Abstract:
A method of synchronizing a network includes transmitting a tone signal to convey time information and setting a local time according to the conveyed time information. The method may also include detecting an occurrence of a predefined aspect of the tone signal, and setting a local time based on the occurrence of the predefined aspect of the signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. Patent Applications entitled “Method and System for Providing Acknowledged Broadcast and Multicast Communication” Ser. No. 11/240,401, “Method and System for Providing an Energy Efficient Exchange of Information in Wireless Networks” Ser. No. 11/239,837, “Method and System for Providing Interference Avoidance and Network Coexistence in Wireless Systems” Ser. No. 11/240,545, “Method and System for Reliable Data Transmission in Wireless Networks” Ser. No. 11/239,836, “Method and System to Reduce Delay and/or Energy Consumption in a Multi-Hop Wireless System” Ser. No. 11/240,436, “Method and System for Providing a Modified Time Division Multiple Access (TDMA) for Reduced Delay” Ser. No. 11/241,639, “Method and System for Providing Reliable Communication with Redundancy for Energy Constrained Wireless Systems” Ser. No. 11/241,300, “System and Method for a Communication Protocol for Wireless Sensor Systems Including Systems with High Priority Asynchronous Message and Low Priority Synchronous Message” Ser. No. 11/241,296, “Method and System to Reconfigure a Network to Improve Network Lifetime Using Most Reliable Communication Links” Ser. No. 11/240,434. The disclosure of each of the foregoing related applications is hereby incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a method and system for time synchronization in a communication networks. 
     BACKGROUND INFORMATION 
     Time synchronization may be important in certain applications, for example, to coordinate different parts in industrial machines, assembly lines, and other sensor and actuator networks. Time synchronization may also be required for Time Division Multiple Access (TDMA) based techniques in communication networks. 
     In communication networks, the clocks of different nodes may drift due to, for example, crystal inaccuracies. In systems that require time synchronization, synchronization may be achieved by periodically exchanging messages between the nodes. The synchronization message has been implemented in the format of a packet which includes the local time of the sender so that the receiver may set its local time (e.g., its clock) as soon at it receives the packet. The synchronization message may be long and the process energy inefficient. In particular, the packet may be long because it includes headers and several bytes of time information. 
     An exemplary embodiment and/or exemplary method of the present invention is directed to method of synchronizing a network, which includes transmitting a tone signal to convey time information, and setting a local time according to the conveyed time information. 
     Another exemplary embodiment and/or exemplary method of the present invention is directed to method of synchronizing a network, in which the signal includes a preamble followed by a start symbol and the local time is set when the start symbol is received. 
     Yet another exemplary embodiment and/or exemplary method of the present invention is directed to method of synchronizing a network, which includes setting a clock of a receiver to a predefined time when one of the tone signal ends and a predefined pattern within the tone signal is received. 
     Still another exemplary embodiment and/or exemplary method of the present invention is directed to method of synchronizing a network, in which the signal is a short tone. 
     Yet another exemplary embodiment and/or exemplary method of the present invention is directed to method of synchronizing a network, in which the network includes wireless sensor nodes. 
     An exemplary embodiment and/or exemplary method of the present invention is directed to a method for synchronizing a communication network, which includes transmitting a tone signal, detecting an occurrence of a predefined aspect of the tone signal, and setting a local time based on the occurrence of the predefined aspect of the signal. 
     Another exemplary embodiment and/or exemplary method of the present invention is directed to a method for synchronizing a communication network, in which the predefined aspect is a start symbol and/or the predefined aspect is an ending of the tone signal. 
     Yet another exemplary embodiment and/or exemplary method of the present invention is directed to a method for synchronizing a communication network, which includes periodically waking up from a sleep cycle to detect the occurrence of the predefined aspect of the tone signal. 
     Still another exemplary embodiment and/or exemplary method of the present invention is directed to a method for synchronizing a communication network, in which the tone signal is transmitted for at least a period of maximum clock skew between a transmitter and a receiver. 
     Yet another exemplary embodiment and/or exemplary method of the present invention is directed to a method for synchronizing a communication network, in which the elements of the network form a chain and/or tree topology and the method is repeated. 
     An exemplary embodiment and/or exemplary method of the present invention is directed to a communication system, which includes a transmitter to transmit a signal to convey time information, and a receiver to receive the signal and set a local time based on the conveyed time information. 
     Another exemplary embodiment and/or exemplary method of the present invention is directed to a communication system, in which the local time is set when a predefined part of the signal occurs or when the signal ends. 
     Yet another exemplary embodiment and/or exemplary method of the present invention is directed to a communication system, in which the signal is a tone. 
     Still another exemplary embodiment and/or exemplary method of the present invention is directed to a communication system, in which the tone includes a preamble followed by a start symbol. 
     Yet another exemplary embodiment and/or exemplary method of the present invention is directed to a communication system, in which the local time is set when the start symbol is received. 
     Still another exemplary embodiment and/or exemplary method of the present invention is directed to a communication system, in which the receiver periodically wakes up from a sleep cycle to receive the signal, and the signal is transmitted for at least a period of a maximum clock skew between the transmitter and receiver. 
     An exemplary embodiment and/or exemplary method of the present invention is directed to an apparatus for synchronizing a communication network, which includes a first arrangement to transmit a tone signal, a second arrangement to received the tone signal, a third arrangement to detect an occurrence of a predefined aspect of the tone signal, and a fourth arrangement to set a local time based on the occurrence of the predefined aspect of the signal. 
     SUMMARY OF THE INVENTION 
     The exemplary embodiment and/or exemplary method of the present invention provides a method and system to synchronize the elements of a communication network with respect to time, which may be efficient and require less resources. 
     According to an exemplary embodiment and/or exemplary method of the present invention, time synchronization is provided using only a signal or tone without the need for additional headers or bytes of time information, although data payload may also be added if needed by the protocol. For example, time synchronization may be provided by transmitting a signal or a tone in which the transmitted signal or tone ends at a predefined time known to the sender and receiver(s). In this regard, some part of the transmission (e.g., beginning, end, or in between) may be used to pinpoint an instant, which corresponds to the predefined time. Accordingly, the receiver(s) may set their clocks to the predefined time, for example, when the signal ends or when a predefined byte in the packet is encountered. Hence, instead of using bytes of information (e.g., data packets) to convey time, a short tone or a predefined event is used, which may reduce the need for long messages. 
     An exemplary embodiment and/or exemplary method according to the present invention may synchronize the clocks of one or more receivers to a single transmitter, which may be particularly desirable in power constrained wireless networks where the duty cycle is limited. For example, according to an exemplary embodiment and/or exemplary method of the present invention, the transmitted signal may end at a predefined time known to the sender and multiple receivers, in which the receivers set their clocks to the predefined time, for example, when the signal ends or when a predefined byte in the packet is encountered. The exemplary method may be repeated to synchronize the nodes in the entire network by forming a chain or tree topology for instance. 
     An exemplary embodiment and/or exemplary method according to the present invention may be applied, for example, in wireless sensor networks, and to any other type of network, for power savings and/or limiting the transmission time. 
     An exemplary method according to the present invention may also remove any uncertainties that may arise due to medium access schemes such as CSMA (Carrier Sense Multiple Access), time needed to switch on the transmitter (variable time usually), etc. 
     According to an exemplary embodiment and/or exemplary method of the present invention, encoding may be used for error correction, as the transmission link may be error prone in wireless networks. However, encoding for error correction may increase the size of the packet and thereby the transmission time and power consumption may increase. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary communication network. 
         FIG. 2  shows an exemplary time line during an attempt to synchronize the network elements of the exemplary communication network of  FIG. 1 . 
         FIG. 3  shows an exemplary communication network, which includes three nodes arranged in a chain configuration. 
         FIG. 4  shows an exemplary communication network, which includes seven nodes arranged in a hierarchical manner. 
         FIG. 5  shows exemplary embodiments of the transmitter node  101  and receiver node  102  of the exemplary communication network  100  of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an exemplary communication network  100 , which includes a transmitter node  101  and a receiver node  102 . The exemplary communication network  100  may be, for example, a wireless communication network, and the transmitter node  101  and the receiver node  102  may be, for example, wireless communication devices. 
     According to an exemplary embodiment and/or exemplary method of the present invention, the transmitter node  101  sends a short pulse (also referred to as a tone) that ends at a predetermined time known to the transmitter node  101  and the receiver node  102 . The tone may include, for example, preamble bytes (e.g., alternate zeros and ones) followed by a start symbol. The receiver node  102  sets its local time to the predetermined time when the tone ends, i.e., when the start symbol is received. The transmitted tone may be brief since additional overhead of bytes for time information and error correction are not required. This may result in a desired savings in transmission time and energy consumption. By contrast, time synchronization packets in other communication networks may require a preamble and start symbol, followed by a header and payload data. However, according to an exemplary embodiment and/or exemplary method of the present invention, the tone may be sufficient to convey the time information, and therefore the exemplary embodiment and/or exemplary method may be more efficient. 
     Using tones instead of packets may be also desired because in certain instances the tones may be heard from a much further distance as compared to data packets. For example, at greater distances from the transmitter, a data packet may not be discernable to a receiver as to the information contained therein, whereas a tone, which may barely audible at these greater distances, is still detectable by the receiver. Hence, at these greater distances where the transmission of the data packet or tone is detectable but the content of the data packet is not discernable, the tone may provide an advantage since the tone need only be detected and examined for its content like a data packet. 
     An exemplary method according to the present invention may be most energy-efficient when the transmitter is not energy constrained. In such systems the transmitter may transmit a tone long enough to cover the maximum clock skew and the required bytes before ending the tone. The receivers are required to receive only from their predefined time until the end of the tone. 
       FIG. 2  shows an exemplary time line during an attempt to synchronize the network elements of the exemplary communication network  100  of  FIG. 1 . More specifically,  FIG. 2  shows an exemplary time line for transmission of a tone by the transmitter node  101  and reception of the tone by the receiver node  102 , during an attempt to synchronize the local time (e.g., clock) of the receiver node  102  with the local time (e.g., clock) of the transmitter node  101 . 
     The predefined time for clock synchronization is t, and δ max  is the maximum clock skew between the transmitter node  101  and the receiver node  102 . The transmitter node  101  transmits the tone for a period of at least δ max  to account for the maximum clock skew. The receiver node  102  wakes up at t-δ max  and listens for the tone. When the tone ends, the receiving node  102  knows that the transmitter node  101  time was t and sets its clock to t. The clock skew δ max  may assumed to be very small if the nodes synchronize often. 
       FIG. 3  shows an exemplary communication network  300 , which includes three nodes  301  to  303  arranged in a chain configuration. In particular, a first node  301  is arranged to communicate to a second node  302 , which is arranged to communicate to a third node  303 . The exemplary communication network  300  may be, for example, a wireless communication network, and the three nodes  301  to  303  may be, for example, wireless communication devices. 
     According to an exemplary embodiment and/or exemplary method of the present invention, the first node  301  sends a short tone that ends a predefined time known to the first node  301  and the second node  302 . The second node  302  set its local time to the defined time when the tone ends. Thereafter, the second node  302  sends a short tone that ends at a subsequent predetermined time known to the second node  302  and the third node  303 . The third node  303  sets its local time to the subsequent predetermined time when the tone ends. 
       FIG. 4  shows an exemplary communication network  400 , which includes seven nodes  401  to  407  arranged in a hierarchical manner. In particular, a first node  401  is arranged to communicate to a second node  402  and a third node  403 , the second node  402  is arranged to communicate with a fourth node  404  and a fifth node  405 , and a third node  403  is arranged to communicate with a sixth node  406  and a seventh node  407 . The exemplary communication network  400  may be, for example, a wireless communication network, and the seventh nodes  401  to  407  may be, for example, wireless communication devices. 
     According to an exemplary embodiment and/or exemplary method of the present invention, the first node  401  sends a short tone that ends at a predetermined time known to the second node  402  and the third node  403 . The second node  402  and the third node  403  set their local time to the predefined time when the tone ends. Thereafter, the second node  402  sends a short tone that ends a subsequent predetermined time known to the fourth node  404  and the fifth node  405 , which set their local time to the subsequent predetermined time known to the fourth node  404  and the fifth node  405 , which set their local time to the subsequent predetermined time when the tone ends. Likewise, the third node  403  sends a short tone that ends at a subsequent predetermined time known to the sixth node  406  and the seventh node  407 , which set their local time to the subsequent predetermined time when the tone ends. In this regard, the tones sent by the second node  402  and the third node  403  may occur at the same subsequent predetermined time or at different subsequent predetermined times. 
     Wireless sensor networks for intrusion detection and fire alarm systems may have stringent requirements. For using the security frequency band at 868 MHz, the European Telecommunication Standards Institute (ETSI) limits the transmission duty cycle to 0.1% and channel bandwidth to 25 KHz. The low bandwidth increases the time needed to transmit a packet, but the low duty cycle (0.1%) limits the transmission time in a given interval. For time synchronization, an exemplary method according to the present invention may better enable systems to meet these conflicting requirements by reducing the transmission time for sending the time information. 
       FIG. 5  shows exemplary embodiments of the transmitter node  101  and receiver node  102  of the exemplary communication network  100  of  FIG. 1 . The transmitter node  101  includes a transceiver device  101   a , a control unit  101   b , a clock  101   c , a processing arrangement  101   d  (e.g., CPU), memory  101   e , and a tone generator  101   f . The receiver node  102  includes a transceiver device  102   a , a control unit  102   b , a clock  102   c , a processing arrangement  102   d  (e.g., CPU), and memory  102   e