ACM: A Transmission Mechanism for Urgent Sensor Information
Tetsuya Kawai
Naoki Wakamiya
Masayuki Murata
Graduate School of Information Science and Technology, Osaka University
Osaka, Japan{t-kawai, wakamiya, murata}@ist.osaka-u.ac.jp
Abstract
The wireless sensor network (WSN) is one of the mostpromising technologies which helps making our society
safe, secure, and comfortable. A WSN as a social infrastructure must transmit critical information faster and morereliable than other information. In this paper, we propose
an autonomous and distributed mechanism, called an “assured corridor” mechanism (ACM), for fast and reliabletransmission for urgent information in WSNs. In ACM, aself-organizing corridor consists of nodes surrounding thepath from the source node to the base station and nodes in
the path. The former refrains from transmitting non-urgent
information to avoid collisions with emergency packets, andthe latter suspends their sleep schedule and keeps awake
to avoid delay caused by sleeping. We conducted simulation experiments with a tree-based and broadcast-based
network. It was shown that ACM improved the delivery ratio and the delay of emergency packets.

1. Introduction
As the development of micro-electromechanical systems(MEMS) technology advances, wireless sensor networks(WSNs) have become popular in the field of informationand communication technology and attracted much attention of many researchers [2]. A WSN consists of a numberof sensor nodes, each of which is equipped with one or moresensors, an analog-digital converter, a radio transceiver, acentral processing unit with limited computational capability, a small 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 an autonomous and distributed manner. Sensor information obtained at nodes is transmitted through a network to a certain
node called a base station (BS) or sink for further processing. WSNs have a wide variety of applications such as agricultural, health, environmental, and industrial purposes.
Among a number of applications, a WSN used as a so-cial 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, whichapparently should not be handled equally. The urgent information, such that for security, disaster, environmental,
and vital conditions, has to be carried through a WSN withhigher reliability and lower delay than other non-urgent information such that for regular monitoring for living andworking space control. It means that a WSN must be capable of differentiating and prioritizing packets depending on
their urgency and importance according to requests from theapplication layer. In addition, it must provide a mechanism
where packets with higher priority are transmitted preferentially.
In this paper, we propose an “assured corridor” mechanism, ACM in short, for fast and reliable transmission ofurgent information in a WSN. In ACM, an assured corridor
is eventually established in an autonomous and distributedmanner, where emergency packets are forwarded preferentially. Transmission of normal packets is suppressed along
the corridor and emergency packets are forwarded by keepawake nodes in the corridor. By suppressing transmission of
normal packets, congestion among emergency and normal
packets is avoided. Therefore, it contributes to reducing thelatency caused by backoff and retransmissions and the loss
probability caused by collisions and retransmission timeout. By keeping nodes awake on the path from the source
node of emergency packets to the BS, the delay required to
wait for a next-hop node to wake up from the sleep mode is
avoided.
There has been some research works to realise reliabletransmission in WSNs, [3, 4, 9] for example. In [3, 4], eachnode relays received packets stochastically according to theforwarding probability, and the reliability of transmission is
improved through multipath or retransmission mechanisms.
The authors assume a collision-free TDMA MAC layer protocol and packet losses caused by random channel error.
However, with a contention-based MAC protocol, which is
widely used for WSNs for the ease of deployment and simplicity of a protocol, collision is one of the most dominant

(a)

(b)

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

causes of packet losses. Collision drastically increases thelatency of the transmission of packets due to backoff and retransmissions, especially when a flooding-based protocol isused. Besides, we focus on achieving as high delivery ratio
as possible for urgent sensor information, instead of achieving desired reliability for all types of information. In [9],the emission rate of source nodes are dynamically updated
according to the feedback from the BS to realize desired reliability. This is a centralized control where the BS makes
every decision, but we emphasize a distributed approach inorder to obtain high scalability of a WSN.
As for delay of sensor information, some protocols havebeen proposed [1, 5, 8]. These protocols are designed tofind the best path in terms of delay. On the contrary, we
aim to design a transmission mechanism which minimize
the delay of critical information independent of any routing
protocol.
Priority-based mechanisms such as a priority queueing
are also helpful to offer a preferential forwarding service tourgent information. ACM works above the network layer
and by combining such lower layer mechanism we can expect more reliable and faster transmission of urgent information.
The rest of the paper is organized as follows. Detailed
description of ACM is discussed in Section 2. Section 3
gives the details of simulation experiments. Then, the results and discussions are presented in Section 4. Finally weconclude the paper in Section 5.

2. Description of the proposed mechanism
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 multihop
routing protocol. For example, a TDMA protocol is also
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 net-

work layer, a multihop scheme with limitation on the radiotransmission energy is usually preferred to avoid contention
among wireless communication and prolong the lifetime ofbatteries.
Examples of an assured corridor are illustrated in Fig. 1
for a tree-based sensor network and a broadcasting-based
sensor network. In the figure, a star corresponds to a node
which detects an emergency and becomes a source node of
emergency packets. Grey circles correspond to nodes on apath from the source node to the BS and they keep awake
during the emergency. Nodes in ranges of radio signals of
those grey nodes are denoted as filled circles, which suppress emission of normal packets.
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 detects an emergency event, its state is changed to the EMG SEND stateand 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 orhears the emergency packet, its state moves to either of the
EMG FORWARD or SUPPRESSED states depending on its
location. If the node 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 andsuppresses the transmission of normal packets in order toavoid 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 firstemergency packet arrives at the BS.
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 the nodes in the WSN are
not aware of the emergency and they remain in their normal
operation.
These mechanisms imply that the reliability and latencyof transmission of emergency packets are improved at sacrifice of the lower delivery ratio and larger transmission delayof non-urgent information and the depletion of a battery of
awake nodes. Although low energy consumption is one of
the most important requirements in WSNs, we should notsacrifice the reliability and latency of transmission of emergency packets for the energy efficiency. Therefore, we donode in this state should suppress transmitting some or
all of normal packets.

Figure 2. State transitions.
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 of a WSN depends on energy
efficiency not in urgent conditions but in normal operation.
If allowed we can introduce a sleep schedule to nodes in acorridor, but it is left as one of our 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, receive and transmit a data
packet, and go back to sleep at regular intervals oftnorm .
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 sourcenode.
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 inforwarding a packet toward the BS. For example, if
the WSN adopts tree topology whose root is the BS, a
preceding node is a child node. On receiving the emergency packet, a node first suspends its sleep schedule.
Then, it sends the received emergency packet to thedesignated next-hop node on the path to the BS, after waiting for the activation of the next-hop node if
it is in the sleep mode. The next-hop node also keeps
awake once it receives the emergency packet. Therefore, following emergency packets sent after the firstemergency 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

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 tothe source node of the emergency packets. On receiving theacknowledgement, the EMG SEND node returns back to the
NORMAL state. On the other hand, the EMG FORWARD
and SUPPRESSED are “soft states.” Entering these states,
a node starts a timer. 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 thetimer is the interval of data gathering in the NORMAL state,i.e., tnorm , since emergency packets are sent more frequent
than normal packets to inform a control center of up-to-dateemergency condition.
Note that an assured corridor is established while the firstemergency 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 the sleep schedule of the data gathering scheme
used for normal operation. After a 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.
Although ACM reduces packet losses due to collisions
between normal packets and emergency packets, a hop-byhop acknowledgment and retransmission scheme can be applied to recover losses due to collisions among emergency
packets in a corridor. The acknowledgment is done by overhearing a packet sent by a next-hop node. If the overheard
packet does not contain the information that the node sent,
the packet is considered lost. Retransmission is repeateduntil it is acknowledged. However, a packet waiting for retransmission is discarded when the next emergency packet
originating at the same source node arrives at the node.

3. Details of simulation experiments
We implemented the proposed mechanism for the ns-2
network simulator package [10] and conducted extensive
simulation experiments. In all of the simulation experiments, 200 sensor nodes are uniformly and randomly distributed in a 20 m × 20 m two-dimensional region with a
BS at its center. IEEE 802.15.4 [6] non-beacon mode isused as the MAC protocol [12] and the transmission rangeof radio signals is set to 2.5 m.
We employ a general broadcast-based or tree-based routing protocol for the underlying network layer. In both routing protocols, we assume that each node knows its own hop
distance from the BS. In the broadcast-based routing, apacket contains the sensor data and the hop distance of the

20

BS
EMG_SEND
EMG_FORWARD
SUPPRESSEDNORMAL

10

Figure 3. The transmission sequences in normal operation.

sender. A node forwards a packet, if the hop distance ofthe sender is larger than that of itself. Otherwise, it simply drops the packet. Since a packet can be received andforwarded by multiple nodes in the sender’s vicinity, thebroadcast-based routing can be categorized into multipath
routing protocols without explicit path establishment. An
example of such a routing protocol is the synchronizationbased data gathering scheme [11]. In the tree-based routing, every node chooses a next-hop node among neighbors
which are closer to the BS by one hop, based on the received signal strength. This is equivalent to choosing thenearest one-hop-closer node in the simulation experiments.
The schedule of transmission is shown in Fig. 3 for normal operation. In the NORMAL state, a node sends packets at regular intervals of tnorm . The instant that a node at
n hops from the BS sends a packet is earlier than the instant that an (n − 1)-hop node sends a packet by δtnorm . It
means that it takes (n − 1)δtnorm for sensor data of n-hop
node to reach the BS. Here, δ is a coefficient which governs
the interval of packet emission between nodes of adjacenthops. Based on the sleep schedule, an n-hop node wakes up
at δtnorm before the timing of its packet emission to receive
packets from preceding (n+1)-hop nodes. It aggregates thereceived data with its own sensor data and then it sends thepacket at the time when (n−1)-hop nodes wake up. Once itoverhears the packet of an (n − 1)-hop node at δtnorm after
the emission, it goes to sleep.
In our simulation experiments, tnorm and δ are set at
10 seconds and 0.1, respectively. Thus the transmission delay is one second per hop in normal situation. In addition,in both normal and emergency situation, a random backoffof 10 ms at maximum is applied in the network layer before
sending a packet to avoid collision.
The size of sensor data is 6 bytes. Since we do not assume data fusion, N sensor data amount to 6N bytes. The
maximum size of the payload in a packet is limited to 78
bytes due to the limitation of IEEE 802.15.4. Sensor data
exceeding this limitation are discarded at each node.
Each simulation experiment lasts for 500 seconds including 300 seconds for initialization without any emergency.
After that, each of randomly chosen 1, 2, 4 or 8 nodes moves
to the EMG SEND state at randomly chosen time in follow-

0

0

10

20

Figure 4. An “assured corridor” in the
broadcast-based network.

ing 10 seconds. They begin sending emergency packets at
the rate of 2 packets/s, i.e., temg = 0.5 seconds. Each ofthem goes back to the NORMAL state at 180 seconds after
it moves to the EMG SEND state. The same experiment isrepeated for 100 times with different node layouts. Fig. 4
shows a snapshot in one of the simulation experiments with
one EMG SEND node.
For comparison purposes, we considered three variantsof the mechanism. One is called as KA (keep awake), in
which only the 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 emissionof normal packets, but lost packets are not recovered by retransmission. The other is called KA+SP+RT, in which retransmission is applied in addition.

4. Results and discussions
In this section, some results of simulation experiments
are presented. Since the performance without ACM fully
depends on underlying routing and data gathering schemes,
we do not present results without ACM. Similarly, emergency packets sent before an assured corridor is establishedare not taken into the results shown in this paper.

4.1. Loss rate of emergency packets
First, we consider the case of one EMG SEND node. The
loss rate of emergency packets against simulation time isshown in Fig. 5. The loss rate is defined as the ratio of thenumber of emergency packets not received by the BS to thenumber of those sent from the source node after a corridor
is completely established.
In the tree-based network without suppression (KA in

0.05

KA
KA+SP
KA+SP+RT

0.04

0.04

0.03

0.03
loss rate

loss rate

0.05

0.02

0.01

KA
KA+SP
KA+SP+RT

0.02

0.01

0

0

300

320

340

360

380

400

420

440

460

480

500

300

320

340

360

380

time (s)

400

420

440

460

480

500

time (s)

(a)

(b)

0.045

0.045

0.04

0.04

0.035

0.035

0.03

0.03

0.025

0.025

delay (s)delay (s)Figure 5. Loss rate of emergency packets in (a) a tree-based network and (b) a broadcast-based
network with one EMG SEND node.

0.02
0.015

0.015

0.01

0.01
KA
KA+SP
KA+SP+RT

0.005
0
300

0.02

320

340

360

380

400
420
time (s)

440

460

KA
KA+SP
KA+SP+RT

0.005
480

500

(a)

0
300

320

340

360

380

400
420
time (s)

440

460

480

500

(b)

Figure 6. Delay of emergency packets in (a) a tree-based network and (b) a broadcast-based networkwith one EMG SEND node.

Fig. 5(a)), about 2 % of emergency packets are lost due
to collision with normal packets. By keeping surrounding nodes quiet (KA+SP), the loss rate becomes almost
zero. In the broadcast-base network (Fig. 5(b)), the loss
rate with KA is larger than in the tree-based network, since
there occur additional collisions among emergency packets
traversing multiple paths. On the other hand, with KA+SP,
although the initial value is the same as that of KA, theloss rate drops gradually in about 20 seconds. The reason
why the loss rate does not become zero is that suppressing
transmission of normal packets can not prevent collisions
among emergency packets in a corridor. With retransmission, i.e., KA+SP+RT, no packet loss occurs. However, thetotal number of emergency packets that EMG SEND and
EMG FORWARD nodes send including retransmission islarger than that of KA+SP by 15 % and this increase leads
to additional energy expenditure.

4.2. Delay of emergency packets
As for the end-to-end delay (Fig. 6), which is defined as
the time taken for an emergency packet emitted by a source
node to arrive at the BS, we can observe that the delay of
KA+SP is a little smaller than that of KA. In KA+SP, sincesurrounding nodes are quiet, the number of backoff due tocontention in the MAC layer is smaller than in KA [7].
The delay of KA+SP+RT becomes larger than the others
for retransmission in the broadcast-based network as shownin Fig. 6(b). However, in the tree-based network (Fig. 6(a)),
such drawback is not observed. As we saw in Fig. 5(a), the

0.3

0.3

0.25

0.25
0.2

KA
KA+SP
KA+SP+RT

Loss rate

Loss rate

0.2
0.15

0.15

0.1

0.1

0.05

0.05

0

KA
KA+SP
KA+SP+RT

0
0

1

2

3

4

5

6

7

8

9

Number of emergency nodes

(a)

0

1

2

3

4

5

6

7

8

9

Number of emergency nodes(b)

Figure 7. Loss rate of emergency packets in (a) a tree-based network and (b) a broadcast-based
network with multiple EMG SEND nodes.

suppression is highly effective in the tree-based network,
which means that there is little need of retransmission. On
the contrary, in the broadcast-based network, there are collisions among emergency packets which incur retransmission. Retransmission is conducted in only 0.4 % cases ofpacket transmission in the tree-based network, while 11 %in the broadcast-based network.
Comparing Fig. 6(a) and Fig. 6(b), we can see that thedelay of KA and KA+SP is larger in the tree-based network than in the broadcast-based network. The reason
is that a random backoff is applied before packet emission in the network layer in the experiments. The BS in
the broadcast-based network receives multiple emergency
packets with the same sequence number which traverse different paths. Among them, the first packet which arrives
at the BS is taken into account in the delay. On the other
hand, in the tree-based network, there is only one path for
an EMG SEND node. Thus the delay in the tree-based network becomes larger than in the broadcast-based network.

4.3. Multiple emergency nodes
Next we consider cases where multiple nodes detect anemergency event and move to the EMG SEND state at thesame time. The loss rate of emergency packets are plottedagainst the number of EMG SEND nodes in Fig. 7. The
more the number of EMG SEND nodes is, the more frequently collisions occur. More than 25 % of emergency
packets are lost in the cases of eight EMG SEND nodes
without retransmission. This is because that emergency
packets originated from different source nodes collide witheach other in the same or merged assured corridor.
In comparing results of KA and KA+SP, the effect ofsuppression of normal packets in reduction of loss rate be-comes slightly smaller as the number of EMG SEND nodes
increases in both the tree-based and broadcast-based networks. With the help of retransmission, in KA+SP+RT, theloss rate is less than 0.3 % and 0.4 % with eight EMG SEND
nodes in the tree-based and the broadcast-based networks
respectively.
Fig. 8 shows the delay of emergency packets against the
number of EMG SEND nodes. For KA and KA+SP, results are close since most of nodes are in EMG SEND or
EMG FORWARD states and suppression of normal packets
is not effective. In addition, the delay slightly decreases as
the number of EMG SEND nodes increases in both the treebased and broadcast-based networks. The reason for thiscan be explained as follows. In calculating the delay, wetake into account only emergency packets that successfully
arrive at the BS. Therefore, there is a bias in favor of emergency packets emitted by EMG SEND nodes closer to the
BS than those of distant EMG SEND nodes, for their lessloss rate. For supporting this, the per-hop delay of KA andKA+SP, which is not shown because of space limitations,slightly increases as the number of EMG SEND nodes becomes larger due to contention in the MAC layer among
more EMG SEND and EMG FORWARD nodes. On thecontrary, the delay of KA+SP+RT increases with the number of EMG SEND nodes reflecting more frequent retransmission due to collisions among emergency packets withina corridor.
In concluding the above results, we can say that suppression of normal packets contributes reduction of the lossof emergency packets. With retransmission, most of emergency packets from a source node can reach the BS at theprobability of higher than about 99.6 %. However, it is accomplished at the sacrifice of the increased delay, which is
proportional to the number of EMG SEND nodes, for con-

0.09

0.08

0.08

0.07

0.07

0.06

0.06

0.05

0.05

Delay (s)Delay (s)

0.09

0.04
0.03

0.03

0.02

0.02
KA
KA+SP
KA+SP+RT

0.01
0

0.04

0

1

2

3

4

5

6

7

KA
KA+SP
KA+SP+RT

0.01
8

9

Number of emergency nodes

(a)

0

0

1

2

3

4

5

6

7

8

9

Number of emergency nodes(b)

Figure 8. Delay of emergency packets in (a) a tree-based network and (b) a broadcast-based networkwith multiple EMG SEND nodes.

transmission of urgent information. Although the forwarding probability is dynamically updated in accordance withthe reception ratio at the BS in [3], we used fixed forwarding probability in comparisons. We conducted a numberof experiments to find forwarding probabilities with which
the stochastic forwarding scheme shows the same level ofreliability as ACM. The suppression of normal packets is
not applied in experiments with the stochastic forwarding
scheme.

Figure 9. The total number of packet emission and the loss rate.

Fig. 9 shows results of the loss rate of emergency
packets and the transmission overhead. The transmission overhead is defined as the ratio of the total number of emergency packets emitted at EMG SEND and
EMG FORWARD nodes to the number of emergency packets successfully delivered to the BS.

tention among emergency packets.

4.4. ACM and stochastic forwarding
Finally, we show results of comparison with a stochastic forwarding scheme in the broadcast-based network, inwhich a node decides whether it forwards or drops a received packet in a probabilistic way. One example of such
scheme is proposed in [3]. In the stochastic forwarding
scheme, if the forwarding probability is equal to one, a node
forwards all packets that it receives. The forwarding probability can be greater than one, which means that a nodestochastically retransmits a packet in addition to its firstemission in order to improve reliability of the transmission.
In our simulation experiments, only EMG SEND and
EMG FORWARD nodes adopt the scheme for reliable

The stochastic forwarding scheme with the forwarding
probability of one, denoted as SF(1.0) in the figure, is equivalent to KA. By increasing the forwarding probability to
1.35, SF(1.35) can attain the same level of reliable transmission of emergency packets as KA+SP. However, the total
number of emergency packets needed is larger by 33 % thanthat of KA+SP. To avoid loss of emergency packets, the forwarding probability for the stochastic forwarding scheme
must be set at 2.0. As a result, the transmission overhead of
the stochastic forwarding scheme becomes larger than thatof ACM by 84 %, whereas KA+SP+RT also introduces additional overhead in retransmission. Although not shown infigure, the delay of emergency packets in SF(1.35) is 10 %larger than in KA+SP, and 7 % larger in SF(2.0) than in
KA+SP+RT.

5. Conclusion
In this paper, we proposed ACM, a fast and reliabletransmission mechanism for urgent information in WSNs.
An assured corridor is eventually established from a source
node to the BS. In the corridor, all nodes keep awake for fasttransmission of emergency packets. Beside the corridor, allnodes refrain from transmission of normal packets to avoid
disturbing transmission of emergency packets in the corridor. The other nodes stay in normal operation. Simulation
experiments showed that the loss rate of emergency packets
was successfully decreased, and the latency of emergency
packets was improved for both a tree-based and broadcastbased networks.
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 andmultiple corridors. We combine several techniques, suchas prioritization among emergency packets and rate control with a backpressure mechanism, with ACM. A WSN
should function properly under this kind of situation and webelieve that this is one of network layer functions needed fora WSN as a social infrastructure.

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 andTechnology, Japan.

References[1] K. Akkaya and M. Younis. Energy and QoS aware routing inwireless sensor networks. Cluster Computing, 8(2–3):179–
188, July 2005.
[2] I. F. Akyildiz, W. Su, Y. Sankarasubramaniam, andE. Cayirci. Wireless sensor networks: a survey. Computer
Networks, 38(4):393–422, Mar. 2002.
[3] S. Bhatnagar, B. Deb, and B. Nath. Service differentiationin sensor networks. In Proceedings of the 4th International
Symposium on Wireless Personal Multimedia Communications (WPMC 2001), Aalborg, Denmark, Sept. 2001.
[4] B. Deb, S. Bhatnagar, and B. Nath. ReInForM: Reliable
information forwarding using multiple paths in sensor networks. In Proceedings of 28th Annual IEEE conference
on Local Computer Networks (LCN 2003), pages 406–415,
Bonn, Germany, Oct. 2003.
[5] T. He, J. A. Stankovic, C. Lu, and T. Abdelzaher. SPEED: astateless protocol for real-time communication in sensor networks. In Proceedings of 23rd International Conference onDistributed Computing Systems (ICDCS 2003), pages 46–
55, Providence, Rhode Island, USA, May 2003.
[6] IEEE 802.15.4. Wireless Medium Access Control and Physical Layer Specifications for Low-Rate Wireless Personal
Area Networks, 2003.
[7] K. Leibnitz, N. Wakamiya, and M. Murata. Modeling ofIEEE 802.15.4 in a cluster of synchronized sensor nodes. In
Proceedings of the 19th International Teletraffic Congress(ICT-19), pages 1345–1354, Beijing, China, Aug. 2005.
[8] A. Mahapatra, K. Anand, and D. P. Agrawal. QoS and energy aware routing for real-time traffic in wireless sensor
networks. Computer Communications, 29(4):437–445, Feb.
2006.
[9] Y. Sankarasubramaniam, B. Akan, and I. F. Akyilidiz.
ESRT: Event-to-sink reliable transport in wireless sensor
networks. In Proceedings of the 4th ACM International symposium on Mobile ad hoc networking and computing (MobiHoc 2003), pages 177–188, Annapolis, Maryland, USA,
June 2003.
[10] The network simulator – ns-2, http://www.isi.edu/
nsnam/ns/.
[11] N. Wakamiya and M. Murata. Synchronization-based data
gathering scheme for sensor networks. IEICE Transactions
on Communications, E88-B(3):873–881, Mar. 2005.
[12] J. Zheng and M. J. Lee. Ns2 simulator for 802.15.4, http://www-ee.ccny.cuny.edu/zheng/pub/.