An Investigation of CTP Use for Wireless Structural Health Monitoring of Infrastructure

Abstract

Structural Health Monitoring (SHM) is a very important research domain whereby civil infrastructure is monitored. Using wireless technologies can boost SHM by providing the level of autonomous operation that is essential for these tasks. Wireless routing, with its subset, geographic routing, is an important procedure that needs to be optimised, in order to lead packets to the basestation. Occasionally, routing is susceptible to interference and collisions due to a large number of connected devices. This fact led to cooperative transmission; cooperative networks are the ones that utilise relays to accomplish the transmission of packets; thus, resulting in link quality as well as throughput increase. In this paper, we investigate the Collection Tree Protocol (CTP) to show that it can be cooperative when used in an SHM for civil infrastructure monitoring applications giving a geographical essence to the routing protocol. We do that by exploiting the fact that the CTP’s mechanism uses its tree formation for a node to transmit to the best link quality parents. An example of a cooperative model to show that it may be applied to the protocol is given. Further, Indriya testbed results of direct and cooperative transmissions are given to strengthen the case of this work, with which a scenario where the CTP exhibits better link quality when using a relay is given. A practical addition is suggested, whereby an extra field in the packet struct is proposed, which will provide the CTP with further strength to changing conditions and direct communication loss.

1. Introduction

Structural Health Monitoring (SHM) constitutes major industrial research areas, with applications in the civil infrastructure domain [1]. SHM in a civil infrastructure could be considered as a network of devices. SHM can be used in many forms by using acousto-ultrasonics [2,3], Acoustic Emissions (AE) [4,5,6,7], and accelerometers [8], among others. SHM has been successfully applied to infrastructure including bridge monitoring [9] with satellite data, fibre-optic sensor SHM of infrastructure as it can be seen in [10] and references therein, buildings assessment [11], as well as tunnels [12].

In [13], the authors recommended and tested a WSN consisting of 45 nodes with IEEE 802.15.4 technology for suspension bridge monitoring. The short-range communication occurred using IEEE 802.15.4 and Code Division Multiple Access (CDMA) for communication with a longer distance basestation. The aim was to exhibit the ability to encapsulate a WSN for bridge monitoring. However, the requirement of interference of multi-hop communication needs to be explicitly mentioned.

Zou et al. [14], suggest the Compressive Sensing (CS) method to tackle lossy links and data loss in WSNs. In particular, they indicate that this method is suitable for data reconstruction and can be compared with reliable communication protocols. This method is implemented in the imote2 platform and this paper targets to provide accurate compensation for stationary and compressible acceleration signals which are retrieved from structural health monitoring (SHM) systems with data loss ratio below 20%. Even though this is a good technique to be utilised, a reliable protocol, such as the Collection Tree Protocol (CTP) [15] can be utilised to tackle the data loss quite effectively.

Zrelli et al. [16] is the closest research to the work presented in this paper. The authors utilise the CTP in SHM; however, they highlight the effect of distance on RSSI and throughput. This is somewhat problematic due to the fact that the distance is difficult to be computed in wireless communications due to reflections and refractions, as well as obstacles in front of the devices. Moreover, devices may not be within Line-Of-Sight, which creates an extra problem with this referencing. The main difference between this work and the work presented here is that we aim to enhance CTP with a cooperative field on the programming struct, in order to perform cooperative transmission when the communication conditions are difficult. Even though the CTP is quite robust and intuitively performs the best parent selection, our approach will ensure that the packet will be forwarded to a specific node in a geographic position via a cooperative link if the upstream node is not the best parent. This will give that opportunity for sensing comparison at a node level, before reaching the basestation.

Strangfeld et al. [17] suggested the monitoring of concrete infrastructures using passive radio frequency identification (RFID)-based sensors. The RFID-based sensors can last for long term and they are protected from harsh environments. The energy supply, as well as the transfer of the data, are being done by the RFID. The authors show monitoring of specific parameters from a large period of time. However, the RFID is not multi-hop meaning that they need to transfer their data locally.

In [18], the authors suggest the implementation of a WSN, which serves as a bridge SHM system. The system comprises the use of IEEE 802.15.4 and 3G for short-range and long-range communication, respectively. The system is tested and debugged and it finally meets the design requirements. The data loss needs to be further investigated, in various conditions such as with high interference.

There is a number of examples a that the CTP with IEEE 802.5.4 transmission can operate to. These examples require that the communication range is not large and multi-hop transmission is promoted. One of these examples is building SHM monitoring with accelerometers and gyroscopes [19]. Moreover, high-speed train systems have been proposed using 802.15.4and Zigbee in practical scenarios [20]. Moreover, another example is the monitoring of bridges [21] and highways [22], with energy-efficient devices that require multi-hop transmission for the data to reach the base station.

The network of devices needs to remain operational for a large period of time; hence power management is of paramount importance and using low power consumption parts is essential, as well as being able to perform time-triggered and event-triggered sampling tasks. IEEE 802.15.4 devices are usually resource-constrained and with low-power listening; hence, they can be encapsulated to applications where range is not a big issue in a single hop. Furthermore, TCP/IP can be substituted with IEEE 802.15.4 to use these energy-constrained devices which operate with dedicated software which is more lightweight, such as the Contiki [23]. Moreover, the devices need to be accurately synchronised, in order to be able to perform correct correlation analyses of the measurements. Since this paper attempts to address monitoring of large infrastructures, the devices may need to communicate between them to forward the data to a dedicated server, potentially using wireless technologies.

Wireless communications have made an impact in the SHM, whereas the data are transmitted and forwarded to the terminal computer. The network of devices communicate with each other or with a basestation/gateway directly depending on the technology used. Moreover, the type of communication is highly correlated with the application of the infrastructure. For instance, using a wireless sensor networks (WSN) on a bridge or a building may utilise low-range wireless technologies as opposed to distant or larger infrastructure. Specifically, WSNs [24,25,26] and their children, namely the Internet of Things (IoT) [27,28] have been playing a pivotal role to the next generation of SHM.

There is a plethora of communication methods that can be applied to infrastructure as it can be seen in [29]. Specifically, wireless technologies, such as IEEE 802.15.4 [30], Wi-Fi [31], and telecommunications [32], perform a metamorphosis of SHM into a wireless network procedure, where enhanced autonomicity is achieved. Other works include the implementation of an Edge-SHM framework through low-power, long-range, wireless, low-cost, and off-the-shelf components [33] to promote energy-efficiency, as well as the development of a new node, which provides powerful hardware and a robust software framework to enable edge computing that can deliver actionable information [34]. The vast number of devices led to specific problems that include the escalation in interference and wireless medium collisions that act negatively towards prolonging the lifetime of a wireless network, due to its limited resources. As such mechanisms that would mitigate interference and maximise successful packet transmissions and throughput are essential. The CTP is a protocol, which is a state-of-the-art in terms of Quality of Service (QoS), since it offers high throughput. The protocol may be used to promote energy efficiency if the transmission power is reduced.

In particular, the IEEE 802.15.4 networking technology can be used to successfully monitor infrastructure. The reader can see a typical wireless node architecture based on IEEE 802.15.4 in Figure 1a. IEEE 802.15.4 has been used in monitoring applications in infrastructure as the reader can see in [35] and references therein. IEEE 802.15.4 is a low-range technology and it operates in a multi-hop function, meaning that in order to reach the terminal computer a packet from a source needs to be forwarded by other nodes. The CTP initially originated for IEEE 802.15.4-constrained devices. This definitely raises issues such as the QoS, whereby the nodes are required to transmit their data successfully, in order to prevent packets with defect values to be lost. The main reason for selecting the IEEE 802.15.4 technology is its low power, low cost, and the fact that they are battery-based, which can last for a very long time.

Energy efficiency in the network is of utmost importance and the main motivation is to enhance the QoS. Often direct communication is prohibited due to problems in the connection; hence, using relay nodes to forward the data in a cooperative manner might be better. This standard along with the Zigbee enhancement is operating in the Indriya testbed nodes. Moreover, the example application provided in Contiki, allows the operation of CTP with the IEEE 802.15.4 and Zigbee. The Indriya testbed contains telosB nodes that operate with the CC2420 communication protocol, which works with the IEEE 802.15.4 standard. Hence, it is available for these devices and the application simply evaluates the best parent of each node to transmit data to, using the ETX as a metric.

Cooperative communication is the solution to promote energy efficiency of a network, as well as maximise its efficiency [36]. The reader can see an example of cooperative communication in Figure 1b, where the yellow line represents an expensive transmission with respect to a link-layer metric. The multi-hop nature of the WSN transmissions allows the utilisation of middle nodes to act as relays (Node B), when the problem of packet reception occurs, which drains the battery of the nodes with retransmissions and packet drops. In fact, the main idea behind communication is relaying, at each hop to transmit data from one node to another. Thus, by sharing resources between nodes, the transmission quality is enhanced.

As we can see in the survey [37] and references therein, cooperative communication has been included in various layers of the wireless stack. In particular, there is a plethora of works that target cooperative communication in the physical and Medium Access Control (MAC) layers. Here, we are interested more in the upper layers (network layer) and especially cross-layer communication that targets them, in order to increase the performance of a network.

Cooperative routing takes advantage of the cooperative transmission of the physical layer. It is a cross-layer approach that combines the physical and the network layers to transmit packets through establishing cooperative links. This cross-layer design approach makes the performance of the routing protocols in wireless networks better. The main objective of cooperative routing is the efficiency in optimising specific Quality of Service (QoS) or energy efficiency metrics. Cooperative routing algorithms have been proposed to target energy efficiency, throughput, packet delivery ratio, outage probability, and collisions of the wireless medium, as we can see in [37] and its references. Cooperative approaches can be found in [38,39,40,41,42,43,44].

In this paper, the CTP is examined, in order to indicate whether it can be cooperative by nature. The CTP tests are initial in the form of an intuitive cooperative example that is left for future work. The objective was to show an extension of CTP, which we believe is of vast importance. The CTP-like protocol is provided in the examples of Contiki and the wireless nodes support the IEEE 802.15.4 standard and the Zigbee enhancement. Note that the CTP is one of the robust protocols in throughput and data transmission [15], since it enables retransmission of the packets and it takes into account the link symmetry, which is the packet reception ratio and the packet acknowledgment ratio. In this way, bidirectional communication is ensured. The retransmissions alleviate the performance since the packet is not dropped with the first loss or non-transmission of acknowledgment. We exploit its parent selection and forwarding mechanism to show that it promotes cooperative communication with a few adjustments. In particular, we show a function that can be used to differentiate the nodes operating under CTP between buyers and sellers. These two sets of nodes cooperate, in order to forward packets to specific nodes using the Expected Transmission Count (ETX) metric. As such, SHM can be performed using IEEE 802.15.4 communication with high success in data transmission, and potential defectfull packets will not be lost. The civil infrastructure will be monitored for a very long time without human intervention. Moreover, note that the terminal computer that all the packets of the network will be destined to, is in a particular geographic region of the infrastructure; thus, a cooperative scheme with geographic parameters may be used in the case that the direct transmission is weak.

The main contributions of this paper are the following: The CTP is suggested with a cooperative mechanism based on ETX, which provides a geographical essence to the routing protocol in SHM applications. Examples of direct and cooperative transmissions are given to strengthen the authors’ point of view. A practical mechanism is suggested, whereby an extra field in the packet struct is proposed, which will provide the CTP with further robustness to changing conditions and direct communication loss.

2. Attempting to Make CTP Cooperative for the Infrastructure Shm

The aim is to show that a highly reliable protocol can be cooperative in order to tackle potential problems in its application in a bridge monitoring system. Hence, it is essential to describe the metric used as well as the cooperative example that, although intuitive can provide flexibility in geographic routing, which can be referred to the infrastructure SHM application.

2.1. Etx as a Function of the Signal-to-Interference-and-Noise-Ratio (Sinr)

There is a plethora of link quality metrics that can be encapsulated towards a solution that maximises throughput, as well as minimising packet drops and retransmissions [45]. Thus, the metric ETX emerged, which is the average number of transmissions of data and ACK packets. A node calculates ETX by obtaining the packet reception ratio of a wireless link l with each of its neighbouring nodes in the data direction, denoted as $PR{R}_{data}$. Thereafter, it calculates the packet reception ratio in the ACK direction, denoted as $PR{R}_{ack}$, ETX is widely known as the inverse of the probability of Packet Success Delivery given as

$$ET{X}_l=\frac{1}{PR{R}_{data}\ast PR{R}_{ack}}$$

(1)

As it is clear from (1), a link is perfect if its ETX value is 1. Moreover, the route ETX is the sum of the ETX of every link in the route. Hence, a two-hop route of perfect links has an ETX of 2. As it can be seen, the larger the ETX value the less reliable the link. ETX has several significant features, such as that it impacts throughput since it depends on delivery ratios. Additionally, it detects link asymmetry by employing bidirectional links, it uses precise link loss ratio measurements, and it penalizes routes with more hops, which have lower throughput due to interference between different hops of the same path [46]. In addition, ETX may implicitly lower the energy consumption per packet, since each node selects the receiver with the less ETX to forward the packet.

At this point, the relationship of ETX with the SINR will be provided. The Rayleigh channel [47] is considered for the transmissions. For a wireless link $(i,j)$, the Packet Reception Ratio (PRR) $PR{R}_{i,j}$ is defined as the ratio of the number of packets received by node j over the number of packets sent by node i. This is the $PR{R}_{data}$ and similarly, $PR{R}_{j,i}$ is the $PR{R}_{ack}$. It can be expressed by approximation as:

$$PR{R}_{i,j}={(1-\xi )}^l,$$

(2)

where l is the packet length in bits. The Bit Error Rate (BER), which we denote as ${\xi }_{i,j}$, is given by the following formula [48]

$${\xi }_{i,j}=\frac{1}{2}\left(1-\sqrt{\frac{{\gamma }_{i,j}}{1+{\gamma }_{i,j}}}\right),$$

(3)

where ${\gamma }_{i,j}$ is the SINR of the transmission from node i to node j. ${\gamma }_{i,j}$ is given by

$${\gamma }_{i,j}=\frac{H_{i,j}{p}_i}{{\displaystyle \sum _{t\ne i,t\ne j}{p}_t{H}_{t,j}+N_0}},$$

(4)

where $H_{i,j}$ and $H_{t,j}$ are the channels gains of the transmitter and the interferer, respectively, $p_i$ and $p_t$ are the transmission powers of the sender and the interferer, and $N_0$ is the noise. As we can see, the SINR increases, then the PRR increases, and the ETX decreases, reaching the value 1, when communication is flawless.

2.2. CTP

In this section, a brief description of the CTP is given. As it can be seen in [49], CTP uses a routing gradient that is based on ETX. ETX is an additive metric that is calculated per node as the ETX of its parent plus the ETX of its link to its parent. Packet retransmissions are included in the calculation as well. For the forwarding of the packets, CTP creates a hierarchical tree-based network, where each node selects its forwarding parent based on the minimum ETX value of a parent node, as we can see in Figure 2. When it comes to the infrastructure monitoring application, this tree can be useful for the transmission of the data to the basestation, which may reside in one end of the infrastructure.

A problem that routing protocols experience is the routing loops. This occurs when a new route that has a much higher ETX than its old one, often in response to losing connectivity with a potential parent. As such a route may include a node that is essentially a descendant and a routing loop takes place. CTP has two mechanisms for handling routing loops. Initially, when CTP receives a data packet with a gradient value lower than its own, it remarks the routing procedure as problematic, in terms of the tree infrastructure. Note that every packet includes the routing gradient as a field. CTP attempts to solve the problem by sending a beacon with the purpose that the node responsible for the loop adjusts its route. The second mechanism of CTP is not to consider routes with an ETX higher than a reasonable constant, which is dependent on the implementation.

The duplication of packets is another problem that CTP is facing. For example, when the data packet is received but the acknowledgment is not. This creates a significant problem as duplication is exponential. Duplicate suppression occurs when packets come from the same origin and has the same sequence number. Additionally, a frame that declares the time that a packet has lived, which becomes incremented in every hop, can result in duplicate messages being dropped.

2.3. Cooperation Example in the SHM Application

A WSN is considered, which is formed by a number of randomly distributed nodes functionally equivalent with respect to means of communication, signal processing, as well as battery demand. The function from [36] is suggested, which essentially proposes a game-theoretic formulation [50] towards link quality enhancement. We denote the $ETX$ value that a source node j requires to transmit a packet directly to an upstream node k by $ET{X}_{jk}$, as it can be seen in Figure 2 between nodes A and C. Additionally, denote the link quality demand of link quality required for a buyer node to transmit a packet to a seller node by $ET{X}_{inter}$, shown in Figure 2 between nodes A and B. Furthermore, denote the link quality value of the ith seller to his own offer to the link quality saving of the cooperative coalition by $c_i$ and the value of the jth buyer to the cooperation of the ith seller by $ET{X}_{ij}$. We denote the value required by the intermediate node to reach the target upstream node as $ET{X}_{frwd}$ (between nodes B and C in Figure 2). At this point, we assume that $c_i=ET{X}_{frwd}$, since a seller node has $ET{X}_{frwd}$ amount of link quality, favouring its corresponding source node. Note that the sellers’ satisfaction will not be fulfilled unless its link quality is compensated. On the contrary, the cooperation between a buyer and a seller and the link quality that will be saved is the requirement of each buyer to form a coalition with a seller. Therefore, we calculate $ET{X}_{ij}$ as

$$ET{X}_{ij}=ET{X}_{jk}-ET{X}_{inter}$$

(5)

if $ET{X}_{inter}>c_i,$ then a value exists that both the sellers and buyers prefer. It should be mentioned that when we refer to link quality saving, this corresponds to selecting the most efficient link quality to transmit each packet. A game-theoretic solution for this cooperative communication can be found in [36]. The objective is to attempt to show that there is a connection of a cooperative scheme with the nature of CTP forwarding. The CTP already performs such a procedure, by utilising the smallest ETX value of node, to which a node forwards the data.

A potential application of the cooperative CTP may originate from the geographic routing paradigm [51]. Here, the nodes are suggested to be part of a bridge monitoring system, whereby they are placed in a geographic manner and wish to transmit their data to a specific direction using the CTP protocol. The geographic routing can be taken care of by CTP since, it operates in a parent–child hierarchical manner. In Figure 3, the reader can see an example of geographic routing, whereby the yellow circles denote the specific nodes which will communicate in a geographic manner. The yellow lines show the link of geographic communication. Moreover, the blue circles are the neighbouring nodes and the blue lines the links of communication. In this example, QoS metrics are not considered and the communication takes place in purely geographic manners. Specifically, there might be routes that are more efficient than the current ones.

The CTP however, with its mechanism of selecting the best parent, may select a different route to forward the packet. The arguments is that such a mechanism can be enforced in CTP with the packet arriving at its geographically set device in a cooperative fashion. Even though this mechanism may resemble intuitive in the CTP operation, it is worth researching to further strengthen the CTP operation. The way the cooperative mechanism can be implemented at the testbed, is by adding a cooperative field in the struct of the packet. In this way, the seller node will be able to transmit the ETX values of forwarding the packet to the destination node. The nodes will geographically transmit the data to a destination node; however, they will essentially obtain an alternative with this cooperative packet field. This is intuitive in the CTP, since it uses the best parent to forward the data; however, the extra field will pose a new enhancement to the CTP in terms of data forwarding.

3. Validation with Tests for SHM Use

Tests have been undertaken on the Indriya [52] testbed using 26 telosb nodes, which operate with the IEEE 802.15.4 standard, operating at the maximum transmission power of 0 db. The application we run is a modified version of the example-collect Contiki application. The data rate was 1 packet per 30 s and the sink is node 1. The CTP gives very good results, due to its tree-based structuring of the nodes and the ETX minimisation process, with which the best parent of a given node is chosen to forward the data towards the sink. We extracted the best neighbour with its respective ETX values, to show the routing process. Note that the experiments have been undertaken to show a potential intervention on the CTP to include a cooperative field, which is left as future work.

A sample of the results has been isolated, whereby we investigated node 12 in terms of transmission and ETX values. The tree infrastructure can be seen in Figure 4, when node 12 is the origin of the data. Note that the bidirectional arrows show that there is a route in each direction, without loops, for simplicity. We list the routes in

Table 1. CTP Routes of node 12 origin.

Origin	1st Hop	2nd Hop	3rd Hop	4th Hop	5th Hop
12	1
12	7	1
12	7	9	1
12	7	9	13	1
12	9	1
12	9	13	7	1
12	10	1
12	10	7	9	13	1
12	10	7	9	1
12	10	7	13	9	1
12	10	7	13	1
12	13	1
12	13	7	9	1
12	13	9	1

.

Direct communication with the sink or utilising intermediate nodes to perform the transmission can be identified immediately. This is the first indication of cooperation, especially if geographical or specific addresses transmission in the nodes is imposed. In order to show our point, a further sample is taken, where cooperation may be more clear. Again, we see that each node transmits its messages to the sink directly or using a longer route. The average ETX values and the routes in Figure 5 are used, as they appear from the field of the programming language struct.

The links $12\to 7\to 1$ have been investigated and the direct transmission $12\to 1$. It can be observed that node 12 has a large ETX value when it is transmitting directly to the sink. The same applies to transmitting to node 7. Additionally, node 12 selected direct transmission 19 times. On the other hand, it selected the best neighbour to be node 7 and 19 times also. Node 7 has a very good ETX with the sink, since it selected it to be its next hop 50 times as we can see in Figure 6.

Furthermore, the sample was taken utilising the ETX values and the best neighbour selection of the direct transmission $9\to 1$ and the link $9\to 13\to 1$. We observed that the average ETX values from the direct transmission and the use of an intermediate node is quite good. Node 9 seems to prefer direct transmission with 49 messages being sent to the sink. On the other hand, it selected node 13 as the best neighbour to transmit its packets 5 times. Node 13 also exhibits good ETX and it sends its messages to the sink 54 times. In fact it is the only node it transmits to. Hence, we see that we have a limited opportunity for cooperation with these three nodes. We can see the number of messages transmitted in the above configuration in Figure 6.

Thereafter, the individual ETX values of all the messages of the links $12\to 1$, $12\to 7$ and $7\to 1$ in Figure 7a were plotted. Most of the ETX values of the direct link are between 28 and 60 with a few values between 10 and 20, while for the potential relay the values reside between 27 and 57 with one value between 1 and 10. Hence, the transmission to the best neighbour fluctuates between the two links since the ETX values are not very far away from each other in terms of quality. This is the reason why the selection as the best neighbour is split in terms of the number of messages sent. On the other hand, the link $7\to 1$ the ETX values are much better, making it a good candidate for the transmission of node 12 data towards the sink.

The individual ETX values of the links $9\to 1$, $9\to 13$, and $13\to 1$ have also been investigated. The direct link exhibits good ETX; thus, it is used as the best link by the CTP as we can see in Figure 7b. Furthermore, the relay node with the sink exhibits good performance. The most significant finding, though, is the fact that the potential relay node 9 is not utilised as the best neighbour, since the ETX performance of the direct link is mostly preferred. Hence, an example is observed that the direct link to the sink is used mostly.

4. Conclusions

In this paper, the robust CTP protocol has been investigated in terms of cooperative communication, with the aim to apply it to an infrastructure SHM task. An infrastructure can be monitored by placing the wireless nodes in a geographic manner and the CTP can take control. Testbed tests have been undertaken and specific links that could be used in terms of cooperation based on the quality of the link have been found. The fact that IEEE 802.15.4 technology can be employed for the SHM task adheres to the necessity of energy efficiency. The CTP, on the other hand, can ensure high transmission reliability; as such, using the transmission power adjustment can optimise the trade-off between energy efficiency and link quality. In this work, the testbed has been attempted to be operated in a pseudo-geographic manner, in order to show cooperative links.

Our future work includes the addition of a practical cooperative scheme to the CTP protocol, in order to further optimise its performance. Our intuition dictates that an addition of a cooperative packet id may produce even better results. Moreover, the testbed needs to be more accurately utilised, to the point possible due to its topology, in order to make the geographic manner of transmission more accurate. The testbed maintains a certain topology, and we did not take into account the nodes’ positions, since they could be affected by many uncontrolled parameters due to their position. This work aims to pave the way towards the cooperative version of CTP. Even though CTP’s mechanism changes the parent by default, that does not necessarily give us a cooperative nature. We only know that the best parent selection will be done. The extra field of the packet struct will provide a buyers sellers approach, which will take under consideration the next hop link quality, as well as that a node must transmit to a specific neighbouring node. Moreover, The experiments in this work are preliminary. Lastly, the aim is to setup a wireless network to our premises to better control the geographic nature of the cooperative CTP.