978-1-4244-4439-7/09/$25.00 c©2009 IEEE

Intelligent Network Synchronization for Energy Saving

in Low Duty Cycle MAC Protocols

Pranesh Sthapit∗ and Jae-Young Pyun∗∗∗

Department of Information and Communication Engineering

Chosun University, Gwangju, Korea

pranesh@stmail.chosun.ac.kr∗, jypyun@chosun.ac.kr∗∗

Abstract

Several MAC protocols such as S-MAC, T-MAC, DS-

MAC, and TEEM have exploited scheduled sleep/listen cy-

cles to conserve energy in sensor networks. These protocols

use periodic SYNC packet in their SYNC period to follow

the same schedule with their neighbors. We have found that

these protocols use around 40% of their listen period for

SYNC period. In an average, unused SYNC periods con-

sume more than 20% of the total energy consumption. In

this paper, we analyze the periodic nature of SYNC packet

and develop a new algorithm, named as intelligent network

synchronization (INS), which exploits the periodic nature

of SYNC packet to reduce energy consumption. The pro-

posed INS makes nodes bypass their own SYNC period by

monitoring sleep/listen cycles of their each neighbor. We

evaluate INS through both mathematical analysis and sim-

ulation. These results show the achievement of up to 25%

of energy saving.

1. Introduction

A wireless sensor network (WSN) is a self-organizing

wireless network consisting of spatially distributed au-

tonomous devices using sensors to cooperatively monitor

physical or environmental conditions at different locations

with less or no mobility. Typically, these nodes coordinate

to perform a common task. These small and inexpensive

devices are self-contained units consisting of a battery, ra-

dio front end, sensors, and a minimal amount of on-board

computing power. Once deployed, changing batteries be-

come difficult or even impossible, thus sensor nodes must

be energy efficient [1][2][3][4].

The radio occupies the largest share of the energy con-

sumption in most of the sensor nodes [6]. The MAC pro-

tocols of WSN save energy by putting the nodes into sleep

∗∗corresponding author

mode, i.e., turning the radio off as long as possible. The

most well-known Sensor-MAC (S-MAC) protocol is one of

them [1]. It reduces energy consumption by using a co-

ordinated sleeping mechanism, similar to the power saving

mechanism of IEEE 802.11 [7]. There are some other MAC

protocols having same working principle of S-MAC and

have certain advancements in each, i.e. TMAC, DSMAC

and TEEM [2][3][4]. For simplicity, we called these proto-

cols collectively as S-MAC family. The S-MAC family uses

contention-based random access method with a periodic

sleep/listen cycle. The sensor nodes can’t communicate

during the sleep mode. Therefore, these protocols locally

manage synchronization and the synchronized schedule can

be controlled by periodic SYNC packet broadcasted to their

neighbors. These protocols have separate parts for data and

SYNC packets in their listen period. RTS and CTS pack-

ets are exchanged before sending data packets. The major

sources of energy wasted in WSN are collision, overhear-

ing, control packet overhead, and idle listening [1]. Some

different methods and techniques have been employed by

each protocol to mitigate these energy wasting attributes.

But, most of the works have been done on the data period

and the SYNC period remains active in all. That is, the po-

tential reduction at SYNC period has been still unexplored.

Hence, we propose intelligent network synchronization

(INS), which lets the node bypass their SYNC period by

exploiting the periodic nature of SYNC packet. INS allows

the nodes to be in sleep state in their SYNC period when

they recognize that nobody has SYNC packet queued in the

current SYNC period. For performance evaluation, we have

implemented our idea in S-MAC and TEEM and compared

it with the original protocols.

2. Related work

This section gives the overview of S-MAC and TEEM,

since we have implemented INS in these protocols. We have

found that among other S-MAC family protocols, these two

Node 0

Node 1

Other nodes

Got CTS DATA SYNC RTS

Got SYNC Got RTS Got CTS

Got DATA ACK

Got ACK

Sleep

Sleep period Listen period

SYNC CTS RTS

Got SYNC Got RTS CTS

Cycle

Time

Figure 1. Sleep/listen cycle of S-MAC.

protocols have considerably different techniques for net-

work synchronization. Therefore, we choose these two pro-

tocols on behalf of all S-MAC family protocols.

2.1. S-MAC overview

S-MAC is a contention-based random access protocol

with a fixed sleep/listen cycle [1]. It uses a coordinated

sleeping mechanism, similar to the power saving mecha-

nism of IEEE 802.11 [7]. A time frame in S-MAC is divided

into two parts: a listen period and a sleep period as shown

in Figure 1. Sensor nodes are able to communicate with

other nodes only in this listen period. Therefore, all neigh-

boring nodes must be synchronized together. Each S-MAC

node periodically exchanges its schedule by broadcasting

a SYNC packet to its neighbors. The period of sending a

SYNC packet is called synchronization period [5]. S-MAC

nodes maintain a table, called neighbor list table (shown

in Table 1), to record the scheduling information of all its

known neighbors [5]. Sensor nodes also periodically per-

form periodic neighbor discovery in which nodes listen for

a whole synchronization period. Nodes never go to sleep

during the entire neighbor discovery period; so that the node

can listen for longer time than usual and have more chance

to hear new or missing neighbors. Every new node per-

forms periodic neighbor discovery before joining the net-

work. In S-MAC, RTS and CTS control packets are used

for data communication similar to IEEE 802.11. The suc-

Table 1. Field definition of neighbor list tableof S-MAC

Field Comment

nodeId ID of this node

schedIdSchedule ID in schedule table that this node

follows

active Flag indicating this node is active recently

state Flag indicating the node has changed schedule

Node 0

Node 1

Other nodes

SYNC rts

CTS

DATA Got CTS

Got CTS

Got DATA ACK

Got ACK

Sleep

Sleep period Listen period

SYNC data

Sleep

Sleep

SYNC nodata

Got SYNC rts

Got SYNC rts

Cycle

Time

Figure 2. Sleep/listen cycle of TEEM.

cessful exchange of RTS/CTS packets between two nodes

implies that they should stay awake in the whole sleep pe-

riod for the completion of their data communication. Again,

all other nodes that are not involved in data communication

can enter a sleep mode. Figure 1 shows the data communi-

cation between node 0 and node 1 in S-MAC. S-MAC has

a fixed timing period for listen and sleep period. The prob-

lem is that, even when nodes have no data or SYNC packet

to send during some time frame, every node still has to be

awake in listen time wasting their energies.

2.2. TEEM overview

Unlike S-MAC, in TEEM [4], the listen period consists

of only two parts, SYNCdata and SYNCnodata, and the time

interval of the listen period is also shorter compared to S-

MAC as shown in Figure 2. The SYNCdata contains data

packets, whereas the SYNCnodata contains SYNC packets.

Both packets are used for synchronization. Instead of us-

ing a separate RTS and SYNC, TEEM combines the RTS

packet with a SYNC packet and sends it in SYNCdata pe-

riod. This combined packet is called SYNCrts. Nodes

having data will contend for medium in SYNCdata pe-

riod. If there is no communication in this period, then

only nodes having SYNC packet contend for medium in

the SYNCnodata period and the winner sends the SYNC

packet. Since the data traffic is transferred in the first pe-

riod of listen time, nodes which are not involved in current

communication can go to sleep immediately. Furthermore,

nodes which are involved in communication can go to sleep

as soon as communication between them is over as depicted

in Figure 2. These procedures make TEEM’s listen period

adaptive and much more energy efficient than S-MAC.

3. Motivation of our work

The S-MAC family protocols deal with data and SYNC

traffic on the sleep/listen cycle. Data traffic depends on the

sensing of some unusual environmental conditions which is

occasional. But, the SYNC packet is sent periodically with

some considerable amount of gap. This periodic SYNC

transmission state is the most dominant state in a typical

WSN. In S-MAC family protocols, even though nodes have

received SYNC packets from all its neighbors and there will

be no more SYNC packets in current synchronization pe-

riod, these nodes still waste their energy in idle listening in

their SYNC period. Such an inefficiency is caused mainly

by the fact that S-MAC family protocols don’t consider the

periodic nature of SYNC packet. This will be more clear

with an example. Let us suppose that there is a cluster of

4 nodes sending SYNC packets with interval of 10 cycles.

Only the SYNC periods of 4 cycles are used to exchange

the SYNC packets, whereas 6 cycles remain in idle listen-

ing. This idle listening for 6 cycles in SYNC period happens

every synchronization period. This observation leads us to

propose a new energy saving technique, INS, which allows

the nodes to be in sleep state in the SYNC period when they

recognize that nobody has SYNC packet queued in the cur-

rent SYNC period.

The rest of this paper is organized as follows. Section 4

shows the INS design, followed by a brief analysis on the

energy performance of INS in Section 5. Section 6 demon-

strates the energy efficiency achieved by INS through nu-

merical and simulations results. Finally, we conclude the

paper in Section 7.

4. Design of proposed intelligent network syn-

chronization

This section describes the design of new algorithm, INS.

Although INS can apply to all sleep/wakeup-based MACs,

we describe the details in our implementation based on S-

MAC which is well-known MAC for WSN.

4.1. Intelligent network synchronization

In order to minimize energy consumption, INS exploits

the periodic nature of SYNC packets. In S-MAC, nodes

Table 2. Field definition of neighbor list table

of proposed INS

Field Comment

nodeId ID of this node

schedIdSchedule ID in schedule table that this node

follows

active Flag indicating this node is active recently

state Flag indicating the node has changed schedule

counterCounts the cycles since last SYNC received from

this node

maintain the list of all their neighbors in the neighbor list

table. Now, by adding one more field in this neighbor list

table, nodes can be intelligent enough to make decision of

either to sleep or wake up in the current SYNC period. Ta-

ble 2 shows neighbor list table including a newly added field

counter. Let us see this with an example. Let X be a node in

the network. Node X has separate counter for each neigh-

bor. Each counter is increased by one at each cycle time.

However, the counter is reset to zero when the node X re-

ceives the SYNC packet from the corresponding neighbor.

That is, the counter is used to calculate the number of cy-

cles elapsed since the last SYNC packet received for each

neighbor. Now the node X can realize whether it will re-

ceive the SYNC packet in the current SYNC period or not

by examining these counters. If the node X finds that any

counter’s value is equal or greater than synchronization pe-

riod, then the node X knows it will get a SYNC packet now.

Thus, it will wake up in current SYNC period or it will go

to sleep in the other case. The more detailed description

of this algorithm has been given in the Table 3. This algo-

rithm is executed at the beginning of every SYNC period

of a node. Note that INS preserves periodic neighbor dis-

covery as shown in Table 3. Therefore, INS will have no

negative impact on the flexibility and scalability of S-MAC.

The difference in S-MAC protocol with and without INS

in a cluster of two nodes is shown in Figure 3. As shown in

the Figure 3(b), nodes with INS do not wake up in SYNC

period after successful exchange of SYNC packets until

the next synchronization period. This figure clearly shows

that INS saves considerable amount of energy when imple-

mented in any S-MAC family protocols. Note that the data

period is left untouched in this new protocol. INS has no

concern with data period and energy conservation is achieve

by managing the SYNC period only. Therefore, INS does

not effect any performance metrics of host protocol. Hence,

the proposed INS can be adopted to all of the S-MAC family

protocols for the advanced energy conservation.

Table 3. Algorithm for INSAlgorithm handleCounterTimer

Let X be a node

Increment the counter value of all neighbors of X by 1

if ((in periodic neighbor discovery) ‖ (counter for any

neighbor in X >= synchronization period)) then

X is expecting the SYNC packet, and wakes up in

SYNC period

else

X will not get SYNC packet in this SYNC period,

thus sleeps

end if

Note:The counter for particular neighbor is reset to 0

every time SYNC packet is received from that node

SYNC Period

DATA Period

. . . . Node 0

Node 1

Send

SYNC

. . . .

After 10

cycles

Send

SYNC Send

SYNC

Send

SYNC

Synchronization period (10 cycles)

(a) Original S-MAC

. . . . Node 0

Node 1

Send

SYNC

. . . .

After 10

cycles

Send

SYNC Send

SYNC

Send

SYNC

Synchronization period (10 cycles)

(b) S-MAC with INS

Figure 3. Exchange of SYNC packets between two nodes in S-MAC and S-MAC with INS.

5. Energy analysis

In this section, we analyze the energy saving with INS.

For the analysis, we take only the SYNC period into con-

sideration and show the consumed energy with and without

INS.

For wireless sensor network applications, the energy is

consumed by receiving, transmitting, listening for messages

on the radio channel, and sleeping. Therefore, the total en-

ergy consumed, E, is given by

E = Ereceive + Etransmit + Eidle listening + Esleep. (1)

In the proposed INS, we are concerned with SYNC period

only. Let us suppose that we have a cluster of N nodes hav-

ing synchronization period of P. Let Erx and Etx be the

energy consumed to receive and transmit the SYNC packet.

Let Eidle be the energy consumed per idle listening in a

SYNC period. Then the energy consumed by the SYNC

periods per synchronization period is given by

Esync period = (N − 1)Erx + Etx + XEidle, (2)

where

X =

{

P − N if N < P

0 elsewhere .

This is because, in a cluster of N nodes, each node will

receive N-1 SYNC packets, and transmit its own SYNC

packet once, thus stay idle listening during rest of the the

synchronization period. Now, INS is applied to this pro-

tocol, then the unnecessary idle listening is removed in a

SYNC period as shown in equation (3). That is, the pervi-

ous idle listening is now changed to sleeping.

Esync period = (N − 1)Erx + Etx. (3)

Since the sleep energy is negligible as compared to listen-

ing, for simplicity, we omit the energy consumed by the

sleep mode in this equation. Comparison of equation (2)

and (3) clearly demonstrates the energy saving introduced

by INS.

6. Performance evaluation

For the performance evaluation, we implement INS in

S-MAC and TEEM and compared with original protocols.

INS is simulated and evaluated on NS-2.32 [8]. In our sim-

ulation model, we used the topology that has a fixed 250m

distance between each node. The transmission range is of

250m. For both the protocols, the simulated nodes are con-

figured using the parameters listed in Table 4. Also, the

synchronization period is 10 cycles and the duty cycle is

10%. In all the simulations, nodes use NOAH static ad-hoc

routing protocol [9].

In our first set of simulation, we took a liner topology

of 5 nodes (4 hops) with first node as source and last node

acting as sink. Our proposed INS has an energy efficiency

feature by managing SYNC period which is regardless of

600 900 1200 1500 1800 2100

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Tot

al e

nerg

y co

nsum

ed [

joul

e]

Simulation time [sec]

S-MAC Simulation

S-MAC with INS Simulation

S-MAC with INS Analysis

Figure 4. Average energy consumed by a

node in the absence of data traffic in S-MACin linear topology.

data traffic. Thus, this simulation demonstrates the energy

efficiency of INS under absence of data traffic as shown

in Figure 4 and 5. Here, nodes exchange SYNC packets

only. Specifically, Figure 4 shows the analytical result for

S-MAC with INS. Table 4 lists the parameters used in the

analysis. The sleep power and the transition power have

not been considered in numerical calculation. The graphs

in Figure 4 show the simulation and analytical results are

in good agreement. The simulation output of INS imple-

mented in S-MAC and TEEM is shown in Figure 5. When

INS is implemented in both protocols, INS saved signifi-

cant amount of energy. The results show that INS reduced

the energy consumption of S-MAC by 25% and TEEM by

23% when simulation was run for 2100 secs.

The energy efficiency of INS in presence of data traffic

under the above liner topology is shown in the Figure 6.

The source node generates total of 50 messages with 100

bytes to be transferred to sink node. Data flows pass through

from source to sink via the intermediate nodes and simula-

Table 4. Parameters for NS-2 simulation.Channel bandwidth 20 kbps

SYNC period (S-MAC) 55ms

SYNC period (TEEM) 53ms

Data period (S-MAC) 88ms

Data period (TEEM) 85ms

Synchronization period 10 cycles

Reception power 14mW

Transmission power 36mW

Idle power 14mW

Sleep power 15µW

Transition power 28mW

600 900 1200 1500 1800 2100

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Tot

al e

nerg

y co

nsum

ed [

jou

le]

Simulation time [sec]

SMAC

TEEM

SMAC with INS

TEEM with INS

Figure 5. Average energy consumed by a

node in the absence of data traffic in lineartopology.

tion ends with the transfer of the last packet. The message

inter-arrival period is varied in order to measure energy con-

sumption in different traffic loads. In our experiment, the

message inter-arrival period varies from 9 to 24 secs. The

average energy consumed by a node under variable traffic

loads is shown in the Figure 6. The experimental result

shows that INS reduced the energy consumption of S-MAC

by 13% when message inter-arrival period was of 9 secs.

However, INS reduced the energy consumption of S-MAC

by more than 15% when the message inter-arrival period

was 24 secs. In the case of TEEM, there was saving of 14%

and 17% under message inter-arrival period of 9 and 24 secs

respectively.

In our second set of simulation, we took grid topology

of 15 nodes arranged in 3 rows with 5 nodes in each row.

Each nodes are at the distance of 250m from each other.

The first and the last nodes of the second row are source

and sink. The other nodes, between source and sink nodes

in the second row, act as intermediate relay nodes and for-

ward the data to sink node. In this set of simulation also

source node generates total of 50 messages with 100 bytes

to be transferred to sink node. Figure 7 shows the aver-

age energy consumed by nodes which are evolved in data

transmission i.e., average energy consumed by nodes in the

second row. The experimental result shows that INS re-

duced the energy consumption of S-MAC by 8% and 10%,

when the message inter-arrival periods were of 9 and 24

secs respectively . In the case of TEEM, there were saving

of 10% and 13%, under message inter-arrival periods of 9

and 24 secs respectively. The simulation result shows that

the amount of energy saved decrease with increase in node

density. The energy saving in this set of simulation is less

as compared to linear topology, the considerable amount of

9 12 15 18 21 24

2.0

2.5

3.0

3.5

4.0

4.5

Tot

al e

nerg

y co

msu

med

[jo

ule]

Message inter-arrival period [sec]

S-MAC

TEEM

S-MAC with INS

TEEM with INS

Figure 6. Average energy consumed by a

node in the presence of data traffic in lineartopology.

energy was saved with INS.

Therefore, we can derive from above simulations that on

the real time scenario, where data are sensed at considerable

amount of gap, large amount of energy can be saved with

the implementation of INS.

7. Conclusion

In this paper, we proposed a new generalized energy sav-

ing technique, INS, which can be implemented in all S-

MAC family protocols. Our scheme increases the energy

efficiency by making nodes go to the sleep state in their un-

used SYNC periods. This energy efficiency is obtained by

maintaining SYNC information of all the neighbors. To ver-

ify INS, we implemented our scheme in S-MAC and TEEM

and simulated it in NS-2. Our experimental results demon-

strated that our scheme works well and saves significant

amount of energy.

Acknowledgment

The authors would like to thank Changsu Suh, one of the

authors of TEEM protocol, for supporting us with TEEM

implementation.

References

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9 12 15 18 21 24

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Tot

al e

nerg

y co

msu

med

[jo

ule]

Message inter-arrival period [sec]

S-MAC

TEEM

S-MAC with INS

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