Patent Publication Number: US-2010110877-A1

Title: System and method for failover of mobile units in a wireless network

Description:
TECHNICAL FIELD 
     Embodiments of the subject matter described herein relate generally to communication between wireless access devices and mobile units. More particularly, embodiments of the subject matter relate to failover between access devices. 
     BACKGROUND 
     A wireless connection to a computer network is vital for mobile units to permit them to exchange information among themselves or with other systems connected the network. Such connectivity is typically provided through wireless access devices. Wireless access devices are usually stationary and provide service to mobile units within range of wireless communication with them. 
     Mobile units are frequently within range of more than one wireless access device. During normal operation, a mobile unit associates itself with one wireless access device for continuous service. Typically the association manifests itself through the mutual use of a designated channel for wireless communication, as well as use by the wireless access device of a unique identifier to prevent ambiguity as to the identity of the transmitting entity or the intended recipient of a particular message. 
     Because no network device is infallible, under certain circumstances, a wireless access device can cease to operate—provide wireless service to mobile units—permanently or temporarily. In the event of such a service discontinuation, a mobile unit associated with the wireless access device will typically spend a period of time attempting to re-establish contact with the failed wireless access device. After a predetermined length of time attempting to contact the non-responsive wireless access device, the mobile unit will attempt to establish service with a new wireless access device. During the period of re-establishment attempts, the mobile unit will be unable to communicate with other network devices, which can impair performance of the mobile unit. 
     BRIEF SUMMARY 
     A method of handling a discontinuation of service from a wireless access device is provided. The first wireless access device has a first basic service set identifier (BSSID) and communicates with a plurality of mobile units using a first channel. The method comprises transmitting a first service notice with a second wireless access device, wherein the first service notice containing information instructing the first plurality of mobile units to transfer to a second channel for service, and providing service to the first plurality of mobile units using the second channel. 
     A method of providing service to a plurality of mobile units is also provided. The method comprises providing service from a first wireless access device using a first channel, the first wireless access device controlled by a system controller and communicating with the plurality of mobile units using a first BSSID, detecting an interruption of service from the first wireless access device with the system controller, transmitting a first channel switch notice with a second wireless device using the first BSSID, the first channel switch notice containing information instructing a first group of the plurality of mobile units to communicate with the second wireless access device using a second channel, and providing service from the second wireless access device using the second channel. 
     A system for providing wireless communication service to a plurality of mobile units is also provided. The system comprises a first wireless access device adapted to provide wireless communication service to the plurality of mobile units, the first wireless access device having a first BSSID and adapted to communicate with the plurality of mobile units using a first channel, a second wireless access device adapted to provide wireless service to the plurality of mobile units using the first BSSID and adapted to communicate with the plurality of mobile units using the first channel and a second channel, and a system controller adapted to detect an operational state of the first wireless access device and to operate the second wireless access device to transmit a channel switch notice in response to detecting a discontinuation of service from the first wireless access device, the channel switch notice conveying information instructing the plurality of mobile units to communicate with the second wireless access device using the second channel. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the subject matter may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures. 
         FIG. 1  is a schematic illustration of a wireless network in a first operating state; 
         FIG. 2  is a schematic illustration of the wireless network of  FIG. 1  in a second operating state; and 
         FIG. 3  is a flowchart that illustrates a method of failover for a wireless network providing service to a mobile device. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the subject matter or the application and uses of such embodiments. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Techniques and technologies may be described herein in terms of functional and/or logical block components and with reference to symbolic representations of operations, processing tasks, and functions that may be performed by various computing components or devices. In practice, one or more processor devices can carry out the described operations, tasks, and functions by manipulating electrical signals representing data bits at memory locations in the system memory, as well as other processing of signals. It should be appreciated that the various block components shown in the figures may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. 
     Certain terminology may also be used in the following description for the purpose of reference only, and thus are not intended to be limiting. For example, terms such as “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     For the sake of brevity, conventional techniques related to signal processing, wireless data transmission, signaling, wireless network infrastructure components, network control, and other functional aspects of the systems (and the individual operating components of the systems) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in an embodiment of the subject matter. 
       FIG. 1  illustrates an exemplary embodiment of a wireless network  100 . The wireless network  100  comprises a system controller  110 , first, second, and third wireless access devices  120 ,  130 ,  140 , and a plurality of mobile units  122 ,  132 ,  134 ,  136 ,  142 . The system controller  110  controls each wireless access device  120 ,  130 ,  140 , through which it can provide wireless network connectivity to each of the mobile units  122 ,  132 ,  134 ,  136 ,  142 . Each wireless access device  120 ,  130 ,  140  has a respective wireless range  121 ,  131 ,  141  within which it can provide wireless service to the mobile units  122 ,  132 ,  134 ,  136 ,  142 . For simplicity and ease of description, the wireless network  100  is depicted with only three wireless access devices. In practice, the wireless network  100  may include any number of wireless access devices, each configured to support any number of mobile units. 
     The system controller  110  is preferably a network device such as a switch, router, or other traffic-directing component. Although the system controller  110  can be embodied as various devices, it preferably includes components necessary to perform the operations and features of the systems and methods described herein. As one non-limiting example, a system controller can comprise a suitably configured processor, memory, and functional modules, such as a wireless transceiver, as well as any other device or component necessary or desirable to enable performance of the role described. The system controller  110  can transmit and receive network signals from devices to which it is coupled. Additionally, the system controller  110  can be adapted to operate other devices, including the each wireless access device  120 ,  130 ,  140 , to which it is coupled. Accordingly, the system controller  110  can monitor, regulate, and control the communication of wireless network signals to and from the mobile units  122 ,  132 ,  134 ,  136 ,  142 . In certain embodiments, the system controller  110  communicates with the wireless access devices  120 ,  130 ,  140  using conventional network interconnects and data communication protocols. For example, the system controller  110  can utilize known Ethernet data communication techniques and suitably configured network cables for communication with the wireless access devices  120 ,  130 ,  140 . 
     Although not shown with additional couplings, the system controller  110  can be coupled to other network components, thereby providing it with network access to the Internet, to an intranet, or to any other network appropriate to the embodiment. The system controller  110  can be embodied as a network component, or a portion thereof, such as a controller submodule of a network component, or a submodule of another device coupled to, and in communication with, a network component, such as those described above. 
     Each wireless access device  120 ,  130 ,  140  is preferably a wireless access point or access port adapted to provide wireless network connectivity to one or more wireless access devices. Regardless of the specific type of wireless access device, each preferably includes components necessary to perform the operations and features of the systems and methods described herein. Accordingly, each wireless access device preferably comprises a suitably configured processor, memory, and functional modules, such as a wireless transceiver, as well as any other necessary device or component. Certain embodiments of wireless access devices can further comprise wired network connections capable of providing network connectivity to fixed connection devices. The first wireless access device  120  can communicate with mobile units through conformity with one or more of the IEEE 802.11 family of standards, or through other standards appropriate to the embodiment. Preferably, each wireless access device  120 ,  130 ,  140  provides a pathway through which the mobile units it services can exchange network signals to remote network participants. Although an approximate wireless range  121 ,  131 ,  141  is shown, respectively, for each wireless access device  120 ,  130 ,  140 , the operating range of a wireless access device need not be symmetric, and the ranges illustrated are for exemplary explanation of service areas, including at least partially overlapping service areas, and the mobile units disposed therewithin, as explained further below. 
     Each wireless access device  120 ,  130 ,  140  can communicate with a mobile unit using a standard wireless channel, such as one specified in a channel list associated with a protocol in the IEEE 802.11 family. In accordance with standard wireless networking practice, each wireless access device  120 ,  130 ,  140  can communicate with an identifier denoting itself in wireless communications. Such an identifier, such as a basic service set identifier (BSSID) can be used by receiving wireless access devices to determine whether information from the transmitting source is useful or appropriate to wireless communication by the receiving device. Other identifiers, designating either a name, description of the wireless device, or other means of distinguishing among the wireless devices can also be used. 
     As one example, which will be explored in greater detail below, one of the group of mobile units  122  can be associated with the first access wireless access device  120 , and can respond to signals from the first wireless access device  120  at least in part because they contain a BSSID identifying the first wireless access device  120 . Additionally, when transmitting a signal destined for a remote network participant, a mobile unit can indicate which wireless access device it is associated with, and receives wireless service from, by including the BSSID of the wireless access device for which the signal is intended. Each wireless access device can operate with a unique BSSID. Devices with different BSSIDs can operate on a shared network. The shared network can be uniquely identified using a service set identifier (SSID). Multiple wireless access devices with distinct BSSIDs can operate on a single network with one SSID. Each wireless access device  120 ,  130 ,  140  can be configurable to use any BSSID, though preferably a different BSSID is used with each device to provide a unique identifier. One example of a unique BSSID for a wireless access device can be the media access control (MAC) address used by the wireless access device to participate on some networks. Additionally, in some embodiments, the wireless access devices  120 ,  130 ,  140  can be configured to identify itself with different BSSIDs on different wireless channels. 
     In certain embodiments, the first, second, and/or third wireless access devices  120 ,  130 ,  140  can be independently-operating wireless service providers. In such embodiments, the wireless access devices  120 ,  130 ,  140  are not coupled to a system controller. Each independent wireless access device can be capable of directing wireless network signals to and from any mobile units supported by the wireless access device. Additionally, each wireless access device preferably can detect and monitor signals from neighboring wireless access devices. Detection can include detection of such information as the channel and the BSSID used by the neighboring wireless access device, including those operating on a same-SSID network, collectively referred to as service information. Thus, although the system controller  110  is present in some embodiments, it is omitted in others where the wireless access devices independently perform at least some of the functions of the system controller  110 . 
     Each wireless access device  120 ,  130 ,  140  can operate in several states, including a normal operating state. Under certain circumstances, however, a wireless access device can experience a service failure. One example of a service failure can be the result of a loss of electrical power to the wireless access device. In another example, the wireless access device could experience a hardware or software failure, inhibiting its proper operation and preventing wireless service to mobile units serviced by the wireless access device. Typically service failure is remedied through external action, such as reconfiguring the device to overcome the hardware or software malfunction or resupplying power to the wireless access device. 
     Another operating state in which a wireless access device can be placed is that of service interruption, during which the wireless access device is functional, but temporarily unable to provide wireless connectivity to mobile units. One example of service interruption can be radiofrequency interference sufficient to interrupt service. Typically, service interruptions do not require action affecting the wireless access device to restore service. 
     The service failure state and service interruption state can be broadly categorized as service discontinuation, wherein wireless service is not provided to mobile units serviced by the wireless access device. After either occurs, wireless service from the wireless access device can resume after the conditions causing the service discontinuation are remedied. Other types of service discontinuation not explicitly listed above are also contemplated. 
     A plurality of mobile units  122 ,  132 ,  134 ,  136 ,  142  is each serviced by one of the wireless access devices  120 ,  130 ,  140 . The mobile units  122 ,  132 ,  134 ,  136 ,  142  can be any sort of wireless access device, including personal digital assistants, mobile computing platforms, integrated multifunction devices, such as bar code scanners with wireless information exchange, and so on. Preferably, regardless of the specific embodiment, each mobile unit preferably includes components necessary to perform the operations and features of the systems and methods described herein. Accordingly, each mobile unit preferably comprises a suitably configured processor, memory, and functional modules, such as a wireless transceiver, as well as any other necessary device or component. Although referred to as mobile units, in some embodiments, they can be stationary wireless network clients cooperating with one of the wireless access devices  120 ,  130 ,  140  for network connectivity. A mixture of device types can also be present in a wireless network. 
     At any given moment in time, a first group of mobile units  122  can be serviced by, or associated with, the first wireless access device  120 , as shown. Although one layout of mobile units  122  is shown, the mobile units or other devices serviced by the first wireless access device  120  can vary over time.  FIG. 1  illustrates communication between mobile units  122  and first wireless access device  120  with a distinguishable line pattern (long dashes). The line pattern indicates that the first wireless access device  120  uses a unique BSSID to interact with the mobile units. As expected, the first wireless access device  120  can only provide service to those mobile units  122  within its wireless range  121 . Although some mobile units  122  are disposed within the wireless range  131  of the second wireless access device  130 , they are associated with the first wireless access device  120 , as shown. 
     The long-dashed line illustrating communication between the first wireless access device  120  and the mobile units  122  indicates, among other aspects, the use of the BSSID “WD 1 ” by the first wireless access device  120  in communications with the mobile units  122 . Although WD 1  is used for exemplary purposes, different BSSIDs can be used by wireless access devices, including the first wireless access device  120 , where desired. As can be seen, one of the mobile units  122  is disposed within the ranges  121 ,  131  of both the first and second wireless access devices  120 ,  130 . The long-dashed line is distinct from the short-dashed line used to illustrate the association of certain mobile units  132  with the second wireless access device  130 , as described in greater detail below. It should be appreciated that the mobile units  122  associated with the first wireless access device  120  preferably use a different channel for communication than those  132 ,  134 ,  136  associated with the second wireless access device  130 . 
     The second wireless access device  130  provides wireless service to three groups of mobile units  132 ,  134 ,  136 , as can be seen by the short-dashed lines illustrating communication therebetween. The first group  132  includes mobile units disposed within the wireless ranges  121 ,  131  of the first and second wireless access devices  120 ,  130 . The second group  134  includes mobile units disposed only within the wireless range  131  of the second wireless access device  130 . The third group  136  includes mobile units disposed with the wireless ranges  131 ,  141  of the second and third wireless access devices  130 ,  140 . All of the mobile units  132 ,  134 ,  136  are associated with the second wireless access device  130 , and utilize it for wireless network connectivity. The third wireless access device  140  is substantially similar to the first wireless access device  120  for purposes of discussion and illustration. 
     While the short-dashed line indicates an association between the mobile units  132 ,  134 ,  136  and the second wireless access device  130 , including the use of the BSSID WD 2  by the second wireless access device  130 , the circle-dashed line indicates the association between the third wireless access device  140  and mobile units  142 . The short-dashed line further indicates the use of a BSSID unique to the second wireless access device  130 , “WD 2 ”, while the circle-dashed line indicates the use of the unique BSSID “WD 3 ” by the third wireless access device  140 . 
     Each mobile unit  122 ,  132 ,  134 ,  136 ,  142  preferably communicates with its respective wireless access device  120 ,  130 ,  140  using a separate channel for each device to prevent interference. The appropriate channel can be indicated as part of the practiced protocol. Similarly, each wireless access device  120 ,  130 ,  140  can operate on its individual channel using a separate SSID. As previously described, for exemplary purposes, the BSSID WD 1  will be used in conjunction with the first wireless access device  120 , and the BSSIDs WD 2  and WD 3  with the second and third wireless access devices  130 ,  140  respectively. 
     Under certain circumstances, a wireless access device can experience a service discontinuation. During the cessation of wireless service, any mobile units associated with the wireless access device will experience a “retry” period during which they attempt to re-establish contact with the wireless access device. Subsequently, they can attempt to establish contact with a new wireless access device to regain network connectivity. This procedure can be longer than desirable for systems and circumstances where reliable data transfer is preferred. 
     The inability of a wireless access device to provide service can occur for a variety of reasons, including power disruption, device failure, interference, and the like. Certain of these events can be detected by the system controller  110  by monitoring the operating state of the wireless access devices to which it is coupled. For example, the system controller  110  can be adapted to continuously monitor the functionality of each wireless access device to which it is coupled. If a wireless access device fails or otherwise experiences a wireless service discontinuation, the system controller  110  can initiate a failover. In those embodiments without a system controller  110 , each wireless access device can monitor or detect the state of its neighboring devices. In the event a wireless access device detects a service discontinuation of a neighboring wireless access device, it can initiate a failover. For exemplary purposes, the second wireless access device  130  will be shown experiencing a service discontinuation. The failover plan can be initiated by the system controller  110  operating the remaining active wireless access devices  120 ,  140 . In those embodiments without a system controller, each wireless access device can operate independently, as appropriate to the embodiment. 
     Thus, if a wireless access device discontinues service, a failover plan can be employed to reduce the interval of interrupted service.  FIG. 2  illustrates some transmissions made to effect such a failover. As can be seen, the second wireless access device  130  has discontinued wireless service to mobile units with which it was associated. Consequently, those mobile units  132 ,  136  which are positioned within the wireless range  121 ,  141  of the first and third wireless access devices  120 ,  140  have had service provided for them by the respective wireless access devices  120 ,  140  to reduce or remove the interval of network access interruption and/or disruption. 
     As can be seen, the first wireless access device  120  communicates with the mobile units  132  within its wireless range  121 . However, as  FIG. 2  indicates, the first wireless access device  120  has contacted the mobile units  132  using the BSSID of second wireless access device  130 , namely, WD 2 . Accordingly, the line indicating the association between the mobile units  132  and the first wireless access device  120  is short-dashed, indicating the contact retains commonality with the previous identity asserted by the second wireless access device  130 . Thus, each mobile unit  132  is addressed as though by the second wireless access device  130 . The first wireless access device  120  can than transmit information to the mobile units  132  instructing them to perform operations as though it was the wireless access device with which they have been associated, instead of a newly-contacting wireless access device, which it is. 
     As part of the service transition whereby the mobile units  132  failover to the first wireless access device  120 , the first wireless access device  120  can take advantage of the multiple available channels for standard wireless connectivity. Under certain protocols, including the IEEE 802.11 family of protocols, different channels can be used by different wireless access devices to reduce or eliminate interference and cross-talk on a channel between multiple wireless access devices. 
     Thus, at the time of service discontinuation, the mobile units  132  are communicating with the second wireless access device  130  using a first channel and with the second wireless access device  130  using its BSSID, WD 2 . As part of the failover, the first wireless access device  120  can transmit a channel switch announcement notice, such as a channel switch announcement primitive from the 802.11h standard. The channel switch announcement notice or service notice can contain information instructing each mobile unit  132  within the wireless range  121  of the first wireless access device  120  that it should switch to a second channel, a different channel than the first channel which had been used by the second wireless access device  130 . 
     Preferably, the channel switch announcement should be transmitted by the first wireless access device  120  using the first channel, and transmitted with the BSSID WD 2 . Accordingly, each mobile unit  132  can receive the notice from the first wireless access device  120  masquerading as the second wireless access device  130 . Additionally, the channel switch announcement or channel switch notice can be repeated over a length of time, at regular or irregular intervals, to increase the likelihood that all mobile units associated with the transmitting—or masquerading—wireless access device receive the notice. As one non-limiting example, the channel switch notice can be repeated every 0.1 seconds for 1 second. After the 1 second has passed, the wireless access device and all mobile units associated with it will begin using the new channel for communication. 
     Thus, in the illustrated exemplary embodiment, the first wireless access device  120  can repeat transmission of the channel switch notice using BSSID WD 2  over an interval, followed by transition of the mobile units  132  to the new channel. The preferred new channel for the transitioning mobile units  132  is the channel used by the first wireless access device  120  for communication with its associated mobile units  122 . The failed-over mobile units  132  would use the same channel as the mobile units  122 , as well as change their association to the first wireless access device  120 . Thereafter, the first wireless access device  120  can communicate with the mobile units  132  using its own BSSID, WD 1 . The first wireless access device  120  preferably continues to provide service to the mobile units  122  associated with it unaffected by the failover sequence, as illustrated by the continued use of the long-dashed lines. 
     From the perspective of each mobile unit  132 , the second wireless access device  130  appears to be instructing it to move to a second channel and switch service to the first wireless access device  120 . In reality, however, the first wireless access device  120  is transitioning the mobile units  132  to a channel preferably used by the first wireless access device  120  in communicating with its own associated mobile units  122 . Subsequently, the first wireless access device  120  can continue to provide wireless service to the mobile units  122 , removing or reducing any lack of network connectivity for the mobile units  122 . Such a channel switch can be similar to that of the IEEE 802.11h protocol for use in radar avoidance. 
     The third wireless access device  140  can perform a failover similar to that performed by the first wireless access device  120 . As shown in  FIGS. 1 and 2 , some mobile units  136  associated with the second wireless access device  130  can be positioned within the wireless range  141  of the third wireless access device  140 . During the service discontinuation of the second wireless access device  130 , the third wireless access device  140  can transmit a channel switch announcement to the mobile units  136  using the BSSID of the second wireless access device  130 , WD 2 , instead of its own, WD 3 . Thus, the communication to the mobile units  136  with the third wireless access device  140  using of the BSSID WD 2  is illustrated through the use of the short-dashed lines. The mobile units  136  can then be serviced on the new channel, preferably the same channel with which the third wireless access device  140  communicates with the mobile units  142  it services, and adjust their association to the third wireless access device  140 . 
     In this way, mobile units  132 ,  136  associated with the second wireless access device  130  are provided wireless service despite its service discontinuation by neighboring wireless access devices  120 ,  140 . The system controller  110  can operate the first and third wireless access devices  120 ,  140  to effect the failover. Some mobile units  134  may be positioned outside the wireless ranges  121 ,  141  of either the first and/or third wireless access devices  120 ,  140 . Accordingly, these mobile units  134  may experience a loss of service or, they may transition to other wireless access devices using a similar failover approach, if such devices are present. 
     After a period of time, the second wireless access device  130  may be restored to service. In those instances where the service discontinuation was temporary, the period of time may be short. On the other hand, if replacing the second wireless access device  130  is required for repair, it may be relatively longer. Regardless, resumption of service by the second wireless access device  130  may be detected by either the system controller  110  or an independent wireless access device, as appropriate to the embodiment. Subsequently, the first wireless access device  120  can initiate a restoration sequence, whereby the mobile units  132  are returned to service by and an association with the second wireless access device  130 . 
     To transition the mobile units  132  back to service with the restored second wireless access device  130 , the first wireless access device  120  can transmit a channel switch notice, similar to the one described above. The channel switch notice can contain information instructing the mobile units  132  to communicate with the transmitting wireless access device—here, represented to be the first wireless access device  120 —using the original channel used by the second wireless access device  130 . The channel switch notice to return mobile units  132  to the second wireless access device  130  can conform to any standard or protocol desired. Preferably, the channel switch notice distinguishes the mobile units  132  which were transitioned from the second wireless access device  130  from those mobile units  122  which had always been associated with the first wireless access device  120 . Thus, only mobile units  132  are channel-switched and returned to association with the second wireless access device  130 , and preferably not all mobile units associated with the first wireless access device  120 . Such a channel switch notice can be part of a proprietary or custom-developed networking protocol, if desired, and need not conform to industry standards. 
     The first wireless access device  120  initiates the transition of the channel switch for the mobile units  132 . Preferably, the first wireless access device  120  or system controller  110  has recorded information uniquely identifying which mobile units  132  have recently been transitioned. The first wireless access device  120  can then address the channel switch notices directly to only those mobile units  132 . Thereafter, upon returning to the original channel, the mobile units  132  can communicate with the second wireless access device  130  in its normal operating state. The third wireless access device  140  can perform a return transition of the mobile units  136  it has failed-over back to the second wireless access device  130  in a similar manner. Subsequent to a return of mobile units  132 ,  134 ,  136  to service from the second wireless access device  130 , the exemplary network appears in the state illustrated in  FIG. 1 . 
     The failover sequence described above can also be implemented in a self-healing network. In a self-healing network, service discontinuation by one wireless access device is remedied in the short term by transitioning affected mobile units to neighboring wireless access devices. The transitioned mobile units can later, as a longer-term solution, be re-associated with the neighboring wireless access device. Accordingly, the need for the failed wireless access device to service the affected mobile units is removed, representing the self-healing aspect of the network. 
       FIG. 3  is a flowchart illustrating the steps of a method practicable by the above-described systems to implement the described failover sequence. The various tasks performed in connection with method  300  below may be performed by software, hardware, firmware, or any combination thereof. For illustrative purposes, the following description of method  300  may refer to elements mentioned above in connection with  FIGS. 1 and 2 . In practice, portions of method  300  may be performed by different elements of the described system, e.g., system controller  110  and/or the wireless access devices  120 ,  130 ,  140 . It should be appreciated that method  300  may include any number of additional or alternative tasks; the tasks shown in  FIG. 3  need not be performed in the illustrated order, and method  300  may be incorporated into a more comprehensive procedure or process having additional functionality not described in detail herein. 
     During normal operation, a first wireless access device provides service to at least one mobile unit using a first channel (task  302 ). The first wireless access device uses a first BSSID to communicate with the mobile unit. Subsequently, the first wireless access device can experience a service discontinuation. The service discontinuation can be detected, either by a system controller coupled to the first wireless access device, or by a remote wireless access device monitoring the service information sent by the first wireless access device (task  304 ). 
     In response to detecting the service discontinuation, a neighboring, second wireless access device can transmit a first service notice using the first channel (task  306 ). Preferably, the second wireless access device transmits the first service notice using the first BSSID. The first service notice contains information instructing the mobile unit to transfer to a second channel, but continuing service. The first (and subsequent) service notice can be transmitted more than once, if desired. 
     After transmitting the first service notice, the second wireless access device can provide service to the mobile unit using the second channel (task  308 ). As part of the transition to the second channel, the mobile unit can alter its association to the second wireless access device. Subsequent communications between the second wireless access device and the mobile unit can be made with a second BSSID, unique to the second wireless access device. During this period, and throughout the operation of the second wireless access device, it can additionally provide wireless service to one or more other mobile units associated with it. Preferably, the second wireless access device uses the second channel for communication with its associated mobile units. Thus, by transitioning the first mobile unit to the second channel, the second wireless access device need only use a single channel to communicate with its own associated mobile units, as well as the new mobile unit from the first wireless access device. 
     At some later point, the first wireless access device may resume service. Preferably such resumption is detected, either by the system controller or by another wireless access device, such as the second wireless access device (task  310 ). In response to detecting the resumption of service, the second wireless access device can transmit a second service notice using the second channel (task  312 ). Preferably, the second service notice is sent addressed only the transitioned mobile unit, and not the mobile units which have always been associated with the second wireless access device. The second service notice contains information instructing the mobile unit originally associated with the first wireless access device to begin using the first, original channel for communication. 
     After the mobile unit returns to the first channel, the restored first wireless access device can resume providing service to it using the first channel and, appropriately, the first BSSID (task  314 ). In this way, the mobile unit associated with the first wireless access device fails over to the second wireless access device for service upon a service discontinuation by the first wireless access device. Thus, the mobile unit can experience as short a service interruption as possible, including eliminating it entirely in some circumstances. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application.