IEICE TRANS. COMMUN., VOL.Exx–??, NO.xx XXXX 200x

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PAPER

Proposal of an Assured Corridor Mechanism for Urgent
Information Transmission in Wireless Sensor Networks
Tetsuya KAWAI† , Student Member, Naoki WAKAMIYA† , Member,and Masayuki MURATA† , Fellow

SUMMARY Wireless sensor networks are expected to play
an essential role as a social infrastructure to realize our safe andsecure living environment. In such a network, critical information must be transmitted faster and more reliably than otherinformation. We propose a distributed transmission mechanism
which enables emergency packets to be carried with high reliability and low latency along a preferential path, which is called an“assured corridor.” In this self-organizing assured corridor mechanism (ACM), which works above the network layer and does notdepend on any specific routing or MAC protocol, a corridor is
gradually established as the first packet containing urgent information propagates to the base station. The nodes surrounding
the corridor suppress the transmission of non-urgent informationand nodes in the corridor are kept awake to forward emergency
packets. ACM avoids packet loss and possible delay caused bycollisions in the wireless transmission and normal sleep scheduling. An acknowledgment and retransmission scheme is incorporated into ACM in order to improve reliability of transmission ofurgent information. Simulation experiments showed that, when
only one node transmitted urgent information, the retransmission contributed to establish a corridor quickly and that ACM
improved the delivery ratio and the delay of the urgent information transmission once a corridor is established. It was provedthat ACM was effective to improve the reliability and the latencyof urgent information as well in the cases where multiple nodes
sent urgent information at once.
key words: sensor networks, urgent information, fastness, reliability

1.

Introduction

Due to advances in the development of microelectromechanical systems (MEMS) technology, wireless sensor networks (WSNs) have become popular in
the field of information and communication technology
and attracted much attention of many researchers [1,2].
A WSN consists of a number of sensor nodes, each ofwhich is equipped with one or more sensors, an analogdigital converter, a radio transceiver, a central processing unit with limited computational capability, asmall amount of memory, and a battery power supply.
Nodes are deployed into a region to be monitored. Theybuild up a network using radio communications in anautonomous and distributed manner. Sensor data obtained at nodes are transmitted through a network toa certain node called a base station (BS) or sink forfurther processing. WSNs have a wide variety of applications such as agricultural, health, environmental,†

Graduate School of Information Science and Technology, Osaka University

and industrial purposes.
Among a number of applications, a WSN used asa social infrastructure to make our life safe, secure,and comfortable is one of the most promising. This
sort of WSNs would carry both urgent and non-urgent
information, which apparently should not be handledequally. The urgent information, such that for security,
disaster, environmental, and vital conditions, has to be
carried through a WSN with higher reliability and lower
delay than other non-urgent information such that for
regular monitoring for living and working space control.
It means that a WSN must be capable of differentiating
and prioritizing packets depending on their urgency andimportance according to requests from the application
layer. In addition, it must provide a mechanism where
packets with higher priority are transmitted preferentially.
Since many factors make radio communication unstable and unreliable, it is a challenging issue to realize
a fast and reliable transmission of urgent information in
WSNs. Among them, collisions are the most influential
and dominant, especially, when Carrier Sense Multiple Access (CSMA) MAC protocol is used. A collision
drastically increases the latency of the transmission of
packets due to backoff and retransmissions. Inserting arandom backoff before transmission is helpful to someextent as it has already been incorporated into many
CSMA algorithms. However, they face the so-called
hidden terminal problem. In addition, collisions cannot be avoided if two or more nodes occasionally start
their carrier sense at the same time.
In this paper, we propose an “assured corridor”
mechanism, ACM in short, for fast and reliable transmission of urgent information in a WSN. In ACM, anassured corridor is eventually established where emergency packets are forwarded preferentially. Transmission of normal packets is suppressed along the corridor
and emergency packets are forwarded by keep-awake
nodes in the corridor. By suppressing transmission of
normal packets, congestion among emergency and normal packets is avoided. Therefore, it contributes toreducing the latency caused by backoff and retransmissions and the loss probability caused by retransmission
timeout. By keeping nodes awake on the path from thesource node of emergency packets to the BS, the delay
required to wait for the next-hop node to wake up from

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(a)

(b)

Fig. 1 An “assured corridor” in (a) a tree-based network and(b) a broadcasting-based network.

the sleep mode is avoided.
Examples of an assured corridor are illustrated
in Fig. 1 for a tree-based sensor network and abroadcasting-based sensor network. In the figure, a starcorresponds to a node which detects an emergency andgrey circles correspond to nodes on a path to the BS.
Nodes in ranges of radio signals of those grey nodes
are denoted as filled circles, which suppress emission of
normal packets.
The rest of the paper is organized as follows. Detailed description of ACM is discussed in Sect. 2. In
Sect. 3, we show an example of application scenario of
ACM to our previous work, i.e., the synchronizationbased data gathering scheme [3, 4]. Section 4 gives thedetails of simulation experiments. Then, the results
and discussions are presented in Sect. 5. Finally weconclude the paper in Sect. 6.
2.

Description of the proposed mechanism

In this paper, we assume a WSN deployed in a residence, a building, or a small area, used for a home security system, a building automation system, or a public
surveillance system for example, in which sensor nodes
are immobile. Although ACM works above the network layer and does not depend on any specific MAC or
routing protocols, we assume a contention-based MAC
protocol and a multihop routing protocol for their wide
range of off-the-shelf products and research and development activities. For example, a TDMA protocol isalso applicable, but we consider that it has many practical problems to be solved such as scheduling overhead
and severe requirement for time synchronization. As
for the network layer, a multihop scheme with limitation on the radio transmission energy is usually preferred to avoid contention among wireless communication and prolong the lifetime of batteries. ACM itself
is not a routing protocol, although it uses routing information of the network layer. An assured corridor
is established along a path that an underlying routing
protocol or data gathering scheme builds for communication or data gathering in normal operation.
In our mechanism, a node follows the state transitions illustrated in Fig 2. A node stays in the NORMAL state in its normal operation. When a node de-tects an emergency event, its state is changed to the
EMG SEND state and it begins emission of emergency
packets. An emergency packet is identified by an emergency flag in its header. When a neighbor node in the
NORMAL state receives or hears the emergency packet,
its state moves to either of the EMG FORWARD or
SUPPRESSED state depending on its location. If thenode is on the path to the BS, in other words, if
the node is a next-hop of the EMG SEND node, it
moves to the EMG FORWARD state. It suspends its
sleep schedule and forwards the emergency packet. If
the node is not involved in forwarding the emergency
packet, i.e., not on the path to the BS, it moves to
the SUPPRESSED state and suppresses transmissionof normal packets in order to avoid collisions with emergency packets in the MAC layer.
Similarly, among neighbor nodes receiving or
hearing an emergency packet forwarded by the
EMG FORWARD node, ones on the path to the BS become EMG FORWARD nodes and the others become
SUPPRESSED nodes. By repeating this process at every hop to the BS, an assured corridor, which consists of
EMG FORWARD nodes forwarding emergency packets
along the path and SUPPRESSED nodes surrounding
the path, is eventually completed when the first emergency packet arrives at the BS. We should note here
that, if an emergency packet is lost due to a collision
for example, NORMAL nodes around stay unaware of
the emergency and keep the NORMAL state.
Once an assured corridor is established, following emergency packets propagate through the corridor
which consists of awake nodes forwarding emergency
packets and surrounding silent nodes. The rest of thenodes in the WSN are not aware of the emergency andthey remain in their normal operation.
These mechanisms imply that the reliability andthe latency of transmission of emergency packets are
improved at sacrifice of the larger transmission delay of
non-urgent information and the depletion of a batteryof awake nodes. Although low energy consumption is
one of the most important requirements in WSNs, weshould not sacrifice the reliability and latency of transmission of emergency packets for the energy efficiency.
Therefore, we do not pay much attention to energy efficiency in our mechanism. We believe that such a design
policy is acceptable, because it is reasonable to assume
that emergency events rarely happen. The lifetime ofa WSN depends on energy efficiency not in urgent conditions but in normal operations. If allowed we canintroduce a sleep schedule to nodes in a corridor, but
it is left as one of future works.
Detailed description of the four states of a node in
ACM is given in the following.
NORMAL As long as there is no emergency event,a WSN operates as usual and nodes are in the
NORMAL state. They periodically wake up, re-KAWAI et al.: ASSURED CORRIDOR MECHANISM FOR URGENT INFORMATION IN SENSOR NETWORKS

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Fig. 3Fig. 2

State transitions.

ceive and transmit a data packet, and go back tosleep at regular intervals of tnorm .
EMG SEND When a node detects an emergency
event, e.g., a fire, it enters the EMG SEND state.
It broadcasts emergency packets with the emergency flag at shorter intervals of temg < tnorm . Every emergency packet sent is given a unique sequence number at the source node.
EMG FORWARD A node which receives an emergency packet for the first time from its preceding
nodes moves into the EMG FORWARD state. A
preceding node is a node for which the node is responsible in forwarding a packet or data toward the
BS. For example, if the WSN adopts tree topology whose root is the BS, a preceding node is achild node. On receiving the emergency packet for
the first time, a node suspends the sleep schedule.
Then, it sends the received emergency packet tothe designated next-hop node on the path to theBS, after waiting for the activation of the nexthop node in the NORMAL state if it is in thesleep mode. The next-hop node also moves to the
EMG FORWARD state and keeps awake once it
receives the emergency packet. Therefore, following emergency packets sent after the first emergency packet by the source node are immediately
relayed by EMG FORWARD nodes toward the BS.
SUPPRESSED A node which receives an emergency
packet from a neighboring node which is not its
preceding node moves into the SUPPRESSED
state. A node in this state should suppress transmitting some or all of normal packets.
We assume that an observatory or a control center receives the urgent information through the BS.
Then, an acknowledgment is sent back to the BS
and it is forwarded to the source node of the emergency packets. On receiving the acknowledgment,
the EMG SEND node returns back to the NORMAL
state. EMG FORWARD and SUPPRESSED are “softstates,” which are controlled not by explicit signaling
but by a timer. Entering these states, a node starts atimer. When the timer expires, it returns to the NORMAL state. The timer is restarted every time when anode receives an emergency packet. A typical length of
the timer is the interval of data gathering in the NOR-

The synchronization-based data gathering scheme.

MAL state, i.e., tnorm , since emergency packets are sent
more frequent than normal packets to inform a control
center of up-to-date emergency condition.
Note that an assured corridor is established while
the first emergency packet is being forwarded to the
BS. Therefore, the transmission delay of the first emergency packet, in other words, the time needed to establish a corridor, depends on a sleep schedule of thedata gathering scheme used for normal operation. Aftera corridor is established, following emergency packets
are forwarded immediately by EMG FORWARD nodes,
which keep awake, thus the delay is minimal and independent of the sleep schedule. In case that the firstemergency packet is lost, any of following emergency
packets succeeds the role of the first emergency packet
to establish a corridor. Here, we call an emergency
packet sent by an EMG SEND node for the first time“the first emergency packet.” In addition, an emergency packet that a NORMAL node or the BS receives
or hears for the first time is also called the first emergency packet. The first emergency packets contributeto establishment of a corridor. Other emergency packets are called “following emergency packets.”
3.

Application to the synchronization-based
data gathering scheme

3.1 Overview
Although ACM does not depend on any specific routing algorithm, we employ the synchronization-based
data gathering scheme [3, 4] to evaluate the behaviorof our mechanism in this paper. The synchronizationbased data gathering scheme is proposed to accomplish
energy-efficient data gathering in a WSN without any
centralized control. Combined with ACM, it enables a
distributed and self-organizing control of fast and reliable transmission of urgent sensor information.
Figure 3 illustrates how sensor data propagate
from all nodes to the BS. A number in each circle,
i.e., node, corresponds to the number of hops from theBS, which is maintained by each node as a level value.
Figure 4(a) shows timings that level n − 2, n − 1, andn nodes wake up, receive and send packets, and go tosleep. Here, the interval of data gathering is given as
tnorm .
In the synchronization-based data gathering
scheme, sensor nodes with the same level value behave
in synchrony. When so-called global synchronization

IEICE TRANS. COMMUN., VOL.Exx–??, NO.xx XXXX 200x4

3.2 In emergency(a) Normal operation.

(b) Successful scenario for transmission of emergency packets.

(c) Scenario with retransmission of the first emergency packet.
Fig. 4 Transmission sequences in the synchronization-based
data gathering scheme.

is accomplished, an interval between packet emissionof level i nodes and that of level i − 1 nodes becomesδtnorm . Now assume that the most distant nodes are atn hops from the BS. First, all level n nodes wake up andthen broadcast a packet at the same time. This broadcasting is scheduled slightly before timing of packet
emission of level n − 1 nodes by δtnorm . At this time,
all level n − 1 nodes wake up. They receive packets
from level n nodes in their vicinity and aggregate orfuse the received data with their own data. Then, theybroadcast a packet containing the aggregated sensor
data slightly before timing of packet emission of leveln − 2 nodes by δtnorm , so that level n − 2 nodes, which
are awake at this time, can receive the packet. At thesame time, level n nodes also receive packets emitted bylevel n − 1 nodes to maintain the synchronization andthen go to sleep. Therefore all sensor nodes need to be
awake for 2δtnorm during the interval of data gathering
as illustrated in Fig. 4(a). For further details of thesynchronization-based data gathering, refer to [3].
In the rest of the paper, we call one-level smaller
nodes of a node as “parent nodes” and one-level larger
nodes as “child nodes”. As easily imagined from theirnames, the mechanism proposed in this paper can directly be applied to a tree-based data gathering scheme
as shown in Fig. 1(a). In the case of MANET-type
schemes, where one or more paths are explicitly built
for communication between a node and a BS, parent
nodes correspond to the next-hop nodes to the BS andchild nodes correspond to the preceding nodes.

Transmission of urgent information in the synchronizationbased data gathering scheme with ACM is depicted inFig. 4(b). When a node detects an emergency event, it
moves into EMG SEND state. It defers emission of thefirst emergency packet for tfirst until its next timing of
packet emission (t = 0, in Fig. 4(b)), since its parent
nodes in the NORMAL state are asleep at the momentof event detection and can not receive any packet until they wake up. To minimize this delay, one possible
way is to have a mechanism to wake up parent nodes
by sending a wake-up signal. ACM can be combinedwith such a mechanism, however, a special hardware
for the wake-up mechanism is needed on every node,
which leads additional production cost.
When the first emergency packet is broadcast at
t = 0, parent nodes move to the EMG FORWARD
state while neighboring nodes of the same or a larger
level move to the SUPPRESSED state. By hop-by-hop
broadcasting of the first emergency packet, all intermediate nodes move to the EMG FORWARD state.
If the first emergency packet is lost at one ofintermediate nodes due to collision for example, thetransmission is delayed for tnorm until the next timing of packet emission. Letting tirelay time taken for an
EMG FORWARD node of level i to wait for level i − 1
nodes to wake up, the total delay of the first emergency
packet originating at level n is given by,
Dn = tfirst +n−1
Xtirelay + ktnorm ,

(1)

i=1

where k is the number of packet loss events.
The maximum of tfirst is tnorm , and all intermediate nodes have to wait for δtnorm , if they are in the
NORMAL state,max(tfirst ) = tnorm ,
Ãn−1
!
X
i
max
trelay = (n − 1)δtnorm .

(2)
(3)

i=1

Thus the maximum of delay Dn for the first emergency
packet originating at an EMG SEND node of level n toreach the BS is given bymax(Dn ) = tnorm + (n − 1)δtnorm + ktnorm .

(4)

The EMG SEND node keeps sending emergency
packets at intervals of temg after its emission of the firstemergency packet. These following emergency packets
are forwarded to the BS immediately along the corridor. If δtnorm is relatively large compared to thetransmission delay of following emergency packets, afollowing emergency packet may catch up the preceding first emergency packet at an intermediate node. In

KAWAI et al.: ASSURED CORRIDOR MECHANISM FOR URGENT INFORMATION IN SENSOR NETWORKS

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this case, the following emergency packet takes over the
first emergency packet and propagates to the BS establishing a corridor as being treated as the first emergency
packet.
3.3 Retransmission
Since a corridor is not established, the first emergency
packet is forwarded to the BS without any prioritization and can get lost. Although a following emergency
packet succeeds the role of a lost first emergency packet
in establishing a corridor, it increases the transmission
delay of emergency packets and can be critical to thesafety and security of our living environment. Following emergency packets can be lost as well, because of
possible collisions among emergency packets.
There are several possibilities to overcome the lossof emergency packets. In this paper, we take a hop-byhop acknowledgement and retransmission scheme at ahigher layer above MAC. Our scheme does not exclude
other techniques and they are helpful to improve the reliability of transmission. For example, we could adopt
a MAC protocol with prioritization [5] or a packet-level
priority control. In [6, 7], the authors consider service
differentiation in terms of delivery ratio based on DiffServ model. Delay-based differentiation is proposed in,for example, [8, 9]. Multipath routing / forwarding isanother possibility to improve the reliability of packet
transmission [6, 7, 10]. We will consider effective andefficient coordination of several techniques for fast andreliable transmission of urgent information in the next
research work.
The synchronization-based data gathering scheme
inherently enables hop-by-hop acknowledgement sincea node receives a packet from a parent node for synchronization. In other kind of schemes, a node can also
expect to receive an emergency packet from its parent
node at the timing of packet emission of the parent
node. A node can confirm the successful transmissionof its emergency packet by observing a packet sent by
one of its parent nodes. If a node does not receive
an emergency packet from its parent node or an emergency packet broadcast by its parent node does not contain urgent information it sent, the emergency packet
is considered lost. Then, it retransmits the emergency
packet. The retransmission scheme of the first emergency packet in the synchronization-based data gathering scheme is shown in Fig. 4(c).
First, level n node which detects an emergency
event immediately moves to the EMG SEND state.
Then, it transmits an emergency packet at the next
timing of packet emission, t = 0. A level n − 1 node,
which is a parent node of the level n node, is expected tomove to the EMG FORWARD state and broadcast the
emergency packet at the next timing of packet emission
at t = δtnorm . However, the first emergency packet canbe lost, due to the collision with a normal packet trans-mitted from a neighbor node or random channel error,
for example. In this case, the level n − 1 node remains
in the NORMAL state and broadcast a normal packet
at its timing of packet emission, t = δtnorm as shownin Fig. 4(c). Receiving a normal packet from its parent
node, the level n node detects the loss and immediately
retransmits the emergency packet with a retransmission
flag in the packet header. Retransmission is repeateduntil it receives the emergency packet from any of its
parent nodes. Therefore, the duration for retransmission is at most δtnorm , i.e., until parent nodes go tosleep. If the next emergency packet originating at thesame source node arrives while an emergency packet is
waiting for retransmission at an intermediate node, thewaiting packet is discarded. This is because that sensor data in the waiting packet is obsoleted by the newdata. It is also possible to merge them and generatea new emergency packet depending on an application’s
requirement.
If a level n − 1 node receives an emergency packet
with a retransmission flag while it is awake, it immediately broadcasts the emergency packet so that leveln − 2 nodes can forward the packet at the next timing of regular emission (t = 2δtnorm in Fig. 4(c)). The
emergency packet sent by the level n − 1 node also confirms the successful reception to the level n node. Since
the other nodes in the vicinity of a node retransmitting
emergency packets do not transmit any packets during retransmission interval, we can avoid collisions andlosses of retransmitted packets.
Now, consider the delay of the first emergency
packet for the worst case scenario with retransmission.
The maximum delay of transmission is δtnorm as far
as retransmission succeeds before a parent node, whichis still in the NORMAL state, goes to sleep (see retransmission from level n node to level n − 1 node in
Fig. 4(c)). If a node fails in retransmission, it must wait
for the next cycle of packet emission at tnorm later because its parent node are asleep until that time. Therefore, the maximum of transmission delay with retransmission, DnR , is given by,max(DnR ) = tnorm +(n−1)δtnorm +k 0 tnorm +t1retrans .(5)where k 0 is the the number of retransmission failures,
which incur delay of tnorm . t1retrans corresponds to thetime for level 1 node to transmit the emergency packet
to the BS. Since the BS is always ready to receive apacket, level 1 node can try retransmitting the emergency packet until the next emergency packet catches
up, which is at most temg after it receives the first emergency packet. Therefore,max(DnR ) = tnorm +(n−1)δtnorm +k 0 tnorm +temg .(6)4.

Details of simulation experiments

We evaluated the synchronization-based data gather-

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20

50

KA, 1nodemax(Dn)45
40

Delay (s)35BS
EMG_SEND
EMG_FORWARD
SUPPRESSEDNORMAL

10

30
25
20
15
10
5
0

0

1
0

Fig. 510

20

An “assured corridor” in a simulation experiment.

ing scheme with ACM on the ns-2 network simulator
package [11]. In all of the simulation experiments, 200
nodes are uniformly and randomly distributed in a 20 m
× 20 m two-dimensional region with the BS at its center and the transmission range of radio signals is set to
2.5 m. The high density of node distribution is for thereason that we assume an indoor application such asbuilding automation. The maximum number of hops
to the BS was twelve in our experiments. The reason why the number is large for a small region is thatpaths were constructed to detour around a void causedby the random distribution of nodes. Figure 5 shows asnapshot in one of the simulation experiments with one
EMG SEND node. A corridor is established from thesource node, which is represented by a circle, towards
the BS, filled square. Nodes in the SUPPRESSED
state surrounding the corridor are illustrated as a open
square.
IEEE 802.15.4 [12] non-beacon mode is used as the
MAC protocol [13]. The interval tnorm of data gathering
is set to 10 seconds. The offset coefficient δ is 0.1, where
each node wakes up δtnorm , i.e., one second before its
packet emission and goes to sleep one second after theemission. Before sending a packet, the random backoffof maximum 10 ms is applied in the network layer inorder to ease the collision situation. The size of sensor
data is 6 bytes and we do not assume data fusion. Thus,
N sensor information amounts to 6N bytes. The maximum size of the payload is limited to 78 bytes due tothe limitation of IEEE 802.15.4 and sensor data beyond
78 bytes are discarded at each node.
Each simulation experiment lasts 20,000 seconds
including 300 seconds for initialization, synchronization, and normal operations without any emergency.
After the initial 300 seconds, nodes are randomly
chosen and moved into the EMG SEND state. The
EMG SEND nodes emit emergency packets at intervals of temg = 2 seconds. They stay in the EMG SEND
state for 40 seconds and then go back to NORMAL
state. After this, 90 seconds are taken to allow
EMG FORWARD and SUPPRESSED nodes to returnto the NORMAL state. Then, after a random interval of up to 10 seconds, the next nodes are randomly

Fig. 6

2

3

4

5
6
7
8
Level of source node

9

10

11

12

Delay of the first emergency packets.

chosen and moved to the EMG SEND state. The sameprocedure is repeated during a single simulation experiment. Under this scenario, we had around 150 emergencies in each simulation experiment. The same experiment is repeated for ten times with different nodelayouts. In Section 5.1 and 5.2, one node is moved to
the EMG SEND state at the same time, whereas multiple nodes detect an emergency in Section 5.3.
For comparison purposes we considered threevariants of the mechanism.
One is called as
KA (keep awake), in which only EMG SEND and
EMG FORWARD states are applied and no suppression of normal packets is conducted. Another is called
KA+SP (suppression), in which an assured corridor
is established by suppressing emission of normal packets, but lost packets are not recovered by retransmission. The other is called KA+SP+RT (retransmission),
which is equivalent to ACM.
When a node detects a loss of packet, it first waits
for a random duration between 0.05 and 0.15 seconds
for the first emergency packet and between 0.1 and 0.2
for a following emergency packet and then retransmits
the packet. If the first retransmission fails, a node waits
for a random duration between 0.2 and 0.3 seconds andthen retries again. After the second retransmission,
the interval between retransmission is doubled, that is,a binary back-off scheme is applied.
5.

Results and discussions

In following subsections, simulation results of delay andloss rate of emergency packets are presented. Note that,in Figs. 7 through 10, the number of samples over level
10 is so small that large variation is observed in thesedata.
5.1 Late arrival ratio and delay of first emergency
packets
In ACM, the first emergency packet propagates to theBS while establishing a corridor. Thus the delay ofurgent information Dn , which is defined here as theduration from the time when a level n node detects an

KAWAI et al.: ASSURED CORRIDOR MECHANISM FOR URGENT INFORMATION IN SENSOR NETWORKS

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0.14

KA, 1node
KA+SP, 1node
KA+SP+RT, 1node

0.8

KA, 1node
KA+SP, 1node
KA+SP+RT, 1node

0.12

Loss rate

Late arrival ratio

0.1
0.6

0.4

0.08
0.06
0.04

0.2
0.02
0

0
1

Fig. 7

2

3

4

5
6
7
8
Level of source node

9

10

11

12

1

Late arrival ratio of the first emergency packets.

Fig. 9

2

3

4

5
6
7
8
Level of source node

9

10

11

12

Loss rate of following emergency packets.

0.12
20
0.1

0.08
Delay (s)Delay (s)

15

10

0.06

0.04
5
0.02
KA, 1node
KA+SP, 1node
KA+SP+RT, 1node

0
1Fig. 8

2

3

4

5
6
7
8
Level of source node

9

10

KA, 1node
KA+SP, 1node
KA+SP+RT, 1node

0
11

1

12

3

4

5

6

7

8

9

10

11

12

Level of source nodeFig. 10

Delay of the first emergency packets.

emergency event to the time when the BS receives an
emergency packet for the first time, indicates the timerequired to establish an assured corridor.
The delay of first emergency packets of KA with
one EMG SEND node is plotted in Fig. 6 for each levelof the source node. A dashed line shows the maximum
delay max(Dn ) without loss, i.e., k = 0 in Eq. (4),given bymax(Dn )k=0 = tnorm + (n − 1)δtnorm .

2(7)

As shown in Fig. 6, the delay exceeds max(Dn )k=0 in
about 15 % of the transmission. In these cases, there
is at least one packet loss on the way to the BS. Although the lost packet is compensated by a following
emergency packet, the loss leads to the increased delay.
The late arrival ratio and the averaged delay offirst emergency packets for each source level for KA,
KA+SP, and KA+SP+RT are shown in Figs. 7 and 8
respectively. The late arrival ratio is defined here as theratio of cases where the delay exceeds max(Dn )k=0 for
KA and KA+SP, and max(DnR )k0 =0 for KA+SP+RT
substituting k 0 = 0 into Eq.(6),max(DnR )k0 =0 = tnorm + (n − 1)δtnorm + temg . (8)As seen in the figures, suppression of transmissionof normal packets has little effect for first emergency
packets because they collide with normal packets in establishing an assured corridor. In KA+SP+RT, in contrast to the others, all of the first emergency packets

Delay of following emergency packets.

reached the BS within max(DnR )k0 =0 in Eq.(8), whichmeans that all first emergency packets are delivered to
the BS within one cycle due to successful retransmission. This is the reason why the delay of KA+SP+RT
is smaller than that of KA and KA+SP. As stated in
Section 3.3, since no normal packets are transmitted
at the time of retransmission of emergency packets, retransmission does not suffer from collisions with normal
packets and is successful unless there is a collision withanother emergency packet.
One might think that the absolute value of the delay is too large. However, this delay depends largely
on a sleep schedule of a data gathering scheme, sincea node must wait for a parent node to wake up in forwarding the first emergency packet. In the case of thesynchronization-based data gathering scheme, we canshorten the delay with smaller δ in Eq.(4) and Eq.(6),but tfirst is unavoidable without a wake-up mechanism.
The results shown in this paper can be regarded as the
expected performance in the worst case scenario. The
typical delay of a fire alarm for a home security system is from several tens of seconds to one minute andthus the delay of our mechanism is acceptable under
the simulation setting. We plan to conduct additional
experiments to see the applicability of the proposal.
5.2 Loss rate and delay of following emergency packets
The loss rate of following emergency packets is shown in
Fig. 9 while changing the level of source node. The loss

IEICE TRANS. COMMUN., VOL.Exx–??, NO.xx XXXX 200x

8

12

KA
KA+SP
KA+SP+RT

10

10

8

8
Delay (s)Delay (s)

12

6

6

4

4

2

2

0

KA
KA+SP
KA+SP+RT

0
0

2

4

6
8
10
12
Number of emergency nodes

14

16

0

2

4

6
8
10
12
Number of emergency nodes

(a)

14

16

(b)

Fig. 11 Delay of the first emergency packets in (a) a broadcast-based network and (b)a tree-based network with multiple EMG SEND nodes.

0.3

0.3

KA
KA+SP
KA+SP+RT

0.25

0.2
Loss rate

0.2
Loss rate

KA
KA+SP
KA+SP+RT

0.25

0.15

0.15

0.1

0.1

0.05

0.05

0

0
0

2

4

6
8
10
12
Number of emergency nodes

14

16

(a)

0

2

4

6
8
10
12
Number of emergency nodes

14

16

(b)

Fig. 12 Loss rate of following emergency packets in (a) a broadcast-based network and(b) a tree-based network with multiple EMG SEND nodes.

rate is defined as the ratio of the number of following
emergency packets not received by the BS to the number of those sent from source nodes after a corridor foreach source node is completely established.
In the experiments, we set the interval tnorm fornormal packets at 10 seconds and the interval temg for
emergency packets at 2 seconds. Therefore, without
suppression of normal packets, one emergency packet
out of five meets normal packets at parent nodes of asource node. Since the density of nodes is the highest
around the BS in our node distribution, the loss rate of
emergency packets in KA is the highest for source nodesof level 1 in Fig. 9. When transmission of normal packets is suppressed, the loss rate decreases to about thehalf. Remaining losses are for collisions among emergency packets traversing different paths. With retransmission, there was no packet loss for about 20,000 following emergency packets.
Figure 10 shows the delay of following emergency
packets. The delay is roughly proportional to the hop
distance from the BS. Retransmission introduces additional delay in waiting for retransmission and the resultant delay becomes larger than the others.

5.3 Multiple emergency nodes
Next we consider cases where multiple nodes detect anemergency event and move to the EMG SEND stateat the same time. In addition to the synchronizationbased data gathering scheme, which is a broadcastbased routing algorithm, we evaluated the performanceof ACM with a tree-based routing algorithm. In thiscase, each node chooses a parent node among neighbors
one hop closer to the BS, based on the received signal strength. This is equivalent to choosing the nearest
one-hop-closer node in the simulation experiments. The
transmission schedule is the same as in the broadcastbased network, which is shown in Fig. 4.
Figure 11 shows the delay of the first emergency
packets against the number of EMG SEND nodes inboth of the broadcast-based and tree-based networks.
Against intuition, the delay decreases as the number
of EMG SEND nodes increases. As stated before, once
an assured corridor is established, emergency packets
are forwarded immediately since all nodes are awake inthe corridor. For example, the first emergency packet
sent from an EMG SEND node which is located inor near a corridor established by another EMG SEND

KAWAI et al.: ASSURED CORRIDOR MECHANISM FOR URGENT INFORMATION IN SENSOR NETWORKS

0.09

0.09

0.08

0.08

0.07

0.07

0.06

0.06
Delay (s)Delay (s)

9

0.05
0.04
0.03

0.05
0.04
0.03

0.02

0.02
KA
KA+SP
KA+SP+RT

0.01
0
0

2

4

6
8
10
12
Number of emergency nodes

14

KA
KA+SP
KA+SP+RT

0.01
0
16

0

(a)

2

4

6
8
10
12
Number of emergency nodes

14

16

(b)

Fig. 13 Delay of following emergency packets in (a) a broadcast-based network and (b)a tree-based network with multiple EMG SEND nodes.

node would be carried through the corridor with shorter
delay. Therefore, the more EMG SEND nodes exists, the larger is the probability that the first emergency packets can use a corridor established by another
EMG SEND node. The smaller delay of KA+SP+RT
compared to KA and KA+SP reflects the smaller loss
rate of the first emergency packets. The delay of KA
and KA+SP is smaller in the broadcast-based network(Fig. 11(a)) than in the tree-based network (Fig. 11(b)),
since multipath effect in the former decreases losses ofthe first emergency packets.
The loss rate and average delay of following
emergency packets are plotted against the number of
EMG SEND nodes in Figs. 12 and 13 respectively. The
effect of suppression of transmission of normal packets
in reduction of loss rate becomes smaller as the number
of EMG SEND nodes increases. The more nodes are in
the EMG SEND state, the more dominant are packet
losses caused by collisions among emergency packets
within a corridor not with normal packets. With retransmission, in KA+SP+RT, the loss rate is less than
0.1 % with 16 EMG SEND nodes in both networks.
Comparing Fig. 12(a) and Fig. 12(b), the contribution of the suppression of normal packets is smallerin the broadcast-based network. This is because, inthe broadcast-based network, some of collisions occur
within a corridor among emergency packets traversing
different paths. The suppression of normal packets can
not avoid collisions within a corridor.
The delay in KA and KA+SP is independent of
the number of EMG SEND nodes, see Fig. 13, since
the increase of delay due to the contention in the MAC
layer among more EMG SEND and EMG FORWARD
nodes is deducted by decrease of the average hop countof emergency packets which arrive at the BS. Thereis a bias in favor of emergency packets originating at
EMG SEND nodes closer to the BS than those of distant EMG SEND nodes, for their less loss rate. For
supporting this, per-hop delay becomes larger with thenumber of EMG SEND nodes, although these results
are not shown because of space limitation. The de-lay of KA+SP is slightly smaller than that of KA.
The reason is that the number of backoff in the MAC
layer is smaller in KA+SP due to suppression of normal packets at surrounding SUPPRESSED nodes. Thiseffect decreases as the number of EMG SEND nodes increases. The more nodes are in the EMG SEND state,
the more nodes move to the EMG FORWARD state,
which means less SUPPRESSED nodes. On the other
hand, the delay increases in proportional to the number of EMG SEND nodes with retransmission. Again,
the reason is collisions among emergency packets within
an assured corridor, which cause more retransmission
and larger delay. The delay for KA+SP+RT in the
broadcast-based network (Fig. 13(a)) is a little larger
than in the tree-based network (Fig. 13(b)) since there
occur more frequent collisions within a corridor which
incur delay due to retransmission.
In summary, for the first emergency packet, suppression of transmission of normal packets along a corridor does not help much by itself. However, by additionally introducing a retransmission scheme, the loss
is completely avoided and the delay is effectively reduced. On the contrary, for following emergency packets, suppression contributes to reduction of the loss.
With retransmission, the loss rate of following emergency packets becomes close to zero at the sacrifice of
the increased delay in proportional to the number of
EMG SEND nodes. The increase in the delay is for collisions among emergency packets. We expect that thedelay can be reduced by introducing other techniquessuch as prioritization among emergency packets. We
are now working on those issues.
6.

Conclusion

In this paper, we proposed ACM, a fast and reliabletransmission mechanism for urgent information in sensor networks, where nodes in a corridor are kept awake
for fast transmission while adjoining nodes are kept
silent for less collisions.
Among data gathering schemes, we adopted the

IEICE TRANS. COMMUN., VOL.Exx–??, NO.xx XXXX 200x10synchronization-based scheme as an example of application scenarios. Although we considered a severe condition where data transmission of nodes were synchronized, data were gathered to the single center point,and sleep scheduling is adopted, simulation experiments
showed that the corridor was quickly established, theloss rate of emergency packets was successfully decreased, and the latency of following emergency packets
was improved.
ACM distinguishes packets in two categories, i.e.,
normal and emergency. In addition, suppressed nodes
completely stop transmitting any packets in the simulation experiments. We now consider to develop a mechanism for more severe conditions with many EMG SEND
nodes and multiple corridors. We combine several techniques, such as prioritization among emergency packets and rate control with a backpressure mechanism,with ACM. A WSN should function properly under
this kind of situation and we believe that this is one of
network layer functions needed for a WSN as a social
infrastructure.

Local Computer Networks (LCN 2003), Bonn, Germany,
pp.406–415, Oct. 2003.
[8] K. Akkaya and M. Younis, “Energy and QoS aware routing in wireless sensor networks,” Cluster Computing, vol.8,
no.2–3, pp.179–188, July 2005.
[9] A. Mahapatra, K. Anand, and D.P. Agrawal, “QoS andenergy aware routing for real-time traffic in wireless sensor networks,” Computer Communications, vol.29, no.4,
pp.437–445, Feb. 2006.
[10] B. Deb, S. Bhatnagar, and B. Nath, “Information assurance in sensor networks,” Proceedings of the 2nd ACM
international conference on Wireless sensor networks andapplications (WSNA 2003), San Diego, California, USA,
pp.160–168, Sept. 2003.
[11] Network simulator ns-2, http://www.isi.edu/nsnam/ns/.
[12] IEEE 802.15.4, “Wireless Medium Access Control andPhysical Layer Specifications for Low-Rate Wireless Personal Area Networks,” 2003.
[13] J. Zheng and M.J. Lee, “Ns2 simulator for 802.15.4,” http://www-ee.ccny.cuny.edu/zheng/pub/.

Acknowledgments
This research was partly supported by “New Information Technologies for Building a Networked Symbiosis
Environment” (The 21st Century Center of Excellence
Program) and a Grant-in-Aid for Scientific Research(A)(2) 16200003 of the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
References[1] I.F. Akyildiz, W. Su, Y. Sankarasubramaniam, andE. Cayirci, “Wireless sensor networks: a survey,” Computer
Networks, vol.38, no.4, pp.393–422, March 2002.
[2] K. Akkaya and M. Younis, “A survey on routing protocols
for wireless sensor networks,” Ad Hoc Networks, vol.3, no.3,
pp.325–349, May 2005.
[3] N. Wakamiya and M. Murata, “Synchronization-based data
gathering scheme for sensor networks,” IEICE Transactions
on Communications, vol.E88-B, no.3, pp.873–881, March
2005.
[4] S. Kashihara, N. Wakamiya, and M. Murata, “Implementation and evaluation of a synchronization-based data gathering scheme for sensor networks,” Proceedings of IEEE International Conference on Communications, Wireless Networking (ICC 2005), Seoul, Korea, pp.3037–3043, May
2005.
[5] L. Chenyang, B.M. Blum, T.F. Abdelzaher, and H. Tian,“RAP: a real-time communication architecture for largescale wireless sensor networks,” Proceedings of the 8thIEEE Real-Time and Embedded Technology and Applications Symposium (RTAS 2002), San Jose, California, USA,
pp.55–66, Sept. 2002.
[6] S. Bhatnagar, B. Deb, and B. Nath, “Service differentiationin sensor networks,” Proceedings of the 4th International
Symposium on Wireless Personal Multimedia Communications (WPMC 2001), Aalborg, Denmark, Sept. 2001.
[7] B. Deb, S. Bhatnagar, and B. Nath, “ReInForM: Reliable
information forwarding using multiple paths in sensor networks,” Proceedings of 28th Annual IEEE conference on

Tetsuya Kawai
received M.Sc. inphysical and inorganic chemistry from Osaka University, Japan, in 1994. In April
2005, he became a Ph.D. student in Graduate School of Information Science andTechnology, Osaka University, after 11
years experience in an electric manufacturer in Osaka, Japan. His research interests include wireless sensor networks. He
is a student member of IEICE.

Naoki Wakamiya
received M.E. and
Ph.D. from Osaka University, Japan, in
1994 and 1996, respectively. He was an
Assistant Professor of Graduate School ofEngineering Science from April 1999 to
March 2002. Since April 2002, he is an
Associate Professor of Graduate School ofInformation Science and Technology, Osaka University. His research interests include overlay networks, sensor networks,
and mobile ad-hoc networks. He is amember of IEICE, IPSJ, ACM, and IEEE.

Masayuki Murata
received the
M.E. and D.E. degrees in Information andComputer Science from Osaka University,
Japan, in 1984 and 1988, respectively. In
April 1999, he became a Professor of Cybermedia Center, Osaka University, and is
now with Graduate School of Information
Science and Technology, Osaka University
since April 2004. His research interests include computer communication networks,
performance modeling and evaluation. He
is an IEICE fellow and a member of IEEE, ACM, The InternetSociety, and IPSJ.