ID-MAC: An Identity-Based MAC Protocol for Wireless Sensor Networks

Transcription

1 ID-MAC: An Identity-Based MAC Protocol for Wireless Sensor Networks Felipe D. Cunha, Ítalo Cunha, Hao Chi Wong, Antonio A. F. Loureiro Leonardo B. Oliveira Gold Member, IEEE Federal University of Minas Gerais, Belo Horizonte, Brazil, {fdcunha, cunha, loureiro, Intel Corporation, CA Abstract Wireless Sensor Networks (WSNs) are comprised mainly of resource-constrained sensor nodes that can be used to monitor areas of interest. In sensor networks, replacement of node batteries is usually infeasible and power consumption must be minimal to increase node lifetime. A large fraction of power consumption is controlled by the MAC layer, which orchestrates access to the wireless medium trading-off power consumption and throughput. Unfortunately, this orchestration requires exchange of additional data and itself ends up consuming energy. In this work we design a novel approach that does not require extra data to be exchanged. More precisely, we present ID-MAC: an identity-based MAC protocol for WSNs. In ID- MAC, sensor nodes compute when they should wake up in order to transmit and receive frames from their neighbors without exchanging coordination messages. Our evaluation shows that, by eliminating coordination messages, ID-MAC saves energy and prolongs the network s operating lifespan. Keywords Energy Efficiency, Identity-Based Protocols, MAC Protocols, Wireless Sensor Networks. I. INTRODUCTION Wireless sensor networks (WSNs) are ad hoc networks comprised mainly of small sensor nodes with limited resources, and one or more well-provisioned sinks that connect the sensor nodes to the rest of the world [1], [2]. WSNs are used for monitoring purposes, providing information about the area being monitored to the rest of the system. Application areas range from battlefield reconnaissance and emergency rescue operations to surveillance and environmental protection. WSNs are often deployed in hostile environments where replacement of node batteries is expensive, if not impossible [3]. This means that nodes are disposable and once their batteries are depleted they are no longer useful. In order to prolong the network s operating lifespan, power consumption must be minimal. A large fraction of energy spent in a WSN is controlled by its communication pattern. Unlike in wired networks, transmission power in WSNs is often significantly higher than CPU power consumption, e.g., approximately 12 times higher for the Telos node [4]. It is paramount to design energy-aware communication protocols tailored to WSNs to increase their lifespan [5], [6]. With regard to the protocol stack used by a WSN, the Medium Access Control (MAC) layer plays a key hole in energy management [5]. MAC protocols control many aspects related to node behavior that in turn have high impact in energy consumption, namely: idle-listening (node is listening but not receiving frames), overhearing (node receives a frame addressed to another node), overemitting (node transmits a frame while the receiver is asleep), collisions, and communication overhead [5], [6]. Broadly speaking, MAC protocols for WSNs can be classified in two different approaches: synchronous and asynchronous. In the synchronous approach, also referred to as the schedule-based approach, nodes agree on a common transmission and reception schedule prior to transmission [5], [7], [8]. This mitigates idle-listening, but on the other hand incur extra frame exchanges to compute and advertise the schedule [5]. In the asynchronous approach (e.g. [9], [10]), senders precede frames with a preamble slightly longer than the sleeping periods of their respective recipients. Whenever receivers wake up, they listen the medium and, if the medium is busy, keep on listening until they identify the frame s destination. This approach is able to decrease idle-listening without resorting to schedules. However, it suffers from overhearing as well as overemitting; both caused by the frame preambles [5]. There are also hybrid approaches that try to combine the advantages of both synchronous and asynchronous approaches [11]. Our contribution. In this work we present ID-MAC, an identity-based MAC protocol for WSNs. ID-MAC is synchronous in that each node knows other nodes schedules. ID-MAC, however, does not require coordination frames to compute and advertise the schedules. ID-MAC computes node schedules non-interactively based on unique node identifiers (e.g., their MAC addresses) and information loaded into nodes prior to deployment. To our knowledge, ours is the first MAC protocol where nodes can derive each other s schedules noninteractively. Our main contributions are the following: 1) the design of ID-MAC; including mechanisms for enabling schedule of broadcast and unicast transmissions without coordination messages; 2) an evaluation of ID-MAC; we compare ID-MAC and

2 S-MAC [7], which is often used as a baseline for WSN MAC protocols [12]; 3) we provide a publicly available 1 implementation of ID-MAC for the ns-2 simulator 2. Organization. The remainder of this work is organized as follows. In the next section we present an overview of MAC protocols for WSNs. In Section III we describe ID-MAC in detail. We explain the protocol s evaluation method and results in Section IV. Last, we discuss related work (Section V) and draw conclusions (Section VI). II. MACS FOR WSNS: OVERVIEW In many WSN applications, it is infeasible to change the nodes power source (e.g., battery). This means that once power is exhausted, the node becomes inoperable. Thus, one of the main goals in WSNs is to minimize power consumption, increasing the network s operating lifespan [3], [13]. A WSN s communication pattern is a key factor to its performance in terms of power efficiency [3], [13]. There are basically three communication patterns: (i) broadcast, (ii) convergecast, and (iii) local gossip. They are used under the following scenarios respectively: (i) when the sink wishes to disseminate information to the entire network 3 ; (ii) when the sensors wish to report back to the sink; and (iii) when the sensor nodes wish to communicate with other nodes in their neighborhood or cluster. Several communication characteristics are dictated by the network s MAC protocol. For instance, listening for frames (i.e., the node being awake and with the radio on) consumes a lot of energy, even if no reception is taking place [5], [6]. Thus, to save energy, WSN MAC protocols try to put the node in sleep mode, with the radio turned off whenever possible. In general, active periods have fixed length, whereas inactive periods depend on the pre-defined duty cycles 4. In addition to frame collisions and communication overhead, WSN MAC protocols have to address the following problems that waste energy during communication [5], [6]: 1) Idle-listening. Happens when a node is listening for frames but no frame is transmitted. 2) Overhearing. Happens when a node receives and decodes frames intended for other nodes in the process of receiving frames that are intended for themselves. 3) Overemitting. Happens when frames are transmitted but the destination is not listening. Thus, an effective and efficient design of a MAC protocol should take into account the problems mentioned above. There are also other design requirements including scalability and adaptability [5]. Changes in network topology, size, and node density should cause the smallest possible impact to the effectiveness and the efficiency of the protocol. A number of critical requirements for traditional networks are secondary for In this context, broadcast denotes something different from frame broadcast, in which network nodes send a frame to all nodes within its communication range. 4 Duty cycle is the fraction of time in which a node has its radio on. WSNs. For instance, throughput and delay are rather important, but energy consumption is even more so. Lastly, fairness in WSNs is not as relevant as in other types of networks; nodes in a WSN usually have the same objective, and it does not matter which nodes worked more to complete the task. Overall, there are two approaches to MAC protocol design in WSNs: (i) synchronous and (ii) asynchronous [5], [6]. Synchronous protocols (e.g., [7], [8]) are schedule-based, where each communicating node has a scheduled time interval reserved for its transmission. Following this schedule, potential recipients stay active to receive frames during the scheduled transmission period. The remaining senders also follow this schedule, and wait for their turn to transmit. Thus, synchronous protocols are able to mitigate both the overemitting and collision problems. On the other hand, these protocols need to exchange control messages to compute and advertise a common schedule, which leads to communication overhead. In addition, the schedule is usually fixed, and this lack of flexibility can lead to bigger delays. Asynchronous protocols are contention-based (e.g., [9], [10]). These protocols also introduced new mechanisms to reduce power consumption in WSNs. In a number of protocols, for instance, the sender sends a preamble which precedes the frame. The preamble is such that its transmission time is slightly longer than the recipient node s sleeping period. When the recipient wakes up and listens to the medium, it detects the preamble, identifies that the sender will transmit a frame, and stays active to receive it. On the other hand, if the potential recipient does not hear the preamble, it will quickly go back to sleep. These protocols have the advantage of lessening the idle listening problem and not requiring synchronization, which decreases communication overhead. However, they are subject to both overemitting and overhearing, since all sensor nodes need to wait for the preamble to verify whether the frame is intended for them. Among current WSN MAC protocols, S-MAC [7] is the most popular and frequently used as a baseline when evaluating new protocols. S-MAC works in transmission rounds. Each round alternates between two types of periods, active and sleeping (see Figure 1). During sleeping periods, sensor nodes turn off the radio and energy consumption is reduced. Following these periods, sensor nodes turn on the radio, and are ready to communicate. The length of these periods varies according to the value of the duty cycle. Active SYNC DATA Sleep Active Sleep Active Fig. 1: Active and sleeping periods in S-MAC. In S-MAC, each sensor node needs to synchronize the start of the rounds with its neighbors. Such synchronization is done at the initial phase, when each sensor node chooses randomly the time to go sleeping, and broadcasts it to its neighbors. This way, virtual clusters of nodes that share the same schedule are Time Time

3 formed. More specifically, each listening period can be divided in two stages in S-MAC: SYNC e DATA (see Figure 1). The SYNC stage is used for synchronization, in which sensor nodes can send schedule updates to their neighbors. The DATA stage is used for communication, in which sensor nodes communicate via broadcast or unicast. S-MAC also uses (RTS-CTS-DATA-ACK) [14] to avoid collisions. Note that S- MAC, like other synchronous protocols, spends a considerable amount of energy on control messages. As we will see below, such messages are unnecessary in ID-MAC. NOTATION n c d q t(c) τ(n, c) f N s TABLE I: Notation DESCRIPTION Node identifier Round counter Round duration Duration of the transmission of a maximum-size frame Start time of round c Transmission time of node n in round c Pseudorandom function Node n s neighbor set III. ID-MAC We now describe our identity-based MAC protocol. We start with an overview (Section III-A), then describe unicast (Section III-C) and broadcast transmissions (Section III-B). We finish with a discussion of implementation and operational details (Section III-D). A. Protocol overview ID-MAC s main advantage over other protocols is that it does not exchange coordination messages between sensor nodes to build a schedule of transmission and listening periods. Instead, each node computes its neighbors transmission and listening schedule non-interactively. This way, ID-MAC combines the advantages of both synchronous and asynchronous protocols, i.e., it mitigates overemitting and overhearing, without the overhead of sending and receiving coordination messages. ID-MAC operates in rounds, fixed-duration time periods. Nodes running ID-MAC maintain a shared round counter, denoted c, that is incremented after each round. The round duration, denoted d, is fixed and common to all nodes. The time instant when round c starts is denoted t(c) and the next round starts immediately after the previous one, i.e., t(c + 1) = t(c) + d. Sensor nodes are synchronized and start round c at the same instant t(c). Each node using ID-MAC has a unique identifier n, e.g., its network card physical address or CPU serial number. During one round, a node n can transmit one frame and receive one frame from each of its neighbors. A transmitting node n computes its transmission time in round c, denoted τ(n, c), using a pseudorandom function. The pseudorandom function is shared across all nodes so that a receiving node can compute when transmissions will occur to wake up and listen. We note that the pseudorandom function need not be distributed while the network is running; it can be preloaded into nodes prior to deployment. Figure 2 illustrates two transmission rounds of a node n. The beginning of each round is reserved for broadcast transmissions. All nodes wake up at the beginning of each round to listen for broadcast transmissions and receive them. Section III-B describes the algorithm we use to choose which node should broadcast at each round. The transmission time of node n during round c, τ(n, c), varies across rounds. For example, node n may transmit right after the broadcast transmission in round c, i.e., τ(n, c) t(c) + q, and at the end of round c + 1, i.e., τ(n, c + 1) t(c + 2) q. The pseudorandom function f receives the node t(c) τ(n, c) q q t(c+1) τ(n, c+1) d broadcast unicast Fig. 2: Illustration of rounds c and c + 1 at node n. identifier n and the current round counter c as input, and outputs a random number between [0, 1). The transmission time of node n in round c, τ(n, c) is proportional to f(n, c) (details in Section III-C). Finally, each node n keeps a set N n with the identifiers of its neighbors, i.e., the nodes inside its transmission radius. We note that a node n has all the data needed to compute the transmission times τ(n, c ) of all its neighbors n N n in any round c. B. Broadcast transmissions ID-MAC reserves the first q milliseconds of each round c to transmit broadcast frames, i.e., the period between t(c) and t(c) + q; where q is the time required to transmit a maximumsize frame and its (possible) acknowledgement. A node n transmits a broadcast frame during round c if f(n, c) < f(x, c) for all x N n and (1) f(n, c) < 1 N n. (2) Eq. (1) distributes available broadcast bandwidth fairly among nodes. Each node receives a fraction of the broadcast bandwidth inversely proportional to the amount of neighbors it has. Because f is a pseudorandom function, nodes eventually obtain the medium to transmit broadcast frames and there is no starvation. Eq. (2) limits broadcast bandwidth utilization so that new nodes joining the network can quickly obtain the medium and broadcast its presence to other nodes in the network. Eq. (2) limits broadcast bandwidth to around 70 75% for neighborhood sizes between 3 and 10 nodes. C. Unicast transmissions Each node can transmit one frame per round. Each node n computes its transmission time in round c, τ(n, c), as τ(n, c) = t(c) + q + (d 2q)f(n, c). (3)

4 We have τ(n, c) > t(c) + q because t(c) + q is the latest instant when the broadcast transmission in round c can end, i.e., the instant when unicast transmissions can start. The d 2q term is the duration of the interval where we can start transmissions during round c. We subtract 2q from the round duration d because the broadcast transmission occupies the first q milliseconds of each round and because we have to start the transmission of a frame before t(c) + d q to guarantee the frame finishes transmission before the next round starts, i.e., τ(n, c) + q must be less than t(c + 1). All neighbors of a node n know its identifier and can compute τ(n, c). Neighbors interested in receiving data from n wake up and listen at instants τ(n, ) to receive any frames n transmits. In particular, in networks where data is forwarded to a sink, the next node in the routing tree from n to the sink has to wake up at instants τ(n, ) to receive n s frames and forward them to the sink. D. Implementation and deployment Network construction. Nodes need to maintain two pieces of shared information: the current round number c and the time when rounds start, t(c). 5 A node joining the network can learn these values from its neighbors. When joining two disjoint networks, we can pick a unique c and t(c) for the joined network using classic leader election algorithms [15]. Synchronization. ID-MAC assumes nodes are kept synchronized. In particular, nodes have to synchronize so they know when a round c starts, t(c). This synchronization can be achieved using Reference-Broadcast Synchronization (RBS) [16]. Operation mode. We described ID-MAC assuming that node n knows which neighbor n it wants to receive from. This operation mode is adequate for networks where nodes forward data to a sink, and reduces collisions as nodes transmit at random times. ID-MAC also works when sensors cannot know what nodes they need to receive from. In this case, we just use Eq. (3) to determine the instant when node n wakes up and listens for data (instead of transmitting). This operation mode is subject to collisions, as two nodes may transmit to n at the same time τ(n, c). Confirmation and retransmission. Receiving nodes running ID-MAC send small acknowledgement frames for each data frame they receive. In case a collision or decoding error happens when a frame is transmitted, the sender will not receive an acknowledgement and tries to retransmit the frame in a later round. IV. PERFORMANCE EVALUATION This section presents results on the performance of ID- MAC. We contrast it against S-MAC by using simulation. We first describe the simulation setup (Section IV-A) and then the simulation results (Section IV-B). 5 Note that the system does not depend on a global shared round counter c. Alternatively, each node n can have its own round counter c n; it suffices that n s neighbors store n s counter c n to compute Eqs. (1), (2) and (3). A. Simulation Setup We consider an event-driven WSNs deployed in an interest area [17]. We use an event model proposed by Mini et al. [18] where events happen at a random location in the network area and are detected once by a single node. The time between the events is defined by a Poisson distribution with λ events per second [19]. Nodes that detect an event collect data and transmit it to the sink node. We assumed a network with 200 static nodes randomly deployed in a field with m 2. The sink node is fixed and located at the bottom left corner, i.e., (0, 0). We run simulations for different values of λ, i.e., 0.03, 0.09, and 0.12 events per second. The radius of influence r is chosen uniformly in the range [2, 10] m 2. The duration of each event is chosen uniformly in the range [5, 50] seconds. The time between consecutive events is set according to the following equation: f(x) = λe λx [20]. We aim at using the aforementioned parameters to generate events with different duration and radius, taking place at different times along the network s operating lifespan. The network is homogenous, i.e., all nodes have the same hardware capabilities and configuration. Each node initially has 25 J of energy. This value is chosen so as to avoid premature node s battery depletion. Nodes communication and sensing range is set to 10 m. The energy dissipation model used follows the one implemented in ns-2 6. According to it, energy consumption is decremented as a function of node operation mode. The model s parameter is set so as to reflect the Mica2 7 configuration. The node s duty cycle is fixed at 20 % and the duration of each transmission round is approximately 140 ms. This value of 20 % was chosen so that all nodes could send data without much overlap in their sending interval. Lastly, we employ a simple routing protocol that creates a routing tree based on the inter-node Euclidian distance [21]. We choose a simple protocol since our focus is on the MAC layer. Table II summarizes the simulation setup and, unless otherwise noted, we use the default parameter values shown there. For a fair comparison, we simulate ID-MAC and S-MAC under the same scenarios. As we mentioned earlier (Section II), S-MAC is often used as a baseline for new MAC protocols. To mitigate the impact of retransmissions in the results, both protocols are set to retransmit a frame only once, if retransmission is required. Besides, we ignore the first 60 s of each simulation run. This is so as to remove the impact of neighbor discovery and routing tree construction from our results our focus is the performance of the MAC protocols in terms of end-to-end data transmission, i.e., from sensor nodes to the sink. We repeat each experiment 33 times with different seeds and show average values and their confidence interval with a confidence level of 95%. In our evaluation, we consider the following performance metrics

5 Cumulative average delay (s) S MAC λ = 0.03 ID MAC λ = Cumulative average delay (s) S MAC λ = 0.09 ID MAC λ = Cumulative average delay (s) S MAC λ = 0.12 ID MAC λ = (a) λ = 0.03 (b) λ = 0.09 (c) λ = 0.12 Fig. 3: Average delay as a function of λ. Average residual energy (J) S MAC λ = 0.03 ID MAC λ = 0.03 Average residual energy (J) S MAC λ = 0.09 ID MAC λ = 0.09 Average residual energy (J) S MAC λ = 0.12 ID MAC λ = 0.12 (a) λ = 0.03 (b) λ = 0.09 (c) λ = 0.12 Fig. 4: Average energy consumption as a function of λ. TABLE II: Simulation setup. Parameters Network topology plain Node hardware homogenous Network size 200 nodes Grid Area m 2 Sink position (0, 0) Battery energy 25 J Transmission power +5 dbm Communication range 10 m Duty cycle 20 % Simulation time 1200 s Radio Energy Consumption (Mica2) Transmit 27 ma Receive 10 ma Sleep mode 1 µa Event range Event duration Event rate λ Events Model Parameters Uniform(2, 10) m Uniform(5, 50) s {0.03, 0.09, 0.12} events/sec 1) End-to-end delay. The interval between transmission of a message by the source node and its reception at the sink. 2) Energy consumption. The total energy consumed by the network along its operating lifespan. 3) Delivery ratio. The fraction of frames that are successfully delivered to the sink. B. Simulation Results The Figure 3 presents the cumulative average delay during the network s operating lifespan. Note the delay increases along the simulation (Figure 3(a)). This is so because messages for events happening far away from the sink start to get received, increasing the average delay. Also note that the higher the λ, the higher the delay; as the network utilization and forwarding queue lengths increase. When we compare the performance of ID-MAC versus S- MAC, we observe that ID-MAC performs better and its delay is lower than S-MAC s. To be precise, ID-MAC s delay is lower than S-MAC s 40%, 39%, and 40% for 0.03, 0.09, and 0.12 values of λ, respectively. Two features of ID-MAC contribute to reducing the delay: (i) the transmissions happen at random moments, which decreases the chance of a collision; and (ii) more than one transmission might happen in a transmission round. We evaluate power consumption by looking at the average residual energy of node batteries. Figure 4 shows the decline of the average residual energy along the simulation for both protocols. We can observe an increase in power consumption when the event frequency increases as the number of transmissions also increase. Further, ID-MAC has power consumption at least 10% lower than S-MAC for all the three event rates. Specifically, ID-MAC s power consumption is 14%, 13%, and 15% lower than S-MAC s for 0.03, 0.09, and 0.12 values of λ, respectively. Three features of ID-MAC protocol contributes to saving energy: (i) it does not use RTS- CTS frames to reserve channel; (ii) the number of collisions (and retransmissions) decreases as transmissions are spread

6 Rx/Tx Ratio S MAC ID MAC Different values of λ Fig. 5: Frame delivery ratio as a function of λ. uniformly by the pseudorandom function; and (iii) the noninteractive nature with which nodes figure out their schedules. Note that these savings in power consumption will ultimately prolong the network s operating lifespan. Figure 5 shows the frame delivery ratio, i.e., the ratio of the number of frames delivered to the sink by the number of frames generated at the nodes. As the event rate increases, network utilization increases and S-MAC s delivery ratio decreases. (In particular, S-MAC s delivery ratio reduces from 75% for λ = 0.03 to first 72% for λ = 0.09 and then 69% for λ = 0.12.) One problem is that higher network utilization leads to more collisions at the MAC layer. Another problem is that S-MAC can transmit only one packet per transmission round, and the duration of transmission rounds depends on the fixed duty cycle of 20%. ID-MAC, on the other hand, spreads transmissions randomly over time, reducing collisions, and can transmit multiple packets per round. Therefore, ID- MAC performs better and can keep the delivery ratio roughly constant as the event rate increases, i.e., around 85%. V. RELATED WORK MAC protocols play a fundamental role in WSNs since they are responsible for the actual data communication between nodes. Several MAC protocols have been proposed for sensor networks [7] [11], [22] [33]. Figure 6 presents a timeline with some of the most important MAC protocols for WSNs. Ye et al. [7] proposed the S-MAC protocol, one of the first MAC protocols for WSNs. S-MAC is a synchronous protocol based on duty cycle, as discussed in Section II, which inspired several other proposals that tried to improve it. One of them is T-MAC [8], which aimed at reducing the idle-listening and the communication latency. T-MAC adapts the duty cycle of sensors according to the data traffic in the network. Before entering sleep mode, a sensor checks whether there is ongoing or imminent communication. If it is the case, the sensor remains active, ready to transmit or receive a message. The disadvantage of this strategy is that the sensor stays active wasting energy if there is no transmission. Following this protocol evolution, Polastre et al. and Lu et al. proposed B-MAC [9] and D-MAC [10], respectively. Both employ the concept of sleep mode to save energy. B-MAC employs an adaptive preamble configured by protocols in the upper layers to decrease the sensor s duty cycle and minimize the idle-listening. D-MAC proposes to improve convergecast by avoiding losing a data transmission to the sink. In D-MAC, each sensor chooses its sleeping moment according to its own location in the routing tree. D-MAC also adjusts the duty-cycle period according to the network traffic. SCP-MAC [26] was designed to work with an extremely low duty cycle. It can work with a duty cycle of 0.1% and even less. Its idea is to optimize the duty-cycle scheduling and poll the communication channel in order to improve channel use. In this protocol, sensors wake up after short periods of time to check the channel activity. SCP-MAC presents a better performance in scenarios of low traffic. SEEDEX [35] and PW-MAC [33] share similarities with our work. In SEEDEX, nodes in ad-hoc networks used the output of a pseudo-random number generator (PRNG) to determine when they transmit, and learned of other nodes schedules by exchanging seeds with each other (thus, still requiring a small amount of communication for coordination). SEEDEX also did not deal with duty cycling, as it mainly focused on avoiding collision and increasing throughput in IEEE protocol. In PW-MAC [33], a solution developed for energy efficiency in WSNs, nodes use PRNG to determine when they wake up to receive packets. Senders use receivers wake-up schedules accordingly, and wake up just in time to send their packets. Like SEEDEX, PW-MAC also incurs overhead for coordination. Senders learn their receivers wake-up schedules (PRNG parameters) on demand, through a request-response exchange using beacons from the receivers. There are also MAC protocols designed for particular scenarios. For instance, Hsu et al. [30] proposed ST-MAC, a MAC protocol for underwater wireless sensor network. And Andrade et al. [34] come up with HMARS, a MAC protocol tailored to WSNs and radio over fiber integration. VI. CONCLUSION Replacement of node batteries is usually impractical in WSNs and power consumption must be minimal. A significant fraction of power consumption is controlled by the MAC layer, which orchestrates transmission and listening periods tradingoff power consumption and throughput. This orchestration, however, requires exchange of coordination messages and consumes energy itself. In this work we presented a novel approach that does not require the exchange of coordination messages. More precisely, we design ID-MAC: an identitybased MAC protocol for WSNs. In ID-MAC, senders and recipients are able to non-interactively compute when they should be awake to transmit and receive frames. Our results show that by not requiring the exchange of coordination messages, ID-MAC reduces power consumption and extends the network s operating lifespan. VII. ACKNOWLEDGMENTS We thank Jie Liu for valuable discussions during the early stages of this work, and Tiago Andrade for his helpful comments. We also thank the anonymous reviewers for their valuable comments and suggestions. This project was financially supported by the CNPq-Brazil. REFERENCES [1] D. Estrin, R. Govindan, J. S. Heidemann, and S. Kumar, Next century challenges: Scalable coordination in sensor networks, in MobiCom 99, pp , 1999.

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