Patent Publication Number: US-10313945-B2

Title: Automatic channel layering in a Wi-Fi communication system

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of priority as a continuation of U.S. patent application Ser. No. 14/866,812, filed Sep. 25, 2015, entitled AUTOMATIC CHANNEL LAYERING IN A WI-FI COMMUNICATION SYSTEM, by Sirivatsan Sankaranarayanan, which is a continuation of U.S. patent application Ser. No. 13/354,264, filed Jan. 19, 2012, entitled AUTOMATIC CHANNEL LAYERING IN A WI-FI COMMUNICATION SYSTEM, by Sirivatsan Sankaranarayanan, which is related to U.S. Pat. No. 7,826,426, issued Nov. 2, 2010, entitled SEAMLESS MOBILIBY IN WIRELESS NETWORKS, issued to Vaduvur Bharghavan et al., the entire contents of both being hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to a Wi-Fi communication system, and more specifically, to automatic channel layering in a Wi-Fi communication system. 
     BACKGROUND 
     In wireless communication using an IEEE 802.11 protocol, wireless stations connect to a wireless communication system using access points (“AP&#39;s”). Each AP services those stations which it is best suited to service, typically those stations which are nearest or otherwise have best signal strength, and for which there is little or no interference with other access points. If the AP&#39;s are spaced too far apart, there might be regions (“coverage holes”) in which coverage by the system is limited, spotty, or even nonexistent. For example, this can be due to difficulty by stations in finding an access point with adequate signal strength. On the other hand, if the AP&#39;s are spaced too closely, there might be regions in which the wireless communication system is operating inefficiently. For example, this can be due to difficulty by access points in communicating without interference. 
     In a system described in the Incorporated Disclosures, a system control element selects, for each station, which AP will communicate with, and service, that station. Multiple AP&#39;s can each communicate with their associated stations in the communication system. This has the effect that multiple AP&#39;s can be disposed nearby, even co-located, and can configured to operate in the same channel without leaving any coverage holes. However, one problem is that channel capacity is limited by interference between the set of devices using the same channel. 
     BRIEF SUMMARY 
     We provide techniques for deploying multiple access points on multiple wireless communication channels, without leaving any significant areas of limited coverage (sometimes referred to herein as “coverage holes”). 
     In a wireless communication system organized having multiple channels, we provide an association between AP&#39;s and those channels. The association maximizes coverage by AP&#39;s servicing stations attempting to use the communication system. This has the effect that each additional channel provides additional communication capability which additional AP&#39;s, and their associated stations, can collectively use once those additional AP&#39;s are assigned to those additional communication channels. This also has the effect that AP&#39;s can be physically close to prevent coverage holes in service, without interfering with each other. 
     The system control element collects information from devices in the wireless communication system, and automatically configures which AP is assigned to which mobile stations. The system control element determines which AP&#39;s are servicing which physical locations, in response to feedback from AP&#39;s and stations in those locations. The system control element assigns, or re-assigns, AP&#39;s in each physical location, with the effect of achieving maximum coverage. This has one effect that the system control element can balance loading on each communication channel by assigning or re-assigning stations to its choice of communication channel. The system control element can, from time to time, collect new information and assign or re-assign AP&#39;s to channels, with the effect of re-achieving maximum coverage even after conditions change. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conceptual diagram of a wireless communication system. 
         FIG. 2  shows a conceptual diagram of a method. 
     
    
    
     DETAILED DESCRIPTION 
     Terms and Phrases 
     The following terms and phrases include their most general applicable form. Each is described in a manner that is illustrative, not limiting. 
     The phrase “access point”, and the term “AP”, and variants thereof, generally refers to any device capable of coupling wireless messages between a wireless station and a wired communication medium. Access points might be coupled directly to a wired medium, such as for example a LAN or other wired network. Alternatively, access points might be coupled indirectly to a wired medium, such as for example using other access points, wireless repeaters, or otherwise. 
     In one embodiment, particular access points might be movable in regions related to a wireless communication system. For example and without limitation, the system might direct those particular access points to be moved to locations where communication is problematic, or to locations suitable for assisting in determining communication problems or remedies therefor. 
     The terms “assignment”, “association”, and variants thereof, when used with respect to AP&#39;s and channels, generally refer to any method or system in which those AP&#39;s are directed to, or otherwise operate to, communicate using those channels. For example, the system control element might assign an AP to a particular channel using a message sent to that AP, or by making an entry in an AP/channel table that the AP reviews, or otherwise. 
     The phrases “wireless communication”, “wireless network”, and variants thereof, generally refer to any method or system in which data or messages are sent, received, or maintained, using a wireless medium. For example and without limitation, the wireless medium might include electromagnetic radiation, such as radio frequencies. In one embodiment, wireless communication is performed using a communication protocol family such as IEEE 802.11, or a variant thereof. However, in the context of the invention, there is no particular requirement for any such limitation. 
     The phrase “wireless station”, and variants thereof, generally refers to any end-user device capable of communicating using wireless messages in a wireless communication system. For example and without limitation, wireless stations might include cellular phones, laptop computers, netbooks (such as the “iPad”), or other devices. Wireless stations might be mobile (and might sometimes be referred to herein as “mobile stations”) or might be stationary. 
     In one embodiment, particular access points might be directed by a wireless communication system to perform as wireless stations, exchanging messages with those access points performing the functions generally associated with access points; in such cases, those particular access points performing as wireless stations might be sometimes treated herein as if they were wireless stations. 
     Generality of the Description 
     Technologies shown or suggested by this description should also be thought of in their most general possible form. This includes, without limitation, the following: 
     The phrases and terms “constantly,” “continually,” “from time to time,” “occasionally,” “periodically” (and similar phrases and terms) generally indicate any case in which a method or technique, or an apparatus or system, operates over a duration of time, including without limitation any case in which that operation occurs only part of that duration of time. For example and without limitation, these terms would include, without limitation, methods which perform an operation as frequently as feasible, on a periodic schedule such as once per second or once per day, in response to an alarm or trigger such as a value reaching a threshold, in response to a request or an implication of a request, in response to operator intervention, otherwise, and to combinations and conjunctions thereof. 
     The phrases and terms “methods, physical articles, and systems,” “techniques” (and similar phrases and terms) generally indicate any material suitable for description, including without limitation all such material within the scope of patentable subject matter, or having ever been considered within the scope of patentable subject matter, or which might colorably be within the scope of patentable subject matter, notwithstanding most recent precedent. 
     The term “relatively” (and similar phrases and terms) generally indicates any relationship in which a comparison is possible, including without limitation “relatively less,” “relatively more,” and the like. In the context of the invention, where a measure or value is indicated to have a relationship “relatively,” that relationship need not be precise, need not be well-defined, need not be by comparison with any particular or specific other measure or value. For example and without limitation, in cases in which a measure or value is “relatively increased” or “relatively more,” that comparison need not be with respect to any known measure or value, but might be with respect to a measure or value held by that measurement or value at another place or time. 
     The term “substantially” (and similar phrases and terms) generally indicates any case or circumstance in which a determination, measure, value, or otherwise, is equal, equivalent, nearly equal, nearly equivalent, or approximately, what the measure or value is recited. The terms “substantially all” and “substantially none” (and similar phrases and terms) generally indicate any case or circumstance in which all but a relatively minor amount or number (for “substantially all”) or none but a relatively minor amount or number (for “substantially none”) have the stated property. The terms “substantial effect” (and similar phrases and terms) generally indicate any case or circumstance in which an effect might be detected or determined. 
     The phrases “this application,” “this description” (and similar phrases and terms) generally indicate any material shown or suggested by any portions of this application, individually or collectively, including all documents incorporated by reference or to which a claim of priority can be made or is made, and include all reasonable conclusions that might be drawn by those skilled in the art when this application is reviewed, even if those conclusions would not have been apparent at the time this application is originally filed. 
     The invention is not in any way limited to the specifics of any particular examples disclosed herein. After reading this application, many other variations are possible which remain within the content, scope and spirit of the invention; these variations would be clear to those skilled in the art, without undue experiment or new invention. 
     FIGURES AND TEXT 
     
       FIG. 1 
     
       FIG. 1  shows a conceptual diagram of a wireless communication system. 
     A wireless communication system  100  includes elements shown in the  FIG. 1 , including a set of wireless communication channels  110 , one or more AP&#39;s  120 , one or more stations  130 , and at least one system control element  140 . 
     The wireless communication channels  110  each include a physical communication layer, such as for example a physical communication layer used by an IEEE 802.11 protocol or a variant thereof. The physical communication layer provides devices coupled to the system  100  the ability to send and receive messages using a wireless substrate, such as for example using electromagnetic radiation in a radio frequency band. As described herein, there are multiple such channels  110  on which communication might occur separately without substantial interference, which might be separable using distinct CDMA codes, distinct frequencies, MIMO separation, distinct TDMA time slots, or otherwise. For example, in the 2.4 GHz ISM band used by IEEE 802.11 protocols there are 3 20-MHz channels, while in the 5 GHz band there are at least 4 40-MHz channels in most countries, and up to 12 40-MHz channels in some countries. 
     Each wireless communication channel  110  is conceptually represented in the figure as a separate network, as if each communication channel  110  were independently accessible by each AP  120 . In one embodiment, each communication channel  110  has an independent FDMA frequency, such as described above with respect to the 2.4 GHz band and the 5 GHz band used by IEEE 802.11 protocols, and each AP  120  includes a radio capable of being tuned to either (1) any one of those FDMA frequencies in the 2.4 GHz band, or (2) any one of those FDMA frequencies in the 5 GHz band. This has the effect that each AP  120  with a radio tunable in the 2.4 GHz band can be assigned, if desired, to any one of the communication channels  110  in the 2.4 GHz band, and each AP  120  with a radio tunable in the 5 GHz band can be assigned, if desired, to any one of the communication channels  110  in the 5 GHz band. 
     This also has the effect that multiple AP&#39;s  120  can communicate concurrently on distinct communication channels  110  without interference. For example, if two AP&#39;s  120  are each transmitting or listening on distinct communication channels  110  (such as for example, distinct frequencies), interference between those two AP&#39;s  120  should be substantially zero, allowing those two AP&#39;s  120  to each act as if the other AP  120  was not present. Thus, if two or more AP&#39;s  120  are substantially co-located, those AP&#39;s  120  might be assigned to distinct communication channels  110  so as to allow them to service stations concurrently without interference. 
     If an AP  120  has more than one radio, each radio can be assigned to a separate communication channel  110 . In one embodiment, this might be treated as if the AP  120  with more than one radio emulates a plurality of co-located AP&#39;s  120 , or might be alternatively treated as if the AP  120  with more than one radio is concurrently assigned to a plurality of communication channels  110 . Similarly, it might occur that a plurality of AP&#39;s  120 , collectively having more than one radio, or otherwise capable of collectively operating on more than one communication channel  110 , might cooperate so as to collectively emulate a plurality of co-located AP&#39;s  120 , or might cooperate so as to collectively act like a single AP  120  which is concurrently assigned to more than one communication channel  110 . 
     In one embodiment, an AP  120  might have multiple radios, each of which is configured to operate on a particular frequency band. For example, one particular frequency band could be either the 2.4 Ghz band or the 5 Ghz band described above, or possibly some additional or other frequency band that might be envisaged in the future. In this example, the AP  120  with multiple radios can be considered as if it included more than one collocated AP  120 , each of which with a single radio capable of operating on its own, separately configured, frequency band. Subsequently, distinct sets of AP&#39;s  120  are formed in every frequency band such that the AP&#39;s  120  from each of those distinct sets operate on the same frequency band. The method described herein can then be applied separately on each of the sets of AP&#39;s  120 , with the effect of arriving at an optimal assignment of radios to channels within each particular frequency band. 
     While this Application primarily describes systems in which each AP  120  has only a single radio and is therefore capable of communication on only a single communication channel  110  at once, in the context of the invention, there is no particular requirement for any such limitation. 
     While this Application primarily describes techniques in which each AP  120  is assigned to a single communication channel  110  (or a single communication channel  110  for each of its radios), in the context of the invention, there is no particular reason for any such limitation. For example, one or more AP&#39;s  120  might be directed to switch between or among two or more communication channels  110 , with the effect that those AP&#39;s  120  might listen on more than one communication channel  110 , although not simultaneously, and with the effect that those AP&#39;s  120  might transmit on more than one communication channel  110 , although not simultaneously. AP&#39;s  120  directed to act in that manner might switch communication channels  110  periodically, or from time to time in response to one or more stimuli. 
     However, it might be the case that not all of the communication channels  110  can be reached by all of the AP&#39;s  120 . For example, as described above, in one embodiment it occurs that some AP&#39;s  120  are only able to reach the communication channels  110  in the 2.4 GHz band, and lack ability to reach the communication channels  110  in the 5 GHz band, while other AP&#39;s  120  are only able to reach the communication channels  110  in the 5 GHz band, and lack ability to reach the communication channels  110  in the 2.4 GHz band. The control element  140  would assign those AP&#39;s  120  only to communication channels  110  they are actually able to reach. For example, the control element  140  might attempt to determine a covering set for the wireless communication system  100  by first restricting its attention to AP&#39;s  120  to a particular frequency band. 
     While this Application primarily describes systems in which each AP  120  has is capable of communication on all the frequencies of a particular frequency band (e.g., the 2.4 GHz band or the 5 GHz band), in the context of the invention, there is no particular requirement for any such limitation. As described below, the control element  140  is generally able to assign AP&#39;s  120  to any communication channel  110 , and will generally make those assignments in response to physical proximity and other coverage factors. 
     The control element  140  determines, from time to time for each station  130 , which AP  120  will communicate with that station  130  and service it. This has the effect that each station  130  sees each channel  110  as a single communication system, through which the station  130  might move about without having to attend to handoffs between AP&#39;s  120 . Even if the station  130  does not physically move, the control element  140  can re-assign the station  130  to a different AP  120  where that might provide improved service to the station  130 . For a first example, in electromagnetic conditions might change, such as due to increased or decreased amounts of interference from other devices. For a second example, the number of stations  130 , or the demand placed on the communication system by stations  130 , in the vicinity of AP&#39;s  120  might change over time, to which the control element  140  can respond by re-associating AP&#39;s  120  with stations  130 , such as possibly to balance the load on AP&#39;s  120  to amounts commensurate with what those AP&#39;s  120  can handle. 
     Each AP  120  reports the RSSI value it has for each station  130  it can hear, even if that station  130  is not then currently assigned to that AP  120 . This has the effect that the control element  140  can determine whether a station  130  would be better off re-assigned to a new AP  120 . In one embodiment, the control element  140  makes the determination of whether to re-assign any particular station  130  in response to the RSSI value for each nearby AP  120 , as well as in response to a measure of loading for each of those nearby AP&#39;s  120 . This has the effect that the control element  140  can assign stations  130  to the AP  120  best able to service them, where “best able to service” can include factors such as how good the communication path they have with that AP  120  and whether that AP  120  has sufficient resources to service them. 
     While this Application primarily describes a communication system  100  in which AP&#39;s  120  report their RSSI values, in the context of the invention, there is no particular requirement for any such limitation. 
     For a first example, AP&#39;s  120  might also report additional information to the control element  140 , possibly including amount of traffic at or near each AP  120 , number of stations at or near each AP  120 , activity by each station  130 , and other information. The control element  140  might use this additional information to determine which AP&#39;s  120  are likely to become overloaded, or which AP&#39;s  120  are likely to involve reassignment from one communication channel  110  to another to effect load-balancing. Load balancing might be desirable either to maintain a relatively even load across communication channels  110 , or to maintain a relatively even load across AP&#39;s  120 , or to maintain a relatively even degree of service to stations  130 , or otherwise. 
     For a second example, the communication system  100  might include additional sensing devices  121  whose purpose is to sense information about operation of the communication system  100  and report that information to the control element  140 . Those additional sensing devices  121  might report similar information as described with respect to AP&#39;s  120 . Additional sensing devices  121  might be relatively simple devices in comparison with AP&#39;s  120 , as they need only listen to traffic they can hear and report statistics regarding that traffic to the control element  140 , without needing to actually service any stations  130 . 
     For a third example, the control element  140  might query stations  130  for RSSI values those stations  130  report with respect to their assigned AP&#39;s  120 , and might obtain similar additional information as described with respect to AP&#39;s  120 . The control element  140  might require all stations  130  to report RSSI values or additional information, or might select only a subset of stations  130  to make such reports, or might distinguish between stations  130  which report more information and stations  130  which report less information. Similarly, the control element  140  might direct some stations  130  to report to their servicing AP&#39;s  120 , might direct some stations  130  to report to non-servicing AP&#39;s  120 , might direct some stations  130  to report to additional sensing devices  121  as described above, or otherwise. 
     Each AP  120  has an associated set of stations  130  serviced by that AP  120 , for which the RSSI value is relatively good, that is, the ability of that AP  120  to communicate with the stations  130  it is assigned to service is relatively good compared with other stations  130 . While stations  130  are often assigned to AP&#39;s  120  in response to their distance from that AP  120 , in the context of the invention, there is no particular reason for any such limitation. For example, it might commonly occur that walls and other obstacles would alter the ability of an AP  120  to hear stations  130  that are physically proximate, with the effect that the group of stations  130  assigned to a particular AP  120  might have an unusual boundary at or near an obstacle. 
     The control element  140  receives status messages from each AP  120 , those status messages reporting at least an RSSI (received signal strength indicator) value for each station  130  that AP  120  can hear. The control element  140  interprets those status messages, or has those status messages interpreted for it by an auxiliary device, and maintains those RSSI values in an data structure  141 . The data structure  141  might be maintained either at the control element  140 , or at another device accessible by the control element  140 . 
     
       FIG. 2 
     
       FIG. 2  shows a conceptual diagram of a method. 
     A method  200  includes a set of flow points and labels and shown in the figure, including at least: a flow point  200 A indicating a beginning of the method, a flow point  210  that the method is ready to assign AP&#39;s  120  to a first channel  110 , a flow point  220  indicating that the method is ready to assign AP&#39;s  120  to a next channel  110 , and a flow point  200 B indicating an end of the method. 
     Beginning of Method 
     The flow point  200 A indicates a beginning of the method  200 . In one embodiment, the method  200  is triggered from time to time, such as (1) whenever the control element  140  determines that the wireless communication system is not best using its channel capacity, (2) whenever a sufficient time passes, or otherwise. 
     At a step  201 , the method  200  initializes its assignment of all AP&#39;s  120  to a 1st communication channel  110 , and initializes a “goodness-index” value to zero for each AP  120 . The “goodness-index” value generally indicates whether the AP  120  is properly assigned to the communication channel  110  to which it is already assigned. 
     As described herein, the method  200  begins with all AP&#39;s  120  generally assigned to the same communication channel  110 . However, in the context of the invention, there is no particular requirement for any such limitation. For example, as noted above, it might occur that some AP&#39;s  120  are limited to only a particular subset of communication channels  110 . It might be advantageous for the control element  140  to begin the method  200  with those AP&#39;s  120  whose operating capability is limited to start assigned to one of the limited communication channels  110 , such as for example, a limited set of operating frequencies, which they are capable of using. 
     At a step  202 , the method  200  collects a set of RSSI values reported by each AP  120  for each station  130  it can hear. The collected RSSI values are maintained by the method  200  in a data structure  141 . In one embodiment, the data structure  141  might be maintained at the control element  140 , such as in memory or mass storage coupled to a processor performing the operations associated in this Application with the control element  140 . In alternative embodiments, the data structure  140  might be maintained external to the control element  140 , but still accessible to the control element  140 , such as in a network attached storage element coupled to the control element  140 , or such as in a Internet-accessible database server with the control element  140  coupled to the Internet. 
     In one embodiment, the AP&#39;s  120  each send one or more messages to the control element  140  reporting the RSSI values they each have to report. In embodiments in which the data structure  141  is maintained at the control element  140 , the control element  140  records information from those messages in the data structure  141 . In alternative embodiments in which the data structure  141  is not maintained physically at the control element  140 , but is accessible by the control element  140 , the AP&#39;s  120  might send messages to a device where the data structure  141  is maintained, or might modify the data structure  141  themselves to add their information. Either way, the data structure  141  maintains the RSSI values reported by each AP  120 . 
     The method  200  proceeds from the flow point  210 . 
     Covering Subset 
     A flow point  210  indicates that the method  200  is ready to assign AP&#39;s  120  to a first channel  110 . 
     As described below, the method  200  determines a subset of the AP&#39;s  120  that are sufficient to provide coverage, on the first channel  110 , to as many stations  130  as possible. Once this subset of AP&#39;s  120  is determined, the method  200  can assign the remaining AP&#39;s  120  to one or more additional channels  110 . 
     At a step  211 , for each station  130 , the method  200  finds the “best” AP  120  for that station  130 , defined for this step as the AP  120  which has the best (that is, highest) RSSI value for that station  130 . Having found the best AP  130  for that station  130 , the method  200  finds a second-best AP  120  for that station  130 , and computes a difference between the best RSSI value and the next best RSSI value. The method  200  assigns the RSSI difference, that is, the difference between the best RSSI value and the next best RSSI value, to the best AP  120  for that station  130 . 
     The method  200  performs this step for each station  130 . This has the effect of determining, for each station  130 , its best AP  120  and the RSSI difference for that best AP  120 . This also has the effect of determining, for each AP  120 , its RSSI difference for each station  130 . 
     At a step  212 , for each AP  120 , the method  200  finds the highest value of its RSSI difference over all stations  130 , that is, the highest RSSI difference found when its RSSI value for all stations  130  are accounted for. This has the result that, if a particular AP  120  is the best AP  120  for only one particular station  130 , its highest RSSI difference will be its RSSI difference for that one station. If a particular AP  120  is the best AP  120  for two or more stations  130 , its highest RSSI difference will be the maximum of its RSSI differences for those two or more stations  130 . 
     For each AP  120 , the method  200  records its highest RSSI difference as a new goodness-index for that AP  120 . Thus, the recorded goodness-index for an AP  120  will be the best RSSI difference the method  200  can find, for all stations  130  reported for that AP  120 . To avoid the possibility of spurious values being reported, which might occur in response to noise or other fluctuations in RSSI values, the method  200  only records a new value for an AP&#39;s goodness-index when it obtains two consecutive new values for goodness-index higher than the most recent goodness-index, and when so, records the lower of those two consecutive new values as the AP&#39;s new goodness-index. 
     At a step  213 , the method  200  marks all AP&#39;s  120  whose goodness-index is at least a threshold value of 3 dB as being assignable to the current channel  110 . While this Application primarily describes a method  200  which uses 3 dB as a threshold value for assigning AP&#39;s  120  to the current channel  110 , in the context of the invention, there is no particular requirement for any such limitation. For a first example, the method  200  might use 2 dB or 4 dB for the threshold value, or any other selected value. For a second example, the method  200  might select the threshold value in response to a number of available communication channels  110 , in response to a best RSSI difference among all AP&#39;s  120 , in response to a number of AP&#39;s  120 , or otherwise. 
     In one embodiment, when the method  200  assigns an AP  120  to a particular communication channel  110 , the control element  140  records an association between that communication channel  110  and that AP  120  in a data structure, and sends a message to that AP  120  directing that AP  120  to use that communication channel  110 . The data structure in which that association is recorded can be the same data structure  141 , or alternatively, another data structure. In alternative embodiments, the AP&#39;s  120  can review that data structure for themselves, with any particular requirement that the control element  140  send messages to those AP&#39;s  120 . 
     When an AP  120  is assigned to a particular communication channel  110 , it generally listens for stations  130  desiring to receive or send messages using that communication channel  110 , and sends its messages using that communication channel  110 . However, the AP  120  might also listen to messages on a secondary communication channel  110  for other reasons. For example, the AP might listen on a secondary communication channel  110  for “emergency” or otherwise high-priority messages, or might listen on a secondary communication channel  110  for messages from other AP&#39;s or from the control element, or might listen on a secondary communication channel  110  for collecting information about traffic in the communication system  100 , or otherwise. 
     At a step  214 , the method  200  finds the one AP  120  with the lowest goodness-index and assigns that AP  120  to the next channel  110 . If there is more than one AP  120  with that lowest goodness-index value, the method  200  selects one of them at random with equal probability. While this Application primarily describes a method  200  which selects one of those AP&#39;s  120  at random, in the context of the invention, there is no particular requirement for any such limitation. For example, the method  200  might select one of those AP&#39;s  120  in response to another factor, such as a proximity to other AP&#39;s  120 , or proximity to stations  130 , or otherwise. 
     At a step  215 , the method determines if there are any remaining AP&#39;s  120  whose goodness-index is less than the threshold value of 3 dB. If so, the method  200  returns to the flow point  210 , and repeats the steps from the flow point  210  to this point, that is, the steps  211 ,  212 ,  213 , and  214 , until there are no more such AP&#39;s  120 . When this occurs, the method  200  has completed assigning AP&#39;s  120  to the current channel  110 , and moves to the next channel  110 . 
     When the step  215  is finished, the subset of AP&#39;s  120  assigned to the current channel  110  include AP&#39;s which can operate on the current channel  110 , and which provide substantially as much coverage as the entire original set of AP&#39;s  120 . With respect to the current channel  110 , those AP&#39;s  120  remaining for the next channel  110  are substantially redundant. The method  200  repeats the same technique for each next channel  110  in turn, until all AP&#39;s  120  have been assigned to a channel  110 . 
     The method  200  proceeds from the flow point  220 . 
     Next Channel 
     A flow point  220  indicates that the method  200  is ready to assign AP&#39;s  120  to a next channel  110 . 
     At a step  221 , with respect to the AP&#39;s  120  that were not already assigned to the first channel  110 , the method  200  repeats the steps between the flow points  210  and  220 , that is, the steps  211 ,  212 ,  213 ,  214 , and  215 , for each channel  110  in turn. 
     Thus, if there are 4 available channels  110 , the method  200  performs those steps for the first channel, selecting a first subset of AP&#39;s  120  to assign to the first channel and a remaining group of AP&#39;s  120  which can be assigned otherwise. The method  200  repeats those steps for the second channel, selecting a second subset of AP&#39;s  120  to assign to the second channel and a remaining group of AP&#39;s  120  which can be assigned otherwise, and so on, until all remaining AP&#39;s  120  are assigned to the last available channel  110 . The latter condition occurs when there are no AP&#39;s  120  in the current group being considered which are redundant and thus which can be moved to a next channel  110 . 
     Redundant AP&#39;s 
     If the method  200  continues to have redundant AP&#39;s  120  but has no more channels  110  to which to move those redundant AP&#39;s  120 , the method  200  can assign those redundant AP&#39;s  120  to support the AP&#39;s already assigned to a channel  110 . For example, the method  200  might (1) assign those redundant AP&#39;s  120  to a channel  110  which could use backup in the event one of its AP&#39;s  120  fails to operate, (2) just leave those redundant AP&#39;s  120  on the channels  110  where they are already located, or (3) retry the channel  110  assignments with a different requirement, such as 2 dB instead of 3 dB. 
     After reading this application, those skilled in the art would notice that the subset of AP&#39;s  120  assigned to the first channel  110  provide as much coverage as the entire original set of AP&#39;s  120 , and that the AP&#39;s  120  which remain for assignment to later channels  110  might not have as good coverage, they can still provide extended capacity by use of those later channels  110 . Similarly, for each channel  110 , the subset of AP&#39;s  120  actually assigned to that channel  110  provide as much coverage as the entire group of AP&#39;s  120  remaining for assignment to that channel  110  or later channels  110 . 
     The method  200  proceeds from the flow point  200 B. 
     End of Method 
     A flow point  200 B indicates an end of the method  200 . In one embodiment, the method  200  is repeated, from time to time, from the beginning at the flow point  200 A.