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Intelligent Transportation Systems are based on information and communication technologies employed in vehicle systems and transport infrastructure. The factors that paved the way for ITS, were rapid growth in population, motorization and urbanization. Various ITS technologies adopted are based on wireless communication, computational technologies, Floating Car Data, sensing technologies, inductive loop detection and video vehicle detection. Intelligent Transport systems aims in improving travel safety eliminate traffic congestion which in turn reduce time travelled and provide low fuel consumptions which are done by effective implementation of most modern technologies. The different application of 'ITS' are Electronic Toll collection, emergency vehicle notification system, congestion pricing, automatic road enforcement, collision avoidance systems and dynamic traffic control systems. As mentioned above wireless communication and sensing technologies are two major ITS technologies in which most research and development are done. This paper proposes a brief study on integration of a wireless sensor network for Intelligent Transport System which helps in assisting drivers with the traffic information. Wireless sensors are small smart and cheap sensors which are integrated with wireless interface. Different sensors communicate each other and form a network. The ITS network described here is based on a sensing unit and a mobile station thereby providing suitable traffic information, to the driver which helps in making decisions.
Keywords: Intelligent Transportation systems (ITS), wireless sensor networks, sensing unit, mobile station.
With the growing number of vehicles and rarified transportation system, assurance of a safe and efficient traffic system is becoming more reliable. Use of advance monitoring technologies such as traffic flow of the network, vehicle density in an area, emergency situations on road, etc can be obtained. Specific communication methods used make it possible to transfer relevant information to drivers in time to take precise judgment and set decision according to known situation, and thus severe accidents can be avoided and this helps to improve the safety and efficiency of the whole transportation system .
Wireless sensor networks help to achieve this goal. There are various sensor nodes along the roads, in which multi in formations such as weather condition, quality of air on road, the number of vehicles passing through a certain time in a place, the situation happening on the road etc, can be monitored rapidly and cooperatively. The use of wireless sensor networks in intelligent transportation system depends on the advantages of these sensor networks. Dense sensors can improve the comprehended results and provide a variety of information by adding together all the information received from the resources, which are then complemented and processed by data processing units. Since a decentralized sensing unit consisting various nodes, it will be inherently more robust against failures that happen in individual sensing nodes and adapts to existing traffic infrastructure.
Most of the studies on wireless sensor networks are based on the development of sensor modules with capabilities of embedded computation and communication, along with routing and networking protocols. Also there are a few studies focused on development of feasible network architectures.
The paper brings concepts of sensor networks with mobile agents into a new two-tiered architecture. The lower tier is a single-hop network in which data's can be transferred to the mobile station from the sensor directly by one hop rather multi-hop and clustered techniques in traditional sensor networks and the upper tier forms a decentralized data fusion network. The sink node in the architecture is the mobile station, i.e. on a vehicle so that it's mobile and collects data in a part of the network. Mobile station performs not only data collection and transfer but also process the data received. The architecture proposed has the following advantages:
High-energy efficiency: The sensor nodes directly transmit their data to mobile station without routing through other nodes and mobile station is the only terminal that receives the data transmitted. Therefore this architecture could save a significant amount of energy compared to the traditional sensor networks having multi-hops
High flexibility and scalability: The various sensor nodes can be distributed decentralized with considerable arbitrariness, so that network can be easily changed or updated. Moreover data fusion and processing functions are done by the mobile stations fitted on each vehicle using the centralized management devices. This helps travelers to join or depart from the system and this makes the network scalable and flexible.
High fault tolerance: Due to the implementation of numerous sensor nodes and the independence of the mobile stations, the system has high robustness. The failure or blockage of a few sensor nodes due to lack of power or any other reason does not affect the network and still performs normally. Similar is the case with a damaged or departed mobile stations on equipped on the vehicles, the system will not be interrupted.
Low cost: As the functions of a sensor node increases the cost of the equipment also increases, but here we need to only invest for the deployment of sensor units, which performs simple functions as sensor nodes. Also there is no change needed on the architecture for existing transportation systems.
Powerful on-vehicle device: Other than the necessary functions such as collection, user interface, data processing, network construction, the on-vehicle mobile stations can be integrated with existing ITS technologies such as guiding system, electronic maps, etc..
The paper provides information about the two-tiered network architecture in section 2. Hardware and software designs, for the mobile station and sensing unit on section 3. Section represents a scenario explaining the working of the system. Finally, conclusions in section 5.
2. Architecture of two-tiered sensor network
a) Network architecture overview
Architecture of a two-tiered sensor network is shown in Fig 1.
Here the sensor nodes are densely positioned along the roads called sensor units. The main function of these sensing units is to detect information about the environment. There are different types of sensing units for collecting various information around the road such as temperature, air quality, humidity as well as traffic situation can be detected and collected using methods like infrared ray, electrical technologies and camera module embedded in the sensing unit. Finally the data is transferred to the mobile stations.
The mobile stations that fitted on the vehicle handle data collection, delivery and processing. The mobile station collects the data detected by sensing unit within its coverage. During the same time the data can either be delivered to or received from other mobile stations to exchange the information from different network parts. These delivered and received data can be analyzed and processed to draw conclusions with the help of software's. The instructions to the drivers are passed through a user interface and this guide the driver to take right decision while driving.
b) Lower tier
The lower tier network consists of sensing units and a mobile station forming a single hop network which is entirely different from the clustered network.
The Fig 2 below shows a traditional clustered sensor network. As u could see the nodes are divided into clusters and in each cluster there is a sink node that collects data from the sensor node either through multi hop or single hop in the same cluster. Similarly the sink node passes the data to base station.
In this architecture proposed the sensor nodes directly transfer the data to the mobile station without routing to other nodes whereas the mobile station takes the role of base station as in case with clustered network. To ensure all the data is collected, the mobile station should be mobile; this enables it to travel covering all the coverage area of the sensor network. But the mobile station does not cover the whole network, so the information collected is limited and this can be solved by the upper tier of the network. Since the sensing unit does not need supporting routing, its communication function is simpler taking the clustered sensor network node into account and for this it cost less energy. The architecture of the lower tier network is shown in the Fig 3.
The network is scalable since the sensing units are decentralized and discretely distributed. The only thing needed to upgrade or enlarge the network is by increasing the sensing units. The communication between sensing unit and mobile station takes place at 2.4 GHz industrial scientific standard (ISM) band with the help of Bluetooth protocol. Bluetooth is an open specification for short range communication of voice and data between stationary and mobile devices. It has a bandwidth of 1 Mb/s and this satisfies the lower tier because the data transmitted from sensing unit to mobile station is not large.
c) Upper tier
The upper tier of the network is shown in the Fig 4 below.
The mobile stations receives and process the data from the sensors within its coverage area and this forms the base station in the lower tier, while works as a P2P network in the higher tier. It receives data from mobile stations and delivers this to other mobile stations at different portions of transportation network. This technique helps in avoiding problems resulting in limited range of certain mobile stations.
There are two operational modes of upper tier. The first mode forms several Ad-hoc strategy sub-networks of wireless local area network (WLAN) in which mobile stations form a netted structure and communicate each other without routing through other mobile stations. The second mode forms an Ad-hoc multi-hop network in which data exchange between mobiles stations take place directly and route through others, therefore a P2P network is created. Thus a mobile station can collect all data from the network with a joint operation of all mobile stations according to the P2P protocol of application layer. As number of mobile station increases efficiency of the network also increases.
Since mobile stations form a P2P network the mobile stations on the vehicle can break or join the network easily and this do not affect the running of the network. This makes the network flexible. 802.11b is the media access control (MAC) protocol applied for the upper tier network which works on 2.4 GHz ISM band but with much wider bandwidth of 11 Mb.
3. Design of sensor unit and mobile station
The network mainly consists of the sensing unit and the mobile station. The hardware and software architecture of them is described below
a) The hardware design of sensing unit
The main job for the sensing unit is to monitor the situation occurring in its surroundings. So the sensing unit is an important factor and selected according to the various tasks to be performed. Other than sensing it must be able to communicate with the mobile station so a 2.4 GHz radio module supporting Bluetooth standard must be incorporated. A processor of low performance is chosen since sensing unit function is simple. Also peripheral components like memory and a power module (onboard battery) is required. The hardware architecture is shown I Fig 5.
b) The software design of sensing unit
The software contains a MAC protocol module and an elementary task module which helps in data storage functions. The software is an operating system independent. The communication function for the sensing unit is simple so there is not much capital involved. The software architecture is shown in Fig 6.
c) The hardware design of mobile station.
Mobile stations are much more complicated than sensing unit. The hardware architecture is shown in Fig 7.
The mobile station central controller should be a performance microprocessor. These mobile stations include two communication modules which separately implement the WLAN 802.11b and Bluetooth protocol. High memory is required and for the integration of mobile stations on vehicles some I/O interfaces are also required. Energy supply is provided by an outer source.
d) The software design of mobile stations
For providing an efficient navigation information for the users different function modules like analyzing, fusing, data processing, and forming P2P networks has to be realized in the software. It should also be able to incorporate other existing modules like electronic map and positioning systems. It should also have an interfacing system in which driver could understand the results drawn from the data processed.
Considering the above functions to be performed an embedded operating system is designed which supports IPv6 in network layer and Bluetooth and WLAN protocol at the MAC layer. The application software is developed including modules such as task management, data analysis and processing, P2P, user interfaces etc. The Fig 8 below shows the software architecture for mobile stations.
4. An illustrative scenario
A situation is shown below in Fig 9, in this scenario, the road is jammed because of a traffic accident.
The proposed two tier architecture informs the driver about the situation and helps him to take a proper decision thereby improving efficiency of traffic system.
A WLAN Ad-hoc mode is assumed here. Consider the four intersections namely a-d in the figure generated by the two-tiered wireless sensor network. Links X and Y are two streets connected to intersections 1 and 2 respectively.
An accident has occurred on the intersection 4, i.e. there is a traffic jam. The sensing unit within coverage area 1 senses some unusual data and these data are then transmitted to available mobile station. So mobile stations on vehicle B receive the transmitted data by some sensing unit within coverage areas 1 and 2. Later mobile stations on calibrate the data and these processed information is stored. This is done by data analysis software on the mobile station.
Consider after 10 minutes, vehicle B reaches the adjacent place of intersection A and enters into the coverage area of the mobile station on vehicle A travelling on link X to the direction shown in Fig 9. They exchange information and vehicle A gets information from the mobile station about the traffic jam on intersection 4.
The aim of vehicle A is to reach link 2 which can realized in two different paths namely 1 and 2. Path 1 is: link X → intersection 1 → intersection 4 → intersection 3 → link Y; and path 2 is: link X → intersection 1 → intersection 2 → intersection 3 → link Y. Obviously shortest path is considered.
Original street construction knowledge is stored in the mobile station of Vehicle A i.e. time 1, which is the time required for vehicle A to reach link Y through path 1 is 10 minute and time 2, the time required for vehicle A to reach link Y through path 2, is 15 minute. Time 1 is obviously lee than time 2.
But after the exchanging the data with mobile station on vehicle B, mobile station on vehicle A refreshes the estimate of time 1. This is added to the wasted time due to the traffic jam. This is estimate by the mobile station onboard software. Let this be 10 minutes. Therefore time 1 becomes 10+10=20 minutes which is definitely larger than the time 2. This result is made into account to the driver by the mobile station and suggests the driver to change the direction to path 2. Eventually the efficiency of transport system is improved.
Two-tiered intelligent transportation system network architecture is proposed to obtain information regarding the environment and guide the driver to take best decision during driving. This is based on single hop wireless sensor and P2P network which is decentralized. The system provides efficient energy use, fault tolerance, flexibility and less capital investment. Components are described and its operation is explained by an illustrative scenario. The study can further be elaborated on detailed issues like overlay protocol performance analysis, data aggregation and optimum estimate means.