Abstract:
A system includes a plurality of networked nodes spatially distributed within a monitored area and adapted to interact with a mobile entity entering and moving through the area, the plurality of nodes including at least one area entrance node adapted to detect the mobile entity upon entering the monitored area, and a transaction node for accomplishing a transaction with the mobile entity; a transaction processing center in communication relationship with the nodes for processing data required for the transaction and for providing the data to the transaction node. A node selection unit is adapted to dynamically select the transaction node among the plurality of nodes based on an estimation of the speed of movement of the mobile entity.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to sensor networks and, more specifically, to sensor networks including sensor nodes spatially distributed in a given area supporting mobile sensor nodes entering the area. 
       DESCRIPTION OF THE RELATED ART 
       [0002]    Several commercial sensor technologies are known that are related to sensor networks, including some combination of sensing, remote signal processing, and communications. 
         [0003]    The advances in integrated circuit technology have enabled construction of far more capable sensors, radios, and processors at low cost, thereby enabling the mass production of sophisticated systems that link the physical world to networks. 
         [0004]    Sensor networks may exhibit both wireless and wired communications capabilities, using a common protocol; the flexibility of the design of the sensor networks enables a wide variety of applications, such as wireless mode vehicular applications. An exemplary application of sensor networks is in the “smart” management of the vehicles mobility, regarding safety, environment and economy. Sensor networks of this kind are called Intelligent Transport Systems (ITSs), and, by combining information technology and telecommunications, are capable of managing traffic-related information (for example, the number of vehicles in a given area, such as a parking area or a highway) and transactions related to the vehicles&#39; mobility (for example, toll collection at the entrance/exit of a parking area and of a highway). 
         [0005]    Many sensor networks have been proposed, with different features and architectures, adapted to satisfy the requirements of the different applications. 
         [0006]    For example, WO 0126335 discloses a method for distributed signal processing in a distributed network. The method comprises monitoring an environment by a plurality of sensor nodes receiving a plurality of signals from the environment. At least one user remotely accesses at least one sensor node of the plurality of sensor nodes by programming using at least one application program interface such that the at least one remote user accesses node processes running below an operating system of the at least one sensor node. 
         [0007]    US 2005270175 describes a traffic visual indicator device for mounting to a surface of a road. The traffic visual indicator device comprises a housing, a power producing source carried by the housing and operable to produce electrical power from energy derived from a source of energy externally located with respect to the traffic visual indicator device, an illumination source carried by the housing and positioned to transmit light out from the housing toward the traffic, a circuit carried by the housing, selectively coupling the power producing source and the illumination source, and a wireless communications subsystem carried by the housing and operable to at least receive wireless communications from an external source remotely spaced from the housing. 
         [0008]    US 2002004741 and US 2003001755 relate to automatic toll collection systems designed to automatically collect tolls through radio communication. In particular, US 2002004741 discloses a system for automatic collection of tolls from a vehicle moving along a roadway, comprising a toll collecting facility, installed on the roadway, collecting the tolls from an in-vehicle unit installed in the vehicle through radio communication with the in-vehicle unit, and an inspecting facility inspecting the in-vehicle unit when an abnormal condition in which it is impossible to collect the tolls from the in-vehicle unit correctly is encountered, said inspecting facility being located outside the roadway. US 2003001755 discloses an apparatus for collecting vehicle tolls in a toll collection environment having an upstream roadway wide area which leads to a downstream plurality of individual lanes. The apparatus comprises a transponder for location in a vehicle entering the upstream roadway wide area, at least one first reader adapted to communicate with said transponder in said wide area, at least one second reader adapted to communicate with said transponder in an individual lane downstream from said wide area, transaction means connected to said first reader responsive to communication with said transponder to calculate a toll associated with said transponder, means for transmitting toll payment status information from said transponder to the first reader, and means associated with the second reader for signaling the payment of a toll. 
         [0009]    US 2005255864 and US 2006015503 disclose methods of localizing a portable device through a wireless system. Particularly, US 2005255864 discloses a wireless position estimation method, comprising: statistically filtering time of flight information resulting from replies to queries to produce a plurality of range measurements, and calculating a position estimate as a result of processing a predetermined collected number of filtered said range measurements. US 2006015503 describes a method for opportunistically tracking the location of a portable device in a wireless infrastructure comprising at least one fixed station operable to communicate wirelessly with said portable device; the portable device provides its unique device identifier to the station when within communication range of said station; association data are generated, comprising the unique identifier with the location of said station, and said associated data are uploaded via a backchannel to a remote database wherein said data is stored. 
       SUMMARY OF THE INVENTION 
       [0010]    The Applicant has observed that while it would be desirable to deploy sensor networks made up of low-cost sensor nodes, of relatively short radio range and low communications throughput, these sensor networks may be unsuitable for the management of vehicular traffic, like for example for the automatic toll collection. Indeed, considering a sensor network including a plurality of sensor nodes distributed, for example, along a highway, where the vehicles move with a relatively high speed, the radio range of the sensor nodes may easily result limited compared to a distance run by a vehicle in the time necessary for example for completing the toll payment transaction with a remote service center. 
         [0011]    A similar problem is encountered in general whenever a sensor network is intended to interact with mobile units that move at a non-negligible speed within an environment monitored by a sensor network, like a parking area. 
         [0012]    In view of the state of the art outlined in the foregoing, at the Applicant has tackled the problem of providing a method of operation of a sensor network capable to dynamically configure itself for a transaction in accordance with a speed of a mobile unit in respect of which the transaction is to be carried out, and with an inherent operation time delay of the sensor network. 
         [0013]    The Applicant has found that the above problem can be solved provided that, based on the speed of the mobile unit, a forecast is made of which node of the sensor network will have to accomplish the transaction, and that node is selected for carrying out the transaction with the mobile unit. 
         [0014]    According to an aspect of the present invention, a system is provided comprising:
       a plurality of networked nodes spatially distributed within a monitored area and adapted to interact with a mobile entity entering and moving through the area, the plurality of nodes including at least one area entrance node adapted to detect the mobile entity upon entering the monitored area, and a transaction node for accomplishing a transaction with the mobile entity;   a transaction processing center in communication relationship with the nodes for processing data required for the transaction and for providing the data to the transaction node.       
 
         [0017]    A node selection unit is provided, adapted to dynamically selecting the transaction node among the plurality of nodes based on an estimation of the speed of movement of the mobile entity. 
         [0018]    According to another aspect of the present invention, a method is further provided of operating of a network of a plurality of sensor nodes spatially distributed within a monitored area and adapted to interact with a mobile entity entering the monitored area to accomplish a transaction, the method including: 
         [0019]    having at least one entrance node of the plurality of sensor nodes detecting the entrance of the mobile entity in the monitored area; 
         [0020]    estimating a speed of movement of the mobile entity; and 
         [0021]    dynamically selecting a transaction completing node among the plurality of sensor nodes based on the estimated speed. 
         [0022]    It is pointed out that, for the purposes of the present description and claims, by “sensor network”, “Wireless Sensor Network” (WSN), or “Wireless Sensor and Actuator Network” (WSAN) it is intended a network, made up of a plurality of (two or more) nodes capable of communicating with each other, and where the generic one of the network nodes may or may not be equipped with a sensor (and an actuator). Thus, the terms “sensor network”, WSN, WSAN are not to be construed as limited to networks made up of nodes equipped with sensors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The features and the advantages of the present invention will be made apparent by the following detailed description of a preferred embodiment thereof, provided merely by way of non-limitative example, description that will be conducted making reference to the attached figures, in which: 
           [0024]      FIG. 1  schematically depicts a sensor network according to an embodiment of the present invention, particularly a sensor network for electronic toll collection; 
           [0025]      FIG. 2  schematically shows a fixed, wireless sensor node included in the sensor network of  FIG. 1 , in terms of the functional blocks relevant to the understanding of the invention embodiment to be described; 
           [0026]      FIG. 3  schematically shows a mobile wireless sensor node, intended to be installed aboard a vehicle and adapted to interact with the sensor network of  FIG. 1 , in terms of the functional blocks relevant to the understanding of the invention embodiment to be described; 
           [0027]      FIG. 4  schematically illustrates a sensor node protocol stack exploited in the sensor nodes of the sensor network of  FIG. 1 ; 
           [0028]      FIG. 5  schematically shows a service center included in the sensor network of  FIG. 1 , in terms of the functional blocks relevant to the understanding of the invention embodiment to be described; 
           [0029]      FIG. 6  is a schematic flowchart describing a transaction procedure performed by the sensor network of  FIG. 1 , according to an embodiment of the present invention; 
           [0030]      FIG. 7  is a flowchart describing a wake-up mobile node procedure performed by the mobile sensor node of  FIG. 3 ; 
           [0031]      FIG. 8  is a flowchart describing a wake-up fixed node procedure performed by the fixed sensor node of  FIG. 2 ; and 
           [0032]      FIG. 9  schematically illustrates a portion of a sensor network numbers according to a further embodiment of the present invention, for pollution monitoring and cross-reference to car. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0033]    With reference to the drawings, in particular to  FIG. 1 , a sensor network according to an embodiment of the present invention is shown, denoted globally with reference numeral  100 , particularly a sensor network for electronic toll collection. 
         [0034]    The sensor network  100  is, in the non-limitative example herein considered, a Wireless Sensor Network Network (hereinafter shortly referred to as WSN), or equivalently a Wireless Sensor and Actuator Network (WSAN), including a number N of fixed sensor nodes  105   1 - 105   N  (ten, in the example shown in the drawing), spatially distributed, in respective, fixed positions, across a selected area, for example along a road  110 , as depicted in the drawing, e.g. a highway or an access/exit lane or ramp of a highway, particularly along an acceleration or a deceleration lane of the highway  110 . The fixed sensor nodes  105   1 - 105   N  are for example, but not necessarily, uniformly spaced apart from each other of about 10 m. 
         [0035]    The fixed sensor nodes  105   1 - 105   N  are devices equipped with a telecommunication system. The specific type of telecommunication system adopted for the sensor nodes of the sensor network  100  is not limitative to the present invention, however a suitable telecommunication system is the known ZigBee™ system, standardized by the ZigBee Alliance. As known to those skilled in the art, ZigBee™ is a standard specification for a suite of high-level communication protocols using small, low-power digital radio interfaces. 
         [0036]    Particularly, in an embodiment of the present invention, each fixed sensor node  105   1 - 105   N  includes a respective transceiver compliant to the ZigBee™ communication standard, which operates at a radio frequency of 2.45 GHz with a relatively low bit rate (250 kbit/s) within a short radio range, limited to about 100 m. Advantageously, the ZigBee™ transceivers are low-power and low-cost devices, that are capable of auto-configuring at set up, by acquiring a respective network address within the WSN  100 ; thus, the fixed sensor nodes  105   1 - 105   N  are adapted to form an ad-hoc network. Preferably, the fixed sensor nodes  105   1 - 105   N  are adapted to fall asleep, entering a very low power consumption mode, when they are inoperative, thus lowering their duty cycle and extending their battery life, as described in greater detail in the following. 
         [0037]    A vehicle  115  moving along the lane of the highway  110  and carrying a mobile sensor node  120  adapted to communicate with the fixed sensor nodes  105   1 - 105   N , for example compliant to the ZigBee™ communication standard, can connect to the sensor network formed by the fixed sensor nodes  105   1 - 105   N . 
         [0038]    When a mobile sensor node, like the mobile sensor node  120 , carried by the vehicle  115 , enters the radio coverage area of the sensor network formed by the fixed sensor nodes  105   i - 105   N , the mobile sensor node is “locked” by the WSN  100 , that assigns a network address thereto. 
         [0039]    As mentioned in the foregoing, the fixed sensor nodes  105   1 - 105   N , albeit having a limited radio range, are adapted to form an ad-hoc network. The WSN  100  is thus a multi-hop wireless network, adapted to ensure a relatively wide radio coverage, wider than the radio coverage of the single sensor node, by multiple hopping over multiple, relatively short wireless links each one corresponding to the distance between two adjacent fixed sensor nodes, as illustrated in the drawing by dashed lines connecting pairs of fixed sensor nodes  105   1 - 105   N ; accordingly, the fixed sensor nodes  105   1 - 105   N  not only act as application endpoints of the WSN  100 , but also as routers, forwarding RF signals in broadcast. 
         [0040]    The WSN  100  also includes a gateway node  125  that is adapted to act as an interface between the WSN  100  and a distribution network  130 . 
         [0041]    The distribution network  130  is a communication network, that may include one or more of a wireless communication network, e.g. a mobile telephony network, and a wireline telephony network. 
         [0042]    The distribution network allows the WSN  100  communicating with a remote service center  135 , which manages the provisioning to customers of the services the WSN  100  is deployed for, as well as the security and the authentication of the fixed and mobile sensor nodes  105   1 - 105   N  and  120  and the access to data required for the application. In detail, the service center  135  comprises a server  140 , connected to the distribution network  130  and thus in communication relationship with the WSN  100 , through the gateway  125 . The server  140  can access a database  145  storing data related to the fixed and mobile sensor nodes  105   1 - 105   N  and  120  and to the services to be provided to the customers. In addition, the service center  135  may include a billing processing module  150 , which, in the exemplary embodiment herein considered, is adapted to processing the data required to accomplish the toll collection. 
         [0043]    Considering  FIG. 2 , a fixed wireless sensor node  105   i  (i is an index taking values between 1 and N) of the WSN  100  is schematically shown, in terms of the functional blocks relevant to the understanding of the invention embodiment to be described. 
         [0044]    The fixed sensor node  105   i  includes a WSN unit  205 , for example, albeit not limitatively, an Integrated Circuit (IC) chip in which a microcontroller (μC)  210  and a μC enable module  215  are integrated. Furthermore, the WSN unit  205  includes a ZigBee transceiver (TRX)  220  and a TRX enable module  225 , and an Analog-to-Digital Converter (ADC)  23 Q (in alternative embodiments of the invention, one or more of the μC  210 , the μC enable module  215 , the TRX  220 , the TRX enable module  225 , and the ADC  230  may be discrete components, instead of being integrated on a same chip). 
         [0045]    The TRX  220  is coupled to an antenna  235  for the reception and the transmission of radio frequency (RF) analog signals. In turn, the antenna  235  is connected to a filter  240 , particularly a pass band filter of the fixed wireless sensor node  105   i , for example a filter having a pass band centered around about 2.4 GHz, for filtering the received RF signal; the output of the filter  240  is coupled to a Low-Noise Amplifier (LNA)  245 . The output of the LNA  245  is coupled to an average signal computing module  250 , adapted to evaluate an average power value of the received, filtered and amplified RF signal. The output of the average signal computing module  250  is coupled to the ADC  230 . 
         [0046]    The fixed sensor node  105   i  further includes a speed assessment sensor  255 , adapted to detect the speed of the vehicle  115  entering the area where the WSN  100  is deployed. The speed assessment sensor  255 , for example, exploits the Doppler effect, by means of a microwave detector for estimating the speed of the vehicle  115  in module and direction. The speed assessment sensor  255  is adapted to provide to the microcontroller  210  data denoted in the drawing by the reference SPEED which correspond to the estimated vehicle speed, and also to provide a wake-up signal WAKE-UP to the microcontroller enable module  215 , adapted to wake the microcontroller up (the wake-up signal WAKE-UP may for example be provided to an interrupt input of the microcontroller). 
         [0047]    It is pointed out that the speed assessment sensor  255  need not be present on every fixed sensor node, being sufficient that it is provided in the first fixed sensor node of the WSN  100  that a vehicle meets when it enters the area covered by the WSN  100  moving along the highway  110 , such as the fixed sensor node  105   1 . Also, the speed assessment sensor might be a component external to the fixed sensor node  105   i , being coupled to it for providing the indication of the estimated vehicle speed, and the wake-up signal. 
         [0048]    A power supply  260 , for example a battery (e.g., a couple of AAA batteries) supplies the fixed sensor node  105   i ; particularly, the power supply  260  supplies the speed assessment sensor  255  and the WSN chip  205 . 
         [0049]    Referring now to  FIG. 3 , the structure of the mobile wireless sensor node  120  carried by the vehicle  115  is schematically shown, in terms of the functional blocks relevant to the understanding of the invention embodiment to be described. 
         [0050]    Similarly to the fixed sensor nodes  105   i  the mobile sensor node  120  includes a WSN unit  305 , for example an IC chip where a μC  310  and a μC enable module  315  are integrated, together with a ZigBee TRX  320 , a TRX enable module  325  and an ADC  330 . 
         [0051]    The TRX  320  is coupled to an antenna  335  for the reception and the transmission of RF signals; the antenna  335  is connected to a filter  340 , particularly a pass band filter (similar to the pass band filter  240  in the fixed sensor node) for filtering the received RF signal. The output of the filter  340  is coupled to an LNA  345  adapted to amplify the filtered RF signal. The output of the LNA  345  is coupled to an average signal computing module  350 , adapted to evaluate the average power value of the received, filtered and amplified RF signal. The output of the average signal computing module  350  is coupled to the ADC  330 . 
         [0052]    The mobile sensor node  120  further includes a vibration assessment sensor  355 , adapted to detect the activity status of the vehicle  115  carrying the mobile sensor node  120 ; in detail, the vibration assessment sensor  355  is adapted to analyse the vibrations of the vehicle  115 , for example by means of an accelerometer, so as to establish if the vehicle is moving or not. The vibration sensor  355  provides an on/off signal ON/OFF to the μC enable module  315  in accordance with the estimated vibrations of the vehicle  115 , so as to enable the turning of the μC  310  on (if the vehicle is moving) or off (if the vehicle is steady). 
         [0053]    A power supply  360  supplies the mobile sensor node  120 , for example, a battery power supply (e.g., a couple of AAA batteries, as in the fixed sensor node  105   i ); particularly, the power supply  360  supplies the vibration assessment sensor  355  and the WSN unit  305 . 
         [0054]      FIG. 4  schematically illustrates protocol layer stacks  405  and  410  implemented in a generic fixed sensor node  105   i , structured according the Open System Interconnection (OSI) architecture. As known to those skilled in the art, the OSI architecture divides functions of a protocol into a series of layers; typically, each layer has the property that it only uses the functions of the layer below, and only exports functionality to the layer above (each layer performs services for the next higher layer and makes requests of the next lower layer). The sensor node  105   1  is structured in a layered fashion to enable the use of standard tools and to facilitate the design of the WSN from network connections to interoperation with the remote database accessed through an external distribution network. 
         [0055]    The protocol layer stack  405  is responsible of the sensing functions, and includes a sensor layer  415  and a sensing management layer  420 ; the protocol layer stack  405  manages the operation of the speed assessment sensor  255  included in (or coupled to) the respective fixed sensor node  105   i  for the detection of the speed of vehicles running along the highway. 
         [0056]    The protocol layer stack  410  is responsible of managing the telecommunication (TLC) functions, and manages the wireless communication with the other fixed sensor nodes  105   i  of the WSN  100  and with the mobile sensor node  120 . The protocol layer stack  410  includes an application layer  425 , an Application Program Interface (API) layer  430  coupled to the sensing management layer  420  of the protocol layer stack  405 , a network layer  435 , a Media Access Control (MAC) layer  440  and a physical (PHY) layer  445  (that defines all the electrical and physical specifications for the communications). 
         [0057]    With reference to  FIG. 5 , the service center  135  is schematically shown, in terms of the functional blocks relevant to the understanding of the invention embodiment to be described (in  FIG. 5  the billing processing  150  is omitted, for the sake of simplicity). 
         [0058]    According to an embodiment of the present invention, the service center  135  includes a distance evaluation module  505  adapted to receive from the fixed sensor nodes of the WSN  100 , particularly from the fixed sensor node  105   1 , the average speed value v m  of a vehicle, estimated by the speed assessment sensor  255  when the vehicle enters the area covered by the WSN  100 . The services offered by the service center  135  may relate to a single type of transaction, or to two or more transaction types; in the former case, the transaction to be carried out in respect of the vehicle entering the area covered by the WSN  100  is known a priori by the service center  135 ; if instead more than one transaction type is supported, the service center (e.g., the distance evaluation module  505 ) receives also an indication of the transaction type T, communicated by the mobile sensor node  120  to the fixed sensor node  105   1 . 
         [0059]    The distance evaluation module  505 , based on indications about the transaction length available to it, estimates a distance D covered by the vehicle  115  during the transaction processing; the estimated distance D is provided to a node identification module  510  of the service center  135 . The node identification module  510  has access to the database  145 , where the configuration of the WSN  100  is stored; in particular, each node address Node 1 -Node N , being the network address that identifies a respective fixed sensor node  105   1 - 105   N , is associated to a respective position indication that indicates the position of that sensor node in the WSN  100 , e.g. the position along the highway access ramp  110 . For example, the fixed sensor node  105   1  has position identified by a distance of 0 m, the fixed sensor node  105   2  by a distance of 10 m from fixed sensor node  105   1 , the fixed sensor node  105   7  by a distance of 40 m from fixed sensor node  105   1 , etc. In accordance with the information stored in the data base  145  and with the distance D estimated by the distance evaluation module  505 , the node identification module  510  is adapted to identify an address Node tgt  of the target fixed sensor node  105   x  which is the sensor node selected for completing the transaction with the mobile sensor node  120 . 
         [0060]    A transaction processing module  515  of the service center  135  is adapted to process the data required for the transaction and to transmit them to the selected, target fixed sensor node  105   x . 
         [0061]    For estimating the distance D, the distance evaluation module  505  exploits an estimated time duration T of the transaction T to be carried out, an estimated time Δt required for accessing the distribution network  130  and the service center  135 , and the estimated speed v m  of the vehicle  115 . In detail, supposing that the vehicle  115  moves with uniform rectilinear motion, the estimated distance D is given by 
         [0000]        D=v   m ·( T+Δt ). 
         [0000]    Based on the estimated distance D that the vehicle covers during the transaction processing, the node identification module  510  is capable of forecasting the position of the vehicle  115  at the time the transaction shall be completed, and thus determining the target fixed sensor node  105   x  in accordance with the respective position along the highway access ramp  110 . For example, if the average speed value v m  is of 50 km/h, the time duration T of the given transaction T of 1 second and the time Δt required for accessing the service center  135  of 2 seconds, then: 
         [0000]    
       
         
           
             D 
             = 
             
               
                 
                   
                     
                       
                         50 
                         · 
                         1000 
                       
                        
                       
                           
                       
                        
                       m 
                     
                     
                       3600 
                        
                       
                           
                       
                        
                       s 
                     
                   
                   · 
                   
                     ( 
                     
                       1 
                       + 
                       2 
                     
                     ) 
                   
                 
                  
                 
                     
                 
                  
                 s 
               
               = 
               
                 41.7 
                  
                 
                     
                 
                  
                 m 
               
             
           
         
       
     
         [0062]    Under the assumption that the fixed sensor nodes  105   1 - 105   N  are uniformly space apart from each other by a distance Δx equal to 10 m, then a node distance D x  of the target fixed sensor node  105   x  from the fixed sensor node  105   1  is obtained by: 
         [0000]    
       
         
           
             
               
                 D 
                 x 
               
               = 
               
                 
                   [ 
                   
                     D 
                     
                       Δ 
                        
                       
                           
                       
                        
                       x 
                     
                   
                   ] 
                 
                  
                 Δ 
                  
                 
                     
                 
                  
                 x 
               
             
             , 
           
         
       
     
         [0000]    i.e., the node distance D x  is the integer part of the amount D/Δx multiplied by the distance Δx between two fixed sensor nodes  105   1 - 105   N . Accordingly, the service center  135  can select the fixed sensor node  105   7 , which has a distance of 40 m from the fixed sensor node  105   1 . 
         [0063]      FIG. 6  is a schematic flowchart describing a transaction procedure, according to an embodiment of the present invention, performed by the WSN  100 . 
         [0064]    Initially, before the vehicle  115  enters the area of the WSN  100 , all the fixed sensor nodes  105   1 - 105   N  of the WSN  100  and the mobile sensor node  120  are in “sleep mode”, i.e. their transceiver  220  and  320  is turned off. 
         [0065]    When the vehicle  115 , running along the highway  110 , enters the area of the WSN  110 , the movement of the vehicle  115  is detected by the fixed sensor nodes  105   1 , equipped with the speed assessment sensor  255  and placed at the entrance of the WSN  100  area (block  605 ). The speed assessment sensor  255  estimates the speed of the vehicle  115 , and asserts the wake-up signal WAKE-UP (block  610 ): the microcontroller  210  is thus woken up. 
         [0066]    The microcontroller  210  further receives the indication SPEED of the estimated average vehicle speed value. The microcontroller  210  assesses whether the estimated average vehicle speed exceeds a predetermined speed threshold value: if the average speed value is higher than the threshold speed value, then the fixed sensor node  105   1  understands that a vehicle is entering the WSN  100  area, and that the whole WSN  100  has to be enabled for a transaction. The transceiver  220  of the fixed sensor node  105   1  is consequently enabled and starts sending wake-up commands to the other fixed sensor nodes of the WSN  100 . In an embodiment of the present invention, the wake-up commands may be in the form of beacon Signals, that the fixed node  105   1  broadcasts to the other nodes of the WSN  100 . The beacon signals that the fixed sensor node  105   1  broadcasts allows waking-up, enabling both the mobile sensor node  120 , carried by the detected vehicle  115 , and all the other fixed sensor nodes  105   2 - 105   N . In fact, the beacon signals are received by the fixed sensor nodes  105   2  and  105   3  and the mobile sensor node  120  which are in the radio range of the fixed sensor node  105   1 ; the TRX enable modules  225  in the fixed sensor nodes  105   2  and  105   3  and  325  in the mobile sensor node  120  assess if the average power of the received beacon signal exceeds a prescribed, minimum threshold value, then the transceivers  220 ,  320  are enabled. The fixed sensor nodes  105   2  and  105   3  that have been woken up, as well as the mobile sensor node  120 , starts in turn broadcasting beacon signals, which are received by the fixed sensor nodes in their radio range, still in sleep mode, and these sensor nodes are thus woken up. Then, when the presence of the vehicle  115  wakes up the first fixed node  105   1  of the WSN  100 , a “wake-up” wave of beacon signals is generated, that progressively wakes up all the fixed sensors nodes  105   1 - 105   N  of the WSN  100  and the mobile sensor node  120 . 
         [0067]    When the mobile sensor node  120  wakes up, the first fixed sensor node  105   1  assigns thereto a respective network address and, accordingly, the mobile sensor node  120  is “locked” by the WSN  100 . 
         [0068]    After the “wake-up” wave is generated, a transaction, such as a toll collection, starts (block  615 ) and, according to the present invention, in the following steps of the transaction procedure the fixed sensor node  105   1 - 105   N  that will have to accomplish the transaction is individuated, for example by the service center  135 , as described above in connection with  FIG. 5 . 
         [0069]    Then, the fixed sensor node  105   1  provides to the service center  135  the estimated average speed of the vehicle  115  (block  620 ); this is done by a multi-hop routing through the fixed sensor nodes  105   2 - 105   6  to the gateway node  125 , and then through the distribution network  130 . 
         [0070]    If the transaction type is not known a priori (block  625 ), then the service center  135  receives also this information (if this information is not automatically provided by the mobile node, the service center may request it—block  630 ). After receiving the indication of the transaction type, the service center  135  determines the corresponding estimated transaction time T (if the transaction type is fixed, the transaction procedure moves directly to the block  635 , because the transaction time T is known a priori). 
         [0071]    The service center  135  exploits the transaction time T, the access time Δt and the estimated average speed value v m  for estimating the distance D covered by the vehicle  115  during the accomplishment of the transaction (block  635 ). The service center  135  also reads from the database  145  the data regarding the distribution of the fixed nodes in the area covered by the WSN  100  (block  640 ). 
         [0072]    The service center  135  then determines the target fixed sensor node  105   x  that will have to complete the transaction with the mobile sensor node  120  and transmits thereto the data necessary for completing the transaction, through the distribution network  130 , the gateway node  125  and a multi-hop routing accomplished by the other fixed sensor nodes (block  645 ). 
         [0073]    The toll collection is accomplished by charging, for example, a credit card account of the driver of the vehicle  115 . 
         [0074]    In case the fixed nodes  105   1 - 105   N  detect an irregularity, such as the entrance into the WSN  100  area of a vehicle without a sensor node, the gateway node  125 , an Optical Character Recognition (OCR) system (not shown in the drawing) may be provided for, adapted to detect a number plate of the vehicle. The number plate is then communicated to the service center  135 , so as to successively identifying the vehicle owner and taking the necessary steps for billing the toll thereto. 
         [0075]    Advantageously, the WSN  100  according to the herein described embodiment of the present invention is able to perform a transaction with a mobile sensor node  120  without the use of any gate, because it is not necessary to have the vehicle slow down and, possibly, stop. In fact, the WSN  100  interacts with the mobile sensor node  120  and accomplishes the transaction despite of the speed of the vehicle  115  within the radio coverage area of the WSN  100 . Accordingly, by means of the WSN  100 , tollgates typically Installed at entrances and exits of highways and closed parking areas may be eliminated. This allows avoiding the related set-up and maintenance costs, as well as the slowing of the traffic. 
         [0076]    In  FIG. 7 , a flowchart describing a wake-up mobile sensor node procedure is illustrated. 
         [0077]    The vibration assessment sensor  355  continuously assess the level of vibrations, and waits for a movement of the vehicle  115  to be detected (block  705 , and decision block  710 , exit branch N). When a movement is detected (block  710 , exit branch Y), then by asserting the on/off signal ON/OFF vibration assessment sensor  355  enables the microcontroller  310  (block  715 ). 
         [0078]    The TRX enable module  325  waits for receiving RF signals from the fixed sensor nodes (block  720 ); in detail, any detected RF signal is filtered by the pass band filter  340  and amplified by the LNA  345  and, successively, the average signal computing module  350  evaluates its average power value, feeding it to the TRX enable module  325 . The TRX enable module  325  assesses whether the average power value of the RF signal is higher than a prescribed minimum threshold, meaning that the mobile sensor node is within an area covered) by a WSN (block  725 ); in the negative case, the mobile sensor node  120  waits for receiving a valid RF signal, otherwise, if the average power value of the RF signal is higher than the threshold value, the TRX enable module  325  enables the TRX  320  (block  730 ). 
         [0079]    Accordingly, the application necessary for performing the transaction with the service center  135  (described with reference to  FIG. 6 ) is started (block  735 ). 
         [0080]      FIG. 8  shows a schematic flowchart describing a wake-up fixed sensor node procedure. 
         [0081]    Initially, the WSN  100  is formed (after the sensor nodes have been positioned and have been powered up), and each fixed sensor node  105   1 - 105   N  acquires automatically a respective network address within the WSN  100  (block  805 ). After the set-up of the WSN  100 , the microcontroller  210  of each fixed sensor node  105   i  puts the sensor node, particularly the transceiver  220  in sleep mode (block  810 ). 
         [0082]    Then, the generic fixed sensor node  105   i  waits until a movement is detected by the speed assessment sensor  255  (block  815 ), if the considered sensor node is equipped with or coupled to it, like in the case of the fixed sensor node  105   1 . In case a detected average speed value is lower than a threshold speed value, or in case the fixed sensor node  105   i  is not equipped with or coupled to the speed assessment sensor  255 , the fixed sensor node  105   i  tests if any RF signal is received (block  820 ). As described for the mobile sensor node  120 , a received RF signal is filtered by the pass band filter  240 , amplified by the LNA  245  and, successively, the average signal computing module  250  evaluates its average power value and provides it to the TRX enable module  235 . The TRX enable module  235  compares the calculated average power value of the RF signal to a threshold value: if the average power value of the received RF signal is lower than the threshold, the fixed sensor node  105   i  waits until a moving vehicle is assessed, or a valid RF signal is received. If the average power value of the RF signal exceeds the threshold value, then TRX  220  is enabled (block  825 ). 
         [0083]    After the TRX  220  is enabled, the fixed sensor node  105   i  starts broadcasting beacon signals (block  830 ), i.e. it generates or propagates the wake-up wave that wakes up the other fixed sensor nodes  105   i . 
         [0084]    Finally, an application is started (block  835 ) for performing the transaction with the mobile sensor node  120  on the vehicle  115 . 
         [0085]    The wake-up procedures according to the described embodiment of the present invention allows greatly reducing a power consumption of the mobile and fixed sensor nodes of the WSN, by enabling the microcontrollers and the transceivers only when required. The microcontroller of the mobile sensor nodes needs to be enabled only when a vehicle carrying it is moving, and the transceiver of the mobile sensor node needs to be enabled only when a RF signal is sensed. Similarly, the microcontroller of the fixed sensor nodes needs to be enabled only when a movement is detected or an RF signal is received. If the mobile and fixed sensor nodes sense an RF signal, then the microcontrollers enable also the transceiver, which in the opposite case are kept turned off. Accordingly, the WSN is energy-efficient, because the microcontrollers and the transceivers are kept off most of the time, and this allows overcoming a problem of power consumption that typically affect sensor networks known in art, especially multi-hop sensor networks, where the nodes act also as routers. 
         [0086]    Considering now  FIG. 9 , a WSN  900  according to another embodiment of the present invention is shown; particularly, the WSN  900  is exploited for a pollution monitoring and a cross-reference to car numbers, for example, in a urban area  902  (in the drawing, reference numerals like those used in  FIG. 1  denote same or similar elements). 
         [0087]    The WSN  900  includes a plurality of fixed sensor nodes  905  arranged within the urban area  902  and along a plurality of roads  910  accessing the urban area  902 . Vehicles  915  running along the roads  910  and equipped with respective mobile sensor nodes  920 , are “locked” by the WSN  900  when they enter in an area covered by the RF signals provided by the fixed sensor nodes  905 , in the same way as in the previously described embodiment. 
         [0088]    Along each road  910  one fixed sensor node has the additional function of gateway node  925  for communicating with a distribution network  930  in turn coupled to a service center (not shown in the drawing) that is adapted to process the data collected by the fixed sensor nodes  905 . Also, within the urban area  902  some fixed sensor nodes operate as gateway nodes  925  for communicating with the distribution network  930 . 
         [0089]    The WSN  900  allows counting the number of vehicles  915  that are within the urban area  902  and, accordingly, to evaluate the pollution versus the numbers of vehicles  915 , as shown in a graph included in  FIG. 9 . When a vehicle  915  approaches the WSN area, the WSN  900  “locks” the respective mobile sensor node  920 . The fixed sensor node  905 , detecting the presence the mobile sensor node  920 , communicates the information to the nearest gateway node  925 , by a multi-hop routing; the gateway node  925  in turn provides the information to the service center through the distribution network  930 , for the data processing. In a similar way, the fixed sensor nodes  905  signal to the service center when a vehicle  915  leaves the urban area  902  or when the vehicle  915  stops within the urban area  902 , for example, by parking or because it is a car of a resident of the urban area  902 . 
         [0090]    Although the present invention has been disclosed and described by way of embodiments, it is apparent to those skilled in the art that several modifications to the described embodiments, as well as other embodiments of the present invention are possible without departing from the scope thereof as defined in the appended claims. 
         [0091]    For example, the sensor nodes could be compliant to a communication standard different from the described ZigBee™ standard. The area covered by the sensor network might be different from a highway or a urban area, and the mobile sensor nodes are not necessarily mounted on vehicles. The architecture of the mobile and fixed sensor nodes might be different and, particularly, the sensor nodes might include other modules with different functions. The sensor network might also be or include wireline connections between the sensor nodes. The same network can perform a plurality of types of transactions and can be exploited for a number of applications simultaneously. The service center can include more than one data base and more than one transaction processing module; furthermore, the transaction processing module can be included in, or directly coupled to, one of the sensor nodes.