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1 of 6 A NEW TDMA BASED SENSOR NETWORK FOR MILITARY MONITORING (MIL-MON) İlker Bekmezci and Fatih Alagöz Bogazici University, TURKEY (ilker.bekmezci, alagoz)@boun.edu.tr ABSTRACT Wireless sensor network (WSN) is a new network family that enables to create smart environments. In this paper, a new TDMA based sensor network for military monitoring (MIL-MON) is proposed. MIL-MON is developed to oper- ate in large areas for acceptable lifetime periods. In order to realize MIL-MON; time synchronization scheme, dis- tributed time scheduling mechanism, topology construction algorithm and rescheduling algorithms are proposed. Simulation results have shown that MIL-MON can operate in large areas, in acceptable lifetime and delay con- straints. Index Terms—sensor networks, TDMA, military monitor- ing. INTRODUCTION Wireless sensor networks (WSNs) may consist of thou- sands of tiny sensors each with capability of detecting am- bient conditions such as temperature, sound, movement, or the presence of certain objects. One of the most common application areas of WSNs is military surveillance. In this paper, a new TDMA based military monitoring sensor network system (MIL-MON) is proposed. Because of the unattended structure of the sensor nodes, the scarcest resource in sensor networks is power. Power consumption can be divided into three domains, as sens- ing, communication, and data processing domains. Domi- nant factor in energy consumption for sensor nodes is communication [1]. Not only transmission but also receiv- ing is the main cause of energy waste. The easiest way to reduce energy consumption is to turn the radio off, when it is not used. Fixed allocation methods, TDMA or FDMA is extremely suitable for this kind of network. There are some sensor networks based on TDMA such as LEACH [2], SMACS [3], two-tiered architecture [4], PACT [5]. LEACH is a self-organizing, adaptive clustering protocol that uses randomization to distribute the energy load evenly among the sensors in the network [2]. Although LEACH can reduce power consumption, there is a prob- lem with the assumptions of LEACH. LEACH assumes that each node can hear each other. So LEACH is not suit- able for using in large areas. SMACS is another sensor network that uses TDMA. In fact SMACS uses TDMA in addition to FDMA. After a series of handshaking signals, neighbor nodes can agree on a frequency and time pair to construct a link. SMACS produces a scalable and reliable flat network. However, SMACS needs FDMA as well as TDMA, but sensor nodes are so tiny and limited that cur- rent sensor nodes cannot meet the requirements of SMACS. PACT uses Unifying Slot Assignment Protocol (USAP), which is a TDMA scheduling scheme for on de- mand ad hoc networks. USAP is adopted for sensor net- works in PACT. However, USAP is originally developed for ad hoc networks and PACT is not fully successful in power consumption for sensor networks. Another TDMA based sensor network proposal is two-tiered structural health monitoring wireless sensor network architecture. According to this structure, there are some fixed cluster heads and sensor nodes. Sensor nodes are clustered around cluster heads. This network is designed to use for monitor- ing buildings. It cannot be used in large areas. In this paper, a new TDMA based sensor network system, which can be used for military monitoring systems in a relatively large area, is presented. The coverage of the network can be in the order of kilometer squares. In order to realize MIL-MON, time synchronization, distributed time scheduling mechanism and topology construction algorithms are developed. In order to enhance delay per- formance, rescheduling mechanism is also proposed. The organization of the paper is as follows. In Section 2, the basic mechanisms and enhancements for TDMA based sensor network will be proposed. In Section 3, the per- formance results of newly proposed algorithms will be outlined. Section 4 states the conclusion and future work. MIL-MON SYSTEM MECHANISMS Overview MIL-MON is proposed to monitor a relatively large area against intruders and send the data about intruders to sink as soon as possible. The main design considerations of MIL-MON are to be able to operate in large areas, to minimize power consumption, to reduce delay. MIL-MON includes time synchronization, distributed time slot assignment, rescheduling and topology construction mechanisms. Before introducing the mechanisms of MIL- MON, basic assumptions of MIL-MON are presented.

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Page 1: [IEEE MILCOM 2005 - 2005 IEEE Military Communications Conference - Atlantic City, NJ, USA (17-20 Oct. 2005)] MILCOM 2005 - 2005 IEEE Military Communications Conference - A New TDMA

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A NEW TDMA BASED SENSOR NETWORK FOR MILITARY MONITORING (MIL-MON)

İlker Bekmezci and Fatih Alagöz Bogazici University, TURKEY

(ilker.bekmezci, alagoz)@boun.edu.tr

ABSTRACT

Wireless sensor network (WSN) is a new network family that enables to create smart environments. In this paper, a new TDMA based sensor network for military monitoring (MIL-MON) is proposed. MIL-MON is developed to oper-ate in large areas for acceptable lifetime periods. In order to realize MIL-MON; time synchronization scheme, dis-tributed time scheduling mechanism, topology construction algorithm and rescheduling algorithms are proposed. Simulation results have shown that MIL-MON can operate in large areas, in acceptable lifetime and delay con-straints.

Index Terms—sensor networks, TDMA, military monitor-ing.

INTRODUCTION

Wireless sensor networks (WSNs) may consist of thou-sands of tiny sensors each with capability of detecting am-bient conditions such as temperature, sound, movement, or the presence of certain objects. One of the most common application areas of WSNs is military surveillance. In this paper, a new TDMA based military monitoring sensor network system (MIL-MON) is proposed.

Because of the unattended structure of the sensor nodes, the scarcest resource in sensor networks is power. Power consumption can be divided into three domains, as sens-ing, communication, and data processing domains. Domi-nant factor in energy consumption for sensor nodes is communication [1]. Not only transmission but also receiv-ing is the main cause of energy waste. The easiest way to reduce energy consumption is to turn the radio off, when it is not used. Fixed allocation methods, TDMA or FDMA is extremely suitable for this kind of network. There are some sensor networks based on TDMA such as LEACH [2], SMACS [3], two-tiered architecture [4], PACT [5].

LEACH is a self-organizing, adaptive clustering protocol that uses randomization to distribute the energy load evenly among the sensors in the network [2]. Although LEACH can reduce power consumption, there is a prob-lem with the assumptions of LEACH. LEACH assumes that each node can hear each other. So LEACH is not suit-able for using in large areas. SMACS is another sensor network that uses TDMA. In fact SMACS uses TDMA in

addition to FDMA. After a series of handshaking signals, neighbor nodes can agree on a frequency and time pair to construct a link. SMACS produces a scalable and reliable flat network. However, SMACS needs FDMA as well as TDMA, but sensor nodes are so tiny and limited that cur-rent sensor nodes cannot meet the requirements of SMACS. PACT uses Unifying Slot Assignment Protocol (USAP), which is a TDMA scheduling scheme for on de-mand ad hoc networks. USAP is adopted for sensor net-works in PACT. However, USAP is originally developed for ad hoc networks and PACT is not fully successful in power consumption for sensor networks. Another TDMA based sensor network proposal is two-tiered structural health monitoring wireless sensor network architecture. According to this structure, there are some fixed cluster heads and sensor nodes. Sensor nodes are clustered around cluster heads. This network is designed to use for monitor-ing buildings. It cannot be used in large areas.

In this paper, a new TDMA based sensor network system, which can be used for military monitoring systems in a relatively large area, is presented. The coverage of the network can be in the order of kilometer squares. In order to realize MIL-MON, time synchronization, distributed time scheduling mechanism and topology construction algorithms are developed. In order to enhance delay per-formance, rescheduling mechanism is also proposed.

The organization of the paper is as follows. In Section 2, the basic mechanisms and enhancements for TDMA based sensor network will be proposed. In Section 3, the per-formance results of newly proposed algorithms will be outlined. Section 4 states the conclusion and future work.

MIL-MON SYSTEM MECHANISMS

Overview

MIL-MON is proposed to monitor a relatively large area against intruders and send the data about intruders to sink as soon as possible. The main design considerations of MIL-MON are to be able to operate in large areas, to minimize power consumption, to reduce delay.

MIL-MON includes time synchronization, distributed time slot assignment, rescheduling and topology construction mechanisms. Before introducing the mechanisms of MIL-MON, basic assumptions of MIL-MON are presented.

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Basic Assumptions

Here are the assumptions of the sensor network system. Sensor nodes are assumed as immobile. Mobile cases can be investigated for further analysis. Radio channel is symmetric. Sink node’s power source theoretically infinite.

The assumptions, which have been mentioned above, are valid for most of the sensor networks. There are some ad-ditional assumptions specifically for MIL-MON. Sink node has high range transmitter as well as low range transmitter. The sink can use its low range transmitter to communicate with its neighbor nodes. In addition to this, it can send broadcasts for all nodes with its high range transmitter. In this way, all the sensor nodes can receive broadcasts of the sink.

Basic Mechanisms

There are some basic mechanisms for operating MIL-MON properly. These are time synchronization, distrib-uted time scheduling and topology construction mecha-nisms.

Time Synchronization

According to assumption of this sensor network, every node in the system can receive the signals of the sink. Sink transmits a broadcast signal to sensor nodes at the begin-ning of each time frame. These broadcast signals synchro-nize the network.

Packet latency error is an important source for time syn-chronization errors. In [6], Kopetz and Schwabl have de-composed message latency into four distinct components: send time, access time, propagation time and receive time. This synchronization schema and TDMA based MAC layer eliminates send time and access time with the peri-odic broadcast of the sink. Because of the close sensor nodes, propagation time is near zero. In this time synchro-nization algorithm, there is no mechanism to estimate re-ceive error, so instead of correction, it accepts receive er-ror. The distribution of receive error between sensor nodes is Gaussian (µ=0, σ=11.1) [7].

Another time synchronization error is clock drift and clock drift is minimized with linear regression as in [7]. In this paper clock drift is assumed as 30 µs.

The last error source is clock instability. The solution for clock instability is to use the most recent broadcast re-ceives times. In this way, old receive times cannot affect the result.

This global time synchronization scheme that is based on the periodic broadcast of the sink is called as Periodic Global Broadcast Time Synchronization (PGB-TS).

Distributed Time Scheduling Mechanism (DTSM)

Almost all sensor network architectures that use TDMA produce its time schedule centrally. Cluster head collects the data about its sensor nodes and produces the time schedule of its cluster. Time schedule is sent to nodes by the cluster head. In most of the centralized time scheduling algorithms, the protocol to collect data about the nodes is contention based. This can be a serious problem for power sensitive systems. However, in distributed time scheduling algorithms, there is no need for communication between nodes and the sink directly. This leads to power saving.

According to DTSM, after sensor nodes are deployed, every node selects a random time slot as its own slot. It listens to all time slots in the first time frame and it trans-mits a special signal only in its own slot. If it receives a jammed signal, it means there is a collision at that particu-lar slot. The node collects all the collision slots. In the next time frame, it transmits a signal at the collision slots. In the same time frame, it listens to its own slot. If it receives a signal, it means it has the same slot with another node’s slot so that their signals are jammed. In this case, it sleeps. If it does not receive any signal at its own slot, it can use that slot and it continues to operate.

This protocol is simple and consumes low power. How-ever, it does not result in a complete solution. Some nodes have to sleep. Fortunately, most of the time, sensor nodes are deployed densely and the non-existence of a small number of nodes can be tolerated.

Rescheduling

Although power consumption is generally the most impor-tant design issue in sensor networks, there are some other design considerations like delay. In military applications, delay is very important.

One of the main problems of TDMA based systems is de-lay. Reducing delay is possible by the help of assigning time slots carefully. The rule is that smaller hop numbered nodes should get higher slot numbers. An example helps to understand the situation clearer. Let us assume that the nodes in Figure 1 are one hop away from its consecutives. In this particular network, time frame has 100 time slots. Let us assume that DTSM algorithm has been run and the assigned time slots are as in the Figure 1(a). Rescheduling algorithm assigns new slot numbers to nodes according to their hop numbers. Figure 1(b) shows a sample slot num-ber assigning after rescheduling.

The relay of an event from D to the sink takes 194 time slots for non-rescheduled network in Figure 1(a). How-ever, it takes only 95 time slots for the rescheduled net-work in Figure 1(b).

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Figure 1. Example network and time slots (a) Before rescheduling. (b) After rescheduling.

In order to realize rescheduling, time frame is divided into m sub time frames. If the whole time frame has n slots, a sub time frame has n/m slots. The slot number assigned to a node with hop number h, must be in (m-((h-1) mod m))th sub time frame. In this way, the slot numbers of consecu-tive hop numbered nodes belong to consecutive sub time frames.

After running distributed time scheduling algorithm, nodes listen to neighbors for an advertisement signal. The adver-tisement signal includes the hop number of the sender. The node can get the number of sub time frames, m, from the sink’s synchronization signal and calculate the sub frame number. It selects a random time slot in its sub time frame and sends a broadcast signal that includes the information about the selected time slot. The node sends this informa-tion in its original time slot that was assigned with DTSM mechanism. All the neighbors of the node listen to its sig-nal and check whether there is collision about its new slot number or not. If there is a collision, the nodes that catch the problem send a signal in their own time slots. The node that tries to reschedule listens to its all neighbors in the next time frame. If it gets a signal from the others, it sleeps. If it does not get any signal related with its new time slot, it means there is no problem and it starts to use its new slot and continues with topology construction pro-cedure.

Topology Construction

After rescheduling, the third step for the proposed sensor network is topology construction. The basic structure of the algorithm lies on handshaking signals. After getting a proper time slot, sensor nodes listen to their neighbors to catch hop number advertisement signal. At the beginning, only the sink sends hop number advertisement signal. Hop number of the sink is zero. If a sensor node can catch ad-vertisement signal or signals, it chooses one of them as its predecessor. The simplest choice is the closest predecessor candidate. In the next time frame, it sends its own adver-tisement signal. If it receives hop number as h from its predecessor, it advertises its own hop number as h+1. In

the same signal, it also sends the node number of its prede-cessor. The predecessor receives this signal. If the prede-cessor receives a hop advertisement signal with its node number, it understands that the owner of the signal has become its child. In the third time frame, it listens to its all neighbors to learn which nodes became its children.

This topology construction algorithm always concludes with a tree. If hop number of a node is h, although it may have multiple connections with the h+1 hop numbered nodes, it has exactly one connection with h-1 hop num-bered nodes. It assures that the result is always a tree.

The pseudo code of the MIL-MON that includes all the phases of DTSM, rescheduling and topology construction is presented as follows: //DTSM Selects a random slot as its own slot; In the first time frame, listens to all time slots and transmits a signal at its own slot; When listening, collects all the slot that has no clear signal; //for collecting all interfering slots. In the second time frame, sends a signal at interfering slots, and listen to its own slot; If it receives a signal in its own slot, it sleeps; //Resheduling of the nodes with hop number h-1 While it has no signal about rescheduling Listens to all its neighbors for rescheduling Wend If it finds a collision in randomly selected slot numbers for re-scheduling, it sends a signal; //Rescheduling of itself While it has no advertisement signal Listens to all its neighbors for an advertisement signal that in-cludes the hop number of the sender; Wend Calculates its own hop number, and selects a random slot num-ber from its own time sub frame; Broadcasts randomly selected slot number in its own slot; Listens to all neighbors for a signal about its random selection; // It checks its neighbors whether there is interference about its new slot or not. If it receives a signal, it sleeps; //Rescheduling of the nodes with hop number h+1 Sends an advertisement signal with hop number h+1 and its par-ent; Listens to all neighbors for their random slot selection; If it finds a collision, it sends a signal in its own slot. //Topology Construction Listens to all neighbors for their advertisement signal that in-cludes their parent data; If it receives a signal that it is the parent of the sender, it accepts the sender as its child;

PERFORMANCE RESULTS

It is very difficult to model the interactions of sensor nodes analytically, even for the limited number of nodes. In order to investigate performance results of MIL-MON, simula-

Sink (1) A (82) B(91) C(6)

Sink (1) A (91) B(82) C(6)

(a)

(b)

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tion method is used. Simulation parameters are listed in Table 1. Energy consumption model is the same as in [8]. Energy of a node is assumed to be supplied with 15 mg. Ni-Cd battery that can support 2 J [9].

Performance results are discussed in four domains. These are time synchronization performance, sleeping node ratio in DTSM and rescheduling, network lifetime and delay.

Performance of Global Time Synchronization

In order to investigate the performance of PGB-TS, a nu-meric simulation is set up. The broadcasts of the sink are assumed to be transmitted in every 10 seconds as in RBS [7]. In the simulation, receive error is assumed as Gaussian (µ=0, σ=11.1) and clock drifts are assumed as 30 µs in one second. Send time, access time and propagation time errors are assumed as zero. Each experiment is run for 6000 sec-onds with 1000 sensor nodes and each experiment is re-peated 20 times.

Under these assumptions precision is below 100 µs. This shows that time synchronization bits must take at least 100 µs. In fact, there are some other time synchronization mechanisms that can produce higher precision, like RBS [7]. However, implementation of RBS is not straightfor-ward and energy to run RBS may be higher than this mechanism.

Sleeping Node Ratio in DTSM Rescheduling

DTSM is a distributed time scheduling mechanism to as-sign proper time slots to sensor nodes. However, DTSM or rescheduling cannot assign time slots for every node.

The nodes that cannot get time slot sleep. The ratio of sleeping nodes is an indicator of performance of DTSM and rescheduling. MIL-MON is simulated for 1000-slotted time frames. 1000-slotted MIL-MON is investigated for non-rescheduled, rescheduled with 5 sub-slotted, 10 sub-slotted and 20 sub-slotted versions. The results are pre-sented in Figure 2. It shows that rescheduling or DTSM

increases sleeping node ratio increases linearly. In every case, rescheduling brings additional sleeping nodes.

Network Lifetime

Network lifetime is one of the most common performance metrics of sensor networks. In this paper, network lifetime is the time that the first sensor node exhausts its energy. Network lifetime of MIL-MON is discussed with the sys-tem load. In this experiment, number of nodes is 2000; number of slots is 1000. The results are in Figure 3. In Figure 3, system load is the number of events in one sec-ond. If there is no event, the first node exhausts after more than 35000 seconds. Network lifetime decreases with the increasing of system load.

05000

10000150002000025000300003500040000

0 0,25 0,50 1

System Load

Tim

e (s

)

Figure 3. Network lifetime for different system loads.

Figure 4 shows a network lifetime comparison between 802.11 [10], S-MAC [11], MIL-MON under low traffic requirement. 802.11 [10] has to listen all the time. Even if it does not transmit any data, its maximum network life-time is 400 s. S-MAC, which is also collision based proto-col, listen the medium for a certain ratio. Maximum net-work lifetime of S-MAC changes according to this ratio.

Table 1. Simulation parameters

Parameter Default values

Power needed for radio elec-tronics (per bit)

50 nJ

Power for receiving (per bit) 50 nJ

Power for transmitting (per bit) 50nJ + 10pJ*d*d (d is distance)

Max. range of nodes 30 m. Power in one node 2 J Simulation area diameter 1000 m. Position of the sink Center of the area. Time for one time slot 1 ms. Bit rate 1 Mbps.

0

2

4

6

8

10

2 2,5 3 3,5 4 4,5 5

Number of nodes (x1000)

% o

f sle

epin

g no

des

1000

1000-5

1000-10

1000-20

Figure 2. Sleeping node ratio for different number of nodes with rescheduling.

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Figure 4 shows that MIL-MON achieves to extend net-work lifetime about 875 times with respect to 802.11. MIL-MON also can extend network lifetime at least 8 times with respect to S-MAC (%1).

0

5000

10000

15000

20000

25000

30000

35000

802.11 S-MAC (%5) S-MAC (%1) MILMON

Netw

ork

Life

time

(s)

Figure 4. Network lifetime comparison between 802.11, S-MAC and MIL-MON.

Delay

Delay is investigated for different distance regions. Net-work area is divided into 10 regions. The first region is 50m. distant from the sink. The second is between 50-100m. distant from the sink, and so on. When distance be-tween the sink and event increases, delay increases dra-matically. MIL-MON is an intruder detection system and early alarm is critical. Rescheduling is proposed to reduce

delay. The experiment is performed for 1000 slot non-rescheduled, 1000 slot rescheduled with 5, 10 and 20 sub-slot MIL-MON. The results are in Figure 5.

Rescheduling improves delay very successfully. Delay in rescheduled with 20 sub-slot is 18 times smaller than non-rescheduled MIL-MON.

CONCLUSION

In this paper, a new TDMA based sensor networks for military monitoring (MIL-MON) is 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. In order to realize MIL-MON, time syn-chronization, distributed time scheduling, topology con-struction and rescheduling mechanisms are studied. Simu-lation performance results for network lifetime suggest that MIL-MON system is very promising for use in mili-tary monitoring. Moreover, delay problem sourced from TDMA structure can be handled by rescheduling mecha-nism and delay is substantially minimized. Unfortunately, MIL-MON mechanism at this stage cannot cope with fail-ure of a sensor node. However, sensor nodes are prone to failure and MIL-MON mechanism should be capable of fixing itself when a node fails. We are currently devising a maintenance algorithm to handle node failures.

REFERENCES

[1] I. F. Akyildiz, W. Su, Y. Sankarasubramaniam and E. Cayirci. “Wireless Sensor Networks: A Survey. Computer Networks” (Amsterdam, Netherlands: 1999), 38(4):393-422, 2002.

[2] W. Heinzelman, A. Chandrakasan, and H. Balakrish-nan, “An Application-Specific Protocol Architecture for Wireless Microsensor Networks”, IEEE Transactions on Wireless Communications, Vol. 1, No. 4, October 2002, pp. 660-670.

[3] K. Sohrabi, J. Gao, V. Ailawadhi, and G. J. Pottie, “Protocols for self-organization of a wireless sensor net-work”, IEEE Personal Communications, vol. 7, pp. 16 - 27, October 2000.

[4] . 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 health monitoring” Pro-ceedings of the SPIE, Volume 5057, 2003, pp. 8-19.

[5] G. Pei and C. Chien, “Low power TDMA in large wireless sensor networks”, MILCOM 2001 - IEEE Mili-tary Communications Conference, no. 1, October 2001, pp. 347 - 351

[6] H. Kopetz and W. Schwabl. Global time in distributed real-time systems. Technical Report 15/89, Technische Universit¨at Wien, 1989.

[7] J. Elson, L. Girod, and D. Estrin, “Fine-grained net-work time synchronization using reference broadcasts” In Fifth Symposium on Operating Systems Design and Im-plementation (OSDI 2002), December 2002.

[8] W. Heinzelman, A. Chandrakasan, and H. Balakrish-nan, “Energy-Efficient Communication Protocols for

0

2000

4000

6000

8000

10000

12000

14000

1 2 3 4 5 6 7 8 9 10

Number of region

Del

ay (m

s)

Normal 1000

Re 1000-5

Re 1000-10

Re 1000-20

Figure 5. Delay for different regions for different number of slots.

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Wireless Microsensor Networks”, Proc. Hawaaian Int'l Conf. on Systems Science, January 2000.

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

[10] ISO/IEC. IEEE 802.11 Standard. IEEE Standard for Information Technology, ISO/IEC 8802-11:1999(E), 1999.

[11] W. Ye, J. Heidemann and D. Estrin, “An Energy-Efficient MAC protocol for Wireless Sensor Networks”, In Proceedings of the 21st International Annual Joint Confer-ence of the IEEE Computer and Communications Societies (INFOCOM 2002), New York, NY, USA, June, 2002.