Efficient Protocols for Railway Accident Monitoring

G.Kiruthika, S.Vinodhini, UG Scholars, SNS College of Technology,Coimbatore

Abstract—Recent advances in wireless sensor networking (WSN) techniques have encouraged interest in the development of vehicle health monitoring (VHM) systems. These have the potential for use in the monitoring of railway signaling systems and rail tracks. Energy efficiency is one of the most important design factors for the WSNs as the typical sensor nodes are equipped with limited power batteries. In earlier research, an energy-efficient cluster-based adaptive time-division multiple- access (TDMA) medium-access-control (MAC) protocol, named EA-TDMA, has been developed by the authors for the purpose of communication between the sensors placed in a railway wagon.

This paper proposes another new protocol, named E-BMA, which achieves even better energy efficiency for low and medium traffic by minimizin

g the idle time during the contention period. In addition to railway applications, the EA-TDMA and E-BMA protocols are suitable for generic wireless data comm

unication purposes. Both analytical and simulation results for the energy consumption of TDMA, EA-TDMA, BMA, and E-BMA have been presented in this paper to demonstrate the superiority of the EA-

TDMA and E-BMA protocols.

Index Terms—Energy efficiency, medium access control (MAC) protocol, railway wagon, vehicle health monitoring (VHM), wire- less sensor network (WSN).

I. INTRODUCTION

Fig. 1. Typical scenario for railway-wagon health monitoring system.

itoring vehicle characteristics in real time from track measure- ment data has been addressed by various research organizations [2]-[7]. Wireless sensor networks (WSNs) are widely used to monitor railway tracks and irregularities, detect abandoned objects in railway stations, develop intrusion detection systems,

secure railway operations, and monitor tunnels [8]-[10]. Seifert envisioned [8] that a network of smart sensors could be

utilized to monitor public spaces for potential invasion to alert the operators at a control center about the event. In addition, a WSN can be deployed to monitor large areas with greater efficacy in video-based intrusion detection systems.

WITH the increased demand for railway services, railway monitoring systems continue to advance at a remarkable

Aboelela et al. [9] proposed a new approach to reduce the accident rate and increase the efficiency of railroad maintenance

pace to maintain reliable, safe, and secure operation. The lack of safety and security monitoring of railway infrastructure runs the risk of train collision, train derailment, terrorist threats, failures in the train wagons, etc. The performance of rail vehicles run- ning on tracks is limited by the lateral instability inherent to the design of the wagon's steering and the response of the railway wagon to individual or combined track irregularities. Railway track irregularities need to be kept within safe operating mar- gins by undertaking appropriate maintenance programs. Track geometry inspection and monitoring enhances train-operating safety and reduced vehicle and track dynamic interaction. Mon-

Manuscript received March 15, 2012; revised August 15, 2012 and October 15, 2012; accepted October 20, 2012. Date of publication November 30, 2012; date of current version May 29, 2013. This work was supported in part by Prof. P. Wolfs and in part by Prof. C. Cole, both from the Center for Railway Engineering, Central Queensland University. The Associate Editor for this paper was B. Ning.

G. M. Shafiullah and S. A. Azad are with the Power Engineering Research Group, School of Engineering and Built Environment, Central Queensland Uni- versity, Rockhampton, Qld. 4702, Australia (e-mail: g.shafiullah@cqu.edu.au; s.azad@cqu.edu.au).

A. B. M. S. Ali is with the School of Information and Communication Tech- nology, Central Queensland University, Rockhampton, Qld. 4702, Australia (e-mail: s.ali@cqu.edu.au).

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TITS.2012.2227315

activities. The protocol adopts a multilayered multipath routing architecture in which each sensor transmits the sensed data to the two nearest cluster heads (CHs). Each CH aggregates the data using a fuzzy logic technique and transfers it to the sink node. Cheekiralla [10] designed a wireless sensor unit for the surveillance of a train tunnel, which measures the vertical displacements along the critical zone of the tunnel during adjacent construction activity.

The potential of WSN technology to monitor the railway- wagon health condition and the vertical displacement of railway wagons due to track irregularities has yet to be fully explored. The limited lifetime of the batteries that power the sensor nodes makes the energy efficiency a major design issue for WSNs [11]. This paper concentrates on developing an energy-efficient WSN MAC protocol to collect data from sensor nodes that are placed inside the railway wagons and send the data to the locomotive for further precautionary actions to prevent any future disastrous events. A prototype of the proposed railway- wagon health monitoring system is given in Fig. 1. Although the proposed energy-efficient protocol is designed with the railway applications in mind, it is applicable to generic wireless data communication purposes. Analytical and simulation models have been developed for the existing and proposed protocols to compare their performances in terms of energy consumption.

II. BACKGROUND OF THE STUDY

Central Queensland University, in collaboration with the Center for Railway Engineering [4], has been working on an autonomous health card device for online analysis of car body motion to perceive track condition and monitor derailment. The health card devices use an accelerometer and angular rate sensors with a coordinate transform to analyze the car body motions into six degrees of freedom [12], [13]. These health card devices inspect every wagon in the fleet using low-cost smart devices [4], [12]. An algorithm was developed, which analyzes signals coming from accelerometers mounted on the wagon body to measure the dynamic interaction between the track and the rail vehicle. The algorithm was validated using collected field data, e.g., accelerations measured at strategic points on the wagon body and the bogies.

Each prototype health card incorporates a 27-MHz mi- crocontroller with 256 kB of onboard RAM, four dual-axis

Fig. 2. Accelerometer locations and axis naming convention [4].

accelerometers, a Global Positioning Satellite receiver, two low-power radios, lithium-ion batteries, and a solar panel. A Rabbit 3000 processor is used, which requires 200 mW of power at 40 MHz. The first generation of the health card con- sumes a total of 400 mW or energy requirement of 9.6 Wh daily. An 80-Wh lithium battery is built into the health card that can provide energy for up to eight days. Data were collected from a ballast wagon in which dual-axis accelerometers were fitted to each corner of the body and each side frame. A personal- computer-based data acquisition system was used to store data. The main purpose of the data acquisition was to provide real data that are represented to the health card device. Data have been used to validate and demonstrate the effectiveness of signal analysis techniques and, finally, to develop a model to monitor typical dynamic behavior and track irregularities [12], [13].

Both the vertical and lateral conditions of the railway wagon have been measured by each accelerometer. The aim of the sensing arrangement was to capture roll, pitch, yaw, vertical, and lateral accelerations of the wagon body. The ADXL202/ADXL210 [14] dual-axis low-power low-cost ac- celeration sensor measured 16 channel acceleration data in g units, with eight channels for the wagon body and eight for the wagon side frame. Four sensor nodes were placed in each wagon body, and the locations of the sensors were front-left body, front-right body, rear-left body, and rear-right body. Sim- ilarly, four sensor nodes were placed in each wagon's side fame [4]. Sensor locations and naming convention are illustrated in Fig. 2. The sampling rate of the accelerometer can be set from 0.01 Hz to 5 KHz through adjustable capacitors, and the clock speed of this health card device was set to 100 Hz. Data were continuously collected from a ballast wagon, which was a conventional three-piece bogie spaced lb = 10.97 m apart. The accelerometers were spaced l = 14.4 m apart. The test run was a normal ballast lying operation, starting with a full load of ballast, traveling to the maintenance site, dropping the ballast on the track, and returning empty via the same route.

This research work is an extension of the existing health card system development endeavor, aiming at improving the energy efficiency of the railway-wagon health monitoring system. In the proposed system, there are five sensor nodes placed inside

each wagon, instead of four in the existing system. One sensor node is used as a CH that collects data from other nodes and sends data to the central control room or base station (BS). In this system, an accelerometer has been placed in each corner of the wagon and one accelerometer at the center of the wagon, which acts as a CH. The BS is placed in the middle of the train for optimal signal transmission range. If there are W wagons in the train, then the BS is positioned between wagons W/2 and W/2 + 1. This feature not only reduces the overall energy consumption of the network significantly but reduces energy dissipation of each CH as well, as they need to transmit data over a shorter range.

Each node forwards packets to the CH in each wagon, and the CH works as a router to send the data to the BS in the middle of the train. The locomotive driver can monitor the sensor data through an audio/visual system and take decisions accordingly. Each node is powered by an internal battery to make this work independently.

The cluster-based WSN deployed in the railway-wagon health monitoring system, as illustrated, must be designed to be very energy efficient and reliable. In a sensor node, power is required for data sensing, communication, and data processing. Energy efficiency is a major issue in designing WSN to prolong the network lifetime as the sensor nodes have limited battery lifetime. The main sources of energy loss are idle listening, col- lision, overhearing, overemitting, and control packet overhead [11], [15].

The wireless modules of the health card were developed using Bluetooth IEEE 802.15.1 standard [16], which is an outdated standard. Bluetooth devices are inefficient in terms of energy dissipation. The data communication range of Bluetooth is only 10 m, which requires more of sensors per wagon for data communication with the locomotive. The absence of energy-efficient features for data collection and communica- tion between wagons to the locomotive makes this system less reliable. Instead of Bluetooth technology, this study con- siders the IEEE 802.15.4/ZigBee standard [17], which is an ultralow-power and low-data-rate radio standard. Due to its simplicity and low cost, ZigBee is the most suitable standard to date for railway applications, e.g., data communication be- tween sensor nodes placed inside the wagons. The CC2431

Fig. 3. Operation diagram of TDMA [22].

System-on-Chip [18] uses an energy-efficient ZigBee-enabled CC2420 RF transceiver [19] with an enhanced 8051 micro- controller, up to 128-KB Flash memory, 8 KB of RAM, and many other powerful features, such as low current consumption, that makes the technology an attractive solution for WSNs. The ZigBee-compliant radio operates on 16 channels in the 2.4-GHz ISM band, and standard data rates are 250 kb/s. This data transfer capability is suitable for the sensor networks placed in the railway wagon.

In general, the maximum length of the railway wagon is 17 m. However, considering redundancy, it is wise to be considering a radio receiver with a 34-m range that covers two wagons. Hence, the transmission range of the receiver is expected to be sufficient for the railway wagon as it covers 35 m of wireless range.

IEEE 802.15.4/ZigBee standard has four states: sleep or shutdown, idle or listening, transmit, and receiving. It was shown in an experiment by Bougard et al. [20] that the Zigbee standard consumes less than 50% of the energy for actual data transmission, and the rest of the energy is consumed for other activities. A significant percentage (25%) of the energy is consumed during the contention procedure. This is due to the multiplicative effect of carrier-sense multiple access/collision detection (CSMA/CD). The waiting for an acknowledgement consumes 15% of the energy. Moreover, 20% of the en- ergy is used for listening for the beacon by the transceiver. Based on the energy breakdown, several ways to improve the overall energy efficiency were proposed by the researchers. Bougard et al. [20] proposed an energy-aware radio activation policy to optimize the PHY and MAC layers' parameters in a dense sensor network scenario. Experimental results showed that PHY level improvements combined with MAC optimiza- tions allow energy-efficient self-powered sensor networks [20].

The traditional wireless MAC and routing protocols do not fulfill the requirements of WSN applications since WSN protocols need to focus on energy-efficient design to ensure minimum power consumption and maximum battery lifetime [21], [22]. Energy-efficient MAC and routing protocol design is currently a prime research area in wireless data communication application.

This paper concentrates on developing an energy-efficient WSN MAC protocol to collect data from sensor nodes that are placed inside the railway wagons and send it to the locomotive for further precautionary actions. The authors have already de- veloped an analytical model of an energy-efficient WSN MAC

protocol EA-TDMA [1], which is the most suitable for medium to high traffic. This paper proposes another energy-efficient WSN MAC protocol, named E-BMA, which achieves even better energy efficiency. Popular MAC protocols are discussed in the following section with their strengths and weaknesses.

III. SCHEDULE-BASED MEDIUM-ACCESS

CONTROL PROTOCOL

The major requirements of a wireless MAC protocol are:

energy efficiency, scalability, latency, fairness, and bandwidth utilization. Contention-based protocols are scalable and adapt- able to node density or traffic load variations. However, these schemes have a major limitation relating to an enormous amount of energy wasted due to collisions, overhearing, and idle listening [11], [21]. Schedule-based protocols are collision free and, hence, trim down the wastage of energy due to colli- sion. However, they lack the flexibility and scalability inherent in the contention-based protocols.

Time-division multiple access (TDMA) is a schedule-based MAC protocol where the transmission channel is divided into several time slots, and each node is assigned a time slot. Each node wakes up and transmits data only in its allocated time slot and remains in sleep mode in the remaining time slots [11], [21]. However, this protocol only uses the node energy effi- ciently when the traffic load is high. Nodes with empty buffers keep their radio turned on during their scheduled slot and, hence, dissipate some of their remaining energy. The energy- efficient TDMA (E-TDMA) reduces energy consumption due to idle listening. Sensor nodes keep their radios off when there is no data to transmit. However, the CH has to keep the radio on all the time and hence waste energy [11], [21], [22]. Fig. 3 illustrates a single round for TDMA protocol.

Low-energy adaptive clustering hierarchy (LEACH) [23], an architecture for wireless microsensor networks, incorporates the features of cluster-based routing and MAC protocol. This protocol achieves energy efficiency and low latency while maintaining application-specific quality. LEACH allows all data from nodes within the cluster to be locally processed in the CH that reduces the data set. Data aggregation was done to combine several correlated data signals into a smaller set of information, and then, the resultant data were sent to the BS using a fixed spreading code and a CSMA approach [23], [24].

The bit-map-assisted (BMA) protocol [25] is another schedule-based protocol that aims at reducing energy wastage

Fig. 4. Operation diagram of the BMA protocol [25].

Fig. 5. Operation and timing diagram of the EA-TDMA protocol [1].

due to collision and idle listening. This protocol deals only

with event-driven networks where sensor nodes forward data to the CH only if a significant event has been detected. The cluster setup phase is similar to the LEACH [23] protocol. In the contention period, each node in the cluster transmits a 1-bit control message to the CH node during its allocated slot if it has data to transmit; otherwise, the transmitter radio remains idle. At the end of the contention period, the CH in the BMA protocol makes a transmission schedule and transmits the schedule only to the source nodes [25], [26]. In TDMA, once a node is allocated a data slot, that allocation persists for all frames in that round regardless of whether the node has enough data packets to send in each frame. Conversely, in BMA, the allocation is done in the contention phase before the starting of each frame, as shown in Fig. 4. Therefore, BMA is more energy

efficient than TDMA and E-TDMA for the cases of low traffic load, relatively few sensor nodes per cluster, and relatively large packet size [25], [26].

In railway applications, the accelerometer data are contin- uously collected while the train is in operation from sensor nodes, and hence, this application is classified as a medium to high traffic load as the sensor collects data at the rate of 25 kb/s. Considering the application requirements, authors developed an energy-efficient protocol, named EA-TDMA [1], which reduces the energy consumption during data transmission. In this protocol, every node wakes up in its allocated slot and transmits data to the CH. If there are no data to send, it turns off the radio immediately. The nodes move into sleep mode instead of idle mode in the absence of data. An operation diagram and a timing diagram of the EA-TDMA protocol are illustrated in

Fig. 6. Operation of the E-BMA protocol.

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EA-TDMA protocol is available in [1]. The energy consump- tion

of EA-TDMA is significantly less than TDMA at low traffic loads, although this gap diminishes at high traffic loads. This protocol also outperforms BMA protocol in all traffic conditions except very low traffic [1]. In this paper, in addition to the analytical results, the superiority of the EA-TDMA protocol has been demonstrated by the simulation model.

The railway-wagon health monitoring system requires the MAC protocol to be capable of handling steady traffic and energy efficient. Although some of the aforementioned proto- cols were customized to achieve energy efficiency, this paper further explores the achievement of better energy efficiency. In addition to the EA-TDMA protocol in this paper, the authors propose a new energy-efficient WSN MAC protocol, named E-BMA. This paper explores both the analytical and simulation model of EA-TDMA and E-BMA protocols to demonstrate the superiority of these protocols compared with other conventional protocols. The proposed protocol achieves better energy effi- ciency for low to medium traffic load, and it is comparable with the EA-TDMA and TDMA protocols for high traffic load. The newly proposed energy-efficient E-BMA protocol is described in the next section.

IV. ENERGY-EFFICIENT E-BMA PROTOCOL

The BMA protocol consumes less energy than TDMA at low

and medium traffic loads, whereas in the energy-efficient ver- sion, EA-TDMA consumes less energy than BMA, unless the traffic load is very low. The contention phase in BMA helps to minimize the idle listening period during the data transmission phase; however, the contention phase itself consumes a certain amount of energy before each frame transmission. The energy consumption in the contention phase is paid off at light traffic loads. However, at high traffic loads, this contention phase turns into an overhead as the probability of data transmission becomes almost certain. In the proposed E-BMA protocol, the source nodes use piggybacking to make the reservation of the corresponding data slot rather than sending a control message during its allocated contention slot, as shown in Fig. 6. Unlike BMA, in the new protocol, a source node does not make the reservation in the contention slot as soon as the data packet becomes available. Instead, it waits for one additional frame

duration to see if there is a successive data packet to send. There is a 1-bit field allocated in each data packet header to indicate whether the source node has a successive data packet to send. If a source node has successive data packets to send in a number of consecutive frames, the reservation is made once for the initial data packet in its allocated contention slot, and the successive confirmations will be made through piggybacking. Note that piggybacking a control message requires only 1-bit extra space in the data packet, and hence, the additional power required for piggybacking a control message on a data packet is negligible. In E-BMA, the transceiver of the source node is turned off during the contention phase when it has no control message to send, whereas in BMA, the transmitter is kept idle in similar situations. This allows the E-BMA protocol to save energy both at low and medium traffic. E-MBA is only outperformed by TDMA and EA-TDMA when the traffic load is extremely high. To achieve energy efficiency, the E-BMA protocol compromises the latency of data transmission. Each data packet has to wait for one additional frame duration before being transmitted to the CH. As there will be few sensor nodes per cluster in the railway-wagon health monitoring system, the frame length will be much shorter, and the latency of E-BMA will be within the acceptable limit.

Operation of the E-MBA protocol is divided into rounds, and each round is comprised of a setup phase and a steady- state phase. The steady-state phase is comprised of a contention phase and a data transmission phase. Both cluster formation and CH selection occur in the setup phase. All non-CH nodes reserve the data slots in the contention phase, whereas data transmission from source nodes to the CH occurs during the data transmission phase.

Setup Phase: Considering the specific application area and its simplicity, it is assumed that the network consists of multiple fixed clusters. In each of the clusters, there is one CH located in the center of the cluster. Based on the application and cluster size, direct transmission for data communication between source nodes and the CH is considered instead of multihop data transmission. In the setup phase, the CH informs all nodes about the start of the current round, frame start/stop time, and number of frames in a round.

Contention Phase: Each node is assigned a specific slot in the contention phase. A node transmits a 1-bit control

message during its scheduled slot to reserve a data slot if it has a data packet to transmit; otherwise, the node remains in sleep mode during that contention slot. After the contention period is completed, the CH sets up and broadcasts a transmission schedule for the source nodes. However, unlike BMA, the source node does not make the reservation immediately after the data becomes available. Instead, the source node keeps the data packet in the buffer, and it waits for one frame duration to see if there is a consecutive data packet to send.

Data Transmission Phase: The data transmission phase con- tains one or more frames. The size and duration of each frame is fixed. Nodes send their data to the CH at most once per frame during their allocated time slot. During the data transmission phase, each source node turns on its radio in its allocated data slot and transmits data to the CH. If there are consecutive packets, the transmitted data packet conveys that information through piggybacking.

After receiving all data from the nodes of a round, data ag- gregation takes place to reduce unwanted data. A considerable amount of energy is saved if the data are locally aggregated in the CH first rather than when sending the raw data to the BS or central controller and aggregating them in the BS. Then, the resultant data are sent from the CH to the BS using a spreading code and a CSMA approach, as used in the LEACH protocol [20]. Once the CH is ready to send the aggregated data, it must sense the channel to see if anyone else is transmitting using the BS spreading code. The CH waits if the channel is busy; otherwise, the CH transmits data to the BS. After a predefined time, the system begins the next round, and the whole process is repeated.

Analytical and simulation models were developed based on the energy model [21], [22] for the TDMA, EA-TDMA, BMA, and E-BMA to compare their performances in terms of traffic load and energy dissipation features, which is presented in the next section.

TABLE I NOMENCLATURE

check its buffer, and turn off its radio is in E-TDMA is Te. The parameters used in the analysis are defined in Table I.

A. Energy Consumption

The energy consumption of the TDMA, EA-TDMA, BMA,

and E-BMA protocols based on the energy model in [20] and [22] is modeled as follows.

Energy Consumption of TDMA Protocol: During the con- tention, the CH and all non-CH nodes keep their radios on, and communication takes place between the CH and all non-CH nodes. In this period, the CH assigns data slots to individual nodes for data transmission and informs all nodes in the cluster. Therefore, energy consumption by the CH to send a control message is PtTc, and energy consumption by each node to receive a control message is PrTc. Therefore, the energy consumption in a contention period is given by

Econt = N PrTc + PtTc. (1) V. ANALYTICAL AND SIMULATION MODELING

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and compare its performance with existing wireless MAC pro- tocols, including EA-TDMA, analytical and simulation models have developed.

This proposed protocol is analyzed in a WSN scenario where there are one CH and N non-CH nodes in each cluster, assum- ing that there are l frames in a round. The data slot duration is assumed to be Td. Let the probability of a node having data to transmit be p. The power consumption in the transmit mode and the receive mode are Pt and Pr, respectively. Energy dissipation of idle listening mode is P i. For simplicity, as stated in [21] and [23], the energy required to turn on the radio by the source nodes for transmission or reception is negligible and, hence, is ignored in the following analysis.

As per definition, Td is the time required to transmit or receive a data packet, and it is assumed that Tc is the time required to transmit/receive a control packet. The time required for the CH to transmit a control message to all non-CH nodes in BMA is Tch. The time required for a node to switch on,

Each node transmits, at most, one packet per frame inter- val. During a frame transmission, energy consumption by a source node is PtTd. The energy consumed by the CH while receiving the data packet is PrTd. A nonsource node turns on its radio and keeps it idle during its scheduled time slots. The energy consumed by a nonsource node is PiTd. As the CH also stays in idle mode when there are no data to receive from the non-CH node during a data slot, the energy consumed by the CH is also PiTd.

In a data slot, a node sends data with probability p and remains idle with probability (1 p). The expected energy consumption during a single frame transmission consisting of N data slots is [pPtTd + (1 p)PiTd + pPrTd + (1p)P iT d]N . The expected energy consumption in a trans- mission round is given by

Etrans = [pPtTd +(1p)PeTe +pPrTd +(1p)PiTd] lN. (2)

As each round is comprised of l frames, the average energy consumption per round in the TDMA protocol can be

formulated as sent by the same source node in the previous frame) and

remains idle in the remaining (N 1) contention slots. ETDMA = [N PrTc + PtTc] The nonsource nodes keep their radio turned off during

+ [pPtTd + 2(1 p)PiTd + pPrTd] lN. (3) the entire contention period. A control message cannot be piggybacked if there is no data packet sent in the

Energy Consumption of EA-TDMA Protocol: Similar to the

TDMA protocol, the energy consumption in a contention period is given by

previous frame by the same node. The probability of a data packet not being piggybacked is p(1 p). If a control message is piggybacked, the source node keeps the radio turned off in the respective contention slot, whereas the CH

Econt = N PrTc + PtTc. (4) node remains in idle listening mode. The expected energy

EA-TDMA differs from TDMA in that every non-CH node in EA-TDMA wakes up in its allocated slot and checks transmit buffers. If there are no data to send, it turns off the radio immediately. Hence, the energy consumed by a non-

consumption during a contention period is given by

Econt = [p(1 p)PtTc + PrTch] N

+ [p(1 p)PrTc + (1 p(1 p)) PiTc] N + PtTch. (10)

CH node that has no data to transmit is PeTe. The energy PeTe is used to switch on, check the transmit buffers, and then turn off the radio module. The expected energy consumption in a transmission round is given by

Etrans = [[pPtTd + (1 p)PeTe]

During a frame transmission, each source node sends the

data packet in its allocated slot, whereas the nonsource nodes keep their radios turned off. Note that piggybacking a control message only requires 1-bit extra space in the data packet. Hence, it is assumed that no additional power is required for piggybacking. The expected energy consump-

+ [pPrTd + (1 p)PiTd]] lN. (5) tion during a frame transmission is given by

As each round is comprised of l frames, the

average energy consumption per round in the EA-TDMA protocol can be

Eframe = [pPtTd + pPrTd]N. (11)

formulated as

EEATDMA = [N PrTc + PtTc] + [[pPtTd + (1 p)PeTe]

+ [pPrTd + (1 p)PiTd]] lN. (6)

Energy Consumption of BMA Protocol: In BMA, there is

The average energy consumption per round in the E-BMA protocol can be formulated as

EEBMA = [[p(1 p)PtTc + p(1 p)PrTc

+ (1 p(1 p)) PiTc + PrTch + pPtTd

a contention period in each session when all nodes keep their radios on. Each source node transmits a control message during its scheduled slot, as well as its remains idle (N 1) slots. Each nonsource node stays idle during

+ pPrTd] N + PtTch]

l.

B. Transmission Latency

(12)

the contention period. During a contention slot, the CH node receives control packets when there is a source node sending a control packet; otherwise, the CH stays idle. The expected energy consumption during a contention period is given by

Econt = [pPtTc +(1 p)PiTc +(N 1)PiTc +PrTch] N

The maximum transmission latency of TDMA and EA- TDMA protocols is given by Tc + N Td as both protocols have similar frame structure. The maximum transmission latency of BMA is Tch + (Tc + Td)N . The maximum transmission la- tency of E-BMA is 2[Tch + 2(Tc + Td)N ] as each data packet has to wait for one additional frame duration before being transmitted.

+ [pPrTc + (1 p)PiTc] N + PtTch. (7) Simulation models have been developed for the TDMA, EA- TDMA, BMA, and E-BMA protocols to verify the correctness

During a frame transmission, each source node sends the data packet in its allocated slot, whereas the nonsource nodes keep their radios turned off. The expected energy consumption during a frame transmission is given by

of the analytical models using Java programming language and SimJava Package version 2.0 [27]. SimJava is a process- oriented discrete-event simulation package developed by the University of Edinburgh. The simulation results represent the general characteristics of the existing and proposed protocols.

Eframe = [pPtTd + pPrTd]N. (8) The simulation for each model was run for 10 000 rounds. The

The average energy consumption per round in the BMA protocol can be formulated as

EBMA = [[pPtTc + pPrTc + 2(1 p)PiTc + (N 1)PiTc

+PrTch + pPtTd + pPrTd] N + PtTch] l. (9)

Energy Consumption of E-BMA Protocol: A source node sends a control message in its respective contention slot

(unless the reservation is done by the preceding data packet

expected energy consumption was calculated, averaging energy consumption over the entire simulation period.

Both analytical and simulation results confirm that E-BMA is more energy efficient than the other three protocols at low to medium traffic. It is only outperformed by the TDMA and EA-

TDMA protocols when the traffic is extremely high. In the following section, detailed analyses of the results are

presented, and protocol performances in terms of energy dissipation are compared.

Fig. 7. Energy dissipation of EA-TDMA, BMA, and TDMA protocols as a function of probability p (N = 10 and l = 2).

VI. RESULTS AND ANALYSIS

This section analyzes the performance of the proposed

E-BMA protocol in terms of energy efficiency and transmission latency. In addition, the performance of the E-BMA protocol has been compared with that of the TDMA, EA-TDMA, and BMA protocols in terms of energy dissipation and transmis- sion latency. As aforementioned, the IEEE 802.15.4 standard and ZigBee wireless module are used for the proposed MAC protocol. The ZigBee-enabled 2.4-GHz CC2420 RF transceiver [19] is used for this analytical and simulation analysis. For analytical and simulation modeling purposes, it is assumed that the power consumption is 50 mW for transmitting, 54 mW for receiving, and 50 mW for idle listening. These power ratings are comparable with that of the CC2420 RF transceiver specification. The data rate is 25 kb/s, and the control packet size is 5 bytes. For simplicity, it is assumed that Te = Td/10 and that Pe = Pi.

A. Energy Consumption

Energy consumption of the protocols has been evaluated and

compared for different parameter settings. For analytical and simulation analysis, four cases have considered the estimation of the energy dissipation of the protocols being analyzed. Case 1: In this case, the average energy consumption of the

aforementioned four WSN MAC protocols has been de- rived for various transmission probabilities. It is assumed that the total number of non-CH nodes is N = 10, the number of frames is l = 2 per round, and the data packet size is 100 bytes.

Fig. 7(a) and (b) shows the average energy consump- tion of the TDMA, EA-TDMA, BMA, and E-BMA proto- cols for the transmission probability varying from p = 0.1 to 1.0. The graphs reveal that the energy consumption of the TDMA protocol is almost constant as the difference be- tween transmission power and idle listening power is very small. The EA-TDMA protocol consumes less energy for low to medium traffic, i.e., from p = 0.1 to 0.5, whereas it is as good as the TDMA protocol for medium to high traffic,1

1Low data traffic refers to transmission probability, i.e., p < 0.3, whereas high data traffic refers to transmission probability, i.e., p > 0.7. Medium traffic refers to the transmission probability in between.

i.e., from p = 0.5 to 1.0. This is because, in EA-TDMA, if a node has no data to send in its allocated slot, the transceiver is turned off to save energy. The lower the traffic, the higher the savings. The BMA protocol is com- parable with the EA-TDMA protocol for low traffic, i.e., p = 0.1; however, it consumes more energy than TDMA and EA-TDMA for medium to high traffic because the contention phase in BMA consumes a certain amount of energy, depending on the traffic load. At high traf- fic loads, the contention phase turns into an overhead for BMA.

The proposed E-BMA protocol outperforms all three protocols significantly. The E-BMA is only outperformed by TDMA and EA-TDMA when p 0.8, i.e., when the traffic load is high. In E-BMA, the transceiver of the source node is turned off during a contention slot when it has no control message to send, whereas in BMA, the transceiver is kept idle in similar situations. This allows the E-BMA protocol to save energy both at low and medium traffic. However, at high traffic loads, the overhead of the con- tention phase surpasses the savings and that why E-MBA is only outperformed by TDMA and EA-TDMA when the traffic load is extremely high. It is to be noted that there is a constant difference between the energy consumption of BMA and that of E-BMA.

Case 2: In this experiment, the average energy consumption of the aforementioned four WSN MAC protocols has been de- rived for a different number of non-CH nodes. It is assumed that the transmission probability is p = 0.4, the number of frames is l = 2, and data packet size is 100 bytes.

Fig. 8(a) and (b) shows the average energy consump- tion of TDMA, EA-TDMA, BMA, and E-BMA protocols for the total number of non-CH nodes varying from N = 5 to 50. As the traffic load is medium (p = 0.4) in this situation, the performance of E-BMA is the best, which is followed by EA-TDMA and TDMA. The BMA protocol has the maximum energy consumption at medium traffic load. The energy consumption of the BMA protocol dra- matically rises as N increases, signifying the overhead due to contention. The proposed E-BMA protocol minimizes this overhead through piggybacking at medium traffic load. There is a moderate increase in the energy consumption in TDMA and E-TDMA as the number of nodes increases

Fig. 8. Energy dissipation of EA-TDMA, BMA, and TDMA protocols as a function of number of nodes in a cluster: N (p = 0.4, l = 10).

Fig. 9. Energy dissipation of EA-TDMA, BMA, and TDMA protocols as a function of number of frames: l (N = 20, p = 0.3).

since the overhead in the contention period is minimal for these two protocols.

Case 3: In this case, the aforementioned four WSN MAC protocols have been evaluated in terms of average energy consumption for various numbers of frames per round. It is assumed that the total number of non-CH nodes is N = 10, the transmission probability is p = 0.3, and the data packet size is 100 bytes.

Fig. 9(a) and (b) shows the average energy consump- tion of TDMA, EA-TDMA, BMA, and E-BMA proto- cols for the number of frames per round changing from l = 2 to 20. As the graphs reveal, for medium traffic and small number of nodes, E-BMA performs the best among these four WSN MAC protocols. In this case, the energy consumption of TDMA is the highest, whereas the energy consumption of EA-TDMA and BMA is in between. Since the number of nodes is small, the contention overhead in BMA moderately increases, and hence, its energy con- sumption is slightly lower than EA-TDMA. The proposed E-BMA protocol has the least contention overhead due to piggybacking.

Case 4: In this experiment, the impact of data packet size on the overall energy dissipation has been measured. It is assumed that the total number of nodes is N = 10, the transmission probability is p = 0.4, and the number of frames is l = 2 per round.

In Fig. 10(a) and (b), it is evident that the E-BMA protocol is the most energy-conservative protocol among the four protocols, whereas TDMA is the most energy- consuming protocol. However, when the data packet size is less than 50 bytes, the energy dissipation of EA-TDMA is similar to that of E-BMA. This is because the overhead of the wakeup period in the EA-TDMA protocol diminishes with the reduction in packet size. Although the energy consumption of BMA is lower than that of TDMA and EA-TDMA, it is as worse as TDMA for packet sizes less than 50 bytes. The reason is the energy wastage due to idle listening in the data transmission period of TDMA exceeds the contention overhead of BMA when the packet size is small.

B. Transmission Latency

The maximum transmission latency of the TDMA, EA-

TDMA, BMA, and E-BMA is presented for different numbers of nodes and packet sizes.

Fig. 11(a) demonstrates that the maximum transmission la- tency in all four protocols increases with the number of nodes, as the length of a frame is directly related to the number of nodes. The packet transmission latency of TDMA and EA- TDMA is the lowest among all four protocols. Due to the exis- tence of the contention period in each frame, the transmission

Fig. 10. Energy dissipation of EA-TDMA, BMA, and TDMA protocols as a function of data packet size (N = 20, l = 10, p = 0.4).

Fig. 11. Transmission latency of EA-TDMA, BMA, and TDMA protocols for (a) different number of nodes and (b) different data packet sizes.

latency of BMA is slightly higher. The transmission latency of E-BMA is twice that of BMA as each packet has to wait for one additional frame duration in E-BMA. Fig. 11(b) demon- strates that the maximum transmission latency in all protocols increases with the data packet size, as the length of a frame is directly related to the data packet size. Similar to the previous case, the packet transmission latency of TDMA and EA-TDMA is the lowest. The transmission latency of BMA is slightly higher, and the transmission latency of E-BMA is twice that of BMA.

Summarizing the analytical and simulation results, the fol- lowing can be concluded.

• The E-BMA protocol is the most energy-efficient protocol, particularly in the case of low and medium traffic applica- tions. However, at extremely high traffic conditions, the EA-TDMA protocol performs better.

• The E-BMA protocol is more energy efficient than the other three protocols for any number of sensor nodes in a cluster when the traffic load is medium.

• The E-BMA protocol dissipates less energy than the other three protocols, regardless of the number of frames per round for medium traffic.

• The performance of the E-BMA protocol is superior to the other three protocols when data packet size is equal to or greater than 50 bytes. For small data packet size (less than 50 bytes), the energy dissipation of EA-TDMA is comparable with E-BMA.

• Although the transmission latency of E-BMA is higher than other protocols, it will not impact the system

performance significantly when the number of nodes is small.

VII. CONCLUSION

The performance of rail vehicles running on railway tracks is

governed by the dynamic behaviors of railway wagons, partic- ularly in the cases of lateral instability and track irregularities. In this paper, considering the traffic conditions of the intended application, an energy-efficient WSN MAC protocol has been investigated to monitor typical dynamic behavior of railway wagons. Simulation and mathematical models have been devel- oped for the proposed E-BMA protocol, and its performance has been compared with the EA-TDMA, TDMA, and BMA protocols in terms of energy efficiency.

Analytical and simulation results show that the E-BMA and EA-TDMA protocols outperform both the TDMA and BMA protocols for all traffic conditions. The results revealed that the E-BMA protocol outperformed other protocols for low to medium traffic, whereas the EA-TDMA protocol outperformed the TDMA and BMA protocols for medium to high traffic. The E-BMA protocol is only outperformed by EA-TDMA and TDMA protocols for high traffic.

ACKNOWLEDGMENT

The authors would like to thank the reviewers for their

valuable and informative suggestions that improved the quality of this paper.

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