Abstract:
A fixed network utility data collection system includes a plurality of endpoints arranged in a tiered and spoke-like configuration relative to a central data collecting device. An RF transmission from the central device transmits out over the endpoints in a spoke. The first endpoint in the spoke to hear the transmission then hops the transmission to the other endpoints in the spoke. Once all of the endpoints in a spoke have received the transmission they respond to the transmission. The response starts with the outer-most endpoint and is transmitted to the next endpoint in the spoke line. That endpoint adds its response and forwards the message to the next endpoint in the spoke line and so on. Upon the inner-most endpoint of the spoke receiving the response, it adds its response and transmits the final collective response to the central device.

Description:
CLAIM TO PRIORITY  
       [0001]     The present application claims priority to U.S. Provisional Patent Application No. 60/565,401, filed Apr. 26, 2004, and entitled “FIXED NETWORK UTILITY DATA COLLECTION SYSTEM AND METHOD.” The contents of the cited provisional patent application is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to radio frequency (RF) communication systems, and more particularly to RF communication schemes used with advanced automatic meter reading (AMR) devices.  
       BACKGROUND OF THE INVENTION  
       [0003]     Automatic meter reading (AMR) systems are generally known in the art. Utility companies, for example, use AMR systems to read and monitor customer meters remotely, typically using radio frequency (RF) and other wireless communications. AMR systems are favored by utility companies and others who use them because they increase the efficiency and accuracy of collecting readings and managing customer billing. For example, utilizing an AMR system for the monthly reading of residential gas, electric, or water meters eliminates the need for a utility employee to physically enter each residence or business where a meter is located to transcribe a meter reading by hand.  
         [0004]     There are two general ways in which current AMR systems are configured, fixed networks and mobile networks. In a fixed network, endpoint devices at meter locations communicate with readers that collect readings and data using RF communication. There may be multiple fixed intermediate readers, or relays, located throughout a larger geographic area on utility poles, for example, with each endpoint device associated with a particular reader and each reader in turn communicating with a central system. Other fixed systems utilize only one central reader with which all endpoint devices communicate. In a mobile network, a handheld unit or otherwise mobile reader with RF communication capabilities is used to collect data from endpoint devices as the mobile reader moves from place to place. The differences in how data is reported up through the system and the impact that has on number of units, data transmission collisions, frequency and bandwidth utilization has resulted in fixed network AMR systems having different communication architectures than mobile network AMR systems.  
         [0005]     AMR systems can include one-way, one-and-a-half-way, or two-way communications capabilities. In a one-way system, an endpoint device typically uses a low power count down timer to periodically turn on, or “bubble up,” in order to send data to a receiver. One-and-a-half-way AMR systems include low power receivers in the endpoint devices that listen for a wake-up signal which then turns the endpoint device on for sending data to a receiver. Two-way systems enable two way command and control between the endpoint device and a receiver/transmitter. Because of the higher power requirements associated with two-way systems, two-way systems have not been favored for residential endpoint devices where the need for a long battery life is critical to the economics of periodically changing out batteries in these devices.  
         [0006]     While conventional fixed networks provide many advantages over manually read meters, they suffer from at least two significant drawbacks. First, conventional fixed networks are generally handicapped by cell size. Because of timing, geographic, and power constraints, central data collection units are limited in the number of meters they may support. Introducing, dedicated intermediate relay units can rectify this problem to a certain degree, but these relay units suffer from similar drawbacks and increase system complexity and cost. Second, conventional fixed network systems are limited by the power consumption and battery life of the individual meters. Configuring the meters to respond to or initiate communications with a central device is a drain on the battery life of the meters. The meters still require frequent manual servicing to change out batteries, defeating the most significant advantage of a fixed network system.  
         [0007]     There is, therefore, a need in the industry for an AMR system that addresses the data collection shortcomings of conventional fixed network systems while providing larger cell sizes and more efficient communication with meter devices.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention substantially meets the aforementioned needs of the industry, in particular a system and method of operating AMR systems that allow for the storage and transfer of meter readings and other data to eliminate the need to physically visit a remote endpoint device and connect directly to the endpoint device for the collection of data.  
         [0009]     In a preferred embodiment, the invention enables communication between a plurality of devices in a fixed network utility data collection system. In one embodiment, the system generally comprises a cell defining a geographical area and includes a central radio device and a plurality of radio-equipped endpoint devices. The central radio device communicates in a “spoke”-like manner with each of the plurality of endpoint devices in the cell. Additionally, the endpoint devices may communicate peer-to-peer within the cell.  
         [0010]     The peer-to-peer communication capability of a preferred embodiment of the invention enlarges the communicative radius of the cells in which the system is implemented and reduces the overall cost of the system. Peer-to-peer communication between endpoint devices arranged in “spokes” enables a larger number of endpoint devices to be under the umbrella of a single central radio device. Further, peer-to-peer communications reduce the power consumption of the devices in the system by reducing the endpoint device wake-up times necessary to communicate with a single central radio device.  
         [0011]     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description that follow more particularly exemplify these embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
         [0013]      FIG. 1  is an exemplary diagram of a cell layout in accordance with one embodiment of the system of the invention.  
         [0014]      FIG. 2  is a diagram of a single cell of  FIG. 1  in accordance with one embodiment of the system of the invention.  
         [0015]      FIG. 3  is a communication path diagram in accordance with one embodiment of the invention.  
         [0016]      FIG. 4  is a timing diagram in accordance with one embodiment of the system of the invention. 
     
    
       [0017]     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     The fixed network utility data collection system and method of the invention provide increased communication capabilities in an enlarged geographical area while reducing device battery consumption. The invention can be more readily understood by reference to  FIGS. 1-4  and the following description. While the invention is not necessarily limited to such an application, the invention will be better appreciated using a discussion of example embodiments in such a specific context.  
         [0019]     An exemplary cell layout  10  is shown in  FIG. 1 . In this example embodiment, cell layout  10  corresponds to a geographic area and utilizes a nine cell reuse pattern. The channels for each individual cell have been chosen to avoid any cell having an adjacent cell with a co-channel or an adjacent channel within the overall cell layout  10 . Each individual cell preferably comprises a central radio device and a plurality of endpoint devices, or meters. Here and throughout this application the term “endpoint device” will be used to generally refer to the meter and communications circuitry as one remote device even though they may in some embodiments be distinct devices, with a reader communicating with the communications circuitry and the communications circuitry in turn communicating with the actual meter using RF.  
         [0020]     The frequency band that the system uses in the United States is 1427.000-1432.000 megaHertz (MHz) in one preferred embodiment. This frequency band is broken into five sub-bands, each having a bandwidth of 1.000 MHz; these sub-bands are as follows in this preferred embodiment:
 
0=1427.000-1428.000 MHz
 
1=1428.000-1429.000 MHz
 
2=1429.000-1430.000 MHz
 
3=1430.000-1431.000 MHz
 
4=1431.000-1432.000 MHz
 
         [0021]     The system can be configured for implementation in varying global regions having different communication standards, for example the U.S. and Europe. The U.S. channels are spaced 100 kHz apart, with the first channel centered 50 kHz above the band edge. The European channels are spaced 60 kHz apart. Preferred frequencies for both Europe and the U.S. are listed in TABLE 1 below.  
                       TABLE 1                       Channel   Frequency In Europe   Frequency In U.S.                   1   868.030   14xx.050       2   868.090   14xx.150       3   868.150   14xx.250       4   868.210   14xx.350       5   868.270   14xx.450       6   868.330   14xx.550       7   868.390   14xx.650       8   868.450   14xx.750       9   868.510   14xx.850       Control   868.570   14xx.950                  
 
         [0022]     Channel 0 is the wake-up or control channel and is the default setting from production in one example embodiment. Transmit deviations and data rates can vary between the U.S. and Europe due to the 100 kHz versus 60 kHz channel spacing. A central radio device in a cell will typically transmit more power than endpoint devices and is positioned higher in the air. Co-channel interference can occur if neighboring central radio devices can “see” each other in the radio frequency (RF) communication scheme. Therefore, in one example embodiment, the endpoint devices are designed to reduce co-channel interference with neighboring central radio devices by transmitting at a lower power level and incorporating a lower antenna height.  
         [0023]     In this example embodiment, the individual cells within the system cell layout  10  are configured as hexagons for determination of area and meter density. Determination of RF coverage and hopping analysis will use circles. The area of a hexagon is defined as follows:
 
 Ah= 12*( r *cos 30* r* sin 30)/2
 
 Ah= 3*cos 30* r{circumflex over ( )} 2, where  r  is the radius of the cell
 
 Ah= 2.598* r{circumflex over ( )} 2
 
         [0024]     Assume for purposes of this analysis and explanation of a preferred system embodiment, that there is one residential meter per 33,508 square feet. This number may vary in a typical installation but serves as an exemplary starting point in the present analysis. TABLE 2 shows that at a range of 1000 feet from the mobile unit in such an area, there might be as many as 78 meters desired to be read.  
                                                         TABLE 2                                   r in feet   Ah in   Ah in   Number of                                        100   25,980   .0009   1           200   103,920   .0037   3           400   415,680   .0149   12           500   649,500   .0233   19           800   1,662,720   .0596   50           1000   2,598,000   .0932   78           1200   3,741,120   .1342   112           1500   5,845,500   .2097   174           1800   8,417,520   .3019   251           2000   10,392,000   .3728   310           2500   16,237,500   .5824   485           3000   23,382,000   .8387   698                      
 
         [0025]     To determine cell coverage and propagation, several characteristics associated with the RF are used for these exemplary calculations:  
                                                       Sensitivity for central device   −110 dBm for 0.01 FER           Sensitivity for endpoint device   −105 dBm for 0.01 FER           Link Margin      20 dB above sensitivity           Transmit Power (central)    +30 dBm or +14 dBm           Transmit Power (endpoint)    +14 dBm           Antenna Gain (central)      3 dBi           Antenna Gain (endpoint)      0 dBi                      
 
         [0026]     Different path loss equations can be used for the loss between the different types of environments in which the system of the present invention may be utilized. The various equations each have a different breakpoint at which the loss changes from a free space loss to a higher exponent loss. The following calculation and TABLE 3 show the amount of loss for a given distance at 1430 MHz rounded to the nearest 0.1 dB:
 
 PL= (10*loss exp)*log(distance)+25−((10*loss exp)−20 )*log(breakpoint)
 
         [0027]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                 Free Space 
                 Urban 
                 Obstructed 
                 Obstructed 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Breakpoint In feet 
                 1 
                 300 
                 100 
                 30 
               
               
                 Loss Exp. 
                 2 
                 2.7 
                 4 
                 5.3 
               
               
                 Distance in feet 
                 PL 
                 PL 
                 PL 
                 PL 
               
               
                 50 
                 59.2 
                 59.2 
                 59.2 
                 66.5 
               
               
                 100 
                 65.2 
                 65.2 
                 65.2 
                 82.5 
               
               
                 200 
                 71.3 
                 71.3 
                 77.3 
                 98.4 
               
               
                 350 
                 76.1 
                 76.6 
                 87.0 
                 111.3 
               
               
                 500 
                 79.2 
                 80.8 
                 93.2 
                 119.5 
               
               
                 800 
                 83.3 
                 86.3 
                 101.4 
                 130.4 
               
               
                 1000 
                 85.2 
                 88.9 
                 105.2 
                 135.5 
               
               
                 1500 
                 88.8 
                 93.6 
                 112.3 
                 144.8 
               
               
                 2000 
                 91.3 
                 97.0 
                 117.3 
                 151.4 
               
               
                 2500 
                 93.2 
                 99.6 
                 121.1 
                 156.6 
               
               
                 3000 
                 94.8 
                 101.8 
                 124.3 
                 160.8 
               
               
                   
               
             
          
         
       
     
         [0028]     The above path loss equation and TABLE 3 provide a basis upon which to determine whether the radio devices in a particular cell can communicate with each other, whether the communication is between a central device and an endpoint device, or communication is between an endpoint device and another endpoint device. Additional factors can influence the equation above, however, and the path loss could vary considerably from what is calculated in this exemplary analysis.  
         [0029]     Some observations can be made from the path loss in TABLE 3 and link margin calculations that provide an indication as to how large a cell can be to enable full communication between the devices within located within the cell. Refer, for example, to TABLE 4:  
                                                         TABLE 4                                   Free Space   Urban Area   Obstructed   Obstructed                                    Breakpoint In feet   1   300   100   30       Loss Exp.   2    2.7    4    5.3       Distance for 118 dB   43,500 feet   11,970 feet   2,086 feet   468 feet       Distance for 102 dB    6,900 feet    3,060 feet     831 feet   233 feet       Distance for 107 dB   12,260 feet    4,685 feet   1,108 feet   290 feet       Distance for 99 dB    4,880 feet    2,368 feet     699 feet   205 feet                  
 
         [0030]     Using a Loss Exp. of 4.0, a central RF device at +30 dBm and +14 dBm could communicate directly with over 96% of the endpoint devices in cells having a radius of 2100 and 1100 feet, respectively. Using a Loss Exp. of 4.0, over 96% of the endpoint devices in a cell could talk directly back to the central device at a range of almost 1100 feet. The percentage reduces to approximately 78% at a range of 2100 feet. Using a Loss Exp. of 4.0, the endpoint devices could talk peer-to-peer at a distance of 700 feet with an approximately 96% probability of successful communications. In a cell having a radius of 2000 feet, the central RF device could communicate directly with approximately 96% of the endpoint devices in the cell.  
         [0031]     To get the last 4%, a peer-to-peer hopping scheme between endpoint devices and then the same path back to the central device could be implemented. Up to three hops out and three hops back may be needed in some system configurations. This method can require some additional time to hop, receive wake-up coordination/synchronization, compensation for real time clock (RTC) drift, and current drain on the battery-powered units. Any combination of hops could be made out and back to meet the required link margin. “Hole fillers,” or repeaters, could be implemented in the system and could be made at a low cost and mounted on houses instead of poles. If desired, an endpoint device could be used as a repeater if it had a suitable power supply. The advantages provided by this embodiment can be further improved in other embodiments of the invention.  
         [0032]     The cell layout  20  depicted in  FIG. 2  is 4000 feet in diameter and can use up to three hops in a communication path. Additional hops can be used, which would shorten battery life. The cell layout  20  is based upon RF levels as opposed to a pure physical representation previously described with the hexagonal cell  10  in  FIG. 1 . A small number of endpoint devices  24  are shown for clarity. Note that coverage goes out from the central device  22  to the endpoint devices  24  in a “spoke-like” manner.  
         [0033]     The central device  22  talks out on the cell channel to one of the spokes  26  in an assigned time slot. In a preferred embodiment, each of the devices  24  in the spoke  26  can hear the central device  22  with the required 20 dB link margin. One, two, or three endpoint devices  24  can be in a single spoke  26 . If any of the endpoint devices  24  hear the central device  22 , the information is “hopped” to them. The Tier  3  ( 28 ) endpoint device  24  then forms its data packet and sends the information back to the Tier  2  ( 30 ) endpoint device  24 . The Tier  2  endpoint device  24  adds its data packet to the information received from the Tier  3  ( 28 ) endpoint device  24  and sends it to the Tier  1  ( 32 ) endpoint device  24 . The Tier  1  endpoint device  24  then adds its data packet to the information received from the Tier  2  ( 30 ) endpoint device  24  and sends the total packet up to the central device  22 . Data rate peer-to-peer and central device  22  to endpoint devices  24  will generally be slower, for example 4.8/9.6 kbaud, than from endpoint device  24  to central device  22 , for example 4.8/9.6/19.2/38.4 kbaud, because of the lower processor power in the endpoint device  24  as compared to the central device  22 .  
         [0034]     After the central device  22  receives the data packet, it waits for the next assigned time slot and repeats the process. Endpoint device  24  density, range peer-to-peer, transmission time and current drain of battery powered devices will be factors in these operations.  
         [0035]     To conserve resources, battery power system devices, for example the endpoint devices  24 , will preferably be in some form of sleep mode until their assigned time slot comes up. The receiver/PLL/uC of the endpoint device  24  is preferably powered up and listening for the central device  22  when the central device  22  transmits. In an example embodiment, the message from central device  22  contains bit/frame synchronization, identification of endpoint device  24  in spoke  26 , the hop order, the command to send the data, and a CRC. Synchronization of the RTCs can also be in the protocol structure in this embodiment.  
         [0036]     In an example embodiment of the system of the invention, the initial synchronization is accomplished by having each endpoint device  24  go into a transmit bubble-up mode as shipped from the factory. The central device  22  will hear the endpoint device  24  with received signal strength indicator (RSSI) information and provide the device  24  with a time slot to listen in. A percentage of endpoint devices  24  could still not be found this way because the central device  22  cannot hear them. Because the geographical location, identified by latitude and longitude, of each installed device  24  will typically be known, an endpoint device  24  close to a “lost” device could be commanded by the nearby and previously identified endpoint device  24  to listen in prescribed time slots for these lost devices. As devices  24  are located and register with the central device  22 , the devices  24  can be sent updates every five to fifteen minutes to keep their RTCs synchronized.  
         [0037]     In a preferred embodiment, the routes or spokes  26  are optimized for efficient system communication. A “Who Can Hear Me” communication is issued to each device  24  in cell  20  and a path loss between each device  22 ,  24  is reported back to central device  22  and then to the Head-End, for example a utility control center, through a wireless area network. A path loss matrix can be formed to optimize spokes  26  using predetermined routing algorithms. Manual routing can be used in special cases.  
         [0038]     In one example operation, central device  22  knows that the read slot for one of spokes  26 , for example spoke  26 A, is coming up. Central device  22  sends out a command that is 60 bytes long to endpoint device  24 A at 9600 baud. This 50 ms burst contains dotting pattern, 3-byte frame sync, endpoint device identification, hop path, the command to read, and a 16CRC. If endpoint device  24 A does not hear the central device  22  command, the endpoint device  24 A goes back into sleep mode. If endpoint device  24 A receives the command, the command is relayed to endpoint device  24 B. Endpoint device  24 B receives the command and next relays it to endpoint device  24 C. Endpoint device  24 C receives the command and forms the return data message. This message is preferably 120 bytes long and takes 100 ms to transmit. Endpoint device  24 B then receives this message from endpoint device  24 C and adds its data to it. The message is now 240 bytes long and takes 200 ms for endpoint device  24 B to send it to endpoint device  24 A in one preferred embodiment. Endpoint device  24 A receives this message and adds its data. These data messages and commands are preferably sent at 9600 baud. Because central device  22  has much more computing power, it is capable of receiving data at 19,200 baud. Endpoint device  24 A therefore sends this 360-byte data message to central device  22  at 19,200 baud, which takes 150 ms. This timing line is shown in  FIG. 4 .  
         [0039]     A full sequence takes 0.6 seconds and collects data from three endpoint devices  24 A-C in this embodiment. Many more combinations of data rate, data length, multiple packets, and hops could be calculated and can be implemented in other various embodiments of the system of the invention. If, for example, the cell  20  had a 2000-foot radius, the cell  20  could contain approximately 310 central devices  24  according to TABLE 2. The entire cell  20  of 310 devices could be read in 62.4 seconds, or a little over one minute, by following the sequence described above 104 times. Provisions can be made for latency, RTC error, retries, second path tries, time between spoke reads, larger packets of data, multiple packets of data, and spokes that have more or fewer then three devices in them. This could double the time required to read cell  20 , meaning that cell  20  could be read every fifteen minutes.  
         [0040]     There will be occasions when an endpoint device  24  will lose synchronization with the central device  22 . In one embodiment, endpoint device  24  can go to the control channel if device  24  has not received communication from central device  22  or other endpoint devices  24  for several time slots. Endpoint device  24  could also go into a transmit bubble-up mode and the central device  22  could listen when not doing reads. If central device  22  hears one of the lost endpoint devices  24 , central device  22  could respond with a new RTC setting and the time slot when endpoint device  24  should wake up for the next read. Some form of this method could also be used to obtain initial synchronization.  
         [0041]     In another preferred embodiment, lost endpoint device  24  goes to the control channel and receives for 10 ms at a rate of every 15 seconds. If device  24  hears central device  22  trying to find it, device  24  will respond. Central device  22  will then send the lost endpoint device  24  a new RTC setting and the time slot when device  24  should wake-up for the next read.  
         [0042]     Data packet sizes will influence system timing because larger packets will take more time to transmit. The nominal size of a data packet in one embodiment of the system is 120 bytes. This data packet will have two bytes of bit synchronization, two bytes of frame synchronization, four bytes of central device  22  identification, twelve bytes of endpoint device  24  identification, two bytes of command protocol, 96 bytes of data, and two bytes of CRC. The 96 bytes of data will allow for 48 IDR times if two bytes/time are allowed in this embodiment. This is enough for four hours of reads with a five-minute interval. This packet could alternatively be made smaller or larger as needed in other embodiments of the system of the present invention.  
         [0043]     Data packet speeds will generally depend upon the receive detection scheme, microcontroller horsepower, and current. In one embodiment, 9600-baud, Manchester encoded data may be decoded in a relatively inexpensive microcontroller. If the data rate can be increased without sacrificing current, the transmitters and receivers in each spoke  26  will require less battery power. This could increase cell  20  read speeds and save battery life. Hardware can also be used to detect and extract the NRZ data from the Manchester encoded data. These configurations could reduce system costs, in particular reduce battery drain.  
         [0044]     The bandwidth of the modulated signal is a function of many things, including the data rate, encoding technique, deviation, data wave shape generation, and base-band filtering. The endpoint device  24  to central device  22  will use some form of FSK (MSK, GMSK, C4FM) modulation with 19.2 kbps Manchester encoded data. Deviation is expected to be ±20 kHz in this embodiment. Using Carson&#39;s rule, the approximate bandwidth is as follows:
 
 BW= 2*Peak Deviation+2*Base-band bandwidth
 
 BW= 2*20 kHz+2*19.2 kHz
 
BW=78.4 kHz
 
 The central device  22  to endpoint device  24  or endpoint device  24  to endpoint device  24  in turn will use some form of frequency shift keying modulation with 9.6 kbps Manchester encoded data. Deviation is expected to be ±20 kHz. Using Carson&#39;s rule, the approximate bandwidth is as follows:
 
 BW= 2*Peak Deviation+2*Base-band bandwidth
 
 BW= 2*20 kHz+2*9.6 kHz
 
BW=59.2 kHz
 
 These bandwidths will comply with the preferred U.S. 100 kHz channels. Deviations or data rates could be reduced for the European system with its 60 kHz channels. 
 
         [0045]     Each endpoint device  24  in a spoke  26  knows when to come up and put itself in receive mode to listen for the central device  22  or endpoint device  24  upstream. If a device&#39;s RTC is out of synchronization, even by only a small amount, the device  24  will miss the read command and the associated RTC update.  
         [0046]     The RTC is preferably running all the time, even during the endpoint device  24  sleep time. This clock and a counter in the microcontroller will tell the receiver when to turn on. Because this clock is preferably at a low frequency to keep the sleep mode current low, a 32 kHz crystal can be used. In one embodiment, the 32 kHz crystal is a “BT” cut with parabolic TC curve having a reference setting at +25 C. Over the −40 C to +80 C temperature range, this crystal could move up to −150 ppm. In a 15-minute period this translates to −135 ms. This amount of time is more than twice as large as the assumed 50 ms period of the initial wake up slot. One way to account for this error is to put the endpoint device  24  in the receive mode for a longer period of time.  
         [0047]     Another correction scheme would set up a timing correction loop between the 32 kHz crystal in the endpoint device  24  and the timing of the slotted wake-up from the central device  22 . Every 15-minute read would reset the RTC counter number in the endpoint device  24  assuming the central device  22  has a much more accurate crystal reference, for example ±0.5 ppm. During the next 15-minute window, the endpoint device  24  will be compensated to the previous 15-minute read window. It is assumed that the endpoint device  24  will change temperature only a small amount during 15 minutes to tighten up the error to ±15 ppm. This would make the timing error in 15 minutes only 13.5 ms. The receive time slot of the endpoint device  24  would then be 77 ms.  
         [0048]     Endpoint devices are generally battery powered. Extending battery life in these devices reduces the overall cost of an AMR system because it also reduces the need for personnel to physically visit each device to change out batteries. In the three-hop read described above, reads are obtained every fifteen minutes and the endpoint devices  24  are either in sleep mode, receive mode, or transmit mode. Battery life of endpoint devices  24  used multiple times in a spoke  26  is reduced because these intermediate devices are transmitting and receiving more frequently. The overall system efficiency and cost savings, however, are still improved when compared to systems that require on-site manual reading by personnel.  
         [0049]     The present invention may be embodied in other specific forms without departing from the essential attributes thereof; therefore, the illustrated embodiments should be considered in all respects as illustrative and not restrictive. The claims provided herein are to ensure adequacy of the present application for establishing foreign priority and for no other purpose.