Analysis of the performance of the network has begun with the statistics of the packets sent by all the nodes during the experiment. Packets are considered dropped when 1 packet has been retransmitted 8 times without receiving the link-level acknowledgement. The obtained results Fig. The small percentage of dropped packets reflects significant efficiency of the XMesh routing protocol in such hostile environments.
It can be also noticed that the huge amount of forwarded packets comes from the nodes 2, 3, 5, 6, 7 and 11, due to the location of these nodes in the stairways. The whole network connectivity is based on the convenient location of these nodes. Nodes in upper decks route their data mainly through stairway nodes, as the radio signal penetration is impossible through metallic ceiling and floors. In order to improve knowledge about radio propagation inside the vessel, the paths followed by packets from source nodes towards the base station have been studied.
As previously stated, the sensor nodes are pre-programmed by the Crossbow XMesh routing protocol. Therefore, a sensor node selects the next hop which minimizes the number of transmissions required to send a packet to the base station. The selected parent node next hop node is then characterized by its closeness to the base station in term of number of hops and its good link quality with the source node.
Hence, the choice of the next hop may be an indicator of the quality of links between a sensor node and its one-hop neighbours. Table 2 shows the parent nodes of each sensor node and the percentage for each one during the test.
Several remarks may be drawn from this table. It can be noticed that node 8 has never selected node 3 or node 7 as parent node despite the small distance between them. This behaviour is in agreement with the statement that two nodes separated by two closed watertight doors cannot be connected in the parking nodes 3 and 7 are located in two stairways with closed doors. However, this connection remains possible when only one closed watertight door separates the two nodes which is the case of nodes 6 and 8.
This was expected, as these three nodes are located in the engine room where the probability of outage between two nodes is very low due to the low path loss exponent. Node 4 has always node 3 as parent node. Connection between the crew's deck and the parking is not possible because the watertight door in the entrance of the parking is closed.
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In spite of the small distance between nodes 4 and 1, the connection between them is impossible since the engine room and the crew's deck are separated by a thick metallic bulkhead. This can be explained by the fact that the two watertight doors of the first stairway between the engine room and the parking and the first stairway between the parking and the passenger deck are opened.
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These two nodes used the intermediate nodes located in the stairways nodes 7 and 2 for the remaining time when the direct connection becomes impossible. The direct connection between nodes 12 and 11 is probably provided by the signal reflection on the metallic tour upside the non-covered passenger deck. Wireless communications between adjacent rooms are possible in the presence of non-conductive materials in the common bulkhead.
Watertight doors are the main source of radio leakage between adjacent rooms. Closing a watertight door induces an attenuation up to 25 dB. These conclusions are used in this section to propose an architecture for a large-scale shipboard WSN.
Wireless Sensor Networks and Applications | SpringerLink
As previously stated, the shipboard monitoring system may contain several thousands of sensors located in all compartments. Some rooms, such as the engine rooms, may contain hundreds of sensors. Using the previously stated concluding remarks, a hierarchical WSN architecture adapted to the particular characteristics of the shipboard environments is proposed. In this architecture, the network will be divided into groups and different nodes levels are defined, based on the functions and resources of nodes.
The radio propagation study has shown that the metallic structure of ships makes each room which is similar to a metallic cube quasi isolated from a wireless propagation point of view. Therefore, it has been decided to divide the network into zones where each metallic room is a zone. Three types of nodes may be found in this architecture: Sensor Nodes SN which collect sensing data from the environment, Border Nodes BN which collect data from SNs, and Gateway Nodes GN which collect data from the BNs and send them through a wired connection to the central processing units.
Wireless Sensor Network Architecture and Its Applications
Two types of wireless communications are distinguished: the intra-zone communications and the inter-zone communications. This level is constituted of SNs distributed in all ship rooms. Different data may be measured by these nodes such as temperature, pressure, humidity, fire, tank level, water level, etc. One SN may be connected to several sensors if their locations are close case of the engine room where hundreds of sensors are located in a small area. If SNs are powered by batteries, their power consumption must be optimized. As the radio unit Tx and Rx consumes the most of the energy, it must be in the sleep mode as much as possible.
Therefore, the number of transmissions must be optimized. In the confined metallic rooms, one-hop communication is sufficient between any nodes placed in the same room. Sensor nodes will not be intended to forward data from other nodes, which can greatly reduce their power consumption. Radio units are then turned on only when sensor nodes want to send their sensing data to the border node.
These data may be periodic or event driven. It is not necessary that a SN sends its data continuously to its BN. Instead, it saves the last sent data and continues to sense its environment. Measured values will be compared firstly to HT. If it exceeds this value higher or lower depending on the application , the data will be sent. If not, the difference between the last value and the measured value will be compared to ST. If the difference exceeds ST, the value will be sent. This procedure reduces the number of transmissions to only urgent cases exceeding HT or to important value changes exceeding ST.
As the IEEE Another contention free mechanism is possible in this standard for critical applications. Border Nodes BNs are the second level of the proposed architecture. Each BN is responsible of all or a part of the sensor nodes in a metallic room. BNs are placed in front of doors borders of each room. More than one BN may be placed in a room if it has several doors, giving multiple choices for SNs to join the network.
SNs send their data to BNs via one-hop intra-zone communication. BNs query and gather sensed measurements from SNs, and aggregate collected data eliminate redundancy before sending to the base station via multi-hop inter-zone communication. Different routing protocols may be adopted for inter-zone communication.
Regarding the critical role of a BN it is responsible of a cluster of sensor nodes , it must be always powered on. BNs may be powered by the mains supply of the ship. Therefore, the inter-zone routing protocol does not have to optimize the energy consumption of these nodes. Instead, the link quality and the number of hops to the base station must be optimized.
XMesh, the used protocol in the network test, or other routing protocols existing in the literature, may be used for inter-zone communications. Gateway Nodes GNs aggregate data from the network, interface to the host, the Ethernet or the Internet through satellites connections. Gateways form bridges to send and receive data between the central repository and the sensor network.
Similarly to BNs, gateways play a vital role in the network. Hence, they are always powered by the mains supply of the ship. Depending on the network size on board the ship and the technology adopted, one or more gateways may be used.
Building Wireless Sensor Networks
In case of multiple gateways, each gateway will form a sub-network using a frequency sub-band and all gateways will be connected to an Ethernet installed on the ship. This mechanism increases the network scalability and decreases the collisions rate. Data aggregated by the gateways are sent to a central repository located usually in the control room or in the wheel house of the ship. Data are analyzed and conclusions concerning the current state of each room are drawn. Central data repository is equipped with a user visualization software and a graphical interface for managing the network and showing measured data.
This section presents the performance analysis of the proposed architecture. It includes the network simulator description, the used standard and the simulation scenarios. OPNET is a discrete-event and object-oriented simulator. Strength of OPNET in wireless network simulations is the accurate modelling of the radio propagation. Different characteristics of physical-link transceivers, antennas and antenna patterns are modeled in detail. In OPNET, the possibility of wireless link between a transmitter and a receiver depends on many physical characteristics of the involved components, as well as time varying parameters, which are modeled in the Transceiver Pipeline Stages.
Parameters such as frequency band, modulation type, transmitter power, distance and antenna pattern are common factors that determine whether a wireless link exists at a particular time or can ever exist. Studying the performance of the shipboard WSN architecture must be preceded by a realistic modelling of the shipboard environments.