Patent Abstract:
A system, method, and apparatus for a synchronized wireless data concentrator are provided for facilitating a precisely synchronized system of nodes in a wireless sensor network for airborne data systems. The wireless data concentrator contains a plurality of IEEE 802.15.4 radio/micro-processor subsystems, which are connected to a local host microprocessor, which is in turn connected to an aircraft data network. The airplane data network also contains a precision clock source and a plurality of specialized network switches, which have a low-jitter data-path routing capability.

Full Description:
BACKGROUND 
       [0001]    The present disclosure relates to synchronized wireless data concentrators. In particular, it relates to synchronized wireless data concentrators for airborne wireless sensor networks. 
       SUMMARY 
       [0002]    The present disclosure relates to an apparatus, system, and method for synchronized wireless data concentrators for airborne wireless sensor networks. In one or more embodiments, the disclosed system for airborne wireless sensor networks includes at least one wireless data concentrator (WDC) operable as a router. The system further includes at least one processor that runs hosted applications related to at least one WDC operable as a router. Also, the system includes at least one network switch that is connected to at least one WDC operable as a router and connected to at least one processor. In addition, the system includes at least one node that is wirelessly in communication with at least one WDC operable as a router. 
         [0003]    In one or more embodiments, at least one WDC operable as a router includes at least one standard router, and at least one node that operates as a standard node. In at least one embodiment, at least one standard router and/or at least one node operable as a standard node employ a Zigbee communications protocol. In some embodiments, at least one standard router transmits and receives signals to at least one node operable as a standard node. In one or more embodiments, at least one node operable as a standard node is powered by battery power, a wired power line, and/or strong harvested energy. In some embodiments, the strong harvested energy is harvested from thermoelectric power, vibration, and/or inductive coupling to a high voltage (e.g., a high voltage produced by generators). 
         [0004]    In at least one embodiment, at least one WDC operable as a router includes at least one green router, and at least one node that operates as a green node. In one or more embodiments, at least one green router and/or at least one node operable as a green node employ the Zigbee communications protocol. In some embodiments, at least one green router receives signals from at least one node operable as a green node. In one or more embodiments, at least one node operable as a green node transmits its state three times sequentially in a row to at least one green router. In at least one embodiment, at least one node operable as a green node is powered by harvested energy. In some embodiments, the harvested energy is harvested from solar power and/or manual actuation power (e.g., the manual action of flipping a switch). 
         [0005]    In one or more embodiments, at least one processor is an application server. In at least one embodiment, at least one network switch is an Ethernet switch (e.g., an IEEE-1588 Ethernet switch). In some embodiments, the disclosed system further includes at least one WDC operable as a coordinator. In at least one embodiment, at least one WDC operable as a coordinator employs the Zigbee communications protocol. In one or more embodiments, at least one WDC operable as a coordinator is in wireless communication with at least one WDC operable as a router. In at least one embodiment, at least one WDC operable as a coordinator coordinates communications with at least one WDC operable as a router. 
         [0006]    In at least one embodiment, the disclosed method for airborne wireless sensor networks involves transmitting state information from at least one node. The method further involves receiving, by at least one wireless data concentrator (WDC) operable as a router, the state information from the node(s). In addition, the method involves transmitting the state information from at least one WDC operable as a router. Additionally, the method involves receiving, by at least one WDC operable as a coordinator, the state information from the WDC(s) operable as a router. Further, the method involves sending the state information, by at least one WDC operable as a coordinator, to at least one processor for processing. In one or more embodiments, the method further involves coordinating, by at least one WDC operable as a coordinator, communications with at least one WDC operable as a router. In at least one embodiment, the state information is sent from at least one WDC operable as a coordinator to at least one processor via a network switch. 
         [0007]    In one or more embodiments, the disclosed wireless data concentrator (WDC) for airborne wireless sensor networks includes at least one router, where at least one node is in communication with the router(s). In addition, the disclosed WDC includes at least one microprocessor, where the microprocessor(s) processes signals received by the router(s) from the node(s). The disclosed WDC further includes at least one clock crystal, where the clock crystal(s) is used for synchronizing communications for at least one router. 
         [0008]    The features, functions, and advantages can be achieved independently in various embodiments of the present inventions or may be combined in yet other embodiments. 
     
    
     
       DRAWINGS 
         [0009]    These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0010]      FIG. 1  shows a high level architectural view of the disclosed system for synchronized wireless data concentrators (WDCs) for airborne wireless sensor networks, in accordance with at least one embodiment of the present disclosure. 
           [0011]      FIG. 2  is a detailed diagram showing the process for how keys are securely managed within the system of  FIG. 1 , in accordance with at least one embodiment of the present disclosure. 
           [0012]      FIG. 3  is a diagram of a two-channel wireless data concentrator (WDC) that is employed by the system of  FIG. 1 , in accordance with at least one embodiment of the present disclosure. 
           [0013]      FIG. 4  is a diagram of a four-channel WDC, in accordance with at least one embodiment of the present disclosure. 
           [0014]      FIG. 5  is a diagram of an eight-channel WDC, in accordance with at least one embodiment of the present disclosure. 
           [0015]      FIG. 6  is a diagram of a sixteen-channel WDC, in accordance with at least one embodiment of the present disclosure. 
           [0016]      FIG. 7  is a detailed diagram depicting the extended precision time protocol (PTP) operation on a two-channel WDC, in accordance with at least one embodiment of the present disclosure. 
           [0017]      FIG. 8  is a table that shows the typical drift rate for the two crystal (Xtal) devices employed by the disclosed system for synchronized WDCs for airborne wireless sensor networks, in accordance with at least one embodiment of the present disclosure. 
           [0018]      FIG. 9  is a diagram depicting a modification of a standard Zigbee/IEEE-802.15.4 software stack which is employed by the disclosed system for synchronized WDCs for airborne wireless sensor networks, in accordance with at least one embodiment of the present disclosure. 
       
    
    
     DESCRIPTION 
       [0019]    The methods and apparatus disclosed herein provide an operative system for wireless data concentrators. Specifically, this system relates to synchronized wireless data concentrators (WDCs) for airborne wireless sensor networks. In particular, the present disclosure teaches a wireless data concentrator (WDC) architecture that significantly advances the flexibility, adaptability, utility, determinism, and security in a large network of wireless sensor network (WSN) devices, which are applied to a commercial aerospace environment. The application space for aircraft WSNs is diverse and poses challenges in reliability, bandwidth management, latency, and security domains. The present disclosure sets forth a broad architecture structure, and a WDC design, which can support the objectives of improved flexibility, adaptability, utility, determinism, and security more than the currently offered solutions. 
         [0020]    Key aspects that are provided by the disclosed system are: 1.) precision time synchronization of nodes in a wireless sensor network to enable a system design pattern of time-based real-time programming; 2.) bandwidth, throughput, and latency management in a large IEEE-802.15.4 wireless sensor network through the use of a wired Ethernet backbone topology; 3.) a distributed trust center, a wired secure key-transport, and key management system; and 4.) parallel WSN channel operation for optimized wireless bandwidth. 
         [0021]    Zigbee is a type of Low Power Wireless Personal Area Network (LP-WPAN) data communication protocol stack, which is used to standardize low data rate transmission between low power wireless devices. Zigbee does not describe the entire software communication stack, but is rather a set of networking framework layers built on top of the IEEE-802.15.4 standard. Systems and environments that typically deploy Zigbee are environments such as home automation, home entertainment, building automation and, most recently, smart energy. Aerospace non-essential systems represent a new area for LP-WPAN deployment so that smart wireless sensors can be distributed throughout the aircraft cabin, structures, and systems; and can provide monitoring, alerting, on-demand services, and non-essential control functions. 
         [0022]    However, when considering employing Zigbee for a large scale architecture adaptable to a wide range of aerospace applications, it is important to understand some of the shortcomings imposed by Zigbee. Although Zigbee is a robust stack, certain design decisions have been made by commercial microprocessor/radio hardware chip manufacturers and Zigbee software stack vendors. These decisions have been made in order to accommodate the size of object code that can fit into current program memory and runtime variables in data memory within various low cost Zigbee/802.15.4 radios in today&#39;s marketplace. Some of these shortcomings are: lack of medium access control (MAC) and network (NWK) layer support for time-slotted or time-based design patterns, the data security is limited to symmetric-key algorithms, lack of a secure key management system, and lack of robust support for energy harvesting sensor devices. 
         [0023]    The system of the present disclosure sets forth an architectural structure and a network topology that significantly improves over the limitations stated above to better address the additional environmental and application requirements of airborne systems. The main features of the disclosed system are: 1.) an introduction of a low cost, local host microprocessor within the wireless data concentrator, which is hardwire connected to both an Ethernet backbone and a plurality of wireless sensor network “router” devices; 2.) an inclusion of a system-wide hierarchical, precision time distribution means to bridge into the wireless sensor network area; 3.) a distributed security trust center mechanism for fast and secure management of network keys; and 4.) parallel Zigbee channel capability that can better handle throughput, latency, and energy harvesting performance demands on the system. 
         [0024]      FIG. 1  shows a high level architectural view of the disclosed system  100  for synchronized wireless data concentrators (WDCs)  110 ,  120  for airborne wireless sensor networks, in accordance with at least one embodiment of the present disclosure. In this figure, the system  100  is shown to include five (5) WDCs. Four (4) of the WDCs  120  are operating as routers, and one WDC  110  is operating as a coordinator. The one WDC  110  operating as a coordinator  115  communicates wirelessly (as denoted by the dashed lines in the figure) with the four WDCs  120 , and coordinates the communications of the four WDCs  120 , which are all situated in a single aircraft zone. In general, one WDC  110  operating as a coordinator is employed per aircraft zone. An aircraft zone is, for example, a specific defined area within the cabin and/or cockpit of an aircraft. The WDC  110  operating as a coordinator is wired (as denoted by the solid line in the figure) to an IEEE-1588 Ethernet switch  160 , and employs the Zigbee communications protocol. 
         [0025]    The four WDCs  120  operating as routers are wired (as denoted by the solid lines in the figure) to the IEEE-1588 Ethernet switch  160 , which is connected to an application server  145 . The application server  145 , which includes at least one processor, is used to run host applications. In addition, a GPS receiver  155  is connected to an IEEE-1588 Grand Master  150 , which uses a GPS signal from the GPS receiver  155  for time synchronization. The Grand Master  150  is connected to the IEEE-1588 Ethernet switch  160 , and passes time synchronization data packets to the IEEE-1588 Ethernet switch  160  through that connection. 
         [0026]    Each of the four WDCs  120  operating as routers is shown to include one Zigbee green router  125  and one Zigbee standard router  130 . It should be noted that in other embodiments, the system  100  may employ WDCs  120  that include various different quantities of Zigbee green routers  125  and Zigbee standard routers  130 . Both the Zigbee green router  125  and the Zigbee standard router  130  employ the Zigbee communications protocol. The Zigbee standard router  130  transmits and receives signals to Zigbee standard endpoint nodes  140 , which contain monitoring sensors and are situated about the aircraft cabin within the specific aircraft zone of the WDCs  110 ,  120 . The signals include information regarding the state of the Zigbee standard endpoint nodes  140 , time synchronization information, as well as acknowledgement (ACK) information (e.g., acknowledgement information sent in ACK data packets) regarding the receipt of the state information. The Zigbee standard endpoint nodes  140  are powered by various means including, but not limited to, battery power, a wired power line, and strong harvested energy. It should be noted that types of strong harvested energy include, but are not limited to, thermoelectric power, vibration, and inductive couple to a high voltage. 
         [0027]    The Zigbee green router  125  receives signals from Zigbee green endpoint nodes  135 , which contain monitoring sensors and are situated about the aircraft cabin within the specific aircraft zone of the WDCs  110 ,  120 . It should be noted that the Zigbee green router  125  does not transmit signals, it only receives signals. The Zigbee standard endpoint nodes  135  periodically transmit their respective state three times sequentially in a row. Since the Zigbee green router  125  cannot transmit signals, the Zigbee green router  125  does not send acknowledgement signals regarding the receipt of state information to the Zigbee green endpoint nodes  135 . The Zigbee green endpoint nodes  135  are powered by harvested energy, which includes, but is not limited to, solar power and manual actuation power. 
         [0028]    The system  100  of  FIG. 1  is applicable to numerous applications within the aircraft. Examples of these applications include, but are not limited to, passenger control of reading lights; window dimming and flight attendant call lights from energy harvesting control buttons in the seats; aircraft systems monitoring functions, such as temperature and air flow within the passenger cabin; and sensors within the aircraft structure, engines, landing gear, wings, tail sections, power systems, hydraulic systems, or any other system within the aircraft that can benefit from prognostic monitoring of aircraft health and system state. The sensors of the endpoints  135 ,  140  are designed to sense various things according to their function for the particular application(s) of the system  100 . Types of things that the sensors are designed to sense include, but are not limited to, temperature, light, power, and air flow. In this figure, a software application server  145  contains certain “hosted functions.” These “hosted functions” are software programs designed to receive information from various sensing elements. The programs then store and process this information into useful operations for passengers, crew, and/or maintenance personnel, as dictated by the requirements of the “function.” The “hosted functions” communicate with various sensors via the Ethernet switch  160 , which is connected to a plurality of WDCs  110 ,  120  strategically positioned throughout an aircraft. 
         [0029]    The disclosed system  100  uses the IEEE-1588 precision time protocol (PTP) as a baseline timing means that is extended to the various WDCs  110 ,  120  through the IEEE-1588 compliant Ethernet switch  160 . In order to utilize IEEE-1588, a suitable PTP time generator, such as the Symmetricon “Timeprovider 5000”, is utilized to provide a grand master time base to the network. The IEEE-1588 grand master  150  typically gets its reference time from a GPS signal to provide better than a  100  nanosecond time synchronization to global Earth time. Precision time packets are distributed through the Ethernet switch  160  to each of the WDCs  110 ,  120 , where the time synchronization is maintained at each WDC  110 ,  120  within the typical performance limits of a typical IEEE-1588 Ethernet network (i.e. &lt;microsecond). An important feature of this system  100  design is the bridging of the PTP protocol through the 802.15.4 Zigbee router devices  125 ,  130  to Zigbee endpoints  135 ,  140  served by each router  125 ,  130  within a WDC  120 . In at least one embodiment, a single WDC  110  coordinator  115  starts the network in a traditional Zigbee protocol, but then can optionally distribute the coordinator function to selected WDCs  120 . This feature helps to improve the performance and management of large number of sensors within the purview of the WDC  110  selected for the distributed coordinator function, when the number of endpoints  135 ,  140  exceeds a predetermined threshold. 
         [0030]      FIG. 2  is a detailed diagram showing the process for how keys are securely managed within the system  100  of  FIG. 1 , in accordance with at least one embodiment of the present disclosure. A typical Zigbee environment will have a single trust center manager (TC M ) designated at the WDC  110  that is operating as a coordinator of the network. The Zigbee address of the trust center manager is usually aligned with the address of the WDC  110  that is operating as a coordinator, but this is generally a programmable register within any WDC  110 ,  120  on a Zigbee network such that an alternate trust center (e.g., TC A , TC B , TC C , or TC D ) at a different WDC  120  may be established. Zigbee Pro only defines support for symmetric encryption keys. Zigbee networks employ three types of keys: a network key, a link key, and a master key. A network key is applicable to every Zigbee WDC device  110 ,  120  in a given personal area network (PAN) within the aircraft (i.e. a Zigbee local network is identified by one unique PAN identification (ID)). A link key is a key established between two WDC devices  110 ,  120  of a Zigbee application. The master key is a key which is used to allow a Zigbee WDC device  110 ,  120  to initially join a network. In a high security mode, as defined in the Zigbee Pro specification, the master key is used to establish link keys, and must be configured on new WDC devices  110 ,  120  “out-of-band.” “Out-of-band” refers to programming or configuring a WDC device  110 ,  120  in an environment different from the wireless network, such as manually typing a key into a WDC device  110 ,  120  at the time of manufacturing. 
         [0031]      FIG. 2  shows a preferred embodiment of a trusted supplier  210  providing device identifiers (“MAC addresses”) to a global universal trust  200 , which will then issue a set of trusted master keys corresponding to each of the WDC devices&#39; MAC addresses. The trusted supplier  210  then pre-configures the WDC device  110 ,  120  with the master key issued by the global universal trust  200 . In particular, as shown in this figure, a trusted Zigbee device supplier/manufacturer  210  sends a request  220  to the global universal trust center  200  for a key for a new WDC device  110 ,  120  that it is manufacturing. The request that the supplier  210  sends to the trust center  200  includes the MAC address for the new WDC device  110 ,  120 . The trust center  200  has a global key manifest  230  that contains a listing of the specific keys that correspond to particular WDC device MAC addresses. The trust center  200  sends to the supplier  210  a key  240 , which corresponds to the WDC device&#39;s MAC address according to the global key manifest  230 . In response, the supplier  210  sends a response  250  to the trust center  200  indicating that the supplier  210  successfully received the key (acknowledgement (ACK)) or did not successfully receive the key (no acknowledgement (NAK)). 
         [0032]    Upon initial commissioning of WDC devices  110 ,  120  on a new aircraft (or for replacement equipment on an existing aircraft), a trusted Internet connection must be made between the application server  145  and the global universal trust  200  (i.e. trust center  200 ). New WDC devices  110 ,  120  that attempt to join the aircraft Zigbee network will cause an aircraft trust center (located at a WDC  110 ,  120 ) to communicate with the trust center manager function (TC MGR)  260 , which will make a request to the global universal trust  200  for a master key for the new WDC device  110 ,  120  requesting to join the network. Once a WDC device  110 ,  120  has been authenticated by the trust center manager  260 , then a key exchange process will occur, and a new encrypted key will be delivered to the new WDC device  110 ,  120  joining the Zigbee network. In particular, as shown in  FIG. 2 , the trust center manager function (TC MGR)  260  sends a request  270  to the global universal trust  200  for a key for the new WDC device  110 ,  120  that is requesting to join the network. The request  270  that the trust center manager function  260  sends to the trust center  200  includes the MAC address for the new WDC device  110 ,  120 . The global universal trust  200  sends  280  to the trust center manager function  260  a key  240 , which corresponds to the WDC device&#39;s MAC address according to the global key manifest  230 . In response, the trust center manager function  260  sends a response  290  to the global universal trust  200  indicating that the trust center manager function  260  successfully received the key (acknowledgement (ACK)) or did not successfully receive the key (no acknowledgement (NAK)). It should be noted that additional keys and data can be exchanged on the network with the new WDC device  110 ,  120 . This includes issuing a network key, which is required for all Zigbee devices  110 ,  120  on a given PAN. 
         [0033]    A feature of this key management method is an optional means to change the master key to a new value once the pre-determined master keys have been used to allow a WDC device  110 ,  120  to join the network. The new master key may be additionally changed at a periodic rate with a last-known master key retained in the event of a master key change error event. If an original master key is lost, after being changed to a new master key, and having rolled past the last-known master key, it is gone forever. Only through a specific trusted new request sequence to the global universal trust  200  may a new pre-determined master key be delivered to a WDC device  110 ,  120  whose master key becomes corrupt or lost. This level of security provides another long term layer of assurance that no rogue devices may be allowed to join an aircraft wireless sensor network. 
         [0034]    Another feature is the use of a distributed trust center scheme. For large networks of many hundreds or thousands of WDC devices  110 ,  120 , having one trust center for the entire network can become unwieldy, and have undesirable latency and memory problems. As such, a distributed trust center allows for a management of subnets (e.g., PANs) by distribution of the trust center key tables  295  efficiently through a secure wired transport. A trust center is also responsible for updating the network key in a normal Zigbee network, and having this distributed trust center function located at the WDC device  110 ,  120  enables a more deterministic behavior to occur during a network key update. The additional security feature of changing the master key requires that a list  295  of master keys and of the last-known master keys is maintained at each trust center responsible for a given network. This updated list is also synchronized with the trust center manager hosted function  260  at the application server  145  level to ensure a coherent backup of the trust center data is maintained should a WDC device  110 ,  120 , acting as a trust center become non-functional or is replaced. Finally, each trust center is designated as a primary or backup trust center on a given PAN. Stated another way, in at least one embodiment, each PAN has a minimum of two trust centers, where each trust center contains a duplicate of the key list  295  for the WDC devices  110 ,  120  within that PAN. 
         [0035]      FIG. 3  is a diagram of a two-channel wireless data concentrator (WDC)  120  that is employed by the system of  FIG. 1 , in accordance with at least one embodiment of the present disclosure. Each WDC  120 , regardless of how many wireless router channels  125 ,  130  are supported, includes a local host Ethernet gateway microprocessor  300 , which contains IEEE-1588 precision time protocol (PTP) hardware support within its TCP/IP MAC layer. Examples of devices that may be employed by the WDC  120  for the local host Ethernet gateway microprocessor  300  include, but are not limited to, a ST Micro STM32F107 device and a ARM Cortex-M3 32-bit RISC core microprocessor. The STM32F107 device, when employed by the local host Ethernet gateway microprocessor  300  for example, acts as the gateway microprocessor  300  and connects to both of the IEEE 802.15.4/Zigbee router microprocessors  125 ,  130  by way of one of the serial peripheral interface (SPI) ports that are configured to clock data at a minimum rate of 4 megabits per second (Mbps). The local host microprocessor  300  also contains a software client  310  to handle the time management functions of the PTP network function, which provides the precise time. The local host microprocessor  300  also distributes a precise hardware interrupt signal to each of the 802.15.4/Zigbee router microprocessors  125 ,  130  to enable the feature of extended precision time protocol, which is described later in the present disclosure. 
         [0036]      FIG. 4  is a diagram of a four-channel WDC  400 , in accordance with at least one embodiment of the present disclosure. In this figure, the four-channel WDC  400  is shown to include one Zigbee green router  125  and four Zigbee standard routers  130 .  FIG. 5  is a diagram of an eight-channel WDC  500 , in accordance with at least one embodiment of the present disclosure. In particular, in this figure, the eight-channel WDC  400  is shown to include two Zigbee green routers  125  and six Zigbee standard routers  130 .  FIG. 6  is a diagram of a sixteen-channel WDC  600 , in accordance with at least one embodiment of the present disclosure. In this figure, the eight-channel WDC  400  is shown to include four Zigbee green routers  125  and six Zigbee standard routers  130 . 
         [0037]      FIG. 7  is a detailed diagram depicting the extended precision time protocol (PTP) operation on a two-channel WDC  120 , in accordance with at least one embodiment of the present disclosure. The microprocessor  300  utilizes a 20.000 megahertz (MHz) (0.5 parts per million (ppm)) clock  700 , which enables a less frequent update period from the PTP master across the Ethernet network than a clock frequency that is less accurate. Also, a low cost 32.768 kilohertz (KHz) watch crystal (Xtal)  710  is used for the Zigbee devices that are nodes (i.e. the Zigbee green endpoint nodes  135  and the Zigbee standard endpoint nodes  140 ). In this case, if a node is battery operated (i.e. a battery operated Zigbee standard endpoint node  140 ), it will be sleeping most of the time at a very low current state. This will require a very low frequency clock source to keep backup time established so that a less frequent synchronization is required. 
         [0038]    In this figure, the Zigbee standard router  130  is shown to be transmitting and receiving time synchronization signals to the Zigbee standard endpoint node  140 . In particular, at time T 1 , the Zigbee standard router  130  sends a synchronization signal  720  (i.e. Sync( 1 )  720 ) to the Zigbee standard endpoint node  140 , and at time T 2 , the Zigbee standard router  130  sends a follow-up signal  730  (i.e. Follow_Up( 2 )  730 ) to the Zigbee standard endpoint node  140 . At time T 3 , the Zigbee standard endpoint node  140  sends a delay request signal  740  (i.e. Delay_Req( 3 )  740 ) to the Zigbee standard router  130 . And, finally, at time T 4 , the Zigbee standard router  130  sends a delay response signal  750  (i.e. Delay_Resp( 4 )  750 ) to the Zigbee standard router  130 . 
         [0039]    The PTP protocol introduces a hierarchical firewall nature of synchronization. To represent this synchronization firewall, a time synchronization firewall  760  (i.e. PTP Time Firewall  760 ) is shown to be present within the WDC  120 . This firewall  760  prevents any downstream extended PTP effect from disturbing the primary Ethernet PTP channels  125 ,  130 . In other words, the time accuracy of the extended nodes  135 ,  140  is strictly governed by the time accuracy and stability of the WDC  120  local host microprocessor  300 . 
         [0040]      FIG. 8  is a table  800  that shows the typical drift rates for the two crystal (Xtal) devices employed by the disclosed system for synchronized WDCs for airborne wireless sensor networks, in accordance with at least one embodiment of the present disclosure. In particular, the table  800  shows that the 20 MHz Xtal has better stability (0.5 parts per million (ppm) +/− spec (i.e. nominal frequency)) than the 32.768 KHz Xtal (5 ppm +/− spec). 
         [0041]      FIG. 9  is a diagram depicting a modification of a standard Zigbee/IEEE-802.15.4 software stack which is employed by the disclosed system for synchronized WDCs for airborne wireless sensor networks, in accordance with at least one embodiment of the present disclosure. In this modification, PTP time stamping support  900  is added to the MAC layer  910  to enable a low latency capture of the time when packets arrive on the 802.15.4 PHY layer  920 . This time stamp information is then communicated directly to the application layer  930  where a special PTP software application  940  is resident to compute the extended PTP synchronization. Once this operation is completed, then other application objects within the Zigbee endpoint nodes  135 ,  140  may take advantage of a high accuracy time stamp. To allow for power down, drift trend information can be captured over time to determine the drift statistics. Referring to  FIG. 8  again, one can see that the maximum drift count of the 32.768 Khz clock would be between  9  and  10  counts per minute. Once the drift is monitored in a real system (after synchronization is complete), then the drift can be managed by compensation based on the long term drift trend. A feature of this is a start up period where during certain periodic times, a higher frequency PTP synchronization occurs to determine the absolute drift during the non-critical time endpoint (i.e. node  135 ,  140 ) operation period. 
         [0042]    Although certain illustrative embodiments and methods have been disclosed herein, it can be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods can be made without departing from the true spirit and scope of the art disclosed. Many other examples of the art disclosed exist, each differing from others in matters of detail only. Accordingly, it is intended that the art disclosed shall be limited only to the extent required by the appended claims and the rules and principles of applicable law.

Technology Classification (CPC): 7