Patent Publication Number: US-11032874-B2

Title: Shadow access point for hierarchical tree network using 802.11 infrastructure nodes in fire detection systems and other systems

Description:
PRIORITY INFORMATION 
     This application is a Continuation of U.S. application Ser. No. 13/826,342, filed Mar. 14, 2013, the contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to monitoring and alarm systems. More specifically, this disclosure relates to a shadow access point for a hierarchical tree network using IEEE 802.11 infrastructure nodes in fire detection systems and other systems. 
     BACKGROUND 
     Fire detection systems and other monitoring and alarm systems routinely include a large number of monitoring nodes distributed in a building or other space. The monitoring nodes monitor their surrounding environment and trigger an alarm when a specified condition is detected. 
     In some systems, monitoring nodes communicate wirelessly with each other and with a centralized monitoring and alarm station. Conventional monitoring and alarm systems that support wireless communications often use battery-operated monitoring nodes with narrowband radios. These monitoring nodes can form a wireless mesh network, which allows the monitoring nodes to monitor a large area. However, in wireless monitoring and alarm systems, redundant communication links are often necessary or desired for providing reliable communications. Providing redundant communication links in a wireless mesh network is typically very easy. In wireless networks supporting other protocols, however, it can be much more difficult to provide redundant wireless communication links. 
     SUMMARY 
     This disclosure provides a shadow access point for a hierarchical tree network using IEEE 802.11 infrastructure nodes in fire detection systems and other systems. 
     In a first embodiment, a system includes first and second access points. Each access point includes one or more 802.11 wireless radios configured to communicate with a wireless node. The first and second access points are both configured to wirelessly receive first data from the wireless node at substantially a same time and forward the first data. The first and second access points are also both configured to receive second data for the wireless node. The first access point is configured to wirelessly is transmit the second data to the wireless node, and the second access point is configured to refrain from transmitting the second data to the wireless node while the first access point is operating properly. 
     In a second embodiment, an apparatus includes one or more 802.11 wireless radios configured to communicate with a wireless node and a controller configured to cause the apparatus to operate as one of: a primary access point and a shadow access point. As the primary access point and as the shadow access point, the apparatus is configured to wirelessly receive first data from the wireless node and forward the first data. As the primary access point, the apparatus is configured to receive second data for the wireless node and wirelessly transmit the second data to the wireless node. As the shadow access point, the apparatus is configured to receive the second data for the wireless node and refrain from transmitting the second data to the wireless node while another access point is operating properly to send the second data to the wireless node. 
     In a third embodiment, a method includes communicating at a first access point with a wireless node, where the first access point includes one or more 802.11 wireless radios. The method also includes selectively operating the first access point as one of: a primary access point and a shadow access point. As the primary access point and as the shadow access point, the first access point wirelessly receives first data from the wireless node and forwards the first data. As the primary access point, the first access point receives second data for the wireless node and wirelessly transmits the second data to the wireless node. As the shadow access point, the first access point receives the second data for the wireless node and refrains from transmitting the second data to the wireless node while a second access point is operating properly and transmits the second data to the wireless node. 
     Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more disclosure, reference complete understanding is now made to the of this following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates an example monitoring and alarm system in accordance with this disclosure; 
         FIG. 2  illustrates an example wireless node in a monitoring and alarm system in accordance with this disclosure; 
         FIGS. 3 and 4  illustrate example backhaul communications involving access points in a monitoring and alarm system in accordance with this disclosure; and 
         FIG. 5  illustrates an example method for is operating a monitoring and alarm system with a shadow access point using IEEE 802.11 infrastructure nodes in accordance with this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 5 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the invention may be implemented in any type of suitably arranged device or system. 
       FIG. 1  illustrates an example monitoring and alarm system  100  in accordance with this disclosure. As shown in  FIG. 1 , the system  100  includes a central station  102  and multiple IEEE 802.11 wireless nodes  104   a - 104   l . The central station  102  supports centralized monitoring of a given area or areas based on data from the wireless nodes  104   a - 104   l . The central station  102  also generates or presents alarms to notify personnel of one or more conditions detected by the wireless nodes  104   a - 104   l . The central station  102  includes any suitable structure supporting monitoring or alarming operations. For example, the central station  102  could include one or more displays for presenting information to one or more users, such as information identifying data from the wireless nodes  104   a - 104   l  and any detected conditions or alarms. 
     Each wireless node  104   a - 104   l  represents a monitoring node having detection or monitoring components allowing the node to monitor its surrounding environment and detect one or more specified conditions. The specified conditions could include the presence of smoke, heat, fire, carbon monoxide, movement, or intruders. The wireless nodes  104   a - 104   l  are also capable of transmitting and receiving wireless signals using wireless radios that support one or more IEEE 802.11 protocols. Each wireless node  104   a - 104   l  could further support additional functions depending on the implementation. Each wireless node  104   a - 104   l , includes any suitable structure for detecting one or more conditions. Each wireless node  104   a - 104   l  also includes one or more IEEE 802.11 wireless radios. The wireless nodes  104   a - 104   l , could represent devices powered internally by local power supplies (such as batteries, solar cells, or fuel cells) or line-powered devices. 
     In this example, communications between the central station  102  and the wireless nodes  104   a - 104   l  occur through various 802.11 access point groups  106   a - 106   n . Each access point group  106   a - 106   n  includes multiple 802.11 access points  108   a - 108   b . The access points  108   a - 108   b  are capable of transmitting wireless signals to and receiving wireless signals from the wireless nodes  104   a - 104   l . The access points  108   a - 108   b  are also capable of communicating with the central station  102  through wired or wireless backhaul links. Each access point  108   a - 108   b  includes any suitable structure facilitating communication between multiple monitoring nodes and a backhaul link, such as one or more IEEE 802.11 wireless radios. 
     An optional gateway  110  could be used in the system  100  to support the exchange of data between the central station  102  and the access point groups  106   a - 106   n . For example, if the access point groups  106   a - 106   n  communicate over wireless backhaul links, the gateway  110  could communicate with the access point groups  106   a - 106   n  over the wireless backhaul links. The gateway  110  could also be coupled to the central station  102  using a wired connection. Here, the gateway  110  could convert wireless-formatted data from the access point groups  106   a - 106   n  into wired-formatted data for the central station  102  (or vice versa). The gateway  110  could also support the exchange of data between access point groups  106   a - 106   n . The gateway  110  includes any suitable structure facilitating communication between different components using different protocols. Note, however, that the use of the gateway  110  is optional. For instance, the central station  102  could communicate directly with the access point groups  106   a - 106   n  over wired or wireless backhaul links. 
     In this example, various network devices (components  104   a - 104   l ,  108   a - 108   b ,  110 ) use IEEE 802.11 wireless radios in infrastructure mode, and these components are arranged in a hierarchical tree configuration. That is, each wireless component in the system  100  (except the top node) communicates with a parent node, and each wireless component in the system  100  can communicate with one or more child nodes. The head of the tree is typically the central station  102  or the gateway  106 . The number of network devices in each level of the tree and the number of levels in the tree can vary depending on the particular implementation. 
     In general, IEEE 802.11 protocols allow wireless radios to operate in the infrastructure mode, where each wireless radio can function as one of an access point or a station (or possibly both at the same time). An access point in the IEEE 802.11 protocols generally represents a device that receives and routes data from one or more stations. A station in the IEEE 802.11 protocols generally represents a device that communicates with other devices via an access point. 
     Unfortunately, the IEEE 802.11 protocols only allow a station to associate with a single access point when operating in 802.11 infrastructure mode. While redundant communication paths can easily exist in wireless mesh networks, this feature of the IEEE 802.11 protocols typically forces a monitoring device with an IEEE 802.11 radio to have a single communication link for wireless communication with a single access point. This makes the overall network much less reliable. This can be problematic in a number of ways. For instance, in a fire detection or security system, the failure of a single access point could prevent multiple monitoring nodes from triggering fire or intrusion alarms. 
     In accordance with this disclosure, the access point groups  106   a - 106   n  provide redundant wireless communication paths even in 802.11-based systems operating in infrastructure mode. Each access point group  106   a - 106   n  includes a primary access point  108   a  and a shadow access point  108   b . The primary and shadow access points  108   a - 108   b  in each group  106   a - 106   n  can be co-located, meaning they are placed physically near one another. All access points in the system  100  can share the same 802.11 service set identifier (SSID), or multiple subnetworks could use different SSIDs with different access points. 
     During normal operation, the primary and shadow access points  108   a - 108   b  in a group  106   a - 106   n  share the same network parameters, authentication credentials, network credentials, and other information. This information could be shared in any suitable manner, such as via the central station  102  or gateway  110 . The primary access point  108   a  in the group  106   a - 106   n  transmits data to those wireless nodes  104   a - 104   l  e operating as child nodes to the primary access point  108   a , while the shadow access points  108   b  in the group  106   a - 106   n  operates in listening mode. Effectively, the shadow access point  108   b  behaves as a “sniffer” and remains ready to assume operation as a primary access point if problems develop with the associated primary access point  108   a . An access point&#39;s transition from shadow mode to primary mode could occur in response to any suitable criteria, such as an indication from the central station  102 . 
     As an example of network operations, the primary access point  108   a  in each group  106   a - 106   n  can be responsible for transmitting beacon signals to the wireless nodes  104   a - 104   l  that are associated with that access point  108   a . The shadow access point  108   b  in that group  106   a - 106   n  could listen for the beacon signals to help verify that the primary access point  108   a  is operating properly. If a primary access point  108   a  fails, its shadow access point  108   b  could immediately begin transmitting the beacon signals using the same network information, such as the same basic service set (BSS) identifier (BSSID). From the perspective of the wireless nodes served by the failed access point  108   a , no change in network operation may be noticed since a single access point is still transmitting beacons using the same BSSID. In the IEEE 802.11 protocols, a BSS represents a single access point and all stations communicating with that access point. In  FIG. 1 , however, since only the primary access point  108   a  in a group transmits data to wireless nodes while the shadow access point  108   b  in the group listens, both access points  108   a - 108   b  can share the same BSSID. 
     As another example of network operations, when a wireless node  104   a - 104   l  transmits data to its primary access point  108   a , the shadow access point  108   b  (operating in listening mode) can receive the same data. As a result, both the primary and shadow access points  108   a - 108   b  can forward the data to the central station  102 , and the central station  102  may support the ability to recognize and discard duplicate data messages. Alternatively, one or both access points  108   a - 108   b  could be used to identify and handle duplicate messages, or a component inserted between the access points  108   a - 108   b  and the central station  102  (such as the gateway  110 ) could identify and handle duplicate messages. From the perspective of the wireless node  104   a - 104   l , again no modification of the IEEE 802.11 protocol is necessary since the wireless node appears to transmit data to a single access point. 
     Similarly, for outgoing data sent to a wireless node  104   a - 104   l , the primary access point  108   a  serving that wireless node can transmit wireless messages to the wireless node. The shadow access point  108   b  can receive the same data to be transmitted to the wireless node, but the shadow access point  108   b  refrains from transmitting that data as long as the primary access point  108   a  is operating properly. Once again, from the perspective of the wireless node, no modification of the IEEE 802.11 protocol is necessary since the wireless node receives data from a single access point. 
     In some embodiments, the central station  102  can configure and reconfigure various components in the system  100 , such as during installation or at other times. For instance, the central station  102  could change the network parameters of the system during operation as needed. The central station  102  can also be responsible for detecting faulty access points and reconfiguring the operational mode (primary versus shadow) for the access points  108   a - 108   b  in each group. For example, primary and shadow access points  108   a - 108   b  could transmit “heartbeat” signals indicating their health to the central station  102 . The heartbeat signals help to maintain the health of the system, and non-receipt of a heartbeat signal (such as for a specified duration) can be used to identify non-functional access points for repair or replacement. The heartbeat signals could be sent at any suitable interval, such as one heartbeat signal per Target Beacon Transmission Time (TBTT) interval, although other intervals (like more than one heartbeat signal per TBTT) can be used. Also, different access points  108   a - 108   b  in a group could transmit heartbeat signals at the same interval or at different intervals. 
     In the event of a malfunction of a primary access point  108   a , the central station  102  can stop receiving that access point&#39;s heartbeat signals. In response, the central station  102  causes the associated shadow access point  108   b  to switch modes and operate in the primary access point mode, and the old primary access point  108   a  can be marked for repair or replacement. In the event of a malfunction of a shadow access point  108   b , the central station  102  can stop receiving that access point&#39;s heartbeat signals. The system can continue to function as usual (since the primary access point  108   a  can still communicate with wireless nodes), and the shadow access point  108   b  can be marked for repair or replacement. 
     In this way, each shadow access point  108   b  supports a communication path that is redundant to the communication path supported by its primary access point  108   a . Moreover, this is done in a manner that is generally invisible to the wireless nodes  104   a - 104   l  communicating with those access points  108   a - 108   b . As a result, compliance with IEEE 802.11 protocols is maintained, allowing standard 802.11 wireless radios to be used. 
     The use of standard IEEE 802.11 wireless radios in various components of  FIG. 1  can provide several advantages. For example, IEEE 802.11 radios are typically less expensive than custom mesh network radios, and IEEE 802.11 radios can offer higher data rates and larger bandwidths. This allows the system  100  to support the creation of a wireless network with larger network capacities, which can support additional services like voice and video transport. Also, IEEE 802.11 radios are becoming more and more power efficient, reducing power consumption and prolonging the operational life of any internal power supplies in network components (assuming the network components are not line-powered). In addition, IEEE 802.11 radios support the creation of secured wireless networks based on standard IEEE 802.11 security protocols, support better interference handling, and increase the ease of implementing and maintaining network-level time synchronization. The overall result allows the system  100  to create a larger-capacity and longer-range wireless network using standard IEEE 802.11-based devices. 
     In some embodiments, access points and wireless nodes communicate using a time division multiple access (TDMA) protocol. For example, a TDMA protocol could divide time into multiple time slots, and communications between an access point and a wireless node occur during the time slot assigned to that wireless node. Also, in some embodiments, a wireless node can periodically wake up and wirelessly transmit data (such as sensed information) to its access point(s)  108   a - 108   b . The transmit address in a data message can be the BSSID of its access point(s)  108   a - 108   b , and the message may be received by both the primary and shadow access points  108   a - 108   b . A wireless node may also wake up periodically to receive beacons or other signals from its primary access point  108   a . The allocation of time slots for wireless transmissions can be done in any suitable manner, such as one-time during network setup and possibly changed later to accommodate new nodes or changing network conditions. 
     Each access point  108   a - 108   b  can be aware of the number of wireless nodes associated with it, and non-receipt of data from a wireless node for a considerable duration (such as a configurable time) can be flagged by the access point to the central station  102  as a possible failure of the wireless node. Wireless nodes can also transmit other information, such as the states of their internal power supplies, to the access points for delivery to the central station  102 . Access points can similarly transmit their health information, as well as information about the wireless network (such as RSSI or interference measurements) to the central station  102 . Such information and statistics can be collected at the central station  102  to support maintenance tasks. 
     In some embodiments, the wireless nodes  104   a - 104   l  may be authenticated with their respective access points (such as their primary access points  108   a ) during network installation, node installation, or other time(s). Authentication could occur using a pre-shared key the first time, and the security credentials can be changed later. Authentication can help to restrict a rogue node from injecting faults into the network. Authentication information received or used by the primary access point  108   a  for communication with a wireless node could be shared with the corresponding shadow access point  108   b , such as over their backhaul links. 
     The use of shadow access points could be beneficial in any suitable monitoring or alarming system. Example systems include fire detection systems, gas sensor systems, and public announcement systems. Any other suitable system could incorporate this functionality. 
     Although  FIG. 1  illustrates one example of a monitoring and alarm system  100 , various changes may be made to  FIG. 1 . For example, the system  100  could include any number of central stations, access point groups, and gateways, and various components (such as the gateway) could be omitted. The system  100  could also include any number of wireless monitoring nodes arranged in any suitable number of hierarchical levels in a tree. Further, the monitoring functionality of a wireless node  104   a - 104   t  could be incorporated into an access point  108   a - 108   b , and/or the routing functionality of an access point  108   a - 108   b  could be incorporated into a wireless node  104   a - 104   l . As a result, the use of a shadow access point  108   b  for a primary access point  108   a  could be incorporated into multiple levels of the hierarchy, including in the nodes  104   a - 104   l . Moreover, each access point group  106   a - 106   n  could include more than two access points, thereby providing more than two communication paths. In addition, note that not every access point in the system  100  needs a backup shadow access point. The use of shadow access points could be limited, for instance, to those primary access points transporting more important or critical data. 
       FIG. 2  illustrates an example wireless node  200  in a monitoring and alarm system in accordance with this disclosure. The wireless node  200  here could represent any of the wireless nodes  104   a - 104   l  or access points  108   a - 108   b  in the system  100  of  FIG. 1 . The wireless node  200  could also represent the gateway  110  or part of the central station  102  in  FIG. 1 . 
     As shown in  FIG. 2 , the node  200  includes a controller  202 , which controls the overall operation of the node  200 . For example, the controller  202  may obtain or generate data to be transmitted, such as data based on one or more sensed environmental conditions. The controller  202  could also generate heartbeat signals or other signals used to indicate proper operation of the wireless node  200 . The controller  202  could further receive data transmitted wirelessly. In addition, the controller  202  in an access point  108   a - 108   b  could control whether the access point operates as a primary or shadow access point. The controller  202  includes any suitable structure for controlling operation of a wireless node. As particular examples, the controller  202  could represent at least one microprocessor, microcontroller, field programmable gate array, application specific integrated circuit, or other processing or control device 
     A memory  204  is coupled to the controller  202 . The memory  204  stores data used, collected, or generated by the node  200 . For example, the memory  204  could store information received at or to be transmitted from the 15 wireless node  200 . The information to be transmitted could originate at the wireless node  200  or be received from another device for relay towards an intended destination. The memory  204  includes any suitable volatile and/or non-volatile storage and retrieval device(s). 
     The node  200  also includes at least one IEEE 802.11 wireless radio  206  and at least one antenna  208 . The wireless radio(s)  206  and antenna(s)  208  can be used to communicate wirelessly with other devices, such as the central station  102 , wireless nodes  104   a - 104   l , access points  108   a - 108   b , or gateway  110 . Each wireless radio  206  includes any suitable structure for communicating using at least one IEEE 802.11 protocol. Each antenna  208  includes any suitable structure for transmitting and receiving wireless signals. 
     If the node  200  represents a monitoring device, the node  200  could also include one or more monitoring components  210 . The monitoring components  210  allow the node  200  to sense one or more environmental conditions around the node  200 . For example, the monitoring components  210  could be used to detect smoke, heat, fire, one or more chemicals, movement, or other conditions near the node  200 . The monitoring components  210  include any suitable structure(s) capable on monitoring or detecting one or more conditions. 
     If the node represents the gateway  110  or an access point  108   a - 108   b , the node  200  could further include a backhaul interface  212 . The backhaul interface  212  allows the node  200  to communicate over a wired or wireless backhaul network, such as an Ethernet network. Among other things, this allows the gateway  110  or access point  108   a - 108   b  to communicate data towards the central station  102 . The backhaul interface  212  includes any suitable structure supporting communications over a backhaul link. 
     Although  FIG. 2  illustrates one example of a wireless node  200  in a monitoring and alarm system, various changes may be made to  FIG. 2 . For example, various components in  FIG. 2  could be combined, subdivided, or omitted and additional components could be added according to particular needs. Also, a “wireless node” represents any device that can transmit or receive data wirelessly, even if the “wireless node” also has the ability to transmit or receive data over a wired connection and/or has the ability to receive power over a wired connection. 
       FIGS. 3 and 4  illustrate example backhaul communications involving access points  108   a - 108   b  in a monitoring and alarm system in accordance with this disclosure. In particular,  FIGS. 3 and 4  illustrate example backhaul communications in monitoring and alarm systems  100 ′ and  100 ″, which are similar in structure to the system  100  of  FIG. 1 . 
     As shown in  FIG. 3 , the system  100 ′ lacks a gateway  110  (although one could be used). Also, the access points  108   a - 108   b  communicate with the central station  102  via wired backhaul links  302 . These backhaul links  302  couple the access points  108   a - 108   b  to the central station  102  and optionally to one another, and the access points  108   a - 108   b  use Ethernet or other wired protocol(s) to communicate. Optionally, power can be provided to the access points  108   a - 108   b  over the backhaul links  302 , such as by using Power over Ethernet (PoE). If collisions are possible when communicating over the backhaul links  302 , various content ion schemes could be used by the access points  108   a - 108   b , such as carrier sense multiple access with collision detection (CSMA/CD). In other embodiments, time slots in a TDMA protocol can be used for communications. 
     In the embodiment shown in  FIG. 3 , no special considerations may be needed for the design of the wireless nodes  104   a - 104   l . These nodes  104   a - 104   l  can operate as regular 802.11 stations. Similarly, when an access point is operating as a primary access point  108   a , no special design considerations may be needed. The access point  108   a  can simply function as a regular 802.11 access point. When an access point is operating as a shadow access point  108   b , the access point  108   b  can behave as a sniffer as described above. The shadow access point  108   b  can receive the same data over its backhaul link  302  that a primary access point  108   a  receives over its backhaul link  302 , although the shadow access point  108   b  may refrain from transmitting that data. Similarly, the shadow access point  108   b  may receive the same wireless messages sent to a primary access point  108   a  from a wireless node, and the shadow access point  108   b  may or may not forward the wireless messages. In addition, the primary and shadow access points  108   a - 108   b  can share authentication credentials and network credentials through their backhaul links  302  (possibly with the central station  102  acting as an arbiter between the access points). As a result, the shadow access point  108   b  can remain ready to immediately assume the primary access point role, such as when indicated by the central station  102 . 
     As shown in  FIG. 4 , the system  100 ″ again lacks a gateway  110 , although one could be used. Also, the access points  108   a - 108   b  communicate with the central station  102  (and possibly each other) via wireless backhaul links  402 . These wireless links  402  represent 802.11 wireless channels that can be used to transport data to and from the access points  108   a - 108   b . Optionally, one or more additional access points  404   a - 404   p  could be used to increase the distance that the access points  108   a - 108   b  can reside from the central station  102 . Note that the access points  108   a - 108   b  could be internally powered or line-powered devices. 
     An access point  108   a - 108   b  with a wireless backhaul link  402  can be implemented as a multi-role or multi-persona device. When communicating with components in a lower level of a tree hierarchy (such as one or more wireless nodes  104   a - 104   l ), an access point  108   a - 108   b  can be configured to operate using a “parent” persona in which the access point functions in the 802.11 access point mode. In this mode, the access point  108   a - 108   b  communicates with child nodes in the lower level of the tree. When communicating with components in a higher level of the tree hierarchy (such as an access point  404   a - 404   p  or the central station  102 ), an access point  108   a - 108   b  can be configured to operate using a “child” persona in which the access point functions in the 802.11 station mode. In that mode, the access point  108   a - 108   b  communicates with its parent node in the higher level of the tree. If desired, the network components in various levels of the tree hierarchy could operate in this manner, where information is propagated up the tree using child-to-parent communications and information is propagated down the tree using parents to-child communications. 
     Various approaches could be used to support multiple personas in a single 802.11 device. For example, in some embodiments, each access point  108   a - 108   b  could include multiple 802.11 wireless radios  206 , where one wireless radio  206  is operated in access point mode and another wireless radio  206  is operated in station mode. In other embodiments, each access point  108   a - 108   b  could include a single 802.11 wireless radio  206 , and the radio  206  could be reconfigured to operate in access point mode or station mode at different times. In still other embodiments, each access point  108   a - 108   b  could include a single 802.11 wireless radio  206  supporting WiFi Direct, a feature where WiFi devices can connect and communicate with one another without requiring an intermediate access point. To support this, each wireless radio  206  could operate in access point mode (with the parent persona) or station mode (with the child persona). Additional details regarding the use of multiple personas in an 802.11 device of a tree hierarchy can be found in U.S. patent application Ser. No. 13/826,709 titled “HIERARCHICAL TREE NETWORK USING TDMA PROTOCOL WITH 802.11 INFRASTRUCTURE NODES FOR FIRE DETECTION SYSTEMS AND OTHER SYSTEMS”, which is hereby incorporated by reference. 
     When supporting wireless communications, the central station  102  (or the gateway  110 ) can support any suitable technique to communicate with multiple access points  108   a - 108   b . For example, the central station  102  could create an 802.11 infrastructure network with its wireless radio  206 , and the infrastructure network could include different orthogonal channels for transmissions to and from different access points  108   a - 108   b . The central station  102  and other network components could also share time using a TDMA protocol. 
     Contentions in transmissions over the wireless backhaul links  402  could be handled using schemes such as Enhanced Distributed Channel Access (EDCA), hybrid coordination function (HCF) controlled channel access (HCCA), or CSMA/CD. 
     In the embodiment shown in  FIG. 4 , no special considerations may be needed for the design of the wireless nodes  104   a - 104   l . These nodes  104   a - 104   l , can operate as regular 802.11 stations. The access points  108   a - 108   b  can be designed to operate using multiple personas as described above. Thus, when an access point is operating as a primary access point  108   a , the access point  108   a  can receive data in one persona and forward it in another persona. When an access point is operating as a shadow access point  108   b , the access point  108   b  could similarly switch back and forth between the parent and child personas. For example, the shadow access point  108   b  can receive the same data over its backhaul link  402  that a primary access point  108   a  receives over its backhaul link  402 , although the shadow access point  108   b  may refrain from transmitting that data. Similarly, the shadow access point  108   b  may receive the same wireless messages sent to a primary access point  108   a  from a wireless node, and the shadow access point  108   b  may or may not forward the wireless messages. In addition, the primary and shadow access points  108   a - 108   b  can share authentication credentials and network credentials through their backhaul links  402  (possibly with the central station  102  acting as an arbiter between the access points). As a result, the shadow access point  108   b  can remain ready to immediately assume the primary access point role, such as when indicated by the central station  102 . The access points  404   a - 404   p  shown here may represent standard 802.11 access points, or they may represent multi-persona devices s that operate in both 802.11 access point mode and 802.11 station mode. 
     Although  FIGS. 3 and 4  illustrate examples of backhaul communications in a monitoring and alarm system, various changes may be made to  FIGS. 3 and 4 . For example, at least one gateway  110  could be used between the access points  108   a - 108   b  and the central station  102  in each figure. Also, a combination of wired and wireless backhaul links could be used in a monitoring and alarm system. 
       FIG. 5  illustrates an example method  500  for 15 operating a monitoring and alarm system with a shadow access point using IEEE 80211 infrastructure nodes in accordance with this disclosure. As shown in  FIG. 5 , operational parameters are shared between primary and shadow access points at step  502 . This could include, for example, a primary access point  108   a  sharing its network parameters (such as SSID and BSSID values), authentication credentials, network credentials, and other information with a shadow access point  108   b    
     Data is propagated up a tree hierarchy at step  503 . Among other things, this includes receiving data from a monitoring node at the primary and shadow access points at step  504 . The same wireless messages can be received from one of the wireless nodes  104   a - 104   l  at both its primary and shadow access points  108   a - 108   b . This also includes transmitting the data towards a central station from at least one of the access points at step  506 . The data can be transmitted from one or both of the access points  108   a - 108   b . Some component (such as the gateway  110  or central station  102 ) could identify duplicative data messages and discard the duplicate messages. Alternatively, only one of the access points, such as the primary access point  108   a , could transmit the data. The data is eventually delivered to the central station at step  508 . This could include providing the data from the access point{s) directly to the central station  102  or indirectly to the central station  102  via the gateway  110 . The data can be used by the central station  102  in any suitable manner. 
     The central station generates data for delivery to the monitoring node at step  510 . This could include, for example, the central station  102  generating data for modifying the behavior of or requesting additional information from a wireless node  104   a - 104   l . The data is propagated down the tree hierarchy at step  511 . Among other things, this includes receiving the data for the monitoring node at the primary and shadow access points at step  512 . The same data can be received at both the primary and shadow access points  108   a - 108   b . This also includes transmitting the data to the monitoring node from the primary access point at step  514 . During this step, the shadow access point  108   b  can refrain from transmitting the data, allowing only the primary access point  108   a  to transmit the data. The data is eventually delivered to the monitoring device at step  516 . The data can be used by the monitoring device in any suitable manner. 
     If the primary access point remains operational and does not fail at step  518 , steps  504 - 516  could occur any number of times. In the event the primary access point fails (such as due to a hardware, software, or power failure), the shadow access point switches to the primary mode of operation at step  520 . This could include, for example, causing the shadow access point  108   b  to assume the role of the primary access point. At this point, steps  504 - 516  could repeat with the access point  108   b  operating as a primary access point. No shadow access point may be present at this point since the previous primary access point has failed. If and when the failed access point is repaired or replaced, that access point could assume operation as a new primary access point or as a shadow access point. 
     Although  FIG. 5  illustrates one example of a method  500  for operating a monitoring and alarm system with a shadow access point using IEEE 802.11 infrastructure nodes, various changes may be made to  FIG. 5 . For example, while shown as a series of steps, various steps in  FIG. 5  could overlap, occur in parallel, occur in a different order, or occur any number of times. Also, various steps in  FIG. 5  could be omitted, such as when communication in a monitoring and alarm system is one-way (like from the monitoring devices to the central station). In addition, the primary access point  108   a  can fail at any point in the method  500  and need not fail at the specific point shown here. 
     In some embodiments, various functions described above are implemented or supported by a computer program that is formed from computer readable program code and that is embodied in a computer readable medium. The phrase “computer readable program code” includes any type of computer code, including source code, object code, and executable code. The phrase “computer readable medium” includes any type of medium capable of being accessed by a computer, such as read only memory (ROM), random access memory (RAM), a hard disk drive, a compact disc (CD), a digital video disc (DVD), or any other type of memory. A “non-transitory” computer readable medium excludes wired, wireless, optical, or other communication links that transport transitory electrical or other signals. A non-transitory computer readable medium includes media where data can be permanently stored and media where data can be stored and later overwritten, such as a rewritable optical disc or an erasable memory device. 
     It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrase “associated with,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like. The phrase “at least one of,” when used with a list of items, means that different combinations of one or more of the listed items may be used, and only one item in the list may be needed. For example, “at least one of: A, B, and C” includes any of the following combinations: A, B, C, A and B, A and C, Band C, and A and B and C. 
     While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.