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               A New TDMA Based Sensor Network for
                  Military Monitoring (MIL-MON)
                                                  İlker Bekmezci, Fatih Alagöz


                                                                    energy consumption is to turn the radio off, when it is not used
    Abstract—Wireless sensor networks (WSN) is a new network         [3]. Fixed allocation methods, TDMA or FDMA is extremely
family that enables to create smart environments. Although WSN       suitable for this kind of network. There are some sensor
has many application areas, military applications of WSN are         networks based on TDMA such as LEACH [4], SMACS [5],
very interesting. In this paper, a new TDMA based sensor
network for military monitoring (MIL-MON) is proposed. MIL-
                                                                     two-tiered architecture [6], PACT [7].
MON is developed to operate in large areas for acceptable               LEACH is a self-organizing, adaptive clustering protocol
lifetime periods. In order to realize MIL-MON, distributed time      that uses randomization to distribute the energy load evenly
scheduling mechanism, topology construction algorithm and            among the sensors in the network [4]. Although LEACH can
rescheduling algorithm is proposed. Simulation results have          reduce power consumption, there is a problem with the
shown that MIL-MON can operate in large areas, in acceptable         assumptions of LEACH. LEACH assumes that each node can
lifetime and delay constraints.
                                                                     hear each other. So LEACH is not suitable for using in large
    Index Terms—sensor networks, TDMA, military monitoring.          areas. SMACS is another sensor network that uses TDMA. In
                                                                     fact SMACS uses TDMA in addition to FDMA. After a series
                        I. INTRODUCTION                              of handshaking signals, neighbor nodes can agree on a
                                                                     frequency and time pair to construct a link. SMACS produces
   n recent years, with the pace of the developing micro-
I  electro-mechanical systems (MEMS) technology, it has
                                                                     a scalable and reliable flat network. However, SMACS needs
                                                                     FDMA as well as TDMA, but sensor nodes are so tiny and
been possible to integrate battery-operated sensor, computing
                                                                     limited that current sensor nodes cannot meet the requirements
power and low power wireless communication components
                                                                     of SMACS. PACT uses Unifying Slot Assignment Protocol
into one small size device. A sensor node collects data from
                                                                     (USAP) which is a TDMA scheduling scheme for on demand
the environment continuously. The data collected by only one
                                                                     ad hoc networks. USAP is adopted for sensor networks in
node are nearly useless, but the collaborative work of
                                                                     PACT. However, USAP is originally developed for ad hoc
thousands of these nodes can be used to collect process and
                                                                     networks and PACT is not fully successful in power
send the data about the environment. The network that is
                                                                     consumption for sensor networks. Another TDMA based
composed of these wireless sensors is called wireless sensor
                                                                     sensor network proposal is two-tiered structural health
network (WSN). The potential applications of wireless sensor
                                                                     monitoring wireless sensor network architecture. According to
networks are highly varied. Environmental monitoring,
                                                                     this structure, there are some fixed cluster heads and sensor
condition based maintenance, smart spaces, military, precision
                                                                     nodes. Sensor nodes are clustered around cluster heads. This
agriculture, transportation, inventory tracking are just some of
                                                                     network is designed to use for monitoring buildings. It can not
the sample application areas [1]. One of the most common
                                                                     be used in large areas.
application areas of sensor networks is military. In this paper,
                                                                        In this paper, a new TDMA based sensor network system,
a new TDMA based military monitoring sensor network
                                                                     which can be used for military monitoring systems in a
system (MIL-MON) is proposed.
                                                                     relatively large area, is presented. The coverage of the network
   Because of the unattended structure of the sensor nodes, the
                                                                     can be in the order of kilometer squares. In order to realize
scarcest resource in sensor networks is power. Power
                                                                     MIL-MON, time synchronization, time slot assignment,
consumption can be divided into three domains, as sensing,
                                                                     topology construction algorithms are developed. In order to
communication, and data processing domains and dominant
                                                                     enhance the network in terms of delay and power usage,
factor in energy consumption for sensor nodes is
                                                                     rescheduling and data indicator slot mechanisms are proposed.
communication [2].. Not only transmission but also receiving
                                                                        The organization of the paper is as follows. In Section 2,
is the main cause of energy waste. The easiest way to reduce
                                                                     preliminary works on TDMA based sensor networks is
                                                                     presented. In Section 3, the basic mechanisms and
   Ilker Bekmezci is with Bogazici University Computer Engineering   enhancements for TDMA based sensor network will be
Department. He is working in Turkish Air Force Academy (e-mail:      proposed. In Section 4, the performance results of newly
i.bekmezci@hho.edu.tr).
   Fatih ALAGOZ is with Bogazici University Computer Engineering     proposed algorithms will be outlined. Section 5 states the
Department. (e-mail: fatih.alagoz@boun.edu.tr)                       conclusion and future work.
                                                                                                                                  2

                 II. PRELIMINARY WORK                              from the sensor nodes and relay its data to central sink.
   TDMA is a technique to share the medium among multiple          Because of these duties, cluster heads have to spend more
users. Time frames are divided into time slots and each slot is    power than regular nodes. The solution of this problem is to
assigned to a user. In this way, when a user wants to access to    change cluster heads periodically. Unfortunately, the solution
the medium, it uses the medium in its own slot. TDMA has           of quick power drain of cluster heads is a cause for spending
many advantages for sensor networks. With the help of              power. Periodic cluster head selection and reorganization of
TDMA, node can know exactly when it should use its radio           network for the new cluster heads may consume considerable
circuit and sensor node can turn off its radio circuit to save     energy.
more power. Node uses transmitter or receiver only when it is         Two tiered architecture for structural health monitoring is
needed. This is why TDMA is very attractive for sensor             another sensor network model that uses TDMA technique.
network applications. There are some sensor network systems        This sensor network architecture is designed for monitoring
based on TDMA, such as LEACH, SMACS, PACT, two-tiered              structural health of buildings. It is also based on clustered
building monitoring system.                                        approach. Sensor nodes are clustered and cluster heads are
   LEACH is a self-organizing, adaptive clustering protocol        special nodes called local site masters (LSM). LSMs are
that uses randomization to distribute the energy load evenly       deployed one by one manually and the power source of LSMs
among the sensors in the network [4]. In LEACH, the nodes          is the power source of the building. So, there is no power
organize themselves into local clusters, around one node acting    limitation on LSM. LSMs collect the data from their sensor
as the local base station or cluster-head. A cluster head is       nodes. LSMs construct a higher level of network and send the
responsible for collecting data from its sensor nodes and          collected data to a center. According to analysis of two tiered
sending the data to a central sink. In this case, cluster heads    sensor network, the expected lifetime of network is about 18
use more energy than the regular nodes. In order to prevent        months [6]. It is very impressive and acceptable. However,
quick power drain of cluster heads, cluster heads are changed      because of LSM limitations, the application of this network in
periodically. The complete algorithm of LEACH is as follows:       open areas is extremely difficult. There must be lots of LSM
                                                                   units and these LSM units should be deployed manually.
  While (there is at least one node with power) do                 Moreover, LSMs must have unlimited source power. Because
   Cluster_Head_Selection_and_Advertisement                        of these limitations, it is not suitable to use this system as
   Cluster_Construction                                            military monitoring system.
   Create_and_Send_Time_Scheduling                                    SMACS is a TDMA based sensor network that produces flat
   While (data transmission period is not exceeded)                topology. In fact, SMACS uses TDMA and FDMA at the same
    Data_Transmission                                              time. After deployment of sensor nodes, they wake up at
   Wend                                                            random times and they start to listen to a certain frequency.
  Wend                                                             This frequency can be called as establishment frequency.
                                                                   After listening to the establishment frequency for a random
   Comparison study shows that LEACH achieves between 7x           period of time, if it can not receive any invitation signal from
and 8x reduction in energy compared with direct                    other nodes, it transmits a short invitation packet that contains
communication and between 4x and 8x reduction in energy            some basic data about the node. This packet is called TYPE1
compared with MTE routing. In addition to reducing energy          message. If a node can receive a TYPE1 message from other
dissipation, LEACH successfully distributes energy-usage           nodes and if it wants to establish a link, it responds TYPE1
among the nodes in the network such that the nodes die             that it will be an invitee. This response is TYPE2 message.
randomly and at essentially in the same rate [8].                  There may be more than one TYPE2 messages for a TYPE1
   Although LEACH achieves power consumption decrease              message. At that particular time, the node that transmits
dramatically, the assumption of LEACH system is not realistic.     TYPE1 message has to decide which node it should choose.
According to LEACH, each node can receive signals from all         After its decision, it sends a response to TYPE2 that includes
the other nodes. However, it is not possible in every              data about the node that is selected. This is TYPE3 message.
environment, especially in larger areas. If sensor network is      At the end, TYPE4 is sent by the invitee. In TYPE3 and
designed to operate for relatively large areas as in MIL-MON,      TYPE4 phases, nodes agree on a certain schedule to
every node cannot receive all signals.                             communicate. This schedule is a time and frequency pair. In
   Another unrealistic assumption of LEACH is about the            this case, the collision probability of schedules can be
existence of data. LEACH assumes that every node has data to       minimized.
send every time. However, in most of the time, there is no data       As times goes on, there will be some subnets in the wireless
to send in sensor network. For example, MIL-MON is                 sensor networks. These subnets unite with each other, when
designed for intrusion detection and the existing of an intruder   new links are established. At the end, all sensor nodes
is not likely to occur most of the time.                           construct a connected network.
   Another disadvantage of LEACH is the need for changing             SMACS is an infrastructure building protocol that forms a
cluster heads. Cluster heads are responsible for collecting data   flat topology for sensor networks. It is a distributed protocol
                                                                                                                                    3

which enables a collection of nodes to discover their neighbors     protocols that are developed for a WSN may not be optimal
and establish transmission/reception schedules, without any         for another WSN [3]. Sensor network systems should be
need for master node [5]. Although flat topology has some           application specific [4]. A sample application scenario can
advantages, it has also some difficulties. Flat topology requires   help to understand the assumptions and mechanisms of
a separate network layer, while cluster based approach has          MIL-MON.
implicit network layer in itself. However, the most serious            In a typical military monitoring scenario, large numbers of
handicap of SMACS is that there is no sensor node that can          unattended, limited powered sensor nodes are deployed near
support the requirements of SMACS. According to SMACS,              the borders of a base. Sensor nodes organize themselves, so
node must support frequency multiplexer and there is no such        that, when sensor nodes detect an intruder, they send their data
a node up to now. A sensor network should be applicable on          to a sink. In this way, soldiers can defend against the intruders.
the existing node models.                                           In most of the cases, sink is a PDA or a laptop.
   PACT (Power Aware Clustered TDMA) is a TDMA based
sensor network that uses passive clustering. Its TDMA                  Here are the assumptions of the sensor network system:
scheduling scheme is based on Unifying Slot Assignment              1) Sensor nodes will be immobile. Mobile cases can be
Protocol (USAP) which is a time scheduling algorithm for on              investigated for further analysis.
demand ad hoc networks. PACT can be considered as a sensor          2) Power consumption model of sensor node is the same as
network version of USAP in terms of time scheduling. This is             described in [9]. This model is one of the mostly used
why it is not fully optimized for power consumption. In PACT,            models in sensor network simulation analysis researches.
every node must listen to all the control slots of a time frame.    3) Radio channel is symmetric.
The number of control slot must be equal to time slot number        4) Sink node’s power source theoretically infinite.
in a time frame and it means every node must spend                     The assumptions, which have been mentioned above, are
considerable amount of power in every time frame. Another           valid for most of the sensor networks. There are some
important question about PACT is time synchronization. Every        additional assumptions specifically for MIL-MON.
TDMA based system must synchronize time and PACT does                  Sink node has high range transmitter as well as low range
not propose a system for time synchronization and it does not       transmitter. In this way, sink can use its low range transmitter
take into account time synchronization overhead.                    to communicate with its neighbor nodes and it can send
                                                                    broadcasts for all nodes. All the sensor nodes can receive
                                                                    broadcasts of the sink.
            III. MIL-MON SYSTEM MECHANISMS                             Sink node has GPS. In this way, clock drift of sink is near
                                                                    zero [10].
  A. Overview
                                                                      C. Basic Mechanisms
   MIL-MON is proposed to monitor a relatively large area
against intruders and send the data about intruders to sink as         There are some basic mechanisms for operating MIL-MON
soon as possible. The main design considerations of                 properly. These are time synchronization, distributed time
MIL-MON are to be able to operate in large areas, to minimize       scheduling and topology construction mechanisms.
power consumption, to reduce delay.                                     1) Time Synchronization
   In order to satisfy design considerations, MIL-MON is               According to assumption of this sensor network, every node
designed as a TDMA based multihop sensor network. The sink          in the system can receive the signals of the sink. Sink transmits
of MIL-MON is supposed to have a high range transmitter so          a broadcast signal to sensor nodes at the beginning of each
that every node can receive its broadcast signal.                   time frame. These broadcast signals synchronize the network.
   MIL-MON includes global time synchronization, distributed        This time synchronization scheme is very similar to LEACH,
time slot assignment, topology construction mechanisms. In          however there are a few differences based on the differences of
addition to these basic mechanisms, there are data slot             system assumptions of MIL-MON and LEACH. The number
indication mechanism to save more power and rescheduling            of cluster in LEACH is a piori in the system. Each cluster head
mechanism to reduce delay. Before introducing the                   synchronizes its own cluster. MIL-MON has only one cluster
mechanisms of MIL-MON, a sample application and basic               and the head of cluster is the sink. Only the sink synchronizes
assumptions of MIL-MON is presented.                                the whole sensor nodes with its broadcast signal.
                                                                        2) Distributed Time Scheduling Mechanism (DTSM)
  B. Sample Application and Basic Assumptions                          Almost all sensor network architectures that use TDMA
   The potential applications of wireless sensor networks are       produce its time schedule centrally. Cluster head collects the
highly varied. Environmental monitoring, condition based            data about its sensor nodes and produces the time schedule of
maintenance, smart spaces, military, precision agriculture,         its cluster. Time schedule is sent to nodes by the cluster head.
transportation, factory instrumentation, inventory tracking are     The main assumption of this system is that when cluster head
just some of the sample application areas. Behaviors of one         transmits a signal, sensor nodes can receive or vice versa.
specific sensor node can be totally different from the others          However, in MIL-MON, although the signal of the sink can
even under the same conditions. In this case, it is clear that      be received by all sensor nodes, the sink can not receive the
                                                                                                                                                           4

signals of all sensor nodes. In this case, sensor nodes can not        its predecessor. The simplest choice is the closest predecessor
send their join request signal. If distributed time scheduling         candidate. In the next time frame, it sends its own
algorithm is used, there is no need to send a signal from sensor       advertisement signal. If it receives hop number as h from its
node to the sink directly.                                             predecessor, it advertises its own hop number as h+1. In the
   In centralized time scheduling algorithm, cluster head              same signal, it also sends the node number of its predecessor.
collects all the requests from sensor nodes in a certain               This signal is received by the predecessor. If the predecessor
protocol. In most of the systems, this protocol is contention          receives a hop advertisement signal with its node number, it
based. If the number of sinks is limited and number of sensor          understands that the owner of the signal has become its child.
nodes for each sink is very high, the traffic from sensor nodes        In the third time frame, it listens to its all neighbors to learn
to sink will be heavily loaded. Contention based protocols can         which nodes became its children. Because in the third time
not achieve high efficiency under heavy traffic. This can be a         frame, the children of the sensor nodes advertise their hop
serious problem for power sensitive systems. However, in               number and predecessor node number.
distributed time scheduling algorithms, there is no need for              Figure 1 shows a sample run of the distributed topology
communication between nodes and the sink directly. This                construction algorithm. Suppose that A has caught an
leads to power saving.                                                 advertisement from X with hop number h. It advertises its own
   According to DTSM, after sensor nodes are deployed, every           advertisement signal with the parameters X and h+1. B catches
node selects a random time slot as its own slot. It listens to all     this signal and other advertisement signals, if any. If A is the
time slots in the first time frame and only in its own slot, it        closest node among the nodes that have sent advertisement
transmits a special signal. If it receives a jammed signal, it         signal, B sends its advertisement signal with parameters A and
means there is a collision at that particular slot. The node           h+2.
collects all the collision slots. In the next time frame, it
transmits a signal at the collision slots. In the same time frame,      A receives adv(Y,h) from X
it listens to its own slot. If it receives a signal, it means it has                                       A sends adv(X,h+1)
the same slot with another node’s slot so that their signals are                                                                B receives adv(X,h+1)

jammed. In this case, it sleeps. If it does not receive any signal      A receives adv(A,h+2) and
                                                                        understands that B is its child.                        B sends adv(A,h+2) and
at its own slot, it can use that slot and it continues to operate.                                                              advertises that A is its
   This protocol is simple and consumes low power. However,                                                                     predecessor.

it does not result in a complete solution. Some nodes have to
sleep. Fortunately, most of the time, sensor nodes are deployed        Fig. 1. A sample run of topology construction algorithm.
densely and the non-existence of a small number of nodes can
be tolerated.                                                             If maximum hop number of the network is H, constructing
     3) Topology Construction                                          the topology takes H+1 time frame for the whole network. It is
   After distributed time scheduling, the second step for the          fast, simple and scalable. It consumes low power and it results
proposed sensor network is topology construction. Many                 in minimum hop paths, which is a good approximation for
TDMA based sensor network systems use star topology. All               minimum power consumption.
the nodes are directly connected to cluster head. However, in              4) Rescheduling
large areas, multi hop structure consumes less power than one             Although power consumption is generally the most
hop. There is a trade off between multihop and one hop                 important design issue in sensor networks, there are some
structures. If the distance is less than 30 meters, direct             other design considerations like delay. In some applications,
transmission is more efficient than multihop transmission.             delay between time of event and time of data arrival to the sink
However if it is longer than 30 meters multihop strategy can           is not crucial. However, if sensor network is designed for
save energy [11].                                                      military purposes, delay can be very important. In a military
   If sensor network is aimed to cover a large area, average           monitoring system, the existence of intruder should be
distance between sensor nodes and the sink is supposed to be           reported as soon as possible.
much longer than 30m. In this case, multi hop strategy is an              If sensor node is h hop away from sink, and one time frame
essential part of MIL-MON architecture.                                is t seconds, the worst delay is t*h and t*h delay can not be
   A distributed topology construction algorithm is proposed to        acceptable in some cases. Suppose that t is one second and
support multi hop strategy. Time scheduling algorithm is also          sensor node that has sensed the arrival of intruder is 7 hops
distributed. In this way, the only scalability constraint of the       away. If the average distance between neighbor nodes is 20 m.,
system is the transmission range of the sink.                          intruder is 140-150 m. away to the sink and the sink can know
   The basic structure of the algorithm lies on handshaking            the existence of intruder after 7 seconds which can be enough
signals. After getting a proper time slot, sensor nodes listen to      for the intruder to harm the soldier that holds the sink.
their neighbors to catch hop number advertisement signal. At              Reducing delay is possible by the help of assigning time
the beginning, only the sink sends hop number advertisement            slots carefully. The rule is that smaller hop numbered nodes
signal. Hop number of the sink is zero. If a sensor node can           should get higher slot numbers. For example, if a sensor node,
catch advertisement signal or signals, it chooses one of them as
                                                                                                                                                                                       5

say A is 3 hops away and its slot number is 320, slot number                                            Node A                             Neighbors of A
of another node with 2 hop number, say B, should be greater
than 320. In this case, node A can send its message in 320 th                     Gets an adv. that
                                                                                  includes hop number
slot and node B can receive at 320th slot. In the same time                       of sender.
                                                                                                             Calculates its sub time frame,
                                                                                                                                                        Receives new time slot
                                                                                                                                                        number and checks
frame, node B can send the data of node A to its predecessor.                                                selects a time slot randomly and           whether there is
                                                                                                             sends its time slot number to its          collision.
   In order to realize to reschedule, time frame is divided into                                             neighbors.
                                                                                                                                                        If there is collision, it
m sub time frames. If the whole time frame has n slots, a sub                                                If it gets a signal from                   sends a signal to A.
                                                                                                             neighbors, it sleeps.
time frame has n/m slots. The slot number assigned to a node                                                 If it does not get, it
                                                                                                                                                        If there is no collision, it
                                                                                                                                                        remains silent.
with hop number h, must be in (m-((h-1) mod m))th sub time                                                   changes its time slot and
                                                                                                             continues with topology
frame. According to this formula, the nodes with 1 hop number                                                construction.
will have a slot number from mth sub time frame. The nodes                       Fig. 3. Rescheduling mechanism.
with 2 hop number will have a slot number from m-1th sub
time frame. In this way, the slot number of consecutive hop                         An example helps to understand rescheduling clearer. Let us
numbered nodes will belong to consecutive sub time frames.                       assume that the nodes in Figure 4 are one hop away from its
Figure 2 shows the structure of this approach.                                   consecutives. In this particular network, time frame has 300
                                                                                 time slots. The first slot is reserved for time synchronization
                                    one time frame divided into                  broadcast of the sink. Let us assume that DTS_SN algorithm
                                    m sub time frames                            has been run and the assigned time slots are as in the Figure
                                                                                 4(a).
     Sub time frame numbers   mth     (m-1)th …….                 1st
               Hop numbers    1         2     …….                 m
                                                                                     Sink, (2)           A, (82)          B, (91)            C, (198)               D, (245)
   Fig. 2. Rescheduling structure. There are m sub time frames in one time
frame, and each sub time frame has n/m slots, if total number of slots in time
frame is n.
                                                                                                                                (a)
   In order to assign proper time slots, node must know its hop
distance to the sink. In this case, rescheduling algorithm can                           Sink, (2)         A, (250)        B, (142)          C, (45)               D, (245)

not be run before distributed time scheduling algorithm.
Rescheduling should be placed between DTSM and topology
                                                                                                                    (b)
construction.                                                                    Fig. 4. Example network and time slots (a) Before rescheduling. (b) After
   After running distributed time scheduling algorithm, non                      rescheduling.
sleeping nodes has valid time slot. Nodes listen to its
neighbors to receive an advertisement signal which includes                         According to rescheduling algorithm, the sink transmits a
the hop number and node number of the sender. When a node                        broadcast signal with hop number 0. A receives this and
receives a hop advertisement signal, rescheduling is started.                    calculates its sub time frame and selects a random time slot in
The advertisement signal includes the hop number of the                          that particular sub time frame. In this example, A must get a
sender. In this way, the receiver of the advertisement signal                    time slot between 300 and 201. Let us assume that A selects
can calculate its hop number. In addition to this, node can get                  250th time slot. It advertises its new time slot in 82nd time slot
the number of sub time frames m from the sink’s                                  (its current time slot. The sink and node B listens to 82 nd time
synchronization signal. The node can calculate the sub frame                     slot for new time slot of A. If the sink and the nodes with hop
number with the formula (m-((h-1) mod m)). It selects a                          number 2 listen to all its neighbors and collect the newly
random time slot in the (m-((h-1) mod m))th sub time frame and                   requested time slots. If they determine a collision for new time
sends a broadcast signal that includes the information about                     slots, they send a warning message in the slots that request the
the selected time slot. The node sends this information in its                   new time slots. In this example, neither the sink nor B receives
original time slot that was assigned with DTSM mechanism.                        a collision for the new time slot of A. Node A does not
All the neighbors of the node listen to its signal and check                     receives a warning signal at 82nd time slot. In this case, new
whether there is collision about its new slot number or not. If                  time slot number of A becomes 250. After rescheduling
there is a collision, the nodes that catch the problem send a                    algorithm, topology construction mechanism is run with the
signal in their own time slots. The node that tries to reschedule                new time slots. So, A sends an advertisement signal with hop
listens to its all neighbors in the next time frame. If it gets a                number 1. The same procedure is run for all nodes. An
signal from the others, it sleeps. If it does not get any signal                 example resulting network is presented in Figure 4(b).
related with its new time slot, it means there is no problem and                    The relay of an event from D to the sink takes 737 time slots
it starts to use its new slot and continues with topology                        for non-rescheduled network in Figure 4(a). However, it takes
construction procedure. Figure 3 shows the signal handshaking                    only 305 time slots for the rescheduled network in Figure 4(b).
required for rescheduling. The figure starts with the node that
has got a slot with DTSM mechanism.
                                                                                                                                                                                     6

                                     IV. PERFORMANCE RESULTS                                  number of nodes and slots. The increasing rate is higher for
   It is very difficult to analytically model the interactions of                             MIL-MON with 500 time slot. Sleeping node rate is
sensor nodes, even for the limited number of nodes. In order to                               acceptable for 1000 and 1500 time slots. However, sleeping
investigate performance results of MIL-MON, simulation                                        node rate is high for 500 time slots.
method is used. Although there are some network simulators                                       The same cases are investigated with non-rescheduled 1000
that can simulate wireless networks, there is no built-in sensor                              time slots, resheduled with 5 sub-slots, 10 sub-slots and 20
network module. In this paper, a new simulator has developed                                  sub-slots. The results are presented in Figure 6.
to simulate MIL-MON.                                                                             Figure 6 shows that rescheduling increases sleeping node
   Simulation parameters are listed in Table1. Energy                                         ratio linearly.
consumption model is the same as in [9]. Energy of a node is
assumed to be supplied with 15 mg. Ni-Cd battery which can                                                                   10
support 2 J [12].                                                                                                             9




                                                                                                       % of sleeping nodes
                                                                                                                              8
                                                  TABLE I                                                                     7                                             1000
                                           SIMULATION PARAMETERS                                                              6                                             1000-5
                                                                                                                              5
                               Parameter                        Default values                                                4                                             1000-10
                                                                                                                              3                                             1000-20
 Power needed for radio                              50 nJ
 electronics per bit                                                                                                          2
 Power for receiving per bit                         50 nJ                                                                    1
 Power for transmitting per bit                      50nJ + 10pJ*d*d (d is distance)                                          0
 Max. range of nodes                                 30 m.                                                                            2   2,5   3   3,5    4   4,5    5
 Power in one node                                   2J
                                                                                                                                          Number of nodes (x1000)
 Simulation area diameter                            1000 m.
 Position of the sink                                Center of the area.
 Time for one time slot                              1 ms.                                     Fig. 6. Sleeping node ratio for different number of nodes with rescheduling.
 Bit rate                                            1 Mbps.

                                                                                                B. Network Lifetime
   Performance results are discussed in three domains. These
are performance of DTSM, network lifetime and delay.                                             Network lifetime is one of the most common performance
                                                                                              metrics of sensor networks. However, network lifetime
  A. Performance of DTSM                                                                      definitions vary. In this paper, network lifetime is the time that
   DTSM is a distributed time scheduling mechanism to assign                                  the first sensor node exhausts its energy. Network lifetime of
proper time slots to sensor nodes. However, DTSM can not                                      MIL-MON is discussed with the system load. In this
assign time slots for every nodes. The nodes that can not get                                 experiment, number of nodes is 2000, number of slots is 1000.
time slot sleep and do not join into network. The ratio of                                    The results are in Figure 7.
sleeping nodes is an indicator of performance of DTSM. MIL-                                      In Figure 7, system load is the number of events in one
MON is simulated for 1000 slotted time frames. 1000 slotted                                   second. If there is no event, the first node exhausts after more
MIL-MON is investigated for non-rescheduled, rescheduled                                      than 35000 seconds. Network lifetime decreases with the
with 5 sub-slotted, 10 sub-slotted and 20 sub-slotted versions.                               increasing of system load. MIL-MON is designed to be used
   Figure 5 shows percentage of sleeping nodes for different                                  for military purposes and the attack of intruder is not so
number of nodes and different number of slots. Percentage of                                  common in the battlefield. MIL-MON can operate for more
sleeping nodes increases linearly with the increasing of                                      than 10 hours. MIL-MON can be used for securing the
                                                                                               environment especially for one night operations.

                          12
                                                                                                                      40000
    % of sleeping nodes




                          10                                                                                          35000
                          8                                                            500                            30000
                                                                                                                      25000
                                                                                                  Time (s)




                          6                                                            1000
                                                                                                                      20000
                          4                                                            1500
                                                                                                                      15000
                          2                                                                                           10000
                          0                                                                                                  5000
                                 2    2,5     3    3,5      4   4,5   5                                                           0
                                     Num ber of nodes (x1000)                                                                               0         0,25           0,50   1
                                                                                                                                                          System Load
Fig. 5. Sleeping node ratio for different number of nodes.
                                                                                              Fig. 7. Network lifetime for different system loads.
                                                                                                                                                              7

   C. Delay                                                                       Rescheduling improves delay very successfully. Delay in
   Delay is very important for military applications. Long                     rescheduled with 20 sub-slot is 18 times smaller than non-
lifetime is not enough, if it gives information too late. MIL-                 rescheduled MIL-MON, especially in the 10th region. (the
MON uses TDMA and constructs a tree. It means delay of                         border of the sensor network area.)
events is a function of distance from event to sink. Delay is
investigated for different distance regions. Network area is                                              V. CONCLUSION
divided into 10 regions. The first region is 10m. distant from                    In recent years, sensor network is one of the hottest topics in
the sink. The second is between 10-20m. distant from the sink,                 wireless communication area. In this paper, a new TDMA
and so on. Figure 8 shows average delay of events for different                based sensor networks for military monitoring (MIL-MON) is
time slot numbers in these regions.                                            proposed. The most important design objectives of MIL-MON
                                                                                are to prolong network lifetime, to reduce delay and to be able
                                                                                to operate in large areas.
                 20000                                                             In order to realize MIL-MON, distributed time scheduling,
                 18000                Normal 500
                                                                                topology construction and rescheduling mechanisms are
                 16000                Normal 1000
                                                                                studied. Simulation results have shown that network lifetime
                 14000                Normal 1500
    Delay (ms)




                 12000                                                          of MIL-MON is long enough to be used as military
                 10000                                                          monitoring. Delay problem sourced form TDMA structure
                  8000                                                          can be handled by rescheduling mechanism and delay is
                  6000                                                          reduced by up to 18 times.
                  4000
                                                                                   When a MIL-MON node exhausts, the network system is
                  2000
                     0                                                          broken in this version of MIL-MON. However, sensor nodes
                         1   2   3     4   5    6    7       8       9    10    are prone to failure and sensor network should be able to fix
                                       Region number
                                                                                itself, when a node fails. As future work, a maintenance
                                                                                algorithm will be developed.
Fig. 8. Delay for different regions for different number of slots.
                                                                                                             REFERENCES
   When distance between the sink and event increases, delay                   [1] Deborah Estrin, Lewis Girod, Greg Pottie, Mani Srivastava,
increases dramatically. MIL-MON is an intruder detection                       “Instrumenting the World with Wireless Sensor Networks”, International
                                                                               Conference on Acoustics, Speech, and Signal Processing (ICASSP 2001), Salt
system and early alarm is critical. MIL-MON should give                        Lake City, Utah, May 2001.
information as soon as possible, even if event is far from the
sink. Even 500 slot is not acceptable. If intruder is 500m.                    [2] I. F. Akyildiz and W. Su and Y. Sankarasubramaniam and E. Cayirci.
                                                                               “Wireless Sensor Networks: A Survey. Computer Networks” (Amsterdam,
distant and if MIL-MON gives information after 5 seconds, it
                                                                               Netherlands: 1999), 38(4):393-422, 2002.
may be too late.
   Rescheduling is proposed to reduce delay. The same                          [3] E. Shih, S.-H. Cho, N. Ickes, R. Min, A. Sinha, A. Wang, and A.
experiment is repeated with 1000 slot non-rescheduled, 1000                    Chandrakasan, “Physical layer driven algorithm and protocol design for
                                                                               energy-efficient wireless sensor networks”, In Proc. ACM MOBICOM, pages
slot rescheduled with 5, 10 and 20 sub-slot MIL-MON. The                       272--286, Rome, Italy, July 2001.
results are in Figure 9.
                                                                               [4] W. Heinzelman, A. Chandrakasan, and H. Balakrishnan, “An
                                                                               Application-Specific Protocol Architecture for Wireless Microsensor
                 14000                                                         Networks”, IEEE Transactions on Wireless Communications, Vol. 1, No. 4,
                                 Normal 1000                                   October 2002, pp. 660-670.
                 12000
                                 Re 1000-5                                     [5] K. Sohrabi, J. Gao, V. Ailawadhi, and G. J. Pottie, “Protocols for self-
                 10000                                                         organization of a wireless sensor network”, IEEE Personal Communications,
                                 Re 1000-10
    Delay (ms)




                                                                               vol. 7, pp. 16 - 27, October 2000.
                  8000           Re 1000-20
                  6000                                                         [6] . Kottapalli, A. S. Kiremidjian, J. P. Lynch, E. C., T. W. Kenny, K. H.
                                                                               Law, Y. Lei, “Two-tiered wireless sensor network architecture for structural
                  4000                                                         health”, SPIE'03.
                  2000                                                         [7] Guangyu Pei and Charles Chien, “Low power TDMA in large wireless
                     0                                                         sensor networks”, MILCOM 2001 - IEEE Military Communications
                                                                               Conference, no. 1, October 2001, pp. 347 - 351
                         1   2   3    4    5   6    7    8       9       10
                                     Num ber of region                         [8] W. Heinzelman, “Application-Specific Protocol Architectures for
                                                                               Wireless Networks”, PhD Thesis, Massachusetts Institute of Technology,
                                                                               June 2000.
Fig. 9. Delay for different regions for different number of slots.
                                                                                [9] Wendi Heinzelman, Anantha Chandrakasan, and Hari Balakrishnan,
                                                                               “Energy-Efficient Communication Protocols for Wireless Microsensor
                                                                               Networks”, Proc. Hawaaian Int'l Conf. on Systems Science, January 2000.
                                                                             8


[10] J. Mannermaa, K. Kalliomaki, T. Mansten, and S. Turunen, “Timing
performance of various GPS receivers” In Proceedings of the 1999 Joint
Meeting of the European Frequency and Time Forum and the IEEE
International Frequency Control Symposium, pages 287–290, April 1999.

[11] Rex Min and Anantha Chandrakasan, “Energy-Efficient Communication
for Ad-Hoc Wireless Sensor Networks”, 35th Asilomar Conference on
Signals, Systems, and Computers, vol. 1, November 2001, pp. 139-143.

[12] J. Frieman. “Portable Computer Power Sources” , In Proceedings of the
Ninth Annual Battery Conference on Application and Advances, pp. 152-158,
January 1994.

				
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