Aggregated traffic flow weight controlled hierarchical MAC protocol for wireless sensor networks

Transcription

1 Ann. Telecommun. (29) 64: DOI.7/s Aggregated traffc flow weght controlled herarchcal MAC protocol for wreless sensor networks M. Abdur Razzaque M. Mamun-Or-Rashd Muhammad Mahbub Alam Choong Seon Hong Receved: 1 September 28 / Accepted: 2 March 29 / Publshed onlne: 1 May 29 Insttut TELECOM and Sprnger-Verlag France 29 Abstract It has been dscussed n the lterature that the medum-access control (MAC) protocols, whch schedule perodc sleep actve states of sensor nodes, can ncrease the longevty of sensor networks. However, these protocols suffer from very low end-toend throughput and ncreased end-to-end packet delay. How to desgn an energy-effcent MAC protocol that greatly mnmzes the packet delay whle maxmzng the achevable data delvery rate, however, remans unanswered. In ths paper, motvated by the manyto-one multhop traffc pattern of sensor networks and the heterogenety n requred data packet rates of dfferent events, we propose an aggregated traffc flow weght controlled herarchcal MAC protocol (ATW-HMAC). We fnd that ATW-HMAC sgnfcantly decreases the packet losses due to collsons and buffer drops (.e., mtgates the congeston), whch helps to mprove network throughput, energy effcency, and end-to-end packet delay. ATW-HMAC s desgned to work wth both sngle-path and multpath routng. Our analytcal analyss shows that ATW-HMAC provdes weghted far rate allocaton and energy effcency. The results of our extensve smulaton, done n ns-2.3, show that ATW-HMAC outperforms S-MAC; traffc-adaptve medum access; and SC-HMAC. M. A. Razzaque M. Mamun-Or-Rashd M. M. Alam C. S. Hong (B) Department of Computer Engneerng, Kyung Hee Unversty, 1, Seocheon-r, Gheung-eup, Yongn-s, Gyeongg-do, , South Korea e-mal: cshong@khu.ac.kr M. A. Razzaque e-mal: m_a_razzaque@yahoo.com Keywords Wreless sensor network Medum-access control Weghted far rate allocaton Multpath routng Energy effcency 1 Introducton Recent advancements n the desgn of sensor node components have enabled the deployment of wreless sensor networks (WSN) capable of supportng a number of applcatons, rangng from reportng very low data rate perodc traffc to comparatvely hgh bandwdth hungry traffc due to sudden mportant events or multmeda data delvery [1, 2]. Each applcaton may produce dfferent classes of traffc and mpose a unque set of goals and requrements based on the sensed event(s). For example, n forest-montorng applcatons, fre detecton events produce more crtcal traffc flows (.e., demandng hgh end-to-end throughput and reduced end-to-end packet delay) than events lke thunderstorms, cyclones, etc. Smlarly, data traffc carryng movng object trackng (e.g., movement of wld anmals n a forest) nformaton should get hgher weght than ts counterpart (e.g., statc or slowly movng anmals). The traffc heterogenety n applcaton-specfc requrements also appears n other emergng WSN applcatons lke ndustral process control, radoactve radaton and leakage detectons, vehcle montorng, etc. Moreover, energy effcency s one of the most mportant desgn crtera for all WSN applcatons. The underlyng medum-access control (MAC) protocol of the network has a sgnfcant role n meetng the above demands. To ths extent, keepng asde the contrbuton of upper-layer protocols (.e., routng or rate control),

2 76 Ann. Telecommun. (29) 64: n ths paper, we concentrate only on the functons of MAC layer. A large number of MAC protocols for sensor networks exst n the lterature [3 9]. They can manly be dvded nto two groups: contenton-based and (TDMA-lke) schedule-based MAC protocols. The contenton-based protocols [5 8] use DCF for medum access and a varety of perodc sleep lsten schedules to conserve energy. However, due to the ntrnsc many-to-one multhop and convergent traffc patterns of sensor networks, the use of DCF causes a huge amount of packet loss, as descrbed n Secton 3 n detal. On the other hand, n schedulebased MAC protocols [3, 4, 9, ], nodes exchange traffc and transmt schedule nformaton n order to predetermne and reserve the tme slots of ther future transmssons. These protocols guarantee collson-free transmsson slots. However, for large-scale multhop wreless networks, schedule-based protocols exhbt nherently hgher delvery delays when compared to contenton-based approaches [3, 9]. Therefore, n today s emergng sensor networks, there s a growng need for mproved MAC protocol that can provde hgher throughput, reduced latency, and energy effcency. In SC-HMAC [11], we frst ntroduced the concept of a herarchcal MAC protocol for WSNs, whch was based on the source count values of the nodes. SC-HMAC was desgned for homogeneous traffc applcatons and sngle-path routng networks only. However, n ths paper, we desgn a more generous MAC protocol that adapts to the applcaton at hand and works n networks ether wth sngle-path or multpath routng, namely, aggregated traffc flow weght controlled herarchcal MAC protocol (ATW-HMAC). ATW-HMAC uses traffc-flow-based nformaton to determne the frequency of medum access of a node (whch s determned by the mnmum contenton wndow, CW mn, value of the node). Nodes usng ATW-HMAC calculate ther aggregated traffc flow weghts (defned n Secton 4.2) and thereby determne ther CW mn values. A number of other works exst n the lterature that adjust the CW mn value of nodes to ncrease the throughput of IEEE DCF [12, 13]. In [12], the backoff procedure of DCF s modfed to maxmze the saturaton throughput of wreless LAN statons. The authors of [13] opened up two approaches n order to control IEEE 82.11e staton s artme usage: by controllng AIFS tme or CW mn and CW max values. Unlke these works, our proposal explots the event traffc rate heterogenety and many-to-one convergent traffc pattern of WSN n dfferentatng the CW mn values of nodes. Our work answers the followng key questons: How can a sensor node, ndependently and n a dstrbuted manner, determne ts approprate CW mn value on the fly? Whch parameter (traffc flow weght or source count value) s more effectve n determnng the CW mn values of WSN nodes for performance mprovements n terms of throughput and delay? How can weghted far data delvery from multple events havng dfferences n data packet generaton rates be ensured? Fnally, the man contrbutons of ATW-HMAC are summarzed as follows: (1) the dstrbuted mantenance of energy-effcent, collson-avoded MAC based on mplct sngle-hop traffc nformaton; (2) hgher throughput and lower end-to-end packet delay; (3) localzed and smple protocol, so that t can be run by nodes wth lmted processng, memory, and power capabltes; (4) capablty to work wth varous applcatons and both sngle- and multpath routng; and (5) weghted far rate allocaton and energy effcency. The performance of ATW-HMAC s compared wth that of S-MAC [6]; traffc-adaptve medum access (TRAMA) [3]; and SC-HMAC [11]. The rest of the paper s organzed as follows: Related works are dscussed n Secton 2, and Secton 3 dscusses the mpact of usng IEEE DCF n sensor networks. The proposed ATW-HMAC protocol s presented n Secton 4, whch ncludes ts network and traffc models and determnaton procedure of nodes CW mn values. Secton 5 presents the probablstc analyss on weghted far rate allocaton and energy consumpton made by ATW-HMAC. Performance evaluaton s carred out n Secton 6, and the paper concludes n Secton 7. 2 Related works A number of MAC protocols exst for WSN amng to adapt wth ts traffc behavor n order to reduce the packet drops and to ncrease the energy effcency. Some of them are detaled as follows: TRAMA protocol [3] tres to maxmze the length of sleep perods by determnng the presence or absence of traffc. It needs a neghbor protocol and a medum-access schedule exchange protocol that has sgnfcant overhead of broadcastng schedule packets perodcally. It also performs an adaptve electon algorthm to reduce the number of unused tme slots. The key problem of ths protocol s that the overall sgnalng overhead of the

3 Ann. Telecommun. (29) 64: above farly complcated algorthms may create a scalablty problem and ncrease end-to-end packet delvery latency, partcularly for large-scale sensor networks. It also suffers from computaton and dstrbuton of global network-wde schedulng nformaton and tme synchronzaton. Later, the complexty and schedulng overhead of TRAMA are crtczed by the same authors n [9], and a new TDMA-based MAC protocol s proposed, namely flow-aware medum access (FLAMA). Unlke TRAMA, FLAMA organzes the tme n perods of random and scheduled access ntervals. However, the mplementaton of FLAMA needs the exchange of two-hop neghborhood nformaton, executon of dstrbuted electon algorthm at every node, and propagaton of schedule nformaton to ensure conflct-free slot assgnments. These communcatons and computatons waste not only the sensor node energy but also the transmsson slots, whch n turn ncrease the packet delvery delay. The funnelng MAC [4] s a snk-orented hybrd MAC protocol that uses both schedule-based TDMA and contenton-based CSMA/CA MAC schemes. Pure CSMA/CA operates network-wde n addton to actng as a component of the funnelng MAC that operates n the hgh-traffc regon. Ths protocol mtgates the funnelng effect by usng local TDMA schedulng for the nodes closer to the snk that typcally carry consderably more traffc than nodes further away from the snk. The swtchng between the CSMA/CA and TDMA modes s controlled by the beacon messages broadcasted from the snk on demand. Ths makes the protocol unscalable and ncurs a lot of message overhead. Moreover, the nstablty of nodes MAC behavor reduces the throughput and ncreases the delay by a sgnfcant amount. Fnally, unlke our proposed ATW-HMAC, funnelng MAC handles congestons only at the regon near the snk. It does not consder the congeston due to lnk contenton at the event detecton area. Probably the most well-known and hghly referenced MAC protocol for sensor network s S-MAC [5, 6]. S-MAC s a contenton-based protocol that avods overhearng and dynamcally sets the duty cycle based on adaptve lstenng. It uses nlne sgnalng for exchangng SYNC packets. The dstrbuted mantenance of clock synchronzaton of nodes by exchangng SYNC packets and ther perodc sleeps sgnfcantly reduces the throughput and ncreases the packet latency. The concept of herarchcal MAC protocol was frst ntroduced n SC-HMAC [11], where each node gets proportonal medum access accordng to ther source count values. It was desgned for homogeneous traffc applcatons and sngle-path routng network only. It outperforms tradtonal MAC protocols n terms of throughput and energy effcency. However, SC- HMAC s not sutable for dverse network applcatons and topologes. In the next secton, we evaluate the performance of IEEE DCF n WSN. 3 IEEE DCF and sensor network The operaton prncple of IEEE DCF s depcted n Fg. 1, where two contendng nodes and j compete to access the medum. It s based on the CSMA/CA, where a node performs a backoff procedure before ntatng the transmsson of a frame. The backoff tme s chosen as a random nteger drawn from a unform dstrbuton over the nterval [, CW 1], where CW s referred to as contenton wndow. If the medum s found to be dle (.e., t does not hear any transmsson from any of ts neghbors) for a DIFS perod of tme, the node starts to decrement ts backoff tme for every slot tme. The node can transmt a data frame wth the basc CSMA/CA procedure, or an RTS frame wth the RTS/CTS procedure, when ts backoff tme s decremented to zero. If the medum becomes busy durng a backoff process, the backoff s suspended. If the medum becomes dle agan for an extra DIFS perod of tme, the backoff process resumes from the pont where t was suspended. The contenton wndow CW has an ntal value CW mn, and s doubled when a collson occurs, up to the maxmum value CW max. For example, n 82.11b, CW mn and CW max are set equal to 32 and 1,24, respectvely. When a frame s successfully transmtted (.e., ACK s receved after a SIFS perod of tme), the contenton wndow s set to ts ntal value CW mn. The value s also reset when the maxmum retransmsson lmt s reached. It s not effcent to apply the above MAC protocol drectly n a sensor network. Snce the typcal traffc Busy Medum Busy Medum DIFS DIFS Suspended Backoff S Node j's I transmsson F S DIFS DIFS 65 Node 's transmsson (a) An example DCF Channel Access for node S I F S A C K DIFS Suspended Backoff DIFS A C K (b) An example DCF Channel Access for node j Fg. 1 IEEE DCF operaton prncples

4 78 Ann. Telecommun. (29) 64: Event 1 2 Event (a) Sngle-path routng S 1 Event Event (b) Mult-path routng Fg. 2 An example network showng how data packets from two dfferent events are routed from the event detecton area(s) to the snk usng sngle-path and multpath forwardng. For b, ntally, nodes 2 and 5 dvde ther traffc equally towards two routes pattern of a sensor network s many-to-one and convergent n nature [4, 11], downstream nodes have to carry more traffc than ther upstreams. 1 The longterm MAC-level farness property of the above CSMA/ CA-based protocol nstgates the bottleneck condton at downstream nodes. Hence, n the presence of a bottleneck node, queue buld-up and packet-drop happen, whch result n waste of the system resources utlzed to delver the packets halfway through the multhop network. For example, n Fg. 2a, nodes 3, 4, and 9 are n the same contenton zone; only one of them can transmt at a gven tme, and the same s true for nodes 1, 2, 3, and 4. Wth CSMA/CA, the upstream sensors 1, 2, 3, and 4 collectvely have more chances to send packets to node 9 than t can forward. The excessve packets receved by node 9 wll eventually cause buffer overflow, and packets are dropped. Another type of congeston happens due to meda contenton, namely, congeston due to packet collson. Ths s caused whenever a number of nodes contend the shared medum usng the same contenton parameter values, as used n the above protocol. Irrespectve of the reason behnd the congeston, t causes many folds of problems. When a packet s dropped/collded, the energy spent by upstream sensors on the packet s wasted. Ths wastage ncreases when the packets drop after travelng more numbers of hops. Fnally, and above all, the data loss due to congeston may jeopardze the msson of the applcaton. We have done smulatons n ns-2.3 [26] for our example network (Fg. 2a) to evaluate the performance 1 We refer the parent/chld relaton n the snk-rooted, tree-based network as a downstream/upstream relaton among the nodes. For example, n Fg. 2a, nodes 3 and 4 are the upstreams of node 9, whch n turn s the downstream of them. S of DCF n a sensor network. The general smulaton setup parameters are descrbed n Secton 6.2. The a/b-lke values n the label of the graphs represent the data packet generatons rates (.e., traffc load) from source nodes detectng less crtcal and more crtcal events (whch we defne n Secton 4.1 as event types A and B, respectvely), respectvely. The graphs of Fg. 3 llustrate that the number of packet collsons and drops greatly ncreases at hgher traffc loads [> 9/18 packets per second (pps)] and, hence, the delvery rato decreases sgnfcantly. Even the ncreasng number of retransmsson lmts cannot ncrease the delvery rato to an acceptable pont n case of hgher traffc loads. Someone may argue that the employment of a congeston/rate control mechansm [2] mght lessen the packet drops, but t can nether ncrease the end-to-end throughput nor decrease the packet delay sgnfcantly. In ths paper, we concentrate on the role of the MAC layer n achevng the goals. 4 Proposed ATW-HMAC protocol As descrbed n Secton 3, the use of an exstng CSMA/CA-based MAC n WSN causes the source nodes to nject as many packets as possble nto the network wth no regard for whether the packets reach ther fnal destnaton. The hgh number of data packets transferred by source nodes overwhelms the capacty of downstream nodes, partcularly the nodes near the snk. To dmnsh ths problem, the MAC protocol for sensor networks should be desgned n such a way that the downstream nodes get more medum access than ther upstreams. More explctly, the frequency of medum access of dfferent sensor nodes should be proportonate to the amount of data traffc carred by them. Ths motvaton drove us to desgn an aggregated traffc flow weght (F agg ) drven herarchcal MAC protocol that gves hgher access to the downstream nodes (wth hgher F agg values) than ther upstreams (wth lower F agg values). In what follows, we frst present the network archtecture and assumptons (consdered for modelng and analyzng the proposed MAC protocol) n Subsecton 4.1. We dscuss the formulaton and computaton procedure of F agg values for all source and forwarder nodes n Subsectons 4.2 and 4.3, respectvely, and then present how to determne CW mn values usng F agg n Subsecton Network archtecture and assumptons We consder a snk-rooted, tree-based network archtecture, where dentcal sensors are unformly dstrbuted

5 Ann. Telecommun. (29) 64: Number of packet collsons 22 6/12 pps 7/14 pps 6/12 pps 7/14 pps 2 8/16 pps 9/18 pps /2 pps 11/22 pps 6 8/16 pps 9/18 pps /2 pps 11/22 pps /24 pps 12/24 pps /12 pps 7/14 pps /16 pps 9/18 pps /2 pps 12/24 pps 11/22 pps Maxmum retransmsson lmt Maxmum retransmsson lmt Maxmum retransmsson lmt Number of buffer drops (a) Packet collsons experenced (b) Packet drops due to buffer overflow (c) Packet delvery rato Fg. 3 Performance of IEEE DCF n WSN (a c). Smulaton s run for s and each data pont value s the mean of results from runs Packet delvery rato n % over the terran. The snk may be located anywhere on the terran. Sensor nodes and the snk are statc after the deployment. A sngle node may be equpped wth multple types of sensors [14] (for example: humdty, temperature, accelerometer, and lght sensors as n mca2, Imote2 nodes [14]),andtherebybeabletodetect multple types of events. However, n the proposed traffc model, we consder that a node can only generate traffc due to one event at a gven tme. All nodes have equal transmsson range (R tx ) and sensng range (R s ). We also assume that all data packets have the same sze, and the buffer sze of a node s counted as the maxmum number of data packets that t can hold. The underlyng routng protocol may use ether sngle-path or multpath forwardng [2, 15]. In some recent research results, we observe that the multpath forwardng can acheve hgher throughput, reduced end-to-end latency, hgher securty, and spatal energy consumpton. Therefore, n the followng secton, we develop a framework of a traffc model for the multpath forwardng case, whch can easly be tuned for sngle-path also. Note that, n case of multpath forwardng, the total traffc load of any node may be dstrbuted over a set of downstream nodes D, whch are the next hop nodes on the routng paths from to the snk. Smlarly, f U represents s set of upstream nodes, each node n U uses as the next hop node on ts routng path. The load dstrbuton polcy of node towards multple paths (defned by D ) depends on the specfc multpath routng algorthm. 2 2 Possble load dstrbuton polces could be as follows: (1) homogeneous dstrbuton the total traffc load of node could be equally dvded amongst the avalable paths and (2) proportonal dstrbuton loads could be dstrbuted proportonal to the mnmum hop count of a path, free buffer space n the downstream node, maxmum resdual energy of the downstream node, successful packet delvery rate of a lnk/path, etc. 4.2 Proposed traffc model Snce all the sensors have equal sensng range, the radus of an event detecton area s equal to the sensng radus of a node. Therefore, when an event occurs, a number of sensor nodes around the vcnty of the event detect t and generate data packets at a constant rate. Then, they forward the sensed data packets toward the snk n a multhop fashon. Based on the mportance of the detected event, each source node assgns a weght value w to the generated data packets, where w 1, 2, 3,..., w. Note that w = 1 corresponds to the least mportant event and w = w corresponds to the most mportant one. Obvously, the upper value of w determnes the maxmum number of traffc classfcatons, whch s determned by the network applcatons. Even though the proposed traffc model and MAC protocol can work wth any values of w, wthout loss of generalty, we assume only two types of events, type A and type B. 3 Hence, the value of w s determned as follows: 1, f node detects an event type A w = 2, f node detects an event type B, otherwse Ths weght value s determned for each event durng the network ntalzaton process and remans statc for the duraton of communcaton. In the case where all the nodes have equal weghts, ths value can be set to 1. Also note that the above traffc classfcaton can easly be extended to more types, f the network applcatons demand. Defnton 1 (Traffc flow): A traffc flow s defned as a stream of data packets from an upstream node to 3 As dscussed n Secton 1, event type B may represent movng object trackng and A may stand for ts counterpart.

6 7 Ann. Telecommun. (29) 64: a downstream node j, denoted by f j. Hence, f j may aggregate data packets from multple flows. Defnton 2 (Aggregated traffc load): In case of multpath routng, each node dvdes ts total traffc nto multple traffc flows, and those flows pass through multple downstream nodes. Therefore, the aggregated traffc load of an ntermedary node (L ) s the total sum of the data packet rates (n pps) of ts all upstream traffcs plus ts own data packet generaton rate (g ), and s expressed as follows: ( ) L = + g, (1) k U r k where r k s the data packet transmsson rate (n pps) of flow f k. Note that the aggregated traffc load of a source only node s smply ts packet generaton rate, g. Defnton 3 (Traffc flow weght): We defne the traffc flow weght of a source node as the rato of ts nstantaneous data packet reportng rate (R ) to ts data packet generaton rate (g ) tmes ts weght value, and s expressed as follows: F = R w, g >, (2) g where R g and ntally set as R = g for all source nodes. The expresson for R s carred out n Subsecton 5.1. Whle g represents the applcatondefned constant packet generaton rate for a partcular event type, R s a dynamcally controllable varable and ts value s determned by an approprate congeston control (CC) or rate control (RC) algorthm. In multpath data forwardng, a CC or RC algorthm may be actvated whenever a certan path/lnk s congested. In ths case, congeston may be reduced ether by dvertng the excessve amount of traffc from an overloaded path to other alternatve lghtly loaded paths or by decreasng the reportng rate R (.e., R becomes less than g ). The frst opton s not applcable whenever all other paths are adequately/hghly loaded so that they are unable to carry addtonal traffc. Then, the actvaton of a second opton becomes mandatory. However, n ths paper, we take advantage of nether of the approaches n support of unbased evaluaton of the proposed ATW-HMAC protocol. 4 4 In our smulaton, we assgn R = g and keep t unchanged durng the whole smulaton perod. Obvously, the consderaton of R opens the door of desgnng an optmal traffc engneerng algorthm for multpath data forwardng n WSN, whch we leave as our future work. Defnton 4 (Aggregated traffc flow weght): Snce, n multpath routng, each downstream node may receve multple flows from ts upstreams, the aggregated traffc flow weght of any ntermedate node s defned as the total sum of the weghted fractons of all flows passng through t plus ts own traffc flow weght. Therefore, node s aggregated traffc flow weght s calculated as ( agg F agg r k F ) k = + F. (3) L k k U These aggregated traffc flow weghts of dfferent sensor nodes are a new way to look nto the heart of the MAC protocol for sensor network (see Secton 4.4). 4.3 Dstrbuted computaton of F agg values We consder that each node k, whle transmttng a data packet to ts downstream node, ncludes r k, L k, and F agg k n the header of every packet. Note that a smple dstrbuted algorthm can compute the aggregated traffc flow weghts of all nodes teratvely, and t works as follows. Intally, sensor sets r k =, F agg k =, and L k =, k U, and r j =, j D. When source nodes start to transfer data packets, ther downstream nodes learn the amount of ncomng traffcs, upstream s aggregated traffc loads, and aggregated traffc flow weghts (as they are embedded nto the packet headers). A downstream node then perodcally computes L and F agg usng Eq. 1 and Eq. 3, respectvely. Note that these computatons are performed only when the value of at least one parameter of the current packet dffers from that of a prevous one. It s obvous that L and F agg values become stable whenever node receves at least one packet from all of ts upstreams. After that, n the next teraton, s downstream nodes ( j) wll compute ther L j and F agg j values n the same way. Ths process repeats untl the aggregated traffc flow weghts of all nodes are calculated. Therefore, the upstream nodes would stablze the values before ther downstreams and the maxmum number of teratons requred before all varables become stablzed s bounded by the length of the longest routng path. More specfcally, a partcular ntermedary node wll requre at most h teratons to stablze ts varables, where h denotes ts hop dstance from the furthest source node for whch t s carryng traffc. Therefore, n our proposed ATW-HMAC protocol, the frequency of these computatons s drven by the event occurrence frequency, whch s typcally very mnmum. Also, note that the aggregated traffc flow weghts of each node along the routng path(s) are updated wthout transferrng

7 Ann. Telecommun. (29) 64: Aggregated traffc flow weghts F 1 agg = 2 F 3 agg = 5 F 4 agg = 3 F 9 agg = 8 F agg = Smulaton tme n seconds Fg. 4 F agg values of nodes over the smulaton tme any addtonal control packets, whch ncurs only a lttle overhead of a few bytes n the packet header. The Fg. 4 plots the aggregated traffc flow weght values of fve ndvdual nodes of Fg. 2b over the smulaton tme, for 8/16 pps traffc load durng.1 to.1 s. Values of the graphs depct that the F agg values of upstream nodes get stablzed earler than ther downstreams, whch s obvous. Also, the F agg value of the nearest node (.e., node ) of the snk s stablzed at last. Note that the duraton of unstable regon s so small (.16 s) that t cannot sgnfcantly affect the performance of the proposed protocol. 4.4 Determnaton of CW mn values In order to gve dfferent levels of medum access to the nodes, we can control the contenton process so that the nodes can access the wreless medum wth dfferent probabltes. The contenton process of nodes can be controlled by allowng them to use dfferent contenton parameters. We can control the value of CW mn used by ndvdual nodes to gve them dfferent opportuntes to access the shared medum. Snce the number of medum access of a node s approxmately nversely proportonal to ts CW mn value [16], we can easly control each node s share of the medum access n a dstrbuted manner usng F agg values. More specfcally, the hgher the aggregated traffc flow weght of a node, the lower ts CW mn value s. To mplement ths noton, our proposed ATW-HMAC protocol gves proportonal medum access to the nodes by assgnng acw mn value of any node as follows: (W 1) C CW mn [] = F agg, (4) where W s usually chosen as a power of 2, and C s a system parameter correspondng to the number source nodes wthn an event radus. It can be calculated as C = ρπr 2 s [11], where ρ s the node densty and R s s the sensng range. Therefore, the value of C lnearly vares wth the node densty of the network. For unform node dstrbuton, the value of C s the same for any event rrespectve of the locaton where the event occurs. For our deployment scenaro (see Secton 6.2), C s value s computed as The motvaton of takng C value nto account for the determnaton of CW mn s descrbed below. When an event s detected, the amount of traffc produced by the sensng nodes ncreases wth the value of C, whch n turn ncreases the contenton of the shared medum, and thus, the collsons become more frequent. Ths problem can be reduced by Eq. 4, whch assgns hgher CW mn values to sensng nodes takng C nto consderaton. In other words, the value of C helps the source-only nodes to choose more approprate CW mn values that mnmze collsons. For all other ntermedary nodes, workng as forwarders of the event traffc, the use of the same C value s justfed n order to buld a herarchcal medum access orderng among the nodes. Choosng the approprate value of W s a crtcal concern snce very hgher values decrease the medum utlzaton and, thereby, the network throughput. Conversely, very lower value mght greatly ncrease the medum contenton, as well as packet losses due to collson. By emprcal evaluatons and analyzng the extensve smulaton results, we found that settng W = 2 4 = 16 produces the best results for our deployment. We also evaluated the effects of dynamc change of the W value durng the data traffc, but t ncreases the oscllatons n the network and volates the farness property. Therefore, we keep ts value fxed durng the whole smulaton perod. Also, note that W s a tunable parameter dependng on the node densty of a network and s propagated from the snk node. Its value may be hgher for low-densty deployment and vce-versa, and ths s requred for better adjustment of CW mn values usng Eq. 4 snce C s value decreases as node densty decreases. For nstance, n our example network scenaro (Fg. 2), C = 4 and we set W = 2 5 = 32. Table1 shows the aggregated traffc flow weghts and CW mn values of nodes 1, calculated usng Eq. 4. Notce that the calculaton of F agg and the assgnment of CW mn valuesasofeq.4tells us that the nodes near the snk carry more traffc than the dstant nodes, and therefore, ther CW mn values are much smaller. 5 C = ρπr 2 s = 1, 1, 1, 22 7 (7)2 15.

8 712 Ann. Telecommun. (29) 64: Table 1 F agg and CW mn values of ndvdual nodes of Fg. 2 (R = g, for all nodes) Sngle-path Routng Multpath Routng Both the events are of type A Event 1 s B type and Both the events are of type A Event 1 s B type and Event 2 s A type Event 2 s A type Node ID w F agg CW mn w F agg CW mn w F agg CW mn w F agg CW mn Ths means that the medum-access orderng s done on a per-hop bass so that any one hop node wll have precedence over any two hop nodes. Therefore, the followng condton holds true: p(h 1 ) p(h 2 )... p(h s ), 1 k s, wherep(h k ) denotes the probablty of medum access of nodes h k hops away from the snk. The equalty condton appears when two nodes have the same aggregated traffc flow weghts. The key dfference of ATW-HMAC from SC-HMAC s as follows. MAC of ATW-HMAC s based on mplct traffc nformaton. However, SC-HMAC does not calculate the amount of traffc load that an ntermedary node has to forward, t merely counts the number of source nodes for whch a forwarder node s carryng traffc. Therefore, t becomes unable to cope up wth the dynamcs of multpath data forwardng. For the same reason, even f we modfy the source count polcy of SC-HMAC a lttle (by countng the source count value 2 for event type-b nodes and 1 for type-a nodes), t lacks approprate traffc nformaton and thereby fals to farly support events wth varyng traffc demands. 4.5 F agg controlled far packet schedulng Ths subsecton brefly dscusses far packet schedulng mechansms requred for far, relable, and guaranteed event percepton. The proposed ATW-HMAC gves proportonate medum access to the nodes accordng to ther F agg values. However, t does not guarantee that an ntermedate node wll schedule a weghted number of packets receved from dfferent upstreams accordng to ther F agg values. The lack of ths property mght hnder the snk node to farly detect an event [11],.e., the snk node may receve qute dfferent numbers of packets from the sensors detectng a partcular event over tme. More specfcally, flows that traverse a larger number of hops may be penalzed wth respect to those havng smaller hops, volatng the farness property. A weghted far queung [17] or weghted round robn algorthm [11] mantanng multple queues can be practcally used to guarantee farness among all transt traffcs, as well as the generated traffc. Unlke other protocols, rather than usng prorty ndex and/or buffer occupancy, n ATW-HMAC, we use F agg values of ncomng traffcs and generated traffc as the weghts for correspondng queues n each forwarder node. Notce that the above approach s also very effectve n balancng end-to-end packet delays experenced by source nodes located at far dstances from the snk and by those closer to the snk. The necessary mathematcal developments have been carred out n the next secton to prove the weghted farness property of the proposed protocol and to measure the energy wastage. 5 Analytcal analyss In the followng subsectons, we present the probablstc analyss on weghted far rate allocaton and energy consumpton made by ATW-HMAC, respectvely. 5.1 Weghted far rate allocaton Property 1 (weghted farness): An algorthm provdes weghted proportonal farness f, for any two nodes and j, F agg j = x F agg, x {1, 2, 3,..., X} the average allocated rate to j s x tmes hgher than that to node. Let q be the probablty that a node has no packets n ts queue awatng transmsson and let τ be the statonary transmsson probablty of node n a randomly chosen tme slot. A collson occurs when more than one contendng nodes transmt n the same slot.

9 Ann. Telecommun. (29) 64: Therefore, the probablty that node s transmsson results n a collson s gven by p c = 1 (1 (1 q )τ j ), (5) j S n, j = where S n s the set of n contendng nodes. Wth P dle denotng the probablty that the medum s sensed dle durng a typcal slot, then, accordng to [18], τ s calculated as τ = b, e ( ) (1 q ) 2 CW mn [] q (1 p c )(1 qcw mn[] ) (1 q)2 P dle, q (6) where b, e s the probablty that a node sees an empty queue after completon of postbackoff. 6 The values of p c and τ can be computed numercally by solvng the above two nonlnear equatons, Eqs. 5 and 6, respectvely. Snce the steady state probablty ndcates the frequency that a node wll be n a partcular state n the long run, τ represents the fracton of data packet transmsson rate of node,.e., R τ. Therefore, for any two nodes and j, the followng condton holds true: τ = R. (7) τ j R j Now, let p s be the probablty that the transmsson of a node n a randomly chosen slot s success, let DATA be the expected amount of data bts transmtted n the slot, and let E[T slot ] be the average duraton of slot tme, θ. Then, we can express the throughput of node as follows: R = p s DATA. (8) E[T slot ] Consderng DATA and E[T slot ] are equal for all nodes n the network and neglectng collsons of more than two nodes, we can express E[T slot ] as follows: E[T slot ]= n =1 + τ n j S n, j = =1 j=+1 n (1 τ j )Tsuc + (1 τ )θ =1 n τ τ j (1 τ k )T, j col, (9) k S n where Tsuc corresponds to the average duraton of successful transmsson from node and T, j col s the duraton of collsons between nodes and j. Accordng to [19], these two tme duratons are calculated as follows: Tsuc = T PLCP + L hdr + DATA + SIFS r +TPLCP + L ACK + DIFS, () r T, j col = T PLCP + L hdr + DATA + DIFS, (11) r where L hdr and L ACK are the szes of MAC header and ACK frames n bts, respectvely, and r s the channel bandwdth. 7 Now, by omttng hgher-order terms of τ (.e., τ 2,τ3,...)fromEq.9, we get n n E[T slot ] τ Tsuc (1 + τ )θ. (12) =1 Takng dervatve of Eq. 12, we have E[T slot ] = Tsuc τ θ T suc. (13) As defned n [2], proportonal far throughputs for the nodes can be acheved by the allocaton that maxmzes n =1 log(r ). The soluton to ths problem can be derved from ( n ) log(r ) =, j. (14) τ j =1 Puttng the value of R from Eq. 8 nto Eq. 14 and takng the dervatve, we get ( 1 τ j S n, = j 1 1 τ =1 ) E[T slot ] n E[T slot] τ j =, j. (15) For τ << 1,, we can approxmate the followng: E[T slot ] E[T slot ] nτ j =, j. (16) τ j For two dfferent nodes j and k, the above system can be rewrtten as E[T slot ] E[T slot ] nτ j nτ k =, j = k. (17) τ j τ k Now, combnng Eqs. 13, 17, 7,and4yelds τ = T j τ j suc T suc = R CW mn[ j] R j CW mn [] = Fagg F agg j. (18) 6 b, e and P dle can be expressed n terms of q, CW mn, p c and maxmum backoff stage t; please see [18] for detals. 7 In ths paper, we assume all nodes are operatng n the same channel,.e., they have equal bandwdths.

10 714 Ann. Telecommun. (29) 64: Note that the above equalty mples the necessary condton for weghted farness as defned n property 1. More specfcally, the followng observatons can bemadefromtheeq.18: frst, the throughput rato of two nodes and j ( R R j ) s equal to the rato of ther aggregated traffc flow weghts ( Fagg ); second, the F agg j throughput of an ntermedary node s the total sum of the fractonal throughputs (passng through t) of ts upstream nodes; and thrd, throughput of an upstream node s 1 tmes that of ts downstream node j, where x x = Fagg j. F agg 5.2 Energy consumpton analyss The longevty of battery-powered sensor networks s an essental performance metrc for WSN protocols. Here, we present a theoretcal analyss of power consumpton made by ATW-HMAC for delverng a packet from the source node to the destnaton snk and compare t wth that of perodc sleep/schedule based MAC protocols (e.g., S-MAC, TRAMA, etc.) Energy consumpton analyss of ATW-HMAC The total amount of energy consumed to delver a sngle packet from source node to snk n ATW-HMAC s derved as follows. As n [21], the amount of energy consumed due to transmsson of a packet from an upstream node to ts downstream s calculated as follows: e trans = (e te + e ta R α tx ) L, (19) where e te s the energy per bt needed by the transmtter electroncs, e ta s the energy consumpton of the transmttng amplfer to send one bt over one unt dstance, L s the packet sze n bts, and α s the path loss factor dependng on the rado frequency envronment and s generally valued between 2 and 4. However, the energy per bt needed by the recever electroncs, e re, does not nclude the energy consumpton due to amplfcaton. Therefore, the energy consumed for each packet recepton s gven as follows: e recv = e re L. (2) A sgnfcant amount of energy s also consumed due to packet overhearng by the neghbor nodes. The energy consumed to read header of the packet by (n 1) neghbor nodes s gven as follows: e recvhdr = e re L hdr (n 1), (21) where L hdr s the length of the header of a packet n bts. For smplcty of analyss, we do not consder the amount of energy consumed by the control packets (e.g., RTS/CTS/ACK packets) snce the node energy s dsspated manly due to the transmsson and recepton of data packets. However, we have to consder that the packet may not be reached at the downstream node successfully n a sngle transmsson attempt due to collson (we do not consder lnk errors). We consder that a lnk layer ARQ mechansm s adopted at each sensor node and the maxmum retry lmt s set to t. If all t transmsson attempts fal, then a node drops the packet. Now, let p s be the probablty that a transmsson attempt of node s successful. The probablstc dstrbuton of gettng frst success n a repeated fnte number of trals (t) s truncated geometrc random [22]. Let q represent the number of transmsson attempts requred to transfer the packet successfully to the next hop. Usng [22], we get the condtonal probablty mass functon of q, wth the condton that the transmsson attempt of the node s successful, as P (q) = (1 p s )q 1 p s. (22) 1 (1 p s )t Usng geometrc seres equatons [23], we fnd the expected number of transmsson attempts requred for sngle hop, E 1 [q], as E 1 [q] = 1 (1 p s )t (1 + tp s ) p s (1 (1. (23) p s )t ) Now, combnng Eqs. 19, 2, 21, and23, we get the expected total amount of energy consumed to successfully transfer a packet only one hop (for nstance, the k th hop) as follows: E k = (e trans E k [q]) + e recv + e recvhdr. (24) If the source node of the packet s h s hops away from the destnaton snk node, then the successful delvery of the packet from source to snk n ATW-HMAC consumes the followng expected amount of energy: h s E successful = E k. (25) k=1 On the other hand, f the packet s dropped at any ntermedate node that s h k hops away from the source node, h k < h s, then the total amount of energy consumed by the packet up to h k hops s wasted, and s measured as follows: E wastage = h k 1 k=1 E k + e trans t. (26)

11 Ann. Telecommun. (29) 64: In Eq. 26, the second part of the rght-hand sde s ncluded because the h k -th hop node would drop the packet only f ts t number of transmsson attempts becomes unsuccessful. The value of ths part must be zero f the packet s dropped due to buffer overflow Comparatve dscusson on energy consumpton Sensor MAC protocols desgned for very low data packet rate applcatons can completely avod the amount of energy consumed due to packet overhearng 8 as opposed to ATW-HMAC, shown n Eq. 21. They can further reduce the energy consumpton due to dle tme lstenng by swtchng off the rado when there s no traffc n the network. However, as the amount of traffc ncreases, nodes get lttle chance to go nto sleep mode (.e., duty-cycle ncreases) and the frequency of swtchng on off the rado ncreases, whch ncurs energy overhead (accordng to [25], the energy consumed for wakng up the rado once typcally ranges from 8 μj to 8 mj). Moreover, these protocols need to perodcally broadcast specal type of control packets (e.g., SYNC packets n S-MAC and schedulng packets n TRAMA) to synchronze the transmsson of neghbor nodes. A greater percentage of energy s wasted by these control packets. In S-MAC, each node mantans a schedule table that stores the schedules of all ts known neghbors and updates t perodcally on hearng of SYNC packets. As data traffc ncreases n the network, more SYNC packets are broadcasted, whch ncreases the collson and collapses the MAC synchronzatons among the nodes. TRAMA executes neghbor protocol (NP) and schedule exchange protocol (SEP) to allow nodes to exchange two-hop neghbor nformaton and ther schedules. It also executes an adaptve electon algorthm (AEA) that uses the above nformaton to select the transmtters and recevers for the current tme slot, leavng all other nodes n lberty to swtch to low-power mode. Agan, the energy cost ncurred by NP, SEP, and AEA ncreases sharply as traffc ncreases. However, ATW-HMAC does not requre any type of addtonal control messages, rather, t uses few bytes n packet headers for exchangng traffc flow nformaton and thereby reduces the protocol energy overhead sgnfcantly. The maxmum amount of energy wastage happens n S-MAC and TRAMA (and n lke protocols) and s 8 In some cases, overhearng s ndeed desrable. Some algorthms may rely on overhearng to gather neghborhood nformaton for network montorng, relable routng, or dstrbuted queres [11, 24]. due to ther lack of consderaton of WSN s many-toone convergent traffc pattern. It causes huge packet drops due to collson and buffer overflow near the snk node, and therefore, the energy expendtures on those packets become useless, as measured by Eq. 26. Agan, ths wastage lnearly ncreases wth the ncreased packet generaton rates of the source nodes. Therefore, for WSN applcatons that demand hgher end-to-end throughput and lower end-to-end packet delay, the energy overhead of perodc sleep lsten MAC protocols s much hgher than that of ATW- HMAC. The next secton shows the supportve results found n our smulaton. 6 Performance evaluaton The effectveness of the proposed ATW-HMAC protocol s evaluated through packet-level smulaton of traffc flows n ns-2.3 [26], wth varyng packet generaton rates. 6.1 Performance metrcs We compare IEEE 82.11DCF, S-MAC [6], TRAMA [3], and SC-HMAC [11] wth ATW-HMAC, n terms of the followng performance metrcs: Throughput: We measure the end-to-end effectve data throughput n klobytes per second for dfferent traffc loads. The results do not count any control packets. Only data packets receved at the snk are counted for the throughput calculaton. The hgher the value s, the better the protocol performance s. End-to-end packet delay: Delay of a sngle packet s measured as the tme dfference between when the packet s receved at snk and ts generaton tme at the source node. Delays experenced by ndvdual data packets are averaged over the total number of ndvdual packets receved by the snk. The lower the value s, the better the performance s. Delvery rato: Delvery rato s the rato of the total number of packets receved by the snk to the number of packets generated by all the source nodes. We express t n percentage. The hgher the value s, the better the protocol performance s. Energy cost per packet: We have measured the total amount of energy dsspated by all source and forwarder nodes durng the smulaton tme and took the average. We then dvde ths per-node average energy consumpton by the number of packets receved by the snk. Therefore, f the same amount

12 716 Ann. Telecommun. (29) 64: of total energy s dsspated by any two protocols, then the protocol that has a hgher delvery rato wll perform better than the other one. Total energy wastage: Whenever a packet s dropped before reachng the destnaton snk, the amount of energy expendtures on the packet by the source and forwarder nodes s consdered as wastage (see Eq. 26). We have added up all these wastages durng the whole smulaton perod to quantfy the performance of the protocols. Protocol operaton overhead: Protocol operaton overhead s the rato of the total number of control bytes 9 transmtted to the total number of data packet payload bytes and control bytes transmtted by all nodes durng the whole smulaton perod. We express t n percentage. Protocol energy overhead: We have measured the total amount of energy dsspated due to the transmsson and recepton of control bytes by all source and ntermedary nodes durng the whole smulaton perod to evaluate and compare the energy overhead of dfferent protocols. Weghted far rate allocaton: We measure the endto-end effectve data throughput for two events wth varyng packet generaton rates to evaluate far rate allocaton. We compare the farness of the proposed protocol only wth MAC and SC- HMAC. 6.2 Smulaton setup The smulaton envronment parameters are lsted n Table 2. Other than the default values n MAC layer mplementaton n ns-2.3, we have done the followng changes: the MAC header sze s ncreased to 184 bts (addtonal 24 bts for nsertng r j, L,and felds n each packet), and the value of CW mn of each node s determned onlne accordng to ATW- HMAC protocol. We establsh p number of alternate braded paths (where 1 p 3) from each node to the snk usng [27]. We consder that paths do not change/fal durng data traffc and each upstream node dstrbutes ts aggregated traffc load equally towards all of ts downstream nodes. In the mplementaton of S-MAC, we use a dynamc duty cycle (1% to 99%); 115-ms-duraton lsten nterval; SYNC packet sze of bytes; and contenton wndow values of 15 and 31 tme F agg 9 SYNC, RTS, CTS, and ACK packets n S-MAC; NP and SEP messages n TRAMA; and RTS, CTS, ACK, and addtonal MAC header bytes n SC-HMAC and ATW-HMAC are consdered for countng the total number of control bytes transmtted by the respectve protocol. Table 2 Smulaton parameters Parameter Value Terran area 1, 1, m Number of sensor nodes 1, Node dstrbuton Unform random Trans. radus, R tx m Sensng radus, R s 7 m Intal node energy 5 J Channel bandwdth, r 512 kbps Buffer sze 3 packets Payload sze 64 bytes PHY, MAC headers 192 and 184 bts, respectvely. RTS, CTS, ACK pkts. 16, 112, and 112 bts, respectvely. T slot, DIFS, SIFS, 34, and μs, respectvely. Max. retry lmt, t 4 Transmt power 7.214e 3 W Rcv. sgnal threshold e W Rado wake up energy 1 mj Sleep power 15 μw Propagaton model TwoRayGround PHY error model Unform random Bt error rate 3 FEC strength 1 Smulaton tme 3 s Snk 1 at locaton [1,, 5] slots for SYNC packets and data packets, respectvely. Smlarly, for mplementng TRAMA, we fx up the maxmum sze of the sgnalng packet at 128 bytes and schedule exchange packets at 21 bytes, as used n [3]. However, unlke n [3], we allow each node to compute SCHEDULE_INTERVAL based on the rate at whch packets are produced by the hgher-layer applcaton, rangng from 5 slots to 8 slots wth 5-slot ntervals. For the sake of far comparson of ATW-HMAC wth SC-HMAC, we also modfy source count polcy of SC- HMAC as stated n the last paragraph of Secton 4.4. Unless otherwse specfed, we execute the smulaton runs for four dfferent event traffcs: two A-type events (A1 and A2) and two B-type events (B1 and B2). The event locaton, duraton, and the data packet generaton rates of event source nodes are lsted n Table 3. Note that the events jon and leave the system at random tme nstants, and some of them are tmeoverlapped. We execute smulaton runs and take the average n order to show more stable values for each data pont n the result graphs. Table 3 Event and traffc settngs Event Locaton Duraton Packet gen. rate A1 (2, 5) 7 th 18 th s 1 8 pps A2 (4, 2) 12 th 25 th s 1 8 pps B1 (7, 15) 5 th th s 2 16 pps B2 (6, 8) 19 th 22 nd s 2 16 pps

13 Ann. Telecommun. (29) 64: Smulaton results Fgure 5 shows the performance comparsons of fve dfferent MAC protocols n terms of throughput, delay, delvery rato, energy cost, energy wastage, and protocol operaton overhead. The a/b-lke values n the X axs of the fgures represent the packet generaton rates of event types A and B, respectvely. We observe from the graphs of Fg. 5a that, for lower data packet generaton rates ( 3/6 pps), the effectve data throughput of the proposed protocol s not sgnfcantly ncreased from that of MAC. Ths s because, at lower rates, the aggregated traffc load of nodes near the snk s not hgh enough to make them congested. However, the gap ncreases wth the ncreased traffc loads. As n S-MAC, TRAMA, and IEEE protocols, the average end-to-end packet delay greatly ncreases wth hgher traffc loads, hence, the throughput decreases. Alternatvely, the herarchcal MAC protocols can operate at hgher-throughput regons snce they can reduce both the packet drops due to funnelng effect [4] and end-to-end packet delay by a consderable amount. At hgher rates, ATW-HMAC defeats SC-HMAC n terms of achevable throughput because nodes n SC- HMAC lack the approprate traffc nformaton (especally for multpath forwardng case) and, hence, fal to choose optmal contenton parameters. For ATW- HMAC, the network reaches saturaton condton at about 7/14 pps traffc load,.e, further ncrease of load cannot rase the throughput, rather, t decreases. Accordng to graphs of Fg. 5b, as source nodes ncrease ther packet generaton rates, the end-to-end packet delay ncreases n all protocols. However, packets n ATW-HMAC reach ther fnal destnaton n mnmum delay (less than 28 ms for 8/16 pps) and more than two tmes faster than those n MAC. The man reason behnd ths result s that the latter protocol gves almost equal medum access to all neghbor nodes (rrespectve of ther traffc loads), whch causes the packets to experence a noteworthy amount of delay at each ntermedate node. It s also observed that SC-HMAC packets are much slower than ATW- HMAC packets for hgher packet rates and the delays experenced by other protocols are really ncomparable. The other reason for obtanng the above results s that, when traffc converges near the snk, collson ncreases greatly n other protocols that force a node to retransmt a sngle packet many tmes, whch n turn ncreases the queung delays of packets. The graphs of Fg. 5c tell us that, as the packet generaton rate ncreases at the source nodes, the delvery rato falls suddenly n case of nonherarchcal MAC protocols. Ths happens due to the funnelng effect [4] near the snk node, whch causes a lot of packet Effectve data throughput n Kbps Avg. energy cost per packet n mj MAC S-MAC TRAMA SC-HMAC ATW-HMAC. 1/2 2/4 3/6 4/8 5/ 6/12 7/14 8/16 Source data packet generaton rates n pps Avg. end-to-end packet delay n ms MAC S-MAC TRAMA SC-HMAC ATW-HMAC 1/2 2/4 3/6 4/8 5/ 6/12 7/14 8/16 Source data packet generaton rates n pps Delvery rato n percentage MAC S-MAC TRAMA 15 SC-HMAC ATW-HMAC 1/2 2/4 3/6 4/8 5/ 6/12 7/14 8/16 Source data packet generaton rates n pps (a) Effectve data throughput (b) Average end-to-end packet delay (c) Packet delvery rato MAC S-MAC TRAMA SC-HMAC ATW-HMAC.55 1/2 2/4 3/6 4/8 5/ 6/12 7/14 8/16 Source data packet generaton rates n pps Total energy wastage n Joule MAC SMAC TRAMA SC-HMAC ATW-HMAC 1/2 2/4 3/6 4/8 5/ 6/12 7/14 8/16 Source data packet generaton rates n pps Protocol operaton overhead n percentage MAC S-MAC TRAMA SC-HMAC ATW-HMAC 4 1/2 2/4 3/6 4/8 5/ 6/12 7/14 8/16 Source data packet generaton rates n pps (d) Avg. energy cost per packet (e) Total energy wastage (f) Protocol operaton overhead Fg. 5 Performance comparsons of fve dfferent MAC protocols (a f)