Mobile Netw Appl (2011) 16:613–628DOI 10.1007/s11036-010-0280-4

Taking Advantage of Overhearing in Low Power ListeningWSNs: A Performance Analysis of the LWT-MACProtocol

Cristina Cano · Boris Bellalta · Anna Sfairopoulou ·Miquel Oliver · Jaume Barceló

Published online: 19 November 2010© Springer Science+Business Media, LLC 2010

Abstract LWT-MAC is a new Low Power ListeningMAC protocol for WSNs designed to rapidly react toinstantaneous increases of the network load. It takesadvantage of overhearing by waking up all nodes at theend of a transmission to send or receive packets withoutneeding to transmit the long preamble before. In thiswork, detailed analytical models of the LWT-MACand B-MAC protocols, for both saturated and unsat-urated conditions, are presented. Moreover, the keyLWT-MAC parameters are optimized in order to min-imize the energy consumption, constrained to obtainthe same throughput as the IEEE 802.11 (CSMA/CA)MAC protocol. From the behavior of the optimalLWT-MAC parameters, a heuristic configuration isproposed. Finally, the LWT-MAC is compared to B-MAC, in both single and multi-hop scenarios, showingimprovements in energy consumption, throughput anddelay.

Keywords WSNs · low power listening · B-MAC ·LWT-MAC · analytical model

1 Introduction

Wireless Sensor Networks (WSNs) consist of smalldevices that sense environmental data and send it to

C. Cano (B) · B. Bellalta · A. Sfairopoulou ·M. Oliver · J. BarcelóNeTS Research Group,Department of Information and CommunicationTechnologies, Universitat Pompeu Fabra, C/ Tanger,122-140, 08018, Barcelona, Spaine-mail: cristina.cano@upf.edu

a central collector. Normally, sensor nodes have re-duced processing, memory and battery resources andare deployed in remote and large areas. The batteryreplacement in these networks is, therefore, too costlyor even impossible, making energy consumption themost important constraint. For this reason, the MediumAccess Control (MAC) protocol is crucial as it directlyinfluences the transceiver operation that is the mostconsuming component of a sensor node. The commonapproach to reduce energy consumption in WSNs isto periodically put the transceiver into sleep mode,working in a low duty cycle operation instead of con-tinuously listening the channel as in traditional wiredand wireless networks without power constraints, likein IEEE 802.11 Carrier Sense Multiple Access withCollision Avoidance (CSMA/CA) [1].

A well-known MAC protocol for WSNs is BerkeleyMAC (B-MAC) [2] in which each node periodically andindependently of the others samples the radio channelto detect activity, what is known as Low Power Lis-tening (LPL) operation. Then, when a node wants tosend a message, it first sends a preamble long enoughto overlap with the listening time (active part of theduty cycle) of the receiver. Using B-MAC the energyconsumption of the sensor nodes is extremely reducedat very low loads. However, as the load increases,for instance, due to events occurrence, the collisionsof preambles become a significant energy waste, evenmore important in large scale WSNs with hidden termi-nal problems.

The Low power listening with Wake up after Trans-missions MAC (LWT-MAC) protocol (presented bythe authors in [3]) was designed to maintain a low en-ergy consumption at low loads while, at the same time,being able to react to instantaneous increases of the

614 Mobile Netw Appl (2011) 16:613–628

network load. A local synchronization after transmis-sions, that ensures that all nodes that have overheardthe last transmission will be awake to receive a newmessage, is adopted, hence without requiring the longpreamble transmission.

Analytical models of WSNs MAC protocols allow toderive performance optimizations of the different para-meters involved: duty cycle, Contention Window (CW)or packet size among others, depending on differentnetwork scenarios. However, existing analytical modelsof WSNs MAC protocols (for instance, the ones usedin [2, 4]) are extremely simple as they only considerthe time to transmit a packet without analyzing thetime and energy wasted in collisions. Although thesesimple analytical models are valid for low traffic loadsthey become inaccurate when the traffic load increases.More detailed analytical models of scheduled MACprotocols such as the Sensor-MAC (S-MAC) [5] andnanoMAC [6] have been presented, however, the LPLoperation has not been studied so exhaustively.

In this work, the LWT-MAC protocol is studied froman analytical point of view. The presented LWT-MACanalytical model considers the energy waste due to col-lisions and overhearing in a single-hop network undersaturated and unsaturated conditions. The analyticalmodel presented is adapted to model the B-MAC pro-tocol and it can also be extended to model other LPLMAC protocols (like the X-MAC protocol [7]). Us-ing the analytical model, the LWT-MAC key networkparameters are identified and optimized to minimizethe energy consumption but maintaining a good per-formance in terms of delay and throughput. From theoptimization process, a heuristic configuration is de-rived. The LWT-MAC with the heuristic configurationis compared with B-MAC in a single-hop network andin a multi-hop scenario with periodic and event-basedtraffic profiles.

The rest of the paper is organized as follows:Section 2 provides a comparison of the proposedapproach with similar existing mechanisms, then, inSection 3 the LWT-MAC is described. The LWT-MACanalytical model and the adaptation to model the B-MAC protocol, as well as their validation are presentedin Section 4. The optimization analysis of the LWT-MAC protocol is performed in Section 5 while theperformance results are discussed in Section 6. Finally,some concluding remarks are given in Section 7.

2 Related work

LPL MAC protocols as B-MAC [2] perform well whenthe traffic load of the network is low, however as

the traffic load increases the continuous collisions ofpreambles considerably decreases the network perfor-mance. The LWT-MAC protocol aims at improvingthe performance by waking up neighboring nodes atthe end of a transmission in order to send or receivepackets [3].

Some similar mechanisms have already been definedin order to address the same problem. For instance, themulti-hop streaming capability defined in the Sched-uled Channel Polling MAC (SCP-MAC) protocol [8]that increases the duty cycle upon a message recep-tion in order to reduce the end-to-end delay. Anotherexample is described in [9] where the sender of amessage activates a send more bit in the header of thepacket indicating that there is more data pending tobe transmitted. The destination of the message staysawake at the end of the transmission to receive thepacket, thus eliminating the need of the long preambletransmission. In scheduled MAC protocols the sameidea can be followed: in S-MAC [4] and Timeout-MAC(T-MAC) [10] each node that overhears an Request ToSend (RTS) or Clear to Send (CTS) stays awake just incase it should forward the message.

All those mechanisms differ from the approach pre-sented in this work in the sense that in LWT-MACevery node that has overheard a transmission is allowedto transmit at the end, not only the sender or the recip-ient of the last transmission. This approach improvesthe performance when the traffic load of the networkincreases. The load of the network can increase, forinstance, due to an event detection, in this situation aset of nearby nodes try to transmit a packet to informthe sink about the event occurrence.

Other approaches like the X-MAC protocol [7]aim at estimating the traffic load of the network andthen adapt the duty cycle accordingly. Those proto-cols provide better performance but they suffer froman increased complexity. The LWT-MAC protocol, incontrast, adapts to the traffic load in a simple man-ner, only by waking up all neighboring nodes after atransmission.

3 Taking advantage of overhearing: the LWT-MACprotocol

The LWT-MAC extends the operation of B-MAC bytaking advantage of the local synchronization of allnodes that overhear a transmission [3]. The LWT-MACdefines that all overhearing nodes wake up simultane-ously at the end of each successful transmission in orderto send or receive packets. Since all nodes that haveoverheard the last transmission will be awake, the long

Mobile Netw Appl (2011) 16:613–628 615

Sender

Receiver

LONG PREAMBLE

ACK

RTSBO

DATA

ACK CTS

Overhearing

Contender

DATA

t

t

t

t

UNSCHEDULED PHASE SCHEDULED PHASE

NAV Timer

NAV Timer

new packet arrival

Cycle

Listen Sleep

Sleep=U(0,SleepTime)

(a) LWT-MAC Protocol

Sender

Receiver

LONG PREAMBLE

ACK

Overhearing

Contender

DATA

t

t

t

t

new packet arrival

Cycle

Listen Sleep

LONG PREAMBLE DATA

ACK

(b) B-MAC Protocol

Fig. 1 Comparison of LWT-MAC and B-MAC medium accessmechanisms

preamble is no longer necessary (the medium accessmechanism is depicted in Fig. 1a).

In this scheduled phase it is mandatory to compute arandom backoff (BO) before attempting transmission,however in the unscheduled phase it is optional. Thisforces all nodes to listen after each transmission at leastthe value of the CW. During the random BO of thescheduled phase nodes should keep listening to thechannel in case any other node starts a transmission.

The use of the RTS/CTS transaction before thepacket transmission in the scheduled phase is recom-mended. The four-way handshake will help in allevi-ating hidden terminal problems and it is useful to letoverhearing nodes sleep during the entire packet trans-mission. The duration of the transmission should beincluded in RTS, CTS and Data messages and will allowthe overhearing nodes to set a Network AllocationVector (NAV) timer (as defined in [1]) and sleep untilthe transmission finishes.

If no message is sent during the listening after trans-missions period, nodes go to sleep for a random time(with a maximum value equal to the sleep time of theduty cycle) moving towards the unscheduled phase.

Observe that, compared to B-MAC (Fig. 1b), thereis a reduction of the energy waste, both in transmissionand reception, due to the suppression of continuouslong preamble transmissions. Moreover, by suppressinglong preambles the delay and the channel occupationdurations are reduced as well. Additionally, at instan-taneous increases of the network load the protocol

reduces the time required to send a packet improvingthe network performance. However, it should be notedthat the performance benefits of LWT-MAC mainlydepend on the probability that the next transmissionreceiver has overheard the last transmission, that inturn depends on the topology and on how the routesto the sink are selected.

If a packet transmission fails, a retransmission pro-cedure is initiated. The details of this procedure aredifferent in the scheduled and unscheduled modes ofoperation.

3.1 Retransmission procedure in the unscheduledaccess

If a transmission failure occurs during the unscheduledaccess, the acknowledgement (ACK) is not received,the sender retransmits the packet by sending an RTSafter waiting a random BO. As the transmission failurecan be caused by collisions of preambles, the retrans-mission increases the probability to receive the RTScorrectly. After that, in case the CTS is not received(there are two consecutive transmission failures), it isassumed that the intended recipient is either involvedin another transmission or waiting for a transmission tofinish. In order to alleviate consecutive collisions, thesender does not immediately retransmit the message.Instead, it waits a Collision Avoidance (CA) timer (setto the duration of a preamble and a frame transmis-sion). During the CA timer the node keeps listeningto the channel, that provides the opportunity to getsynchronized with the current ongoing transmission (ifany) after overhearing a message involved. If that isthe case, it can sleep until the transmission finishesand retry transmission using the scheduled method.Otherwise, if the CA timer expires, the node retriestransmission using the long preamble again. The CAtimer has been added as an optional mechanism toreduce hidden terminal problems, however, it can bedeactivated when needed, for instance in single-hopnetworks in which all nodes are inside the coveragerange of the others.

The RTS/CTS mechanism can also be activated inthe unscheduled mode [2], i.e., an RTS is sent afterthe transmission of the long preamble. In this case, ifa transmission failure occurs (CTS not received) thesender assumes that the recipient is involved in anothertransmission and waits the CA timer as previouslydescribed. Note that, after a collision among hiddenterminal nodes, the receiver will hopefully receive atleast one of the RTS sent. By activating the RTS/CTSin the unscheduled mode the use of the NAV timer

616 Mobile Netw Appl (2011) 16:613–628

will also allow to let overhearing nodes to sleep aspreviously explained.

3.2 Retransmission procedure in the scheduled access

If a transmission fails during the scheduled access itcan be either because the CTS or the ACK are notreceived. In case the RTS fails (CTS not received), it isassumed that the recipient has not overheard the pasttransmission and, therefore, it is sleeping (notice thatthe long preamble has not been previously sent). In thiscase, the node will retry to send the message by sendingthe long preamble first in order to wake up the receiver.If the packet transmission is not acknowledged thesender waits a CA timer as already explained in theunscheduled access.

The detailed specification of the protocol behavior isdepicted in Fig. 2.

3.3 Performance benefits

For illustration purposes the energy consumption andthroughput of LWT-MAC compared with the resultsobtained using IEEE 802.11 [1] and B-MAC in a 10-node single hop network are shown in Figs. 3 and 4 (theparameters used are depicted in Table 1). Observe thatthe throughput is significantly improved if compared toB-MAC at the cost of a slightly higher energy consump-tion at low loads, where idle listening after transmissionperiods occur. However, the energy consumption isreduced at high loads due to the suppression of the longpreamble transmission.

3.4 Addressing collective Quality of Service (QoS)

The LWT-MAC wake up after transmissions capabilitymakes it a good candidate to be used in event-based

Send PreambleSet Mode = Unscheduled

no yes

activated?Is the RTS/CTS

Send RTS

Set CTSTimer

Send Data

Set ACKTimer

CATimer Expires

Listen(Tlisten)

Set State = PeriodicSleep

Init

Sleep(Tsleep)

Periodic Sleep

Channel Idle

Channel Busy

Periodic SleepSet State = Non

Listen

Stop CATimer(if set)

Packet received

Packet overheard

Receive Preamble Receive RTS Receive CTS Receive Data

Send CTS Stop CTSTimer

Send Data

Set ACKTimer

Send ACK

Receive ACK

Stop ACKTimerPktTx

Stop CATimer(if set)

Set NAV Timer

Sleep(NAVTimer)

Is PktTx > 0yes no

Listen(Tlisten+CW•slot)

Sleep(Random(Tsleep))

Set Mode = UnscheduledSend RTS

Set Mode = Scheduled

Set CTSTimer

Channel Idle Channel Idle

Send Data

Set ACKTimer

Set CATimer

Channel Idle

Send PreambleSet Mode = Unscheduled

CTSTimer Expires

Mode == Scheduled?yesno

Backoff

Backoff

Channel Busy

New packet to tx

PktTx++

State == Periodic

yes

Set State = NonPeriodic Sleep

Sleep?

Backoff

ACKTimer Expires

Mode == Scheduled?

Send RTS

Set CTSTimer

Set CATimer

yesno

Channel Idle

Backoff Channel Busy

Channel Busy Channel Busy

Fig. 2 Flowchart of the LWT-MAC protocol. The Mode value is initialized to Unscheduled and PktTx is initialized to zero

Mobile Netw Appl (2011) 16:613–628 617

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.80

2000

4000

6000

8000

10000

12000

Lambda (packets/s)

Thro

ughput (b

ps)

IEEE 802.11B–MACLWT–MAC

Fig. 3 Throughput of IEEE 802.11, B-MAC and LWT-MAC ina 10-node single hop network

WSNs where instantaneous increases of the traffic loadoccur due to event detection at nearby sensor nodes.The QoS observed by event-based messages is crucialin order to assure the correct and fast event detectionat sink. However, this QoS differs from the traditionaldefinition in which the QoS measurement is madepacket by packet. In event-based WSNs the QoS shouldrefer to the group of messages related to an event, it isknown as collective QoS. Collective QoS is defined asthe QoS (delay, bandwidth, packet loss, etc.) of the setof packets related to a specific event [11]; i.e., the delay

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.82000

4000

6000

8000

10000

12000

14000

16000

Lambda (packets/s)

Energ

y C

onsum

ption (

J)

IEEE 802.11B–MACLWT–MAC

Fig. 4 Energy consumption of IEEE 802.11, B-MAC and LWT-MAC in a 10-node single hop network

Table 1 Default parameters

Parameter Value Parameter Value

r (data rate) 20 kbps Tlisten/Tsleep 24.5/75.5 msσ (empty slot) 1 ms Ldata (packet size) 1,000 bitsDIFS 10 ms Lrts, Lcts, Lack 64 bitsSIFS 5 ms Etx 24.75 mWCW 64 Erx, Eidle 13.5 mWK (queue size) 100 pkts Esleep 0.015 mWR (retry limit) 7 T (time) 1 · 105 s

of the individual messages is not crucial but the latencyfrom the event generation until the event detection atsink is critical. A MAC protocol that efficiently reactsto event-based traffic will increase the collective QoS ofthe messages involved.

The LWT-MAC protocol is designed in order toimprove the collective QoS of event messages whilemaintaining a low energy consumption. As far as theauthors know, there is not any other MAC protocoldesigned keeping in mind collective QoS metrics. Thosewhich are focused on QoS are based on end-to-endtraditional QoS metrics instead [12–17].

The behavior of the protocol with periodic andevent-based traffic profiles and its ability to increase thecollective QoS was studied in [18].

4 LWT-MAC analytical model

In this section an analytical model of the LWT-MACprotocol in single-hop WSNs is described. The analyti-cal model assumes ideal channel conditions (no channelerrors or hidden terminal problems) and that each nodecomputes a random BO (between 0 and CW) beforeeach transmission attempt.1 For simplicity reasons thesensor nodes are considered to be homogeneous (equaltraffic profiles and capabilities). Table 2 provides thedescription of some relevant variables used.

The extension of the analytical model to a multi-hopnetwork is a challenging task since collisions can hap-pen due to hidden terminal problems [19]. Moreover,in WSNs, where hidden terminals can wake up at anymoment during an ongoing transmission, the multi-hopanalysis becomes even more difficult than in traditionalwireless networks where nodes are always listening tothe channel. Therefore, the multi-hop analysis is left forfuture study.

1The effect of the CA timer previously described has not beenstudied, it is considered that collisions move the system to theunscheduled mode.

618 Mobile Netw Appl (2011) 16:613–628

Table 2 Notation

Notation Description

n Number of nodesLdata Packet size (bits)λ Rate of packet generation (packets/s)K Queue size (packets)Lrts,cts,ack RTS,CTS,ACK packet size (bits)Lp Preamble size (bits)r Transmission rate (bits/s)T Time (s)S Throughput (bits/s)M Average number of transmission attempts per packetB Average number of slots in BOpsch Scheduled probabilitypw Probability to wake up after a transmissionρ Queue utilizationτ Transmission probability in a given slotCW Contention window of the random BOσ Empty slot duration (s)ζ Refers to the metrics: S, M, B, psch, pe, ρ

Tsleep Sleep time of the duty cycle (s)Tlisten Listen time of the duty cycle (s)

4.1 Network performance metrics

The LWT-MAC analytical model is based on the onedescribed in [20] where an IEEE 802.11 (CSMA/CA)analytical model to compute traditional metrics such asthroughput, delay and queue occupation is presented.

A sensor node is modeled as a single queue of lengthK packets. Each node generates packets following aPoisson distribution with rate λ packets/s and averagepacket length Ldata bits. From these assumptions thefollowing metrics can be computed:

A = λX, ρ = A(1 − Pb), Pb = (1 − A)AK

1 − AK+1(1)

where A is the offered load, ρ is the queue utilization,Pb denotes the blocking probability and X refers to theservice time (the time since the packet arrives at thehead of the queue until it is released from it, assumingthat it follows an exponential distribution).

The service time can be calculated as:

X = (M − 1) (Bα + Tc) + Bα + Ts (2)

where M is the average number of required transmis-sion attempts per packet, B is defined as the averagenumber of slots selected before each transmission at-tempt, α is the average slot duration and Tc and Ts arethe durations of a collision and a successful transmis-sion respectively.

The average number of attempts per packet success-fully transmitted or discarded (M) due to maximumretry limit (R) reached is computed as:

M = 1 − pR+1

1 − p(3)

where p is the conditional collision probability (as-sumed to be constant for all transmission attempts):

p = 1 − (1 − τ)n−1 (4)

Being n the total number of nodes in the networkand τ the steady state probability that a node transmitsin a random slot given that it has a packet ready to betransmitted:

τ = ρ

B + 1(5)

Assuming that the BO is uniformly distributed in therange [0-CW], B can be obtained as shown in Eq. 6.

B = CW − 1

2(6)

The average slot duration (α), is calculated consider-ing the duration of the slot depending on the channelstate (Eq. 7). As the channel is assumed to be error-free, it can only be in empty, successf ul or collisionstates with their corresponding probabilities pe, ps, pc.

α = peσ + ps(Ts + σ) + pc(Tc + σ) (7)

where σ is the empty slot duration.The channel state probabilities are related to the

stationary probability that the rest of the nodes (exceptthe one that is in BO) try to transmit in a given randomslot. These are given by:

pe = (1 − τ)n−1

ps = (n − 1)τ (1 − τ)n−2

pc = 1 − pe − ps (8)

Finally, the throughput per node can be computedas:

S = ρLdata

X(1 − pd) (9)

where pd is the probability to discard a packet due tomaximum retry limit reached:

pd = pR+1 (10)

The channel occupation durations (Eqs. 11 and 12)can be computed considering the length of the messagesinvolved and the time intervals between them. Notethat in the unscheduled mode the preamble is sent be-fore each packet transmission attempt. The analytical

Mobile Netw Appl (2011) 16:613–628 619

model presented in this work only considers the trans-mission of packets preceded by RTS/CTS messages,although the model is easily adaptable to also considerthe basic access method. Observe also that it has beenconsidered the use of the DIFS, SIFS and EIFS timeperiods as in the IEEE 802.11 (CSMA/CA) [1].

Tc = DIFS + (Lp · punsch) + Lrts

r+ EIFS (11)

Ts = DIFS + (Lp · punsch) + Lrts + Lcts + Ldata + Lack

r

+ 3SIFS (12)

where Lp, Lrts, Lcts and Lack are the lengths of thepreamble, RTS, CTS and ACK messages respectively,while r refers to the transmission rate. The probabilityof transmitting a packet in scheduled mode (psch) isthe probability that after a successful transmission, anyother node has still data to be transmitted. On theother hand, the probability to transmit a packet in theunscheduled mode (punsch) is obtained as the comple-mentary of the former:

psch = pss

1 − pes(1 − (1 − ρ)n), punsch = 1 − psch (13)

where pss and pes are the probabilities of successful andempty slots from the network point of view:

pes = (1 − τ)n, pss = nτ(1 − τ)n−1 (14)

The analytical model is solved using a fixed pointapproximation. With the metrics obtained the energyconsumption of a sensor node during a certain amountof time can be computed.

4.2 Energy consumption

The total energy consumption of a sensor node can bedivided in four parts: (i) the energy spent to transmitand (ii) receive messages, (iii) the energy wasted inoverhearing, and (iv) the energy spent in duty cycle(sleeping and waking up in inactive periods):

e = etx + erx + eov + edc (15)

Let Ns be the total number of messages a nodesuccessfully sends during a time T, it can be derivedusing the throughput S and the packet length Ldata as

Ns = T(

SLdata

).

The energy spent to transmit Ns messages is com-puted taking into account the energy needed to success-fully transmit a packet (es,tx) and the energy spent incollisions for each unsuccessful attempt (ec,tx):

etx = Ns(es,tx + (M − 1)ec,tx

)(16)

The values of es,tx and ec,tx can be obtained consid-ering the energy spent to transmit and receive the mes-sages involved and the empty time intervals (Eqs. 17and 18). Moreover, the empty slots of the BO proce-dure must be considered (note that the busy slots of theBO countdown are part of the receiving or overhearingenergy consumptions).

ec,tx = Eidle

(DIFS + Bσ pe + 2SIFS + Lcts

r

)

+ Etx(Lp · punsch) + Lrts

r(17)

es,tx = Eidle(DIFS + Bσ pe + 3SIFS + Te)

+ Etx(Lp · punsch) + Lrts + Ldata

r

+ ErxLcts + Lack

r(18)

where Etx, Erx and Eidle denote the energy consump-tions of being in transmission, reception and idlemodes.

Notice that, after a successful transmission, all nodesin the network keep listening to the channel in caseanother node has something to transmit. If there are nomore data to send, this time remains empty and eachnode stays in the idle mode during a Tlisten (added toavoid synchronization problems) plus the CW, this is:

Te = (1 − ρ)n(Tlisten + σCW) (19)

For simplicity reasons, it has been considered thateach node receives Ns packet destined to it. Therefore,the total energy consumption to receive those messagesis computed as:

erx = Ns · es,rx (20)

The colliding packets have not been considered here.It is assumed that the unsuccessful transmissions of anode collide with those that are destined to it and thatthe probability to collide with more than one packet canbe neglected.

The energy consumption to receive a packet is:

es,rx = psch · eb + punsch · ep + Eidle(3SIFS + Te)

+ Erx

(Lrts + Ldata

r

)+ Etx

(Lcts + Lack

r

)(21)

where eb is the energy of a busy listening after transmis-sions period, i.e., the idle time interval before sendinga packet in the scheduled mode. Observe that, thosenodes without data to send must also wait the remain-ing BO of the other nodes.

620 Mobile Netw Appl (2011) 16:613–628

This time has been approximated by Bσ as shownin Eq. 22. This approximation does not affect when thetraffic load is low (the scheduled probability is small)or high (the probability that a node has no data totransmit is negligible). However, it affects the resultswith moderate traffic load and its effect depends onthe number of nodes. When n is high the energy con-sumption is overestimated (as the probability that anode has a small remaining BO increases) while withsmall n the energy consumed is underestimated (sincethe probability to transmit a packet that arrives atthe queue during the listen after transmission periodincreases). Then:

eb ≈ Eidle(DIFS + Bσ(1 − ρ)) (22)

The parameter ep refers to the energy spent receiv-ing the long preamble. If a node has something totransmit it will be listening to the channel, thereforeit will receive the entire long preamble of any othertransmission in the medium. Otherwise, the node willbe in duty cycle mode and on average it will wake up inthe middle of the other’s long preamble transmissionsplus its own Tlisten:

ep = ρ

(EidleDIFS + Erx

Lp

r

)

+ (1 − ρ)

(EidleTlisten + Erx

Lp

r

2

)(23)

Similarly, the energy consumption due to overhear-ing is computed as:

eov = Ns(n − 2)

(es,ov + M − 1

2ec,ov

)(24)

where es,ov and ec,ov are the energy consumptionto overhear a successful transmission or a collisionrespectively:

ec,ov = psch · eb + punsch · ep + ErxLrts

r

+ Eidle

(2SIFS + Lcts

r

)(25)

es,ov = psch · eb + punsch · ep + EidleTe + ErxLrts

r

+ Esleep

(Lcts + Ldata + Lack

r+ 3SIFS

)(26)

where Esleep denotes the energy of being in sleep mode.Finally, the rest of time (Tinactive), that can be ob-

tained using the equations above computing the timeinstead of the energy, and the total time T, each node

performs a low duty cycle operation, listening andsleeping according to the duty cycle:

edc = Tinactive

(Eidle

Tlisten

Tci+ Esleep

Tsleep

Tci

)(27)

where Tci is the check interval:

Tci = Tlisten + Tsleep (28)

4.3 B-MAC analytical model

The previous analytical model can be adapted to modelthe behaviour of the B-MAC protocol. To achieve thataim two modifications are needed: (i) the probabilityof being in scheduled mode (psch) has to be fixedto 0, meaning that the protocol always works in theunscheduled mode and (ii) the empty listen time after asuccessful transmission has not to be considered (Te =0) when computing the energy consumption of a sensornode since the listen after transmissions capability doesnot apply in B-MAC.

4.4 Analytical model validation

The SENSE simulator [21] has been used to validatethe results obtained with the analytical model. Thescenario consists of n nodes randomly placed in anarea smaller than the maximum coverage range, thus,assuming ideal channel conditions, a full connectivityamong them is assured. The default parameters usedfor the evaluation are shown in Table 1 (see Section 3).

Figure 5 shows the analytical and simulation resultsof B-MAC and LWT-MAC with different number ofsensor nodes. Observe how the scheduled probabilityof LWT-MAC (Fig. 5a) increases with the traffic loaduntil a certain point at which the number of collisions(that move the system to the unscheduled mode) isnoticeable. With a higher number of nodes, the sched-uled probability increases faster (since the traffic loadis higher), however it also becomes constant sooner asthe collision probability is also higher. The total energyconsumption of LWT-MAC (Fig. 5b) is strongly re-lated to the scheduled probability. Note how the modelunderestimates the energy consumption approachingsaturation for two and five nodes and overestimates itwith ten nodes. The reason for this discrepancy is theaforementioned approximation on the value of eb. Incontrast, for B-MAC, since it always works in unsched-uled mode, its accuracy is not affected by the eb approx-imation. It is also interesting to observe the throughput(Fig. 5c) that slightly increases, in both protocols, withthe number of nodes, however the saturation through-

Mobile Netw Appl (2011) 16:613–628 621

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LWT–MAC Model n=2LWT–MAC Sim n=2LWT–MAC Model n=5LWT–MAC Sim n=5LWT–MAC Model n=10LWT–MAC Sim n=10

(a) Scheduled Probability

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(c) Throughput

Fig. 5 Scheduled probability, total energy consumption and throughput with different number of sensor nodes

put decreases with ten nodes as the collision probabilityincreases.

Figure 6 shows the results with ten nodes anddifferent check intervals (the Tlisten value is maintainedwhile Tsleep changes to fit the given check interval). Thescheduled probability of LWT-MAC (Fig. 6a) remainsequal for all the check intervals at high loads, howeverat lower loads it increases faster with higher valuesof the check interval. This difference occurs becausewith longer check intervals the probability that a nodehas received something to transmit during an ongo-ing transmission increases. In the energy consumption(Fig. 6b) it can be seen, for both protocols, that at lowloads the use of short time intervals notably penalizesthe energy consumption as nodes wake up unnecessar-ily more often. However, as the load increases, to havelonger check intervals in LWT-MAC implies higherenergy consumption caused by the collisions that movethe network to the unscheduled mode (moreover, aftera collision nodes wait awake listening the entire pream-ble transmission). However, the obtained energy con-sumption values at high loads are substantially lowerthan the ones obtained using B-MAC. In the through-

put (Fig. 6c), it can be observed that lower saturationvalues are obtained when longer preambles are used,in both cases: using B-MAC and LWT-MAC. In thiscase, once again, it can be seen how the LWT-MACimproves the B-MAC performance increasing notablythe saturation throughput.

5 Optimization analysis

In this section, a performance optimization is done inorder to derive the best parameter configuration of theLWT-MAC protocol for a single-hop scenario. A firstgoal is to identify how the different parameters thatdefine the LWT-MAC operation affect its performanceand how they can be tuned to minimize the energy con-sumption without significantly reducing the throughputand delay.

5.1 Key parameters optimization

One of the parameters of crucial importance is thesleep time of the duty cycle (Tsleep) as it directly affects

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(c) Throughput

Fig. 6 Scheduled probability, total energy consumption and throughput with different values of the check interval

622 Mobile Netw Appl (2011) 16:613–628

the performance of the network. High values of theTsleep allow to decrease the energy consumption but atthe cost of reducing the throughput and increasing thedelay. Additionally, it is possible to define a probabilityof waking up after successful transmissions (pw), basedon which sensor nodes decide to wake up at the endof a successful transmission or go to sleep remainingin the unscheduled access. Observe that, an additionalmechanism should be implemented to inform nodes towake up at the end of each transmission based on thisprobability, otherwise receivers can be sleeping whennodes send packets without using the long preamble.The sender can, for instance, notify sensor nodes towake up at the end of the transmission in the dataor RTS messages. At low traffic loads, setting thatprobability to small values allows to reduce the energyconsumption as fewer idle listening after transmissionperiods will occur, however, as load increases, highervalues of pw reduce the energy waste since the numberof long preamble transmissions are reduced. Note thatby defining this probability Eqs. 13, 18, 21 and 26should be rewritten as shown in Eqs. 29, 30, 31 and 32,respectively:

psch = pwpss

1 − pes(1 − (1 − ρ)n), punsch = 1 − psch

(29)

es,tx = Eidle(DIFS + Bσ pe + 3SIFS + pwTe)

+ Etx(Lp · punsch) + Lrts + Ldata

r

+ ErxLcts + Lack

r(30)

es,rx = psch · eb + punsch · ep + Eidle(3SIFS + pwTe)

+ Erx

(Lrts + Ldata

r

)+ Etx

(Lcts + Lack

r

)(31)

es,ov = psch · eb + punsch · ep + Eidle pwTe + ErxLrts

r

+ Esleep

(Lcts + Ldata + Lack

r+ 3SIFS

)(32)

Other parameters such as the CW, used to computethe random BO, also affect the performance, but com-pared to the Tsleep and pw their influence is limited.Parameters like the packet length or the traffic load areconsidered fixed.

5.2 Optimization function

The main goal of the optimization process is to min-imize the energy consumption (e) but constrained

to achieve the same throughput as the IEEE 802.11(CSMA/CA) MAC protocol. The IEEE 802.11 hasbeen chosen as a reference as it does not implementthe duty cycle operation, resulting in an upper boundin terms of performance (given that the other com-mon parameters, such as the CW and the RTS/CTSoption are equally configured in both approaches). Theoptimization analysis considers the throughput as theonly constraint since the average packet transmissiondelay will necessarily increase to allocate space for thelong preamble transmission, which is the price that theLWT-MAC pays to obtain a lower energy consumptionthan the IEEE 802.11 without reducing its throughput.The optimization function is shown in Eq. 33.

[T∗sleep, p∗

w]= arg min

Tsleep∈[0,0.5],pw∈[0,1],S≥S802.11

e(n, Ldata, ζ, T, Tsleep, pw

)

(33)

Observe that, the metrics referred by ζ (see Table 2)are a function of n, λ and Ldata and can be obtainedusing the previously described analytical model.

5.3 Optimization results

By applying Eq. 33 the optimal sleep time (T∗sleep)

and wake up probability (p∗w) can be obtained for

a given scenario. The optimal values are those thatminimize the energy consumption and achieve the samethroughput as the IEEE 802.11. To see how the optimalparameters change for different traffic loads (A) andnumber of nodes (n), a single-hop network with theparameters shown in Table 1 (see Section 3) has beenevaluated.

Results are shown in Fig. 7. Note that, once thethroughput of IEEE 802.11 cannot be achieved themetrics are not longer depicted. The minimum energyconsumption (Fig. 7b) for the achieved throughput isdepicted in Fig. 7a. Observe, in Fig. 7c, how the optimalprobability to wake up after a successful transmission(p∗

w) increases rapidly from 0 to 1 when the traffic loadstarts to be noticeable. In contrast, the T∗

sleep (Fig. 7f)takes high values at low loads, meaning that, for theload requirements, sensor nodes can be sleeping duringa longer time in each duty cycle. As the load increases,the value of T∗

sleep decreases, however it shows aninflection point and begins to increase. This effect iscaused by the listening after transmissions probabilitythat suddenly increases to 1 reducing the offered loadof the network as it reduces the time needed to senda message. After this behavior, the T∗

sleep continuesdecreasing in order to maintain the throughput but also

Mobile Netw Appl (2011) 16:613–628 623

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eep (

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Fig. 7 Performance metrics with optimum T∗sleep and p∗

w in a single-hop network

to reduce the duration of collisions. Moreover, as ithas been considered that after a collision nodes alsokeep listening to the channel, thus receiving the entirenext long preamble transmission, the consumption withhigh traffic loads increases with the value of the sleeptime (that makes the long preamble to increase). Thevalues T∗

sleep and p∗w directly influence the delay and the

scheduled probability. The delay, depicted in Fig. 7d,shows a small increase at very low loads caused by thelong sleep time and remains more or less constant untilthe queues become saturated. Regarding the scheduledprobability (Fig. 7e), it is directly affected by p∗

w thatbounds its value at low loads.

From these results, it can be concluded that, if theload can be estimated, the best configuration is: a) atlow loads set the Tsleep to long values and pw equal 0and b) at high loads decrease the Tsleep value and setpw equal 1.

5.4 Heuristic configuration of the LWT-MACparameters

The estimation of the traffic load is a difficult task, evenmore in event-based WSNs where the traffic profilesdiffer from the traditional ones, showing sporadic and

instantaneous increases of the traffic load due, for in-stance, to events occurrence. Moreover, sensor nodesare devices with limited capabilities in terms of process-ing and memory resources making the load estimationan arduous task. However, if the load can be estimatedin a fast and reliable way, the best option will be touse the LWT-MAC with the optimal values for Tsleep

and pw. Otherwise, from the observations made in theoptimization analysis performed in the previous sec-tion, a heuristic parameter configuration to provide lowenergy consumption, high throughput and small delaythrough the entire load range can be made.

The major disadvantage of LWT-MAC is that itconsumes more energy than B-MAC at low loads dueto idle listening after transmission periods, however amoderately long value of Tsleep can help to decreasethe energy consumption at low loads. To benefit fromthe advantages of LWT-MAC, if the Tsleep is fixed toa long value, the pw probability should always be setto one. With this configuration, the LWT-MAC willconsume less energy at low loads and it will maintainits capability to react to instantaneous increases of thenetwork load.

Figure 8 shows for different number of sensor nodesthe value of the energy consumption and throughput

624 Mobile Netw Appl (2011) 16:613–628

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(f) Throughput n = 2

Fig. 8 Performance metrics for different Tsleep and pw = 1 in a single-hop network

with different values of Tsleep and varying the trafficload. It can be seen that for Tsleep = 50 ms the en-ergy consumption at low loads is significantly higher.However, it decreases substantially with Tsleep = 100ms and, still a bit more, with Tsleep = 200 ms. How-ever, with Tsleep = 300 ms the reduction of energy con-sumption at low loads is extremely small, compared tothe obtained with 200 ms, and at the cost of a lowerthroughput. Therefore, a Tsleep around 200 ms providesa considerably reduction of the energy consumptionat low loads maintaining an acceptable value for thethroughput. However, once the network saturates theenergy consumption increases with higher values ofTsleep since it is considered that all nodes keep listeningto the channel after a collision, thus receiving the entirelong preamble of the retransmissions. Nevertheless, inthe normal operation, the network is expected to workfrom low to moderate load conditions, not in saturation.

Thus, the suggested heuristic configuration is:Tsleep = 200 ms and pw = 1.

6 Performance evaluation

In this section, the heuristic configuration of Tsleep =200 ms and pw = 1 is evaluated and compared to B-

MAC with Tsleep = 75 ms in a single-hop and a multi-hop network. The results of LWT-MAC with Tsleep =75 ms and pw = 1 have also been included to keep them

Sink

Sensor Node

Periodic Messages

Fig. 9 Single-hop scenario

Mobile Netw Appl (2011) 16:613–628 625

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Fig. 10 Performance metrics with recommended heuristic Tsleep and pw in a single-hop network with five and ten sensor nodes

as a reference. For a fair comparison the RTS/CTSprocedure is used in both B-MAC and LWT-MAC (forthe scheduled and unscheduled accesses), therefore anRTS is sent immediately after the long preamble trans-mission. After overhearing an RTS or CTS message,nodes go to sleep for the ongoing transmission durationin both B-MAC and LWT-MAC. In the multi-hop caseevent-based traffic profiles will be also considered.

The SENSE simulator [21], as explained before, hasbeen used to obtain the results. In this case the simula-tor has also been extended with the B-MAC protocoland the event-based traffic profile. The channel hasbeen considered error-free.

6.1 Single-hop network

The single-hop scenario consists of n nodes randomlyplaced in an area smaller than the maximum coveragerange. All sensor nodes generate messages following aPoisson distribution and send them to the sink (Fig. 9).The default parameters used for the evaluation areshown in Table 1 (see Section 3). In this scenario theCA timer is deactivated.

Observe that this scenario provides the best condi-tions for the LWT-MAC protocol since without hid-

den terminals and channel errors all sensor nodes willsynchronize to ongoing transmissions and therefore,will be able to send their packets after them withoutmaking use of the long preamble. Moreover, since allthe nodes are inside the coverage range of the others,receivers are always awake in the scheduled phase, i.e.,they overhear all transmissions taking place. The only

Sensor Node

Sink

Periodic MessagesEvent Data

Event Radius

Fig. 11 Multi-hop scenario with periodic and event-based trafficprofiles

626 Mobile Netw Appl (2011) 16:613–628

Table 3 Default parameters

Parameter Value Parameter Value

r (data rate) 20 kbps Tlisten 24.5 msσ (empty slot) 1 ms Ldata (packet size) 240 bitsDIFS 10 ms Lrts, Lcts, Lack 64 bitsSIFS 5 ms Etx 24.75 mWCW 64 Erx, Eidle 13.5 mWK (queue size) 10 pkts Esleep 0.015 mWR (retry limit) 5 T (time) 5 · 105 s

limitation of this scenario is the occurrence of collisions.It has been assumed that collisions move the system tothe unscheduled phase in which nodes should use thelong preamble before a data transmission. However,after the initial transmission in the unscheduled phasethe system will immediately move to the scheduledphase again.

Results (Fig. 10) show that the energy consump-tion of LWT-MAC with the heuristic parameterconfiguration is similar to the one obtained by B-MACat low loads as can be observed in Fig. 10a and d.However, at high loads the suppression of the longpreamble reduces the energy consumption of the LWT-MAC protocol. The suppression of the long preamble

transmission is the cause of the higher throughput ofthe LWT-MAC compared to the B-MAC as depictedin Fig. 10b and e. However, the increase of the Tsleep tomaintain the energy consumption makes the delay toslightly increase at low loads as can be seen in Fig. 10cand f.

Compared to the LWT-MAC with Tsleep = 75 ms,the heuristic configuration provides slightly worse re-sults in terms of throughput and delay but considerablyreduces the energy consumption at low loads.

6.2 Multi-hop network

The multi-hop scenario consists of a multi-hop event-based WSN with 100 nodes randomly placed in a 100 ×100 m2 area. The radio range of each node is 43 mand the Floyd algorithm has been used to compute theshortest path between any pair of nodes. Each sensornode generates two kinds of traffic profiles: (i) mes-sages generated following a Poisson distribution and(ii) event-based messages (see Fig. 11). A random eventgenerator selects randomly the event position and no-tifies the sensor nodes that are inside the coverage ra-dius of the event in order to send event-based messages

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Fig. 12 Performance metrics with recommended heuristic Tsleep and pw in a multi-hop network with periodic and event-based trafficprofiles

Mobile Netw Appl (2011) 16:613–628 627

Table 4 Collective reliability for periodic interarrival time equals10 and 25 s

MAC protocol 10 s 25 s

B-MAC Tsleep = 75 ms 0.509 0.995LWT-MAC Tsleep = 75 ms 0.992 0.999LWT-MAC Tsleep = 200 ms 0.891 0.999

to the sink. Events have a constant coverage radius of30 m, the time between events follows an exponentialdistribution with mean 600 s and the number of eventmessages needed at sink to reliably detect an event hasbeen set to 5. In this scenario the CA timer is activated.The parameters used are shown in Table 3.

The results obtained are shown in Fig. 12. It isobserved that at low loads the energy consumptionof the LWT-MAC with the heuristic configuration isconsiderably lower than using the B-MAC (Fig. 12a).This effect appears due to the longer Tsleep but also dueto the CA timer that avoids continuous collisions ofpreambles and better manages hidden terminal prob-lems. The LWT-MAC provides also better results inthroughput, depicted in Fig. 12b, with a higher value ofthe saturation throughput and delay (Fig. 12c).

The collective QoS metrics are shown in Fig. 12d,e and Table 4. The LWT-MAC with heuristicconfiguration achieves lower collective delay (Fig. 12d),defined as the time span between the event occurrenceand the event detection at sink [22]. It also provides bet-ter collective bandwidth for the event-based messagesas shown in Fig. 12e and better collective reliability (seeTable 4) for a message interarrival period equals 10 and25 s. For other loads the reliability is almost 100% in allthe cases. Collective bandwidth refers to the bandwidthrequired to detect an event while collective reliabilityis the fraction of correctly detected events among allevents generated [18].

Observe that, the LWT-MAC with Tsleep = 75 msprovides, as seen in the single-hop scenario, betterresults in throughput and delay but also in the collectivemetrics. However, the heuristic configuration allows tonoticeably decrease the energy waste at low loads byobtaining better collective and individual metrics thanB-MAC.

7 Concluding remarks

In this work an analysis of the LWT-MAC protocolhas been performed. A LWT-MAC analytical modelthat computes the network performance metrics andthe energy consumption taking into account collisionsin both saturated and unsaturated conditions has been

presented. Moreover, the optimal configuration for theprobability to wake up after a successful transmission(pw) and the sleep time of the duty cycle (Tsleep) havebeen obtained depending on the number of nodes andthe load of the network in a single-hop scenario. Fromthe optimization results, a heuristic configuration forthe Tsleep and the pw is suggested. The use of theproposed heuristic parameter configuration avoids thecomplexity of other mechanisms that, for example,adapt the parameters based on the traffic load estima-tion, which can be unfeasible in WSNs, and providesa near-optimal performance in a wide range of situa-tions. Additionally, the LWT-MAC with the heuristicparameter configuration has been compared with B-MAC in a single-hop network as well as in a multi-hopscenario with event-based traffic. Results show that theenergy consumption of the sensor nodes is maintainedsimilar or lower to the consumed by B-MAC and thatthe other performance metrics, specially those regard-ing to collective QoS, are substantially improved. Al-though the heuristic parameter configuration has beenobtained for a single-hop scenario, it has been shownthat it is also valid in a multi-hop network.

Acknowledgements This work has been partially supported bythe Spanish Government under the projects TEC2008-0655/TEC(GEPETO, Plan Nacional I+D) and CSD2008-00010 (COMON-SENS, Consolider-Ingenio Program) and by the Catalan Govern-ment (SGR2009#00617)

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