A novel technique for low latency data gathering in Wireless Sensor Networks

Itziar Marı́nElectronics and Communications Dept.

Tekniker Foundation, Spainimarin@tekniker.es

Aitzol Zuloaga Iker LosadaElectronics and Telecommunications Dept.

University of the Basque Country, Spainaitzol.zuloaga@ehu.es iker@bips78.bi.ehu.es

Abstract

This paper proposes a brand new medium access con-trol (MAC) protocol specifically designed for wireless sen-sor network applications that require low data latency. Wi-reless sensor networks use battery-operated computing andsensing devices and its main application is environmentalmonitoring. The proposed protocol uses novel techniquesto offer a low end-to-end data transmission latency fromthe furthest away nodes to the sink in a unique working-cycle while offering a low-duty-cycle operation. Key featu-res of this protocol include a synchronised sleep schedule toreduce control overhead along with a mechanism to avoidoverhearing unnecessary traffic and elude collisions.

1. Introduction

Wireless sensor networks are formed by several scatte-red nodes, from hundreds to thousands, in a sensor field.Each node contains both processing and communicationelements and has event-oriented environment monitoring asmain functionality. Collected data from the environment aresent to the base station to be processed. Thanks to the highnode density in this kind of networks, collaboration amongthem, allows creating a high quality and fault-tolerant envi-ronment monitoring system [17,19,22].

Recent advances in low power radio electronics, inmicro-electromechanical systems (MEMS) and in wirelesscommunications, have contributed to the development ofhardware affordable sensor nodes. Because of tremendousadvances in semiconductor integration, wireless sensor ne-tworks have emerged as an ultra-low power and reducedcost technology that can be applied to a number of appli-cations, in both civilian and military scenarios, includingmonitoring and surveillance of large, remote or inaccessibleareas over extended periods of time. Unlike standard wire-less/ad hoc networks, wireless sensor networks (WSN) areseverely resource constrained and energy conservation is ofparamount importance. The lifetime of the sensor network

is hence limited by the lifetime of the node battery. Need-less to say, low power consumption is a major requirementin the design of communication protocol for wireless sensornetworks. As the wireless radio-communication interfaceconsumes a significant fraction of node energy, energy effi-ciency can only be achieved through the design of energy-aware communication protocols.

Organization of the paper is as follows. Related work ispresented in Section 2. The proposed MAC protocol is des-cribed in Sections 3-5 and the performance of the protocolis evaluated in Section 6. Finally, the paper is concluded inSection 7.

2 Related Work

Most MAC protocols for wireless sensor networks havebeen based on conventional wireless protocols, especiallyIEEE 802.11 [1], and IEEE 802.15.4 [2,13]. Low duty cy-cle MAC protocols, whose objective is battery consumptionreduction, works in a similar way: nodes are sleeping mostof the time and only wake up whenever is required for recei-ving or transmitting information. This approach that tradeslatency or throughput for energy saving, is quite effective instationery and latency-tolerant networks.

The starting point of our research was the protocol calledS-MAC [25]. It is the first real MAC protocol for WSNthat offers low power consumption thanks to a sleep/awakescheme. Inspired by PAMAS [20] and IEEE 802.11, at thebeginning of each active period, nodes get synchronised andduring the remaining active period, data may be transferredusing control packets. In order to reduce the probability ofcollisions, it uses RTS/CTS to ask for the medium access.Hence, nodes not only negotiate a transmission but also tryto avoid the hidden terminal problem [27].

S-MAC authors in [26] introduce a modification of theprotocol with an enhanced packet forwarding capacity, andGSA-FPA [10], that offers lower latency and energy con-sumption. At the same time, some other protocols, based onS-MAC, tried to solve some shortcomings: DSMAC [11]tries to reduces latency varying the working cycle; and T-

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DOI 10.1109/DSD.2008.39

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Figure 1. Working cycle scheme.

MAC [23] decreases idle listening period by increasing thelatency.

Following a similar philosophy to S-MAC, Wei Ye et al.presented in [24] an alternative, called SCP-MAC. This pro-tocol is mainly based on WiseMAC [7] and proposes a pro-grammed polling medium access. It offers a different ap-proach: instead of being the receiver responsible of swit-ching on its radio interface when the emitter is going totransmit, it is the transmitter which must wait until the re-ceiver is ready and wakes up.

TRaffic-Adaptive Medium Access (TRAMA) protocol[16] tries to solve the collision and low consumption pro-blem with a TDMA strategy: it does not assign transmissionslots to those nodes that have nothing to send, which do notwake up and keep on sleeping saving energy.

However, none of the presented MAC protocols offersa global solution for latency-aware data transmission fromeach node in the network to the sink through several hops orefficient topology management. Although several routingprotocols have focused their research in low latency datacollection [5,6,14,21], MAC protocol implementations havenot offered good enough integrated solutions yet.

DMAC [12], DMTCS [9] (Distributed Minimal TimeConvergecast Scheduling) and MERLIN [18] are three re-presentative examples of protocols oriented to optimise ne-twork latency. MS-MAC [15] is a MAC protocol appliedto an infrastructure with mobile sensors. All of them weresimulated and no real implementation of them is available.

The DMAC protocol offers an information routing me-chanism for WSN that leads traffic from the branches to thesink. Nodes wake up sequentially in a chain reaction fas-hion, in order to transmit information from one to anotheralong the different hops towards the destination.

However, DMAC protocol does not implementRTS/CTS flow control mechanism. All the nodes, at thesame network level (that is, at the same hop number) havethe same time scheme. Consequently, all of them willtransmit at the same slot. MERLIN protocol introduces thesame disadvantage when tha data is routed to the sink. Both

Figure 2. Control interval scheme.

approaches, DMAC and MERLIN, suffer from the hiddenterminal problem and collisions between transmittingnodes are very often in the network.

On the other hand, DMTCS is a TDMA solution with nocollision or hidden terminal possibility but takes too manyassumptions and there are no clear explanations of the per-formance. Finally, MS-MAC protocol is based upon S-MAC and its objective is adapting that protocol to a sensornetwork with certain mobility, offering some kind of chan-ging topology management.

3 Low Latency Design Overview

The presented protocol is aimed to a specific applicationof WSN: periodic data collecting from all the nodes to thesink through multi-hop paths (commonly known as conver-gecast) as fast as possible, in order to have the values of thenetwork with similar age, as stated before. It offers a brandnew performance planning for nodes to transmit informa-tion divided into time slots, apart from topology manage-ment, energy consumption reduction and global network la-tency improvements.

The network is supposed to be formed by a number ofnodes scattered in the working area. All of them are sensingthe environment and sending that information to the sink viaa multi-hop route.

In order to understand the nomenclature, some parame-ters must be defined:

• Tc: Working cycle period.

• tslot: A time slot is the required time for sending onepacket with a MAC ACK included. Current optimisa-tion is 10 milliseconds.

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• tsleep: Each node sleeps during this period of time.

• C: control interval adjustment (ratio Data intervals /Control intervals).

• M : maximum number of hops of the network. Mustbe defined before deployment.

• N : maximum number of nodes in the network (exclu-ding the Base Station). Must be defined before deploy-ment.

• Pi: maximum number of children of each node of thehop i. Each node in hop m can accept Pm children,each node in hop number n can accept Pn children andso on. Must be defined before deployment.

• Si: maximum offspring of each node of the hop i.Each node in hop m has Sm descendants, each node inhop number n has Sn descendants and so on. It is im-portant to remark the difference between child/childrenand offspring. Its children are the nodes, belonging tothe next hop, that are directly connected to it. Howe-ver, its offspring are the nodes of the next hops that areconnected to it or to its children or to the children ofits children and so on.

Similar to most of the presented protocols in Section 2,the working cycle Tc is divided into two periods: ActivePeriod (AP) and Sleep Period (SP). The AP is also dividedinto two intervals: Control interval and Data interval. In theControl interval, topology information is shared, and in theData interval, collected information is routed to the desti-nation. A general overview of the working cycle is shownin Figure 1. However, Control interval in this protocol isquite different from most of the presented protocols, as itdoes not share the medium access, although all the nodesare listening all the time. It is formed by four sub-intervals,named as follows:

1. Clock Adjustment (∆t): Allows fixing desynchroniza-tion among the clocks of the nodes.

2. Node Advertisements (NA): Each node publishes itsown advertisement in this sub-interval.

3. Child Adoption Request (CAR): This sub-interval willonly be used for nodes that want to join the networkbecause they are new to it or due to a parent loss orchange.

4. Child Confirm (CC): This sub-interval is used by theparents to confirm to the newly adopted children.

Advertisement messages include all the information nee-ded for the nodes to get synchronised, know their hop num-ber and choose a parent. The second and third sub-intervals

Figure 3. Working Cycle Scheme with C=2.

(CAR and CC) only have traffic when any node decides tochange its parent relationship (due to topology changes ornode movement) or new nodes appear in the network. Alt-hough each node must wait for its time slot to publish itsown information, every node is listening to every transmis-sion in the air. Figure 2 shows the Control interval.

Each one of these sub-intervals (NA, CAR and CC) is di-vided into M non-uniform divisions. As there are differentamount of nodes in each hop, there are different amountof time slots reserved in each hop division. Km representsthe number of time slots included in hop m. Equation 1offers the distribution of the N + 1 time slots inside eachsub-interval. In each sub-interval, each node plus the BaseStation has a unique time slot assigned (N + 1 time slots).

M∑m=0

m∏i=0

Pi =M∑

m=0

Km = N + 1 (1)

The Data interval is divided into M divisions (one foreach hop number), and each of them divided into N timeslots subdivisions, each one for each node transmission, asit will be further discussed in Section 5.

Equation 2 represents the Working Cycle Period and li-mits the collected data resolution, as environment data issampled every Tc.

Tc = [(M + 3) ·N + 3] · tslot + tsleep (2)

4 Synchronised Performance

As a sensor network is formed by a great amount ofnodes scattered in a field and they are not supposed to beswitched on in a predefined order: base station first, thenthe surrounding nodes and so forth. The protocol has beendesigned in order to overcome random start-up proceduresand nodes get adapted to the network as long as they listento advertisements.

The basic behaviour of every node is quite simple: eachnode has to publish its advertisements to the rest of the

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Figure 4. Data interval scheme.

nodes, receive information from its children and retransmitit to its parent. When a node wakes up for the first time, itis not synchronised and it has no real parent. As long as anode does not have a functional parent, it can send neitheradvertisements nor data because it knows neither when norwho send them to. Moreover, orphan nodes keep awake andlistening all the time, waiting for an advertisement.

Base Station behaviour is a bit different: wakes up withan operative parent (the sink), so it does not have to waitfor any advertisement. As expected, the Base Station isthe node that establishes the working cycle (Tc) and the ac-tive/sleep scheme from the beginning.

4.1 Changing Topology Management

As stated in Section 3, every node in hop i could be pa-rent of a number of Pi children, and it will address themfrom #0 to #Pi − 1. Each time that it receives a CAR, itwill check if it has enough free space to adopt that child. Ifso, the parent will allocate that child with the smallest ordi-nal number, beginning from #0. In that moment, that nodewill be the child number #0 for that parent, regardless of itsID. The next child to be adopted by the same parent will bechild number #1 and so on, up to Pi children.

As indicated before, when a node wakes up for the firsttime it has no real parent. In order to update its parent-relationships, every time the Control interval is executed,all the nodes analyse every received advertisement. All thepackets are analysed in order to determine if the currentparent is the best one of the available parents. Once thenode decides which one is the most suitable parent (the bestlink quality), it will ask for its child adoption in the CARsub-interval. Following the process explained before, theparent-to-be decides whether it can adopt that child or not,and if so, it will answer a CC message. When a parent hasits maximum number of allocated children, he will not sendan answer.

In the CC message, the parent notifies to its newly adop-

Figure 5. Tmote Sky mote

ted child the assigned data slot number. With this informa-tion, the child can calculate the exact moment when it hasto transmit the data to its parent and its own hop number. Ifa child does not receive a CC message in the same Controlinterval that it had asked for, that child keeps the currentparent and forgets about the parent-to-be.

In every working cycle, each node sends its own data toits parent in the time slot number that had been assignedbefore. A parent change (and therefore, a topology change)can be completely done within a unique Control interval.

The former parent will not listen to the current transmis-sions of the recently moved node. Therefore, if a parentdoes not receive data from one child for 3 consecutive cy-cles, it decides to delete it from its childhood.

4.2 Cycle Synchronisation

As explained in Section 3, advertisements also offer in-formation for synchronisation purposes: a synchronisedtime-stamp and the control interval adjustment parameter(C).

Every child processes advertisements received from itsparent in order to extract the synchronisation information.Once it has updated its own variables, it will include thatinformation on its own advertisements. Due to the timeslot order, every node always transmits updated informa-tion. The first node to talk is the base station and its chil-dren hear it (NA sub-interval, BS division). Then, in thenext division (hop 0 division), base station’s children talkwith its own information, updated with the information re-ceived from the base station and so on, as shown in Figure2. The synchronised time stamp allows every node to adjustits own clock to its parents. With this synchronisation met-hod, all the nodes in the network wake up simultaneouslyat the beginning of a working cycle and listen to each otherat the right time. Further explanation of the synchronisationmethod can be found in [8].

It must be stressed that the synchronisation, as well astopology management, is performed with the informationattached in advertisements so it can only be done every Con-

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Table 1. Parameters of implementationParameter Value

Time slot (tslot) 10 msDuration of packet transmission (ttx) 5 msDuration of packet reception (trx) 5 msDuration of radio module start up (ton) 5 msCycle repetition (Tc) 30 sTransmission current (Itx) 12.5 mAReception current (Irx) 22 mASleep current (Isleep) 6 µAStart up current (Ion) 2.5 mA

trol interval. C is the ratio that indicates how many times aData interval exists for one Control interval. Figure 3 pre-sents an example of this modified performance for C = 2.It can be seen that in the second Tc, when it was supposed tobe a Control interval, node keeps on sleeping until the Datainterval arrives.

If C is high, there is a Control interval for many Dataintervals and this means that topology information mightnot be updated as often as necessary. For this reason, Cmust be carefully chosen for each network’s requirementsof topology dynamics.

5 Low Latency Mechanism

The Data interval is not a simple TDMA period oftime. Time slotting has been carefully analysed in orderto achieve minimum latency performance at a low compu-tational cost. As mentioned in Section 3, Data interval isdivided into M divisions, and each one of them is, in turn,divided into N time slots sub-divisions. Each node will talkto its parent in the assigned time slot within the correspon-ding division. The Data interval is presented in Figure 4.

The information from the nodes is transmitted hop-by-hop from the furthest hop to the sink. The first nodes totalk belong to the highest hop number (in the division M −1), then the following hop M − 2 nodes and so on. Everyparent listens to all its children, each in a different time slotand then store that information. If a parent is in the hop1, it will listen to its children in the hop 2 in the time slotspreassigned. In the next hop division (hop 1), the parentwill send its own data along with the stored data from itschildren in the previous hop division to its parent, and so on.It must be stressed that no data aggregation mechanism hasbeen implemented in order not to increase node complexity.

A B

C D E

BS

HOP 1

HOP 0

F G H

Figure 6. Tested topology.

6 Protocol Implementation

The purpose of our implementation is to demonstrate theefficiency of the proposed protocol and to compare it withother similar MAC protocols. We use Tmote Sky motes 1

in our development platform and testbed. These motes arerunning TinyOS, an efficient event-driven operating systemfor sensor nodes [4].

Tmote Sky motes have the low-power TI MSP430F1611microcontroller with 48 kB of flash and 10 kB of RAM me-mory. These motes are equipped with the Chipcon CC2420radio transceiver and a whip antenna for working in 2.4 GHz(see Figure 5). Some important parameters of our protocolimplementation are listed in Table 1 [3].

The goal of the experimentation is to reveal the improve-ments achieved in latency in comparison with energy con-sumption, synchronisation and network topology manage-ment. However, our most important parameter latency, andalmost every parameter, cannot be directly compared withsome others MAC protocols results as they consider onlyone packet source and the rest of the nodes as routers of thatinformation, so there is no increase in the amount of data totransmit as hop number decreases. Most implementationsso far, have only tried to reduce latency in packets (one byone) from one unique source to the sink [12,15,18,25], whe-reas our protocol offers low latency for each and every pa-cket from all the nodes in the network.

Protocol experimentation was made with a non-linearhierarchy multi-hop network with eight nodes (N = 8) andtwo hops to the sink (M = 2). The election of the valuesof the parameters Pi, M and N was not made randomly:the best latency-performance-complexity trade-off for ourenvironment was selected. Our work was tested in the ne-twork scheme presented in Figure 6. We assume that nodeswill listen to any advertisement in a reasonable lapse of timefrom its initial awakening. Moreover, all the Data and Con-trol packets that belong to the same Tc are numbered the

1[Online] Available: http://www.sentilla.com

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Figure 7. Network performance.

same, to better identify.

6.1 Network Synchronisation

All the schemes are based upon global synchronisationof the network for hearing advertisements and for transmit-ting (and correctly receiving) data. To measure synchroni-sation, each node is wired to a common oscilloscope. Upontransmission of data packet and upon listening to children,we toggle a hardware pin. Using the oscilloscope, we cap-ture the sequence of the happenings, as shown in Figure 7.

Measurements along 6 hours of performance, demons-trated that synchronisation among nodes is in the bounda-ries of ± 200 µs. One-Sample Kolmogorov-Smirnov Test isa non-parametric test that studies the normality of a distri-bution. This test indicates that 95% of the samples of eachnode satisfies the Equation 3 with m = 6.3 (mean value) andσ = 33.67 (standard deviation), obtaining a value of syn-chronization 6.3± 67.34 (µs), which offers a deviation ofapproximately 75 µs around Base Station’s Tc.

Synchronisation (µs) = m± 2 · σ (3)

The measured syncronisation value is negligible in com-parison with the time slot duration (10 ms—see table 1), as-suring therefore the correct performance of the protocol.

6.2 End to End Latency

When topology is established, in this case N = 8 andM = 2, the latency of all the transmissions of each nodewill be the same regardless of whether all the topology isfulfilled or not. At this point, it’s important not to confuselatency with Tc. Latency is the time that a message spendsfrom its source to the sink and the working cycle (Tc) is theperiod of repetition and determines the data resolution (datais taken every Tc).

Using the end-to-end latency equation defined in Sec-tion 5, the theoretical maximum latency is Lend = 80ms.Figure 7 presents a real measurement of the performanceof the network and the latency achieved. Since node Cstarts transmitting until the Base Station receives its mes-sage, there’s a gap of 79.95 ms. It can be seen that the reallatency fulfils theoretical estimation.

6.3 Energy Consumption

Node lifetime is determined by its overall energy con-sumption. To measure the consumption of the radio, whichis, by far, the most consuming part of the node, we measurethe amount of time the radio is in sleep, transmitting or re-ceiving modes. The energy consumption in each mode isthen calculated by multiplying the time in each mode by therequired power to operate the radio in that mode. We mea-sure the energy directly but in a non-intrusive way, thanksto a Mobile Communication DC Source 66319B by AgilentTechnologies that offered high resolution and sample rate(0.5 µA and 64 kHz, respectively).

In wireless sensor network applications, like in mostburst communications, the consumed current can be expres-sed by Equation 4. In our MAC implementation, data sam-pling is assumed instantaneous and it does not add extrapower consumption to the total amount. In the followingequations, V is the power supply voltage.

Ic =Ectrl + Erx + Etx + Esleep

V · Tc(4)

Ectrl is the consumed energy during the Control interval,in which all the nodes switch the radio module once, keepon listening all the time, and, in an average situation, all thenodes will listen to the same amount of advertisements andwill have to send its own one. For this general analysis, it isassumed that no traffic will be in CAR and CC sub-intervals(see Section 3).

Ectrl = V · (Ion · ton + Itx · ttx + Irx · (tctrl − ttx)) (5)

Erx depends on the offspring of each node, as every nodelistens to all the packets its own children send which includethe packets of the rest of the offspring below those children.Every node starts up the radio module as many times aschildren has and keeps on listening during as many timeslots as descendants has.

Erx = V · (Ion · ton · Pi + Irx · tslot · Si) (6)

Etx depends on the number of packets to send that inturn depends on the offspring of each node. For each trans-mission, node must listen to a MAC acknowledge message.Every node switches on the radio module once and trans-mits as many packets as descendants has.

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0 5 10 15 20 25 30 35 40 450

1

2

3

4

5

6

7

8

9

10

X: 12Y: 3.071

Data Resolution (s)

X: 12Y: 1.48

X: 36Y: 1.845

X: 36Y: 1.519

Life

time

(yea

rs)

C=1 Minimum Offspring

C=1 Maximum Offspring

C=6 Minimum Offspring

C=6 Maximum Offspring

Figure 8. Lifetime versus data resolution.

Etx = V ·(Ion ·ton+(Itx ·ttx+Irx ·(tslot−ttx)) ·(Si+1))(7)

Esleep is the current consumption during the time thenode is doing nothing but sleep. It depends on Tc and thetransmission, reception and Control intervals duration.

Once studied every part of Equation 4, it is important toanalyse the contribution of each of them to the total amountof consumed current. This way, Ectrl is the most consu-ming part of every Tc with the 87.71% of the global con-sumption, whereas Data (Erx +Etx) consumes the 12.21%and the sleep period just draws the 0.08% of the current.

6.4 Life Time versus Data Resolution

As shown in Section 6.2, after establishing the networktopology, the latency for each packet is fixed and only de-pends on the maximum number of hops (M ) and nodes (N )in the network. However, power consumption (and con-sequently, node lifetime) does change depending on otherparameters such as the control interval adjustment (C), thenode offspring or the desired datarate. Figure 8 shows therelationship between nodes lifetime with two AA batteriesand the lapse of time between samples (data resolution—seeEquation 2). The longer these wait periods are, the less da-tarate will be required to send the information and, hence,nodes will remain longer is sleep slots. Thus, power con-sumption is reduced and lifetime, increased. On the otherhand, low values of C require that more sleep intervals areoccupied with control slots, which produces higher powerconsumptions. The position occupied by the node in the ne-twork topology (its offspring, Si) will also alter the amountof energy consumed by that node: those nodes, which areplaced closer to the sink will have more traffic to process,

5 10 15 20 25 30 35 400

0.5

1

1.5

2

2.5

3

End

-to-

end

late

ncy

(s)

Number of sources

LL−MAC

DMAC

S−MAC, MS−MAC

MERLIN

DMTCS

Figure 9. End-to-end latency comparison.

which means that will consume more power than much furt-her placed nodes, and therefore, they will have a shorter li-fetime.

In the selected topology, the furthest node (in hop 1) hasno offspring while the closest (in hop 0) has three descen-dants. For the selected data resolution (36 seconds) andC = 1, Figure 8 offers a lifetime of 1.519 years for theclosest node to the sink and for the furthest node, almosttwo years of life with a completely synchronised and lowlatency performance. As in most wireless sensor networksapplications, the achieved duty cycle with this data resolu-tion is around 1% for every node in the network.

6.5 Control Interval Adjustment

In Section 6.4, data resolution is constrained by the nodelifetime. On the other hand, Section 6.3 presented that morethan the 85% of the total consumption of every node is dueto the Control interval of every Tc. If the contribution of theControl interval to the total consumption could be reduced,data resolution would be increased while maintaining thesame lifetime of the network.

As explained in Section 4.2, C parameter represents theratio between Data periods and Control periods. The cu-rrent consumption due to the inclusion of parameter C isslightly different to Equation 4. Every C · Tc, sleep period,data transmission and data reception phases are repeated Ctimes but Control interval is executed once. The remainingC − 1 times that Control interval is not present, the node issleeping and consumes the Isleep current.

Assuming a fixed lifetime (one year and a half), if Cis high, Control interval influence in the total consumptiondecreases and data resolution could get improved. Needlessto say, reducing the number of Control intervals decreases

509

5 10 15 20 25 30 35 400

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Number of sources

Life

time

(yea

rs)

LL−MAC C=1

LL−MAC C=2

LL−MAC C=3

LL−MAC C=4

LL−MAC C=5

LL−MAC C=6

S−MAC

MS−MAC

Figure 10. Topology management compari-son.

the speed reaction to network topology changes. If C is 6and lifetime 1.5 years, a data resolution of 12 seconds canbe achieved, if network dynamics allows a topology updateof 72 seconds (C · Tc). With this configuration, data canbe captured and collected three times faster (from 36 to 12seconds) than in Section 6.4 but with the double of topologymanagement period (from 36 to 72 seconds), as also showsFigure 8. These results outperform any others presented sofar, taking into account that the data latency is just 80 msfor the furthest node.

6.6 Protocol Results Comparison

As stated in Section 2, the only four MAC protocols thatare aimed to our two main goals are D-MAC, MERLIN,DMTCS and MS-MAC. Besides, S-MAC has been added tothe comparison as the reference MAC protocol in WSN. Ne-vertheless, S-MAC and MS-MAC results have been adaptedto a tree-topology, as they were designed for linear topolo-gies.

End-to-end latency is one of our major objectives. Inevery presented protocol, latency depends on the number ofdata sources in the network. However, in our protocol, thereis no additional delay due to collisions and retransmissionsas every node has its own time slot to transmit. Moreover,there is no possibility of appearance of the hidden terminalproblem.

In Figure 9, end-to-end latency for one message from thefurthest node to the sink in a 100 nodes network is presen-ted. For any quantity of sources, our protocol suffers froma lower latency, but as the number of nodes in the networkincreases, the difference becomes more obvious. This is

Table 2. Working cycles in percentageWorking Cycle (WC) (%)

S-MAC 10MS-MAC 10D-MAC 10MERLIN 6DMTCS 2

due to the collisions and retransmissions that S-MAC, MS-MAC, D-MAC and MERLIN have to face and lead to ahigher end-to-end latency. Moreover, the last graph repre-sents the latency of the whole network with the presentedprotocol, explained in Section 6.2. Although DMTCS offerssimilar results, the end-to-end latency can only be compa-red with our protocol’s global latency results, not with ourprotocol’s end-to-end latency. Except for the DMTCS, a pa-cket of any of the rest of the presented MAC protocols willtake twice as long as our protocol to reach the sink.

On the other hand, topology management is the othergoal of the presented protocol. Our protocol is comparedwith S-MAC and MS-MAC, assuming a common data re-solution of 5 seconds. For S-MAC and MS-MAC, as statedbefore, the values have been estimated for a tree-topology.The obtained results are depicted in Figure 10. As it can beseen, D-MAC, MERLIN and DMTCS are not included inthis comparison as they do not offer topology managementcapabilities.

C = 1 is the only combination that offers a shorter life-time than S-MAC, but its topology management update (2minutes) is not comparable with our protocol ( 5 s). Takinga data resolution of 5 s with C = 6, topology is updatedevery 30 s, the same period as MS-MAC. However, if Cis reduced, data resolution is maintained the same but to-pology can updated more often. Even with C = 1 ( 5 s ofdata resolution and 5 s of topology update repetition), thelifetime is longer than MS-MAC. So, for the same topologyupdate period, the presented protocol offers the best resultsfor lifetime.

Finally, Working Cycle comparison is shown in the Table2. S-MAC. MS-MAC and D-MAC offer a very high valuefor a Wireless Sensor Network MAC protocol. MERLINis a little bit lower but still higher than desired and onlyDMTCS has a good value. However, our protocol offers thelowest value: 1.6 %.

7 Conclusions

This paper presents a new medium access control pro-tocol specifically designed for wireless sensor networks.Apart from energy efficiency, low end-to-end latency and

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efficient topology management are the main goals ofthe protocol design. Together with control messages(RTS/CTS) avoidance, the presented protocol outperformsother sensor networks MAC protocols in latency reduction,topology management and energy consumption, thanks to ameticulous time slot structure.

Moreover, our protocol evades hidden terminal problemand improves channel utilisation, becoming a resilient pro-tocol to packet collision and network dynamics, apart fromreducing dramatically power consumption and global la-tency.

The protocol has been implemented on the Tmote Skyhardware and experimental results have verified the designgoals.

References

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[3] CC2420 RF Transceiver Datasheet.http://www.chipcon.com, 2007.

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