intelligent network synchronization for energy saving in low duty cycle mac protocols
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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
TEEM with INS
Figure 7. Average energy consumed by a
node in the presence of data traffic in gridtopology.
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