as a percentage of v) requred usng varyng values of n. B. The Adaptaton Algorthm Adaptaton s performed durng network ntalzaton and topologcal changes, whch allows each node to compute: () ts forwardng set F ; () duty-cycles α j requred of each Algorthm 1 Computng F and α j by n each round. 1: Input: set of undetermned canddates ℵ u, set of determned canddates ℵ d, set of canddates ℵ = ℵ u ℵ d, progress d j j ℵ, duty-cycle α j j ℵ d 2: per-hop rate of progress V = ; forwardng set F = ℵ d ; temporary set of undetermned canddates Q u = ℵu ; dutycycles of undetermned canddates α j = j ℵ u (ϕ) 3: ϕ = F ; current forwardng set F = F, whch contans the ℵ d determned canddates and next ϕ ℵu canddates wth largest progress 4: whle Q u and [(V < V mn ) (P n > β)] do 5: Compute V, mn α (ϕ) j and P n usng F (ϕ) 6: undetermned canddate wth next largest progress b = argmax j 7: ϕ = ϕ + 1; F (ϕ) 8: end whle 9: φ = argmn ϕ d j, j Q u = F (ϕ) {b}; Q u = Q u \ {b} α (ϕ) j ; α j = α (φ) j j ℵ u ; F (φ) = F forwarder j F ; and () ts own duty-cycle α based on the requrements from ts neghbors. Intally, the duty-cycles of all nodes are consdered to be undetermned; for brevty, we refer to such nodes as undetermned nodes. Each executon of the adaptaton algorthm proceeds n bphase rounds. Algorthm 1 summarzes how, n the frst phase of every round, a node wth undetermned canddate nodes computes: () ts forwardng set; and () duty-cycle requrements of each forwarder j. These computatons are based on the two lemmas presented n Secton IV-A. In each teraton of the whle loop, the undetermned canddate node that has the largest progress s added to the (current) forwardng set and the new duty-cycle s computed. The loop exts when the local constrants are met (Lne 4). A key feature of s that t explots hgher duty-cycles of determned nodes to reduce the requred duty-cycles of addtonal (undetermned) nodes 1. The fnal forwardng set F and duty-cycle α j that s requred from ts neghbors n the current round s the confguraton that provdes the mnmum duty-cycle requrements. In the second phase of every round, each undetermned node computes ts nterm duty-cycle based on the duty-cycle re- 1 Consderng the network topology n Fgure 4, once the larger α 4 value of.5556 s selected, α 3 can be reduced slghtly from.5526 to.551.

5 5 TABLE I SIMULATION PARAMETERS Parameter Transmttng I tx Recevng I rx Idle I dle Sleep I sleep Tme Slot Length τ Cycle Length L cycle Value 11. ma 19.7 ma.426 ma.1 ma 2 ms 2 s energy (J) d max (s) Fg. 7. (a) Energy tme to frst node falure (s) (p) d max (s) (b) Tme to Frst Node Falure Delay tradeoff under varyng delay constrants. qurements from ts neghbors (computed from the frst phase). The undetermned node wth the largest nterm duty-cycle among all ts undetermned neghbors then fxes ts duty-cycle to be that of the computed nterm, and thereafter s known as a determned node. The next round then commences, untl all the nodes n the network become determned. The algorthm s guaranteed to termnate, as at least one undetermned node becomes determned n each round. V. SIMULATION EVALUATION We evaluate the performance of wth: () X- MAC [1], a well-known energy-effcent asynchronous uncast MAC protocol; and () [9], whch s optmal among approaches usng the same duty-cycle for all the nodes, usng GloMoSm [17]. Results shown are the average of 2 runs. The common smulaton parameters are lsted n Table I 2. All traffc arrvals follow a Posson dstrbuton. The routng protocol used to forward each 6-byte packet s MFP (Maxmum Forward Progress). The snk s placed at the top rghthand corner of the terran of sze 25 m 25 m, and the transmsson range s approxmately 6 meters. Sectons V-A to V-C assume that energy expended n packet transmsson s neglgble as compared to energy expended through long perods of dle lstenng. In Secton V-D, we consder hgher traffc loads where transmssons ncur sgnfcant energy. A. Delay Tradeoffs We vary the delay constrant d max from 2s to 6s and study the tradeoffs of the three protocols (, X-MAX and ) n a network o5 randomly placed nodes wth average node degree of d v 2 n Fgure 7. As d max ncreases, nodes sleep longer, leadng to lower duty-cycles and per-node energy consumpton n Fgure 7(a). acheves better energy-delay tradeoffs partcularly for smaller values of d max, due to ts complementary use of random wakeup schedules, anycast, as well as adaptve mechansms. In contrast, and assgn the same (maxmum) duty-cycle to all the nodes, whch ncreases energy consumpton. As does not globally optmze the tme to the frst node falure, t performs slghtly worse than (whch s optmzed for ths aspect) for hgher d max values n Fgure 7(b). In and, nodes are assgned the same duty-cycles and fal at the same rate; n, nodes 2 Based on Chpcon CC242 RF Transcever specfcatons. percentage connectvty (%) (a) tme (s) (a) Percentage Connectvty. percentage coverage (%) tme (s) (b) Percentage Coverage. Fg. 8. Percentage connectvty and coverage wth d max = 2s. fal gracefully over tme. Consequently, the tme to network partton for - denoted as (p) - exceeds the tme to frst node falure of by 2% to 5%, as the network remans connected even when some nodes n the (typcally dense) sensor network has faled. B. Connectvty and Coverage Fgure 8 llustrates the network connectvty and coverage over tme wth d max = 2s. The percentage connectvty s the rato of nodes that reman alve and connected to the snk relatve to total number of nodes. The percentage coverage s the rato of the terran wthn the range of any connected and alve node relatve to the ntal coverage area. In, nodes do not explot the redundancy of neghbors to reduce duty-cycles; hence the percentage connectvty deterorates very quckly over tme. Although utlzes anycast to mnmze duty-cycles, ts network connectvty stll deterorates quckly as all nodes use the same maxmum requred duty-cycle. Fgure 8(a) shows that has the best percentage connectvty as t: () mnmzes the local maxmum duty-cycle; and () adaptvely assgns (dfferent) duty-cycles to each node based on ts local topology. We note that the percentage of alve nodes n - denoted as (a) - s hgher than the percentage connectvty; ths ndcates that there are nodes that are alve but have lost connectvty to the snk. They are potentally useful as they can transmt data to the snk when the network s repared, or through technques such as message ferryng. The hgher network connectvty n allows t to acheve better coverage than and n Fgure 8(b). Notce that n, a small percentage of the nodes reman connected and cover a small proporton of the network for a long tme. These nodes are close to the snk and have few upstream nodes, resultng n extremely low energy consumpton.

6 6 energy (J) node degree d v (a) Energy consumpton per node. energy (J) tme to frst node falure (s) (p) node degree d v (b) Tme to frst node falure. Fg. 9. Performance wth varyng network denstes and d max = lnk error rate (%) (a) Energy Consumpton Fg. 1. throughput (bps) lnk error rate (%) (b) Throughput Performance wth ntermttent lnk connectvty. C. Random Topology wth Varyng Network Denstes The network sze n Fgure 9 s vared from 1 to 3 nodes such that d v vares between 15 to 45. Fgure 9(a) ndcates that energy expended ncreases wth ncreasng d v. As employs a uncast mechansm, t does not explot the avalablty of ncreased redundancy (or neghbors), resultng n hgh duty-cycles and energy consumpton. By utlzng larger forwardng sets as d v ncreases, and A 2 - MAC can acheve low energy consumpton. The latter expends the least energy as t selects neghbors that provde more progress as forwarders, and allows nodes to adapt (lower) ther duty-cycles accordng to local topologes. In Fgure 9(b), A 2 - MAC and utlze more forwarders wth ncreasng d v, resultng n lowered duty-cycles and longer tmes to frst node falure. has better performance and longer tme to network partton as ts anycast and adaptve mechansms maxmze the beneft of network redundancy. D. Random Topology wth Intermttent Lnk Connectvty In Fgure 1, 5% of the 1 nodes n the network are randomly selected as data sources and 1% of the lnks are randomly selected to have error rates from % to 5%. d max s set to 3ms when there s no lnk error. As utlzes a smlar anycast approach as, ts performance n ntermttently connected networks s smlar to and not shown. Generally, the number of retransmssons requred ncreases wth lnk errors. Ths results n the correspondng ncrease n energy consumpton n Fgure 1(a). Durng packet losses, retransmts unsuccessfully over the same poorqualty lnk, resultng n hgh energy consumpton and low throughput. By dynamcally selectng the next-hops based on prevalng network condtons, s more reslent to ntermttent lnk falures; hence t can acheve hgher and more consstent throughput n Fgure 1(b). We have also evaluated the performance of the MAC protocols under varyng traffc loads and delay constrants d max. The results show that the anycast and adaptve mechansms n mnmzes excessve overheads and energy consumptons, wthout compromsng on the end-to-end latency. VI. CONCLUSION Severe energy lmtatons n sensors accentuate the need for energy-effcent MAC protocols. However, duty-cyclng ncurs hgher latences as transmtters have to wat for forwarders to be awake before communcaton can commence. s an adaptve, anycast-based MAC protocol that utlzes an asynchronous random wakeup schedule, anycast mechansm as well as adaptve forwardng set selecton and duty-cycle selecton. It adapts ts duty-cycle and forwardng set based on local network topology and a gven delay constrant to acheve energy effcency wth low latences. can also acheve better connectvty and coverage, and sgnfcantly outperforms exstng asynchronous sensor MAC protocols. REFERENCES [1] M. Buettner, G. Yee, E. Anderson, and R. Han, : A Short Preamble MAC Protocol For Duty-Cycled Wreless Networks, n Sen- Sys, 26. [2] W. Ye, J. Hedemann, and D. Estrn, An Energy-Effcent MAC Protocol for Wreless Sensor Networks, n INFOCOM, 22. [3], Medum Access Control wth Coordnated Adaptve Sleepng for Wreless Sensor Networks, IEEE ACM Transactons on Networkng, vol. 12, no. 3, 24. [4] T. van Dam and K. Langendoen, An Adaptve Energy-Effcent MAC Protocol for Wreless Sensor Networks, n SenSys, 23. [5] G. Lu, B. Krshnamachar, and C. 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