Abstract:
A wireless apparatus may receive scheduling definitions from a networking device. The wireless apparatus may determine, from the scheduling definitions, downlink transmission times and to start a timer at one of the determined downlink transmission times. The wireless apparatus may discontinue reception at expiration of the timer. The wireless apparatus may also, in response to reception of a downlink transmission, transmit an acknowledgement and to discontinue reception before the timer has expired.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/537,085, filed Aug. 6, 2009, which is a continuation of U.S. patent application Ser. No. 12/253,130, filed Oct. 16, 2008, which issued as U.S. Pat. No. 7,979,096 on Jul. 12, 2011, which is a continuation of U.S. patent application Ser. No. 12/174,512, filed Jul. 16, 2008, which issued as U.S. Pat. No. 7,623,897 on Nov. 24, 2009, which is a continuation of U.S. patent application Ser. No. 10/328,566, filed Dec. 23, 2002, which issued as U.S. Pat. No. 7,421,257 on Sep. 2, 2008, which is a continuation-in-part of U.S. patent application Ser. No. 09/998,946, filed Nov. 30, 2001, which issued as U.S. Pat. No. 7,020,501 on Mar. 28, 2006, all of which are hereby incorporated by reference as if fully set forth. 
    
    
     This application is related to U.S. patent application Ser. No. 12/537,010, filed Aug. 6, 2009, which issued as U.S. Pat. No. 7,979,098 on Jul. 12, 2011. This application is also related to U.S. patent application Ser. No. 13/442,109, filed Apr. 9, 2012, now abandoned. 
     FIELD OF THE INVENTION 
     The present invention relates generally to ad-hoc, multi-node wireless networks and, more particularly, to systems and methods for implementing energy efficient data forwarding mechanisms in such networks. 
     BACKGROUND OF THE INVENTION 
     Recently, much research has been directed towards the building of networks of distributed wireless sensor nodes. Sensor nodes in such networks conduct measurements at distributed locations and relay the measurements, via other sensor nodes in the network, to one or more measurement data collection points. Sensor networks, generally, are envisioned as encompassing a large number (N) of sensor nodes (e.g., as many as tens of thousands of sensor nodes), with traffic flowing from the sensor nodes into a much smaller number (K) of measurement data collection points using routing protocols. These routing protocols conventionally involve the forwarding of routing packets throughout the sensor nodes of the network to distribute the routing information necessary for sensor nodes to relay measurements to an appropriate measurement data collection point. 
     A key problem with conventional sensor networks is that each sensor node of the network operates for extended periods of time on self-contained power supplies (e.g., batteries or fuel cells). For the routing protocols of the sensor network to operate properly, each sensor node must be prepared to receive and forward routing packets at any time. Each sensor node&#39;s transmitter and receiver, thus, conventionally operates in a continuous fashion to enable the sensor node to receive and forward the routing packets essential for relaying measurements from a measuring sensor node to a measurement data collection point in the network. This continuous operation depletes each node&#39;s power supply reserves and, therefore, limits the operational life of each of the sensor nodes. 
     Therefore, there exists a need for mechanisms in a wireless sensor network that enable the reduction of sensor node power consumption while, at the same time, permitting the reception and forwarding of the routing packets necessary to implement a distributed wireless network. 
     SUMMARY OF THE INVENTION 
     Systems and methods consistent with the present invention address this need and others by providing mechanisms that enable sensor node transmitters and receivers to be turned off, and remain in a “sleep” state, for substantial periods, thus, increasing the energy efficiency of the nodes. Systems and methods consistent with the present invention further implement transmission and reception schedules that permit the reception and forwarding of packets containing routing, or other types of data, during short periods when the sensor node transmitters and receivers are powered up and, thus, “awake.” The present invention, thus, increases sensor node operational life by reducing energy consumption while permitting the reception and forwarding of the routing messages needed to self-organize the distributed network. 
     In accordance with the purpose of the invention as embodied and broadly described herein, a method of conserving energy in a node in a wireless network includes receiving a first powering-on schedule from another node in the network, and selectively powering-on at least one of a transmitter and receiver based on the received first schedule. 
     In another implementation consistent with the present invention, a method of conveying messages in a sensor network includes organizing a sensor network into a hierarchy of tiers, transmitting one or more transmit/receive scheduling messages throughout the network, and transmitting and receiving data messages between nodes in adjacent tiers based on the one or more transmit/receive scheduling messages. 
     In a further implementation consistent with the present invention, a method of conserving energy in a multi-node network includes organizing the multi-node network into tiers, producing a transmit/receive schedule at a first tier in the network, and controlling the powering-on and powering-off of transmitters and receivers in nodes in a tier adjacent to the first tier according to the transmit/receive schedule. 
     In yet another implementation consistent with the present invention, a method of forwarding messages at a first node in a network includes receiving scheduling messages from a plurality of nodes in the network, selecting one of the plurality of nodes as a parent node, and selectively forwarding data messages to the parent node based on the received scheduling message associated with the selected one of the plurality of nodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, explain the invention. In the drawings, 
         FIG. 1  illustrates an exemplary network consistent with the present invention; 
         FIG. 2  illustrates an exemplary sensor network consistent with the present invention; 
         FIG. 3  illustrates the exemplary sensor network of  FIG. 2  organized into tiers consistent with the present invention; 
         FIG. 4  illustrates exemplary components of a sensor node consistent with the present invention; 
         FIG. 5  illustrates exemplary components of a monitor point consistent with the present invention; 
         FIG. 6A  illustrates an exemplary monitor point database consistent with the present invention; 
         FIG. 6B  illustrates exemplary monitor point affiliation/schedule data stored in the database of  FIG. 6A  consistent with the present invention; 
         FIG. 7A  illustrates an exemplary sensor node database consistent with the present invention; 
         FIG. 7B  illustrates exemplary sensor node affiliation/schedule data stored in the database of  FIG. 7A  consistent with the present invention; 
         FIG. 8  illustrates an exemplary schedule message consistent with the present invention; 
         FIG. 9  illustrates exemplary transmit/receive scheduling consistent with the present invention; 
         FIGS. 10-11  are flowcharts that illustrate parent/child affiliation processing consistent with the present invention; 
         FIG. 12  is a flowchart that illustrates exemplary monitor point scheduling processing consistent with the present invention; 
         FIGS. 13-16  are flowcharts that illustrate sensor node schedule message processing consistent with the present invention; 
         FIG. 17  illustrates an exemplary message transmission diagram consistent with the present invention; and 
         FIG. 18  illustrates exemplary node receiver timing consistent with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
     Systems and methods consistent with the present invention provide mechanisms for conserving energy in wireless nodes by transmitting scheduling messages throughout the nodes of the network. The scheduling messages include time schedules for selectively powering-on and powering-off node transmitters and receivers. Message datagrams and routing messages may, thus, be conveyed throughout the network during appropriate transmitter/receiver power-on and power-off intervals. 
     Exemplary Network 
       FIG. 1  illustrates an exemplary network  100 , consistent with the present invention. Network  100  may include monitor points  105   a - 105   n  connected to sensor network  110  and network  115  via wired  120 , wireless  125 , or optical connection links (not shown). Network  100  may further include one or more servers  130  interconnected with network  115 . 
     Monitor points  105   a - 105   n  may include data transceiver units for transmitting messages to, and receiving messages from, one or more sensors of sensor network  110 . Such messages may include routing messages containing network routing data, message datagrams containing sensor measurement data, and schedule messages containing sensor node transmit and receive scheduling data. The routing messages may include identification data for one or more monitor points, and the number of hops to reach each respective identified monitor point, as determined by a sensor node/monitor point that is the source of the routing message. The routing messages may be transmitted as wireless broadcast messages in network  100 . The routing messages, thus, permit sensor nodes to determine a minimum hop path to a monitor point in network  100 . Through the use of routing messages, monitor points  105   a - 105   n  may operate as “sinks” for sensor measurements made at nearby sensor nodes. Message datagrams may include sensor measurement data that may be transmitted to a monitor point  105   a - 105   n  for data collection. Message datagrams may be sent from a monitor point to a sensor node, from a sensor node to a monitor point, or from a sensor node to a sensor node. 
     Sensor network  110  may include one or more distributed sensor nodes (not shown) that may organize themselves into an ad-hoc, multi-hop wireless network. Each of the distributed sensor nodes of sensor network  110  may include one or more of any type of conventional sensing device, such as, for example, acoustic sensors, motion-detection sensors, radar sensors, sensors that detect specific chemicals or families of chemicals, sensors that detect nuclear radiation or biological agents, magnetic sensors, electronic emissions signal sensors, thermal sensors, and visual sensors that detect or record still or moving images in the visible or other spectrum. Sensor nodes of sensor network  110  may perform one or more measurements over a sampling period and transmit the measured values via packets, datagrams, cells or the like to monitor points  105   a - 105   n.    
     Network  115  may include one or more networks of any type, including a Public Land Mobile Network (PLMN), Public Switched Telephone Network (PSTN), local area network (LAN), metropolitan area network (MAN), wide area network (WAN), Internet, or Intranet. The one or more PLMNs may further include packet-switched sub-networks, such as, for example, General Packet Radio Service (GPRS), Cellular Digital Packet Data (CDPD), and Mobile IP sub-networks. 
     Server  130  may include a conventional computer, such as a desktop, laptop or the like. Server  130  may collect data, via network  115 , from each monitor point  105  of network  100  and archive the data for future retrieval. 
     Exemplary Sensor Network 
       FIG. 2  illustrates an exemplary sensor network  110  consistent with the present invention. Sensor network  110  may include one or more sensor nodes  205   a - 205   s  that may be distributed across a geographic area. Sensor nodes  205   a - 205   s  may communicate with one another, and with one or more monitor points  105   a - 105   n , via wireless or wire-line links (not shown), using, for example, packet-switching mechanisms. Using techniques such as those described in co-pending patent application Ser. No. 09/999,353, entitled “Systems and Methods for Scalable Routing in Ad-Hoc Wireless Sensor Networks” and filed Nov. 15, 2001 (the disclosure of which is incorporated by reference herein), sensor nodes  205   a - 205   s  may organize themselves into an ad-hoc, multi-hop wireless network through the communication of routing messages and message datagrams. 
       FIG. 3  illustrates sensor network  110  self-organized into tiers using conventional routing protocols, or the routing protocol described in the above-described co-pending patent application Ser. No. 09/999,353. When organized into tiers, messages may be forwarded, hop by hop through the network, from monitor points to sensor nodes, or from individual sensor nodes to monitor points that act as “sinks” for nearby sensor nodes. As shown in the exemplary network configuration illustrated in  FIG. 3 , monitor point MP1  105   a  may act as a “sink” for message datagrams from sensor nodes  205   a - 205   e , monitor point MP2  105   b  may act as a “sink” for message datagrams from sensor nodes  205   f - 205   l , and monitor point MP3  105   n  may act as a “sink” for message datagrams from sensor nodes  205   m - 205   s.    
     As further shown in  FIG. 3 , monitor point MP1  105   a  may reside in MP1 tier 0  305 , sensor nodes  205   a - 205   c  may reside in MP1 tier 1  310 , and sensor nodes  205   d - 205   e  may reside in MP1 tier 2  315 . Monitor point MP2  105   b  may reside in MP2 tier 0  320 , sensor nodes  205   f - 205   h  may reside in MP2 tier 1  325 , sensor nodes  205   i - 205   k  may reside in MP2 tier 2  330  and sensor node  205   l  may reside in MP2 tier 3  335 . Monitor point MP3  105   n  may reside in MP3 tier 0  340 , sensor nodes  205   m - 205   o  may reside in MP3 tier 1  345 , sensor nodes  205   p - 205   q  may reside in MP3 tier 2  350  and sensor nodes  205   r - 205   s  may reside in MP3 tier 3  355 . Each tier shown in  FIG. 3  represents an additional hop that data must traverse when traveling from a sensor node to a monitor point, or from a monitor point to a sensor node. At least one node in any tier may act as a “parent” for nodes in the next higher tier (e.g., MP1 Tier 2  315 ). Thus, for example, sensor node  205   a  acts as a “parent” node for sensor nodes  205   d - 205   e . Sensor nodes  205   d - 205   e  may relay all messages through sensor node  205   a  to reach monitor point MP1  105   a.    
     Exemplary Sensor Node 
       FIG. 4  illustrates exemplary components of a sensor node  205  consistent with the present invention. Sensor node  205  may include a transmitter/receiver  405 , an antenna  410 , a processing unit  415 , a memory  420 , an optional output device(s)  425 , an optional input device(s)  430 , one or more sensor units  435   a - 435   n , a clock  440 , and a bus  445 . 
     Transmitter/receiver  405  may connect sensor node  205  to a monitor point  105  or another sensor node. For example, transmitter/receiver  405  may include transmitter and receiver circuitry well known to one skilled in the art for transmitting and/or receiving data bursts via antenna  410 . 
     Processing unit  415  may perform all data processing functions for inputting, outputting and processing of data including data buffering and sensor node control functions. Memory  420  may include random access memory (RAM) and/or read only memory (ROM) that provides permanent, semi-permanent, or temporary working storage of data and instructions for use by processing unit  415  in performing processing functions. Memory  420  may also include large-capacity storage devices, such as magnetic and/or optical recording devices. Output device(s)  425  may include conventional mechanisms for outputting data in video, audio and/or hard copy format. For example, output device(s)  425  may include a conventional display for displaying sensor measurement data. Input device(s)  430  may permit entry of data into sensor node  205 . Input device(s)  430  may include, for example, a touch pad or keyboard. 
     Sensor units  435   a - 435   n  may include one or more of any type of conventional sensing device, such as, for example, acoustic sensors, motion-detection sensors, radar sensors, sensors that detect specific chemicals or families of chemicals, sensors that detect nuclear radiation or sensors that detect biological agents such as anthrax. Each sensor unit  435   a - 435   n  may perform one or more measurements over a sampling period and transmit the measured values via packets, cells, datagrams, or the like to monitor points  105   a - 105   n . Clock  440  may include conventional circuitry for maintaining a time base to enable the maintenance of a local time at sensor node  205 . Alternatively, sensor node  205  may derive a local time from an external clock signal, such as, for example, a GPS signal, or from an internal clock synchronized to an external time base. 
     Bus  445  may interconnect the various components of sensor node  205  and permit them to communicate with one another. 
     Exemplary Monitor Point 
       FIG. 5  illustrates exemplary components of a monitor point  105  consistent with the present invention. Monitor point  105  may include a transmitter/receiver  505 , an antenna  510 , a processing unit  515 , a memory  520 , an input device(s)  525 , an output device(s)  530 , network interface(s)  535 , a clock  540 , and a bus  545 . 
     Transmitter/receiver  505  may connect monitor point  105  to another device, such as another monitor point or one or more sensor nodes. For example, transmitter/receiver  505  may include transmitter and receiver circuitry well known to one skilled in the art for transmitting and/or receiving data bursts via antenna  510 . 
     Processing unit  515  may perform all data processing functions for inputting, outputting, and processing of data. Memory  520  may include Random Access Memory (RAM) that provides temporary working storage of data and instructions for use by processing unit  515  in performing processing functions. Memory  520  may additionally include Read Only Memory (ROM) that provides permanent or semi-permanent storage of data and instructions for use by processing unit  515 . Memory  520  can also include large-capacity storage devices, such as a magnetic and/or optical device. 
     Input device(s)  525  permits entry of data into monitor point  105  and may include a user interface (not shown). Output device(s)  530  permits the output of data in video, audio, or hard copy format. Network interface(s)  535  interconnects monitor point  105  with network  115 . Clock  540  may include conventional circuitry for maintaining a time base to enable the maintenance of a local time at monitor point  105 . Alternatively, monitor point  105  may derive a local time from an external clock signal, such as, for example, a GPS signal, or from an internal clock synchronized to an external time base. 
     Bus  545  interconnects the various components of monitor point  105  to permit the components to communicate with one another. 
     Exemplary Monitor Point Database 
       FIG. 6A  illustrates an exemplary database  600  that may be stored in memory  520  of a monitor point  105 . Database  600  may include monitor point affiliation/schedule data  605  that includes identifiers of sensor nodes affiliated with monitor point  105 , and scheduling data indicating times at which monitor point  105  may transmit to, or receive bursts of data from, affiliated sensor nodes.  FIG. 6B  illustrates exemplary data that may be contained in monitor point affiliation/schedule data  605 . Monitor point affiliation/schedule data  605  may include “affiliated children IDs” data  610  and “Tx/Rx schedule” data  615 . “Tx/Rx schedule” data  615  may further include “parent Tx”  620  data, “child-to-parent Tx” data  625 , and “next tier activity” data  630 . 
     “Affiliated children IDs” data  610  may include unique identifiers of sensor nodes  205  that are affiliated with monitor point  105  and, thus, from which monitor point  105  may receive messages. “Parent Tx” data  620  may include a time at which monitor point  105  may transmit messages to sensor nodes identified by the “affiliated children IDs” data  610 . “Child-to-Parent Tx” data  625  may include times at which sensor nodes identified by “affiliated children IDs”  610  may transmit messages to monitor point  105 . “Next Tier Activity” data  630  may include times at which sensor nodes identified by the “affiliated children IDs” data  610  may transmit messages to, and receive messages from, their affiliated children. 
     Exemplary Sensor Node Database 
       FIG. 7A  illustrates an exemplary database  700  that may be stored in memory  420  of a sensor node  205 . Database  700  may include sensor affiliation/schedule data  705  that may further include data indicating which sensor nodes are affiliated with sensor node  205  and indicating schedules for sensor node  205  to transmit and receive messages. 
       FIG. 7B  illustrates exemplary sensor affiliation/schedule data  705 . Sensor affiliation/schedule data  705  may include “designated parent ID” data  710 , “parent&#39;s schedule” data  715 , “derived schedule” data  720 , and “affiliated children IDs” data  725 . “Designated parent ID” data  710  may include a unique identifier that identifies the “parent” node, in a lower tier of sensor network  110 , to which sensor node  205  forwards messages. “Parent&#39;s schedule” data  715  may further include “parent Tx” data  620 , “child-to-parent Tx” data  625  and “next tier activity” data  630 . “Derived schedule” data  720  may further include “this node Tx” data  730 , “children-to-this node Tx” data  735 , and “this node&#39;s next tier activity” data  740 . “This node Tx” data  730  may indicate a time at which sensor node  205  forwards messages to sensor nodes identified by “affiliated children IDs” data  725 . “Children-to-this node Tx” data  735  may indicate times at which sensor nodes identified by “affiliated children IDs” data  725  may forward messages to sensor node  205 . “This node&#39;s next tier activity”  740  may indicate one or more time periods allocated to sensor nodes in the next higher tier for transmitting and receiving messages. 
     Exemplary Schedule Message 
       FIG. 8  illustrates an exemplary schedule message  800  that may be transmitted from a monitor point  105  or sensor node  205  for scheduling message transmit and receive times within sensor network  110 . Schedule message  800  may include a number of data fields, including “transmitting node ID” data  805 , “parent Tx” data  620 , and “next-tier node transmit schedule” data  810 . “Next-tier node transmit schedule”  810  may further include “child-to-parent Tx” data  625  and “next tier activity” data  630 . “Transmitting node ID” data  805  may include a unique identifier of the monitor point  105  or sensor node  205  originating the schedule message  800 . 
     Exemplary Transmit/Receive Scheduling 
       FIG. 9  illustrates exemplary transmit/receive scheduling that may be employed at each sensor node  205  of network  110  according to schedule messages  800  received from “parent” nodes in a lower tier. The first time period shown on the scheduling timeline, Parent Tx time  620 , may include the time period allocated by a “parent” node for transmitting messages from the “parent” node to its affiliated children. The time periods “child-to-parent Tx”  625  may include time periods allocated to each affiliated child of a parent node for transmitting messages to the parent node. During the “child-to-parent Tx”  625  time periods, the receiver of the parent node may be turned on to receive messages from the affiliated children. 
     The “next tier activity”  630  may include time periods allocated to each child of a parent node for transmitting messages to, and receiving messages from, each child&#39;s own children nodes. From the time periods allocated to the children of a parent node, each child may construct its own derived schedule. This derived schedule may include a time period, “this node Tx”  730  during which the child node may transmit to its own affiliated children. The derived schedule may further include time periods, “children-to-this node Tx”  735  during which these affiliated children may transmit messages to the parent&#39;s child node. The derived schedule may additionally include time periods, designated “this node&#39;s next tier activity”  740 , that may be allocated to this node&#39;s children so that they may, in turn, construct their own derived schedule for their own affiliated children. 
     Exemplary Parent/Child Affiliation Processing 
       FIGS. 10-11  are flowcharts that illustrate exemplary processing, consistent with the present invention, for affiliating “child” sensor nodes  205  with “parent” nodes in a lower tier. Such “parent” nodes may include other sensor nodes  205  in sensor network  110  or monitor points  105 . As one skilled in the art will appreciate, the method exemplified by  FIGS. 10 and 11  can be implemented as a sequence of instructions and stored in memory  420  of sensor node  205  for execution by processing unit  415 . 
     An unaffiliated sensor node  205  may begin parent/child affiliation processing by turning on its receiver  405  and continuously listening for schedule message(s) transmitted from a lower tier of sensor network  110  [step  1005 ] ( FIG. 10 ). Sensor node  205  may be unaffiliated with any “parent” node if it has recently been powered on. Sensor node  205  may also be unaffiliated if it has stopped receiving schedule messages from its “parent” node for a specified time period. If one or more schedule messages are received [step  1010 ], unaffiliated sensor node  205  may select a neighboring node to designate as a parent [step  1015 ]. For example, sensor node  205  may select a neighboring node whose transmit signal has the greatest strength or the least bit error rate (BER). Sensor node  205  may insert the “transmitting node ID” data  805  from the corresponding schedule message  800  of the selected neighboring node into the “designated parent ID” data  710  of database  700  [step  1020 ]. Sensor node  205  may then update database  700 &#39;s “parent&#39;s schedule” data  715  with “parent Tx” data  620 , “child-to-parent Tx” data  625 , and “next tier activity” data  630  from the corresponding schedule message  800  of the selected neighboring node [step  1025 ]. 
     Sensor node  205  may determine if any affiliation messages have been received from sensor nodes residing in higher tiers [step  1105 ] ( FIG. 11 ). If so, sensor node  205  may store message node identifiers contained in the affiliation messages in database  700 &#39;s “affiliation children IDs” data  725  [step  1110 ]. Sensor node  205  may also transmit an affiliation message to the node identified by “designated parent ID” data  710  in database  700  [step  1115 ]. Sensor node  205  may further determine a derived schedule from the “next tier activity” data  630  in database  700  [step  1120 ] and store in the “derived schedule” data  720 . 
     Exemplary Monitor Point Message Processing 
       FIG. 12  is a flowchart that illustrates exemplary processing, consistent with the present invention, for receiving affiliation messages and transmitting schedule messages at a monitor point  105 . As one skilled in the art will appreciate, the method exemplified by  FIG. 12  can be implemented as a sequence of instructions and stored in memory  520  of monitor point  105  for execution by processing unit  515 . 
     Monitor point message processing may begin with a monitor point  105  receiving one or more affiliation messages from neighboring sensor nodes [step  1205 ] ( FIG. 12 ). Monitor point  105  may insert the node identifiers from the received affiliation message(s) into database  600 &#39;s “affiliation children IDs” data  610  [step  1210 ]. Monitor point  105  may construct the “Tx/Rx schedule”  615  based on the number of affiliated children indicated in “affiliated children IDs” data  610  [step  1215 ]. Monitor point  105  may then transmit a schedule message  800  to sensor nodes identified by “affiliated children IDs” data  610  containing monitor point  105 &#39;s “transmitting node ID” data  805 , “parent Tx” data  620 , and “next-tier transmit schedule” data  810  [step  1220 ]. Schedule message  800  may be transmitted periodically using conventional multiple access mechanisms, such as, for example, Carrier Sense Multiple Access (CSMA). Subsequent to transmission of schedule message  800 , monitor point  105  may determine if acknowledgements (ACKs) have been received from all affiliated children [step  1225 ]. If not, monitor point  105  may re-transmit the schedule message  800  at regular intervals until ACKs are received from all affiliated children [step  1230 ]. In this manner, monitor point  105  coordinates and schedules the power on/off intervals of the sensor nodes that is associated with (i.e., the nodes with which it transmits/receives data from). 
     Exemplary Message Reception/Transmission Processing 
       FIGS. 13-16  are flowcharts that illustrate exemplary processing, consistent with the present invention, for receiving and/or transmitting messages at a sensor node  205 . As one skilled in the art will appreciate, the method exemplified by  FIGS. 13-16  can be implemented as a sequence of instructions and stored in memory  420  of sensor node  205  for execution by processing unit  415 . The exemplary reception and transmission of messages at a sensor node  205  as illustrated in  FIGS. 13-16  is further demonstrated with respect to the exemplary messages transmission diagram illustrated in  FIG. 17 . 
     Sensor node  205  (“This node”  1710  of  FIG. 17 ) may begin processing by determining if it is the next parent transmit time as indicated by clock  440  and the “parent Tx” data  620  of database  700  [step  1305 ]. If so, sensor node  205  may turn on receiver  405  [step  1310 ] ( FIG. 13 ) and listen for messages transmitted from a parent (see also “Parent Node”  1705  of  FIG. 17 ). If no messages are received, sensor node  205  determines if a receive timer has expired [step  1405 ] ( FIG. 14 ). The receive timer may indicate a maximum time period that sensor node  205  (see “This Node”  1710  of  FIG. 17 ) may listen for messages before turning off receiver  405 . If the receive timer has not expired, processing may return to step  1315 . If the receive timer has expired, sensor node  205  may turn off receiver  405  [step  1410 ]. If messages have been received (see “Parent TX”  620  of  FIG. 17 ), sensor node  205  may, optionally, transmit an ACK to the parent node that transmitted the messages [step  1320 ]. Sensor node  205  may then turn off receiver  405  [step  1325 ]. 
     Inspecting the received messages, sensor node  205  may determine if sensor node  205  is the destination of each of the received messages [step  1330 ]. If so, sensor node  205  may process the message [step  1335 ]. If not, sensor node  205  may determine a next hop in sensor network  110  for the message using conventional routing tables, and place the message in a forwarding queue [step  1340 ]. At step  1415 , sensor node  205  may determine if it is time to transmit messages to the parent node as indicated by “child-to-parent Tx” data  625  of database  700  (see “child-to-parent Tx”  625  of  FIG. 17 ). If not, sensor node  205  may sleep until clock  440  indicates that it is time to transmit messages to the parent node [step  1420 ]. If clock  440  and “child-to-parent Tx” data  625  indicate that it is time to transmit messages to the parent node, sensor node  205  may turn on transmitter  405  and transmit all messages intended to go to the node indicated by the “designated parent ID” data  710  of database  700  [step  1425 ]. After all messages are transmitted to the parent node, sensor node  205  may turn off transmitter  405  [step  1430 ]. 
     Sensor node  205  may create a new derived schedule for it&#39;s children identified by “affiliated children IDs” data  725 , based on the “parent&#39;s schedule”  715 , and may then store the new derived schedule in the “derived schedule” data  720  of database  700  [step  1435 ]. Sensor node  205  may inspect the “this node Tx” data  730  of database  700  to determine if it is time to transmit to the sensor nodes identified by the “affiliated children IDs” data  725  [step  1505 ] ( FIG. 15 ). If so, sensor node  205  may turn on transmitter  405  and transmit messages, including schedule messages, to its children [step  1510 ] (see “This Node Tx”  730 ,  FIG. 17 ). For each transmitted message, sensor node  205  may, optionally, determine if an ACK is received [step  1515 ]. If not, sensor node  205  may further, optionally, re-transmit the corresponding message at a regular interval until an ACK is received [step  1520 ]. When all ACKs are received, sensor node  205  may turn off transmitter  405  [step  1525 ]. Sensor node  205  may then determine if it is time for its children to transmit to sensor node  205  as indicated by clock  440  and “children-to-this node Tx” data  735  of database  700  [step  1605 ] ( FIG. 16 ). If so, sensor node  205  may turn on receiver  405  and receive one or messages from the children identified by the “affiliated children IDs” data  725  of database  700  [step  1610 ] (see “Children-to-this Node Tx”  735 ,  FIG. 17 ). Sensor node  205  may then turn off receiver  405  [step  1615 ] and processing may return to step  1305  ( FIG. 13 ). In this manner, sensor nodes may power on and off their transmitters and receivers at appropriate times to conserve energy, while still performing their intended functions in network  100 . 
     Exemplary Receiver Timing 
       FIG. 18  illustrates exemplary receiver timing when monitor points  105  or sensor nodes  205  of network  100  use internal clocks that may have inherent “clock drift.” “Clock drift” occurs when internal clocks runs faster or slower then the true elapsed time and may be inherent in many types of internal clocks employed in monitor points  105  or sensor nodes  205 . “Clock drift” may be taken into account when scheduling the time at which a node&#39;s receiver must be turned on, since both the transmitting node and the receiving node may both have drifting clocks. As shown in  FIG. 18 , T nominal    1805  represents the next time at which a receiver must be turned on based on scheduling data contained in the schedule message received from a parent node. A “Rx Drift Window”  1810  exists around this time which represents T nominal  plus or minus the “Max Rx Drift”  1815  for this node over the amount of time remaining until T nominal . If the transmitting node has zero clock drift, the receiving node should, thus, wake up at the beginning of its “Rx Drift Window”  1810 . 
     The clock at the transmitting node may also incur clock drift, “Max Tx Drift”  1820 , that must be accounted for at the receiving node when turning on and off the receiver. The receiving node should, thus, turn on its receiver at a local clock time that is “Max Tx Drift”  1820  plus “Max Rx Drift”  1815  before T nominal . The receiving node should also turn off its receiver at a local clock time that is “Max Rx Drift”  1815  plus “Max Tx Drift”  1820  plus a maximum estimated time to receive a packet from the transmitting node (TRx  1825 ). TRx  1825  may include packet transmission time and packet propagation time. By taking into account maximum estimated clock drift at both the receiving node and transmitting node, monitor points  105  and sensor nodes  205  of sensor network  110  may successfully implement transmit/receive scheduling as described above with respect to  FIGS. 1-17 . 
     CONCLUSION 
     Systems and methods consistent with the present invention, therefore, provide mechanisms that enable sensor node transmitters and receivers to be turned off, and remain in a “sleep” state, for substantial periods, thus, increasing the energy efficiency of the nodes. Systems and methods consistent with the present invention further implement transmission and reception schedules that permit the reception and forwarding of packets containing routing, or other types of data, during short periods when the sensor node transmitters and receivers are powered up and, thus, “awake.” The present invention, thus, increases sensor node operational life by reducing energy consumption while permitting the reception and forwarding of the routing messages needed to self-organize the distributed network. 
     The foregoing description of exemplary embodiments of the present invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. For example, while certain components of the invention have been described as implemented in hardware and others in software, other hardware/software configurations may be possible. Also, while series of steps have been described with regard to  FIGS. 10-16 , the order of the steps is not critical. 
     No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. The scope of the invention is defined by the following claims and their equivalents.